Author: Richard Johnston

The FEV1 and FEV1/FVC ratio seems to have become the predominant, if not the sole factor for determining the presence of airway obstruction. It is true that a reduced FEV1/FVC ratio provides a strong and reliable signal for this purpose but its limitations have also been recognized for quite a while. The most obvious one is that the FEV1/FVC ratio will be falsely elevated when the FVC is underestimated. This is the primary factor driving the interest in FEV6 and the FEV1/FEV6 ratio. Less well appreciated is the fact that there are many causes and sites within the airways that can be involved in airway obstruction and that the focus on the FEV1/FVC ratio may cause certain forms of airway obstruction to be overlooked.

The FEF25-75 (aka MMEF) was originally proposed as way to determine the presence of small airways disease but it has since been shown to be an unreliable indicator. Most of the pulmonary physicians I work with have expressed doubt that there is such a thing as small airways disease but that doesn’t mean that some patients don’t have mild airway obstruction that is not evident when assessed solely by the FEV1/FVC ratio.

One of the ways to think about what happens during an exhalation is a concept called time constants. It is all too easy of to think of the lung as a single unit when of course it actually has five lobes, tens of thousands of acini and millions of alveoli. Airway obstruction does not occur homogeneously across the lung, but in different amounts in different areas. An area of the lung that empties quickly has a short time-constant. One that empties slowly, because of airway narrowing or a loss of elasticity, has a long time-constant.

Even the healthiest lungs will have a few areas with long time constants, but when the lungs as a whole have a short time-constant then they will empty quickly and this will be shown by a normal flow-volume loop and volume time curve. As lung disease progresses the number of lung units with a long time-constant start to predominate and the exhalation time increases. This is shown by a progressively more obstructive pattern on the flow-volume loop and volume-time curve.

It is the terminal expiratory flow rates, those that occur after the FEV1, that are affected earliest when the number of lung units with long time-constants increases. It is for this reason that a number of investigators have proposed using the FEV3/FVC ratio both as a substitute for the FEF25-75 and as an indicator of early and mild airway obstruction.

Normal values for the FEV3 and FEV3/FVC ratio were first proposed several decades ago but more recently Hanson et al re-analyzed the NHANESIII database and developed a set of Adult reference equations for the FEV3/FVC ratio:

Gender	Ethnicity	FEV3/FVC	LLN
Males	White	100.63 – (0.1692 * age)	95.00 – (0.1692 * age)
	Black	100.99 – (0.1699 * age)	96.03 – (0.1699 * age)
	Latin	101.02 – (0.1773 * age)	96.58 – (0.1773 * age)
	All	100.86 – (0.1756 * age)	95.56 – (0.1756 * age)
Females	White	102.41 – ( 0.1826 * age)	96.56 – ( 0.1826 * age)
	Black	100.86 – (0.1568 * age)	95.16 – (0.1568 * age)
	Latina	101.74 – ( 0.1740 * age)	96.78 – ( 0.1740 * age)
	All	101.83 – (0.1782 * age)	96.02 – (0.1782 * age)

Studies have indicated that over 10% of patients with a normal FEV1/FVC ratio have an abnormal FEV3/FVC ratio. Investigators have also shown that these patients tend to show an overall pattern of mild airway obstruction. The FEV1 and FEV1/FVC ratio, although within normal limits, tends to be at the lower limits of normal. The RV and RV/TLC ratio tends to be mildly elevated and the DLCO tends to be at or slightly below the normal cutoff.

My own experience with the FEV3/FVC ratio is somewhat equivocal because during a trial period several years ago I rarely saw a patient with a reduced FEV3/FVC ratio that didn’t also have a reduced FEV1/FVC ratio. In retrospect however, it is evident I was comparing apples to oranges. Specifically, my lab was not using the NHANESIII reference equations at that time and we use an FEV1/FVC ratio that is 95% of predicted as the cutoff for normalacy. The NHANESIII LLN for the FEV1/FVC ratio is roughly 89% of the predicted FEV1/FVC ratio which means that if we used the LLN we would have characterized fewer patients as being obstructed and I would likely have found more patients that met the criteria I was looking for.

During the trial period several years ago I was also unable to create any interest in the use of the FEV3/FVC ratio among the department’s physicians but this is likely due at least in part to the fact that only one significant paper on the use of the FEV3/FVC ratio existed at that time. Since then several more papers have been published, each of which has expanded and concurred with the original premise that a reduced FEV3/FVC ratio is an indicator of airway obstruction.

I think the FEV3/FVC ratio concept shows value but I also need to point out that it suffers from the same limitation that the FEV1/FVC ratio does which is that it depends on the accuracy of the FVC. For this reason I would like to see a study that looked at the FEV3/FEV6 ratio. I suspect that because the FEV6 is also a function of terminal flow rates that any decreases in the FEV3/FEV6 ratio may be too subtle to be of value but it would be useful to know nonetheless.

I would also like to see one or more longitudinal studies as well. This would help make it clearer whether the FEV3/FVC ratio is truly an indicator of early airway obstruction or not.

The FEV3/FVC ratio looks to be a useful adjunct towards assessing airway obstruction. It is hard to find fault with the premise that it is more sensitive to reductions in terminal expiratory flow than the FEV1/FVC ratio and therefore more sensitive to early or mild airway obstruction as well. Whether or not there is such a thing as small airways disease I think the FEV3/FVC ratio is clearly superior to the FEF25-75 and should replace that value both on reports and in the assessment process.

References:

Hansen JE, Sun X-G, Wasserman K. Discriminating measures and normal values for expiratory obstruction. Chest 2006; 129: 369-377.

Lam DCL, Fong DYT, Yu WC, Ko FWS, Lau ACW, Chan JWM, Choo KL, Mok TYW, Tam CY, Ip MSM, Chan-Yeung MM. FEV3, FEV6 and their derivatives for detecting airflow obstruction in adult Chinese. Int J Tuberc Lung Dis 2012; 16(5): 681-686.

Lutfi MF. Acceptable alternatives for forced vital capacity in the spirometric diagnosis of bronchial asthma. Int J Appl Basic Med Res 2011 1(1): 20-23.

Mehrparvar H, Rahimian M, Mirmohammadi SJ, Gheidi A, Mostaghaci M, Lotfi MH, Comparison of FEV3/FEV6, FEV1/FVC3 and FEV1/FEV6 with usual spirometric indices. Respirology 2012; 17: 541-546.

Morris ZQ, Coz A, Starosta D. An isolated reduction of the FEV3/FVC ratio is an indicator of mild lung injury. Chest 2013; 144(4): 1117-1123.

PFT Blog by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

I used to think that spirometry and diffusion capacity tests were hard and that lung volumes were easy. That may have been true in terms of getting patients to do the tests but I’ve long since come to the conclusion that it is easier to assess the quality of spirometry and diffusing capacity tests and know whether you have reasonably accurate results than it is to do this for lung volumes regardless of which lung volume measurement technique you use.

I was reviewing a set of plethysmographic lung volume tests when I noticed something very odd about the reported results. I usually look at just the VTG loops and the volume-time graphs in order to assess test quality. The testing software automatically selects and averages all VTG efforts and when I reviewed them there were a couple loops that were poor quality and I manually de-selected them. I was reviewing this report because the reported lung volume results didn’t quite match what the spirometry results were saying so this time I also took a close look at the numbers after I removed the low-quality loops. That’s when I realized that the reported TLC was larger than the two tests it was averaged from.

This actually made a difference in how the test results would be interpreted. The largest TLC of the two test efforts was 79% of predicted, slightly below the 80% cutoff we use for the normal range. The averaged TLC was 81% of predicted, slightly above the cutoff, so although it was suspiciously low, it was WNL.

How did this happen?

A close look at the numbers showed me that when the software averages two (or more) different tests, it averages the FRC and ERV but then uses the largest VC. This is completely in line with the ATS-ERS recommendations but also highlights one of the problems in interpreting results.

All lung volume tests, regardless of technique, actually measure FRC, the volume in the lungs after an unforced exhalation. A slow vital capacity test (SVC) is performed either immediately before or immediately after the FRC measurement. When the SVC results are compared to the location of FRC it is then possible to determine the Inspiratory Capacity (IC) and the Expiratory Reserve Volume (ERV). Lung volumes are then derived from:

RV = FRC – ERV

TLC = RV + VC

FRC volume is the result of the balance of forces between the lung and the rib cage. It can be affected by an individual’s breathing pattern and tends to vary slightly over time. This is normal and strictly speaking changes in FRC should not affect TLC or RV, but will instead primarily affect IC and ERV.

The first and second lung volume test efforts had mostly similar results. The FRC was only 0.18 L different, the TLC was only 0.06 L and the RV were only 0.17 L different, all of which are well within the guidelines for reproducibility. The biggest difference was between the SVC and ERV results which were 0.23 L and 0.36 L different, respectively.

Although this averaging process follows the ATS/ERS recommendations is this really the correct way to calculate TLC? I understand that because FRC can vary from test effort to test effort that it makes sense to average FRC results and that because IC and ERV are affected by changes in FRC is also makes sense to average them. I also understand that the largest SVC, since it is presumably the most accurate reflection of the patient’s vital capacity, should be used. What this overlooks is that ERV is as much affected by the quality of the SVC as it is by the FRC.

My general experience is that patients can usually perform the IC port of the SVC maneuver correctly almost every time. ERV on the other hand is the effort-dependent part of the SVC and depends on how much the patient is willing or able to push themselves. In this case a not-so-good ERV from the first effort was averaged with a significantly better ERV from the second effort. This blunted the quality of the reported ERV measurement and then when the largest SVC effort was added to it, it actually produced a higher reported TLC than in the test efforts it was averaged from.

I think that since RV is the amount of air left in the lung after a maximal exhalation the lowest value measured in any lung volume test effort could be considered the most accurate value. If this premise is accepted then when lung volume tests are averaged the lowest value of RV and the highest value of SVC should be used to calculate TLC. If this approach was taken, then for this example at least there wouldn’t have been a significant difference between test efforts and reported results. That doesn’t make it the correct approach, however.

The accuracy of any single lung volume measurement test is always suspect. This is why reproducibility is so important and why test efforts are averaged. The reported TLC is affected not only by the quality of the FRC and SVC measurements but also by the quality of the ERV measurement. Different assumptions in the averaging process will produce different TLC and RV measurements from the same set of test efforts. The problem is that assumptions are just that, assumptions.

At the moment I have no evidence that using the lowest RV rather than averaging ERV results leads to a more accurate TLC and in most instances this probably wouldn’t make a significant clinical difference. In this particular case however, the minor differences in TLC straddled the normal cutoff and it does make a difference.

Are you beginning to see why I think that spirometry and DLCO tests are usually easy in comparison?

References:

Brusasco V, Crapo R, Viegi G. ATS/ERS Standardisation of Lung Function Testing: Standardisation of the measurement of lung volumes. Eur Respir J 2005; 26: 511-522.

PFT Blog by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

The MIP test is used to assess the strength of the inspiratory muscles and is commonly performed in patients with neuromuscular disease. Results however, are often low because the sensation involved in performing a maximal inspiratory effort against an occluded airway is unpleasant and because the MIP requires coordination, cooperation and motivation to be performed correctly. In addition patients with neuromuscular disease frequently lack the muscular strength necessary to grip the mouthpiece and can therefore leak around it. For these reasons although a normal value will rule out significant muscular weakness, a low value can be difficult to interpret.

Sniff Nasal Inspiratory Pressure (SNIP) is an alternative way to measure inspiratory muscle strength that does not require a mouthpiece and uses a fairly natural maneuver. To perform the test one nostril is blocked with a soft probe attached to either a manometer or a transducer. The patient is asked to perform a normal exhalation to FRC and then inhale forcibly (sniff) with their mouth closed. The sniff effort is short since the maximum nasal inspiratory pressure is reached in a half a second or less.

Although soft nasal probes for SNIP measurements are now available from a couple of suppliers, probes can be made from re-purposed equipment such as an eartip intended for auditory evoked potentials or the nasal cushions from sleep apnea equipment. Prior to testing air leakage around the nasal probe should be checked by blocking the contralateral nostril while the patient attempts to breathe in. If there is a leak then either a different sized probe should should be used or the probe size adjusted with something like earplug wax.

There is a learning effect involved in performing the SNIP test. The number of SNIP trials needed to obtain the maximum SNIP value from a given patient has varied, with some investigators using only five while others have shown that up to twenty should be performed. There is a general consensus among most investigators that at least 10 SNIP efforts should be made and that if the maximum SNIP is obtained on the tenth effort then further trials should be made. SNIP efforts should be separated by at least 30 seconds.

In adults SNIP has been shown to be primarily related to gender and age. Height, weight and BMI are not factors and there is no significant difference in SNIP values between supine and upright positions. Normal values for adults are:

Males = 126.8 – (0.42 * age), LLN ~80 cm H2O

Females = 94.9 – (0.22 * age), LLN ~70 cm H2O

For children the results are more complex. SNIP was shown to correlate with height, age and weight with boy but not with girls. The reasons for this are not clear and because of this it may be better to compare results to the overall means and LLNs for children.

Boys (6-12 yrs): mean 99, LLN 63 cm H2O

Boys (13-17 yrs): mean 117, LLN 66 cm H2O

Girls (6-12 yrs): mean 92, LLN 56 cm H2O

Girls (13-16 yrs): mean 97, LLN 54 cm H2O

Investigators have noted that a SNIP below 40 cm H2O is significantly related to nocturnal hypoxemia and that a patient with a SNIP less than 30% of predicted is at risk for hypercapnia.

SNIP has been shown to reflect the sniff esophageal pressure (Pes) in most patients and SNIP is usually higher than MIP. Patients with COPD, however usually show a reduced SNIP and MIP when compared to Pes and this has been attributed to dampening of the pressure waves due to the increased airway time constants in COPD and the shortness of the sniff effort. Somewhat surprisingly this effect seems to be independent of the degree of airway obstruction.

Despite the fact that the SNIP test usually has somewhat higher and more reproducible results than the MIP, some patients are still better able to perform a MIP than a SNIP. Although sniff Pes is considered by some to be the gold standard for assessing respiratory muscle strength it was found to be no more sensitive or accurate than when both MIP and SNIP tests were performed in the same patient. For this reason an initial assessment of respiratory muscle weakness should probably include both SNIP and MIP.

Spirometry continues to be an important means of assessing the status and course of neuromuscular disease. Vital capacity however, can remain more or less normal until there is profound respiratory muscle weakness. SNIP has been found to be a better predictor of mortality in ALS than spirometry or MIP, but this may be associated at least in part with the fact that the SNIP can be performed in patients with advanced stages of the disease while MIP and FVC often cannot. Having said that investigators have reported that ALS patients with a SNIP less than 40 cm H2O was associated with a median survival rate of six months and a SNIP less than 30 cm H2O was associated with a median survival rate of three months.

SNIP nasal probes and instruments with software designed to measure both SNIP and MIP are available from at least a couple of manufacturers but there is no reason that (with the exception of manometers without recording needles) many instruments designed to measure MIP and MEP shouldn’t be able to be re-purposed towards measuring SNIP. Given the limitations of reporting software however, results may need to be clearly labeled or otherwise identified in testing notes.

The SNIP test produces information that is as relevant to the assessment of respiratory muscle strength as the MIP, MEP, FVC and Pes tests. Comparatively however, the SNIP is much easier to perform and much better tolerated by most patients. In addition patients that are unable to perform a MIP or FVC can usually perform a SNIP. This makes the SNIP far more suitable for children and for adults with progressive neuromuscular disease than the other tests. If you’re already MIP-ping, why aren’t you also SNIP-ping?

References:

ATS/ERS Statement on Respiratory Muscle Testing. Am J Respir Crit Care Med 2002; 166: 518-624.

Chaudri MB, Liu C, Watson L, Jefferson D, Kinnear WJ. Sniff nasal inspiratory pressure as a marker of respiratory function in motor neuron disease. Eur Respir J 2000; 15: 539-542.

Faroux B, Aubertin G, Cohen E, Clement A, Lofaso F. Sniff nasal isnpiratory pressure in children with muscular, chest wall or lung disease. Eur Respir J 2009; 33: 113-117.

Fitting J-W, Paillex R, Hirt L, Aebischer P, Schluep M. Sniff nasal pressure: A sensitive respiratory test to assess progression of Amyotropic Lateral Sclerosis. Ann Neurol 1999; 46: 887-893.

Heretier F, Rahm F, Pasche P, Fitting J-W. Sniff nasal inspiratory pressure. A noninvasive assessment of inspiratory muscle strength. Am J Respir Crit Care Med 1994; 150: 1678-1683.

Koulouris N, Mulvey DA, Laroche CM, Sawicka EH, Green M, Moxham J. The measurement of inspiratory muscle strength by sniff esophageal, nasopharyngeal and mouth pressures. Am Rev Respir Dis 1989; 139: 641-646.

Lafoso F et al. Sniff nasal inspiratory pressure: what is the optimal number of sniffs? Eur Respir J 2006; 27: 980-982.

Lyall RA, Donaldson N, Polkey MI, Leigh PN, Moxham J. Respiratory muscle strength and ventilatory failure in amyotrophic lateral sclerosis. Brain 2006; 124: 2000-2013.

Maillard JO, Burdet L, van Melle G, Fitting JW. Reproducibility of twitch mouth pressure, sniff nasal inspiratory pressure, and maximal inspiratory pressure. Eur Respir J 1998; 11: 901-905.

Morgan RK, McNally S, Alexander M, Conroy R, Hardiman O, Costello RW. Use of sniff nasal-inspiratory force to predict survival in Amyotrophic Lateral Sclerosis. Am J Respir Crit Care Med 2005; 171: 269-274.

Polkey MI, Green M, Moxham J. Measurement of respiratory muscle strength. Thorax 1995; 50: 1131-1135.

Steffanutti D, Fitting J-W. Sniff nasal inspiratory pressure. Reference values in Caucasian children. Am J Respir Crit Care Med 1998; 159: 107-111.

Steffanutti D, Benoist M-R, Scheinman P, Chaussain M, Fitting J-W. Usefulness of sniff nasal pressure in patients with neuromuscular or skeletal disorders. Am J Respir Crit Care Med 2000; 162: 1507-1511.

Stier J et al. Respiratory Muscles. The value of multiple tests of respiratory muscle strength. Thorax 2007; 62: 975-980.

Terzi N et al. Measuring inspiratory muscle strength in neuromuscular disease: one test or two? Eur Respir J 2008; 31: 93-98.

Uldry C, Fitting J-W. Maximal values of sniff nasal inspiratory pressure in health subjects. Thorax 1995; 50: 371-375.

Uldry C, Janssens JP, de Muralt B, Fitting JW. Sniff nasal inspiratory pressure in patients with chronic obstructive pulmonary disease. Eur Respir J 1997; 10: 1292-1296.

PFT Blog by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

The distance attained during a 6-minute walk (6MWD) has been used extensively to assess the functional capacity of patients with a variety of diseases and conditions. It is relatively easy to perform and requires a minimum of equipment. Changes in 6MWD before and after rehabilitation, surgery, or medications are often used to signal the success or failure of these therapies. For the last dozen years every drug and device research study my lab has been involved with has used the 6MWD as one of their outcomes.

The 6-minute walk distance has been noted to depend on the age, gender, height and weight of the individual. There are, however, relatively few studies to choose from when it comes to selecting normal values for the 6MWD and each of these studies differs not only in the degree of importance it assigns to these variables but in the predicted 6MWD. Despite the clinical significance of changes in the 6-minute walk distance it is far from clear what a normal 6-minute walk distance actually is.

It is hard to criticize the studies that have attempted to determine a normal 6MWD. The studies have all adhered to the ATS 6-minute walk test guidelines and most have used similar exclusion factors. Patients with an active involvement in a competitive sport, a chronic disease that would affect exercise capacity, a BMI > 35, a FEV1/FVC ratio below 0.70 or the use of beta blocker or calcium channel blocker medications have been excluded. Most subjects performed more than one 6MWT separated by at least a couple of days and the highest 6MWD was used in the analysis. The most obvious difference between studies were the ages of their subjects. For males, these ages and equations are:

Equation	Year	Author	No.	Ages
A	1998	Enright et al	117	43-77
B	1999	Troosters et al	29	50-85
C	2006	Chetta et al	48	20-50
D	2009	Iwama et al	61	14-84
E	2009	Alameri et al	127	18-50
F	2011	Casanova et al	238	40-80

Reference equations:

A	(ht x 7.57) – (5.02 x age) – (1.76 x wt) – 309
B	269.31 + (ht x 5.14) – (age * 5.78) – (wt x 2.29)
C	518.853 + (ht x 1.25) – (age x 2.816)
D	622.461 – (age x 1.846)
E	(ht x 2.81) – (age x 0.79) – 28.5
F	361 + (ht x 2) + (% max hr/pred max hr) – (age x 4) – (1.5 x wt)

Because females have been noted to walk a shorter distance than males it is apparent that gender does make a difference but in general the reference equations handle gender quite simplistically. With only one exception the same reference equations for men are used for females and a constant subtraction factor (which is applied to all ages, heights and weights) is included:

Equation	Factor
A	(ht x 2.11) – (wt x 2.29) – (age * 5.78) +611
B	-51.31
C	-39.07
D	-61.503
E	0
F	-30

It seems obvious that the other factors affecting the 6MWD would be age, height and weight. There is universal agreement that age is an important factor, but a look at the 6MWD regression equations shows that there is a fair amount of disagreement concerning its significance since the weighting factor ranges from 0.79 to 5.78. This, however, does not appear to be due to the different age ranges used in the studies since studies with similar age ranges can have quite different age factors and studies with quite different age ranges can have similar age factors.

It would also seem intuitive that height would affect stride length and would therefore affect an individual’s 6MWD but at least one study did not find height to be a significant factor. Among the remaining studies the height factor ranges from 1.25 to 7.57. The effect of these factors on individual reference equations are most apparent at the extremes of the normal height range.

  

Numerous studies have shown that the 6MWD is lower when subjects are obese, yet despite this half of the reference equations do not include body weight. This may be due to the fact that obese patients have been excluded from most reference studies so that differences in weight among the remaining participants is no longer statistically significant. This may be why the weight factor shows the least variation amongst the reference equations with a range of 1.5 to 2.29.

Several researchers have calculated a value called the 6MWD work (6MWDW or 6MWork) by multiplying the subject’s body weight (in kg) times the distance (in meters). Although obese patients have a lower 6MWD they usually have a larger 6MWork value than their leaner counterparts. A threshold value of 25,000 kg-M has been proposed where patients below this value have a significantly higher mortality risk. This is an interesting concept but at this time there are no reference equations for 6MWork.

Casanova et al studied the 6MWD from populations around the world and noted that there were significant differences between geographic areas that could not be explained by height, weight or age. The difference between reference equation C, which is from an Italian study group and E, an Arabian study group with reasonably similar ages, heights and weights is quite dramatic and this alone makes it clear that attempting to determine whether or not a 6MWD is normal is very problematic.

There are marked differences between the various 6MWD reference equations and the reasons for these differences are far from clear. At least part of this may be due to the relatively small number of participants in each research study which makes statistical analysis of the results more dependent on small differences in the study group. Gender, age, height and weight are obvious factors in determining a normal 6MWD but other unknown factors appear to be just as important.

In 2002, the ATS statement on the 6-minute walk test stated that “Optimal reference equations from healthy population-based samples using standardized 6MWT methods are not yet available.” Over a decade later this statement still applies and it still appears that the best and most appropriate use of the 6MWD is in serial comparisons for the same patient.

Update:

This topic was extensively updated with numerous additional reference equations in 6MWT re-visited. Now with the MCID!

References: 

Alameri H, Al-Majed S, Al-Howaikan. Six-min walk test in a health adult Arab population. Resp Med 2009; 103: 1041-1046.

ATS Statement: Guidelines for the six-minute walk test. Amer J Respir Crit Care Med 2002; 166: 111-117.

Casanova C, et al. The 6-min walk distance in health subjects: reference equations from seven countries. Eur Respir J 2011; 37: 150-156.

Carter R, Holiday DB, Nwasuraba C, Stocks J, Grothus C, Tiep B. 6-minute walk work for assessment of functional capacity in patients with COPD. Chest 2003; 123: 1408-1415.

Chetta A, et al. Reference values for the 6-min walk test in healthy subjects 20-50 years old. Respir Med 2006; 100: 1573-1578.

Cote CG, et al. Validation and comparison of reference equations for the 6-min walk distance test. Eur Respir J 2008; 31: 571-578.

Enright PL, Sherril DL. Reference Equations for the six-minute walk in healthy adults. Amer J Respir Crit Care Med 1998; 158: 1384-1387.

Iwama AM, Andrade GN, Shima P, Tanni SE, Godoy I, Dourado VZ. The six-minute walk test and body weight x walk distance product in healthy Brazilian subjects. Braz J Med Biol Res 2009; 42: 1080-1085.

Larsson UE, Reynisdottir S. The six-minute walk test in outpatients with obesity: reproducibility and known group validity. Physiother Res Int 2008; 13(2): 84-93.

Troosters T, Gosselink R, Decramer M. Six minute walking distance in healthy elderly subjects. Eur Respir J 1999; 14: 270-274.

PFT Blog by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

The late 1800’s through the early 1900’s saw the birth and spread of amusement parks, penny arcades and nickelodeons. Although this was due in part to an increase in the number of people living in cities and to an increase in disposable income, it was also in large part due to the invention of mass transit. In a bid to increase ridership many railroad, trolley and subway lines built or sponsored amusement parks.

At the amusement parks, along with the carousels, ferris wheels and roller coasters there was the penny arcade and in amongst the penny arcade’s slot machines, strength testers, music machines, scales, gumball machines and electric shockers were the coin-operated spirometers. Coin operated spirometers started off with simple dials and quickly became elaborate amusements unto themselves.

I have been able to find sixty different patents for spirometers or lung testers between 1860 and 1915. Nineteen of these patents are for spirometers that in one way or another were intended for medical use. Four are for lung “exercisers” that also claimed to measure lung capacity. Of the remaining thirty-seven patents, twenty-five are for coin-operated arcade spirometers, seven are for spirometer “toys” and five are for practical jokes.

Arcade spirometers used a variety of mechanisms for measuring vital capacity, the most common of which was a spring loaded piston inside cylinder. Very few of these penny arcade spirometers still exist but they do come up for auction occasionally.

Despite the fact that many of the arcade spirometers called themselves “hygeinic” they were probably far from that.  The mouthpieces of the free-standing coin-operated spirometers were not cleaned in between uses and the insides of the spirometers were likely never cleaned.

PFT Blog by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

A writer posed an interesting question on the AARC Diagnostics forum several weeks ago and that was how to market their PFT Lab. I don’t think they got much of a response but I have been thinking about this since then.

I think that any good lab manager wants to see their lab succeed and grow. I’ve always felt that pulmonary function testing is an essential component of preventive care but that despite this PFT Labs are underutilized. In order to market your PFT Lab effectively you need to understand your customers and target your message accordingly. You also need to understand that you can’t get something for nothing. Marketing requires that you expend resources, whether it is just your time or includes lab budget money, in order to get any payback.

There are three target audiences for your marketing; patients, physicians and administrators. Each audience has a different question you must be able to answer. For patients the question is going to be “why do I need pulmonary function testing?”. For physicians it is going to be “why should I send my patients to your lab?” and for administrators it is “why should I devote resources to your lab?”.

I think there are two specific ways in which you can market to patients. The first and most direct way is to perform spirometry at a health fair. This most definitely requires a commitment of budgeted lab resources. You will need a portable spirometer and a printer. You will need the time of one or more technicians. You will also need disposable filter mouthpieces and nose clips (this is not a place to get cheap since a health fair is not the place you want to be perceived as being un-hygeinic).

My experience with health fairs is that once you attract the first person the action and coaching involved in spirometry tends to attract other people rapidly. At a well-attended health fair you can quickly become overwhelmed so you may want to consider sending more than one technician to perform spirometry. Even if you only have one spirometer, the second person can get the name, height and date of birth of the next person in line and be ready to enter it into the spirometry system as soon as the report from the previous system is being printed out.

It is worth giving some thought to the spirometry report you use at a health fair. It should have your Lab’s address and phone number of course, but I have mixed feelings about including a computer diagnosis. Spirometry interpretation algorithms are still quite simplistic and the quality of spirometry at health fairs tends to be somewhat limited. Moreover, you may not want to be involved in a prolonged discussion with an individual when their results are abnormal. Despite this, a computer-generated interpretation can be an important talking point between a physician and a patient. Regardless of whether you do or do not include one, I think it is important that somewhere on the report, in a large and bold typeface is a disclaimer to the effect that “This test was performed for informational purposes only and should not be relied upon to diagnose any medical conditions”.

Another set of critical items that should brought to a health fair are informational brochures and this is the second way to market to patients. Your lab should have a variety of informational brochures about different lung diseases. At the very least you need to cover the most common lung diseases such as asthma, COPD, lung cancer and pulmonary fibrosis as well as brochures about smoking and air pollution. There are a variety of organizations that you can get brochures from but be forewarned that almost all of them charge a fee for their brochures. In addition these brochures are also intended to market the organizations they come from, not your lab. If you use these kinds of brochures you should get a rubber stamp or a label with a “provided by” and your lab’s contact information and make sure this information is added to each brochure.

Should you create your own brochures? It is relatively easy to create tri-fold brochures using a standard word processor and photocopying is a lot less expensive than buying brochures would be (at least as long as it is just text or simple illustrations, color photocopying is still relatively expensive). Creating brochures can also be an interesting educational project for you and your staff and in addition they have the potential to be tailored to the needs your specific patient base (age, ethnicity, region or disease mix), but be careful about plagiarizing content. If you do “borrow” content, be sure to acknowledge it in a footnote somewhere on the brochure. Never take credit for content you did not create because this can come back to bite you in any number of ways.

Physicians are interested in quite different things than patients, and much of this interest has to do with how well your lab is run. Physicians deal frequently with a number of different medical professionals and so a large part of marketing to physicians will be establishing and maintaining a professional and consistently reliable laboratory.

First, physicians want to be able to schedule appointments promptly. I will agree that there is probably no such thing as an “emergency” PFT but if your lab is constantly booking several weeks out then how likely are physicians to send patients to you? On average at least 10 percent of my lab’s patients are same-day add-ons. Patients often come a distance (in time if not in miles) and if you cannot see them as part of their physician visit, then you are unlikely to get them otherwise. For this reason, when our schedule was tight, which has happened mostly when resources (equipment or technicians) were scarce for one reason or another, no matter how much it “hurt” we always kept at least one and usually two appointment slots open and only filled them with same-day patients. We’ve found that these slots usually get filled and because they usually get filled with “extra” patients this always helps the bottom line for the lab.

Physicians want reports to be prompt as well. I realize that many aspects of reports (in particular the interpretation and physician signature) are outside the PFT Lab’s ability to control. This doesn’t mean that you can’t offer to email or fax preliminary results to the ordering physician, or to print a report for the patient to take with them to their next appointment. My lab does this automatically for all oncology and neurology patients, and we offer it to all other physician offices when a patient’s appointment is made. This is something that is easy to do and helps demonstrate both your lab’s flexibility and that you understand the physician’s needs.

How well your staff treats patients will matter to physicians, too. The baseline assumption is going to be that patients are treated well. Physicians will understand that patients may not like having to perform pulmonary function tests but if patients also consistently report that your staff were rude, condescending or unpleasant this will reflect negatively on your lab’s professional image.

For direct physician marketing the most I would suggest would be to create a standard “Dear Doctor” letter with your lab’s contact information (phone number for scheduling, manager’s name, phone number and email address), your lab’s location (mailing address, physical location, nearest parking lot) the tests you perform (highlighting anything out of the ordinary such as supine spirometry, methacholine challenges, HAST) and any special services (emailing or faxing preliminary reports, providing oxygen to patients during testing). I would send this letter to any new physicians in your service area that have the potential to refer patients to your lab.

You should be keeping an email and/or a snail-mail address list of the physicians you serve. You can send an update about your services (new equipment, new tests, new locations, changes in hours etc) to all the physicians you serve but only when you have a real update. Sending updates too frequently with inconsequential information is a good way to get ignored.

Although administrators are responsible for many things, the most important interaction with the PFT lab is going to be the budget. Any hospital or clinic has limited resources and an administrator is going to be under constant pressure to allocate these resources effectively. The PFT Lab must compete with other departments for these resources so marketing the PFT Lab to an administrator has to start by showing you are able to speak their language. That means you must know your expenses and your revenue, and you should be able to perform a cost analysis on any service you provide. Since this is an area where your administrator has access to the same information you do you will not be able to hide problems but the more you understand your lab’s finances the more likely you will be able to put a positive spin on events.

Regularly marketing your PFT Lab to your administrator is necessary to maintain support for the lab, but it becomes critical when you are considering expanding services, increasing staffing or looking to acquire capital equipment. The capital budget is often a very idiosyncratic (and political) process and your administrator can often help – or hurt – your ability to navigate through the minefields. Keep in mind that you have several things in your favor:

• PFT Labs almost always primarily serve outpatients, and outpatient revenue is quite different (and better!) than inpatient revenue.
• As a testing facility, PFT Labs tend to have a relatively high revenue to expense ratio.
• Hospitals and clinics are being rewarded for keeping patients out of the hospital and a PFT Lab is a cost effective use of resources in the management of patients with lung disease. 

Use these facts when marketing your administrator and don’t forget to mention any of the marketing efforts you are making towards patients and physicians.

A final thought about marketing is to ask what kind of a presence your PFT Lab has on the internet. Everyone is more and more reliant on the internet to find the information they are looking for. My lab has its contact information (lab and manager), hospital location (building and room numbers) and a listing of the tests it performs on the hospital website. It can be reached both through a straightforward menu structure (departments → Pulmonary → PFT Lab) and through a search function. The same cannot be said of many other hospitals. There are several PFT Labs that I personally know to exist but even after a diligent search I have been unable to find any listing whatsoever on their hospital’s website. Part of marketing is making it easy for customers (patients and physicians) to find you so don’t ignore your place on the internet as part of this.

Marketing is an essential cornerstone of business. A Pulmonary Function lab is not exactly a business but there are enough similarities that marketing is important to not only maintain the health of your lab but to see that its services are available to all who need them. Part of marketing your lab is going to be outreach, part is going to be maintaining a professional, high-quality service and part is going to be talking the right language. Marketing is something you will need to devote your (limited and valuable) time and your lab’s (limited and valuable) resources to but the payback for doing this will be seeing your lab thrive.

 

PFT Blog by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

The beginning of pulmonary function testing can best be dated to 1846 when John Hutchinson invented the spirometer. Although similar devices had been used by previous scientists he turned it into a precision instrument and approached the study of lung function systematically. His work inspired innumerable other researchers and inventors, and within a few years improved versions of his spirometer were appearing across Europe and America. By my count within twenty years there were at least four other methods for measuring expiratory volume (at least one of which is still in use) had been applied to a new generation of spirometers.

The initial focus of spirometry was entirely on exhaled volume and the use of spirometers for any other purpose did not begin to change until the 1930’s when the MVV test was proposed and formalized. During this time the accuracy of spirometers improved and many serious researchers laid the groundwork for our current understanding of pulmonary medicine and physiology. This was also a period of time in which what we now call quack medicine flourished. A quick glance in almost any of the newspapers or magazines of that time will show a variety of ads extolling the virtues of different medicines, treatments and devices, most of which now appear to be silly, ineffective or just plain dangerous. As much as we might like to ignore or belittle this sordid period in our medical history, the truth is that spirometers were also one of these quack medicine devices.

The longest-lived one of these was the Barnes Spirometer which was first invented in 1865 by Aaron P. Barnes.

A description from 1866: “The Lung Tester of A.P. Barnes (to be had of Messrs. Codman & Shurtleff, Boston) is the simplest and cheapest of all spirometric instruments. It consists of a cylindrical bag of India-rubber cloth, closed at each extremity by a disk of wood, and furnished with two metallic tubes; one tube enters laterally at the bottom, and is about three inches long; the other, vertical, is about twelve inches long, and graduated, and inserted in the centre of the upper disk. A flexible tube of proper length, with a mouthpiece is stretched over the outer aperture of the lower metal tube, and through this a forced expiration is made; the expired air fills, more or less, the bag and the vital capacity is recorded on the upper tube, which is forced up as the bag expands. The bag is enclosed in a tin cylinder, shut at both ends with two holes for the tubes.”

It was, needless to say, not terribly accurate. Several academics of that time pointed this out and one suggested that its only use was to track lung function for a single individual (much like personal peak flow meters now). It was sold, however, as a device for exercise or treatment and not as a scientific instrument. We now know that expiratory muscle training benefits only a small number of patients so its ability to provide any benefit to its users, particularly its claim to prevent or cure consumption (tuberculosis) were pretty much nonexistent.

It is hard to blame the people of this time period for buying these and many of the other quack devices, medications and treatments. The practice and understanding of medicine was still primitive and there were no really effective treatments for any respiratory diseases so I’d guess that any chance was better than none. (And in all honesty, tuberculosis and asthma were poorly understood for a long time and a wide variety of causes and cures were proposed at various times by many notable medical practitioners. If physicians were confused, how could laymen do any better?)

There were a couple other spirometers also sold as exercise devices and cures for a variety of ailments. The most notable of these were the Simplex Spirometer and the Lewis Spirometer.

 

The Barnes Spirometer both pre-dated and outlived them all and was still being sold and still promising many of the same benefits of its use up until the mid-1920’s.

None of these however come close to making the totally outrageous claims that were made by Matthieu Souvielle, a Canadian con man par-excellence. His “spirometer” cured everything and he had the testimonials to prove it. Here are two parts of a full-column ad from a Geneva, NY newspaper from 1880:

I have found similar ads in newspapers from Lowell, Massachusetts, Toronto, Ontario and Hamilton, Bermuda, and I am sure they were placed in many other newspapers as well. Even more outrageous than the claims he made is that while the other spirometer manufacturers at least made a pretense of measuring exhaled lung volume Souvielle’s device was nothing more than a box with two holes (which he was somehow able to patent!):

Souvielle made tens of thousands of dollars (a fortune at that time) and did this not only by preying on the people that bought his treatment but by bilking a number of people that had invested large sums of money in his business venture of building and selling “spirometers”. As best as I have been able to determine, Souvielle eventually moved to Florida to escape his creditors and was never prosecuted.

Spirometers have had a varied and complex history in the last century and a half. Quack medicine was a part of this history. Although there is not much positive that can be said about the use of spirometers in this fashion, it is as at least possible that they did in a small way increase public awareness about the concept of lung function testing and thereby made it just a little bit easier for pulmonary function labs to come into existence. 

PFT Blog by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

The Slow Vital Capacity (SVC) maneuver is usually performed as part of lung volume measurements. It is not unusual for the SVC to be larger than the FVC, particularly in patients with airway obstruction. This can have a bearing on the FEV1/FVC ratio and in fact the ATS-ERS recommendations for PFT interpretation say that the largest vital capacity value regardless of which test it comes from should be used to calculate the FEV1/VC ratio. When I review a full panel of tests (FVC, lung volumes, DLCO) I always check to see if the SVC or IVC (from the DLCO test) are larger than the FVC and then re-calculate the FEV1/FVC ratio and its percent predicted if they are. Test results that at first glance look normal will instead show airway obstruction often enough when this has been done that the time spent going through this process is worthwhile.

This only works however, when I have a full panel of tests to extract other vital capacities from. Patients that show airway obstruction when their FEV1/VC ratio is re-calculated have often had only spirometry performed on prior visits and their spirometry results were considered to be within normal limits at those times. Our lab software lets us select and report the “best” FVC and FEV1 from a series of spirometry efforts so this raises an interesting question and that is when and how often should a SVC maneuver be performed instead of a FVC maneuver during a spirometry session in order to get and report the largest VC?

The FEV1/FVC ratio is an important component in determining whether or not airway obstruction is present and to some extent in characterizing the degree of obstruction when it is present. Although it is dependent on the quality of both the FEV1 and FVC determinations my experience has been that the FVC tends to be a lot more variable and more dependent on effort than the FEV1. This is one reason why the FEV1/FEV6 ratio has been proposed as a substitute for the FEV1/FVC ratio. Although the FEV1/FEV6 ratio may be easier to perform and more reliable to obtain one reason this approach has not caught on is that the indication for airway obstruction is subtler and not as clearly evident as the FEV1/FVC ratio, particularly for older patients.

The ATS-ERS statement on spirometry says that a forced vital capacity maneuver should be at least 6 seconds long but at the same time acknowledges that this is a minimum standard and technicians and patients should be encouraged to exceed this whenever possible. Even so, I suspect that a lot of technicians are in the habit of terminating a spirometry effort when 6 seconds has been reached often simply because that is what the testing software says is adequate.

The reality is that older patients and patients with airway obstruction often need more than 6 seconds to exhale completely. Of course, the need for a complete exhalation needs to be counterbalanced with the patient’s safety and I will be the first to admit that knowing when to stop pushing a patient can be difficult to determine. The SVC maneuver is usually significantly less stressful than the FVC maneuver so patients can often exhale longer and produce a larger vital capacity for this reason. Additionally, a forced maneuver often causes a degree of airway collapse and the steady effort of an SVC maneuver can often produce a larger vital capacity in the same time period as a corresponding FVC effort. For these reasons I’d suggest that a SVC maneuver should be performed whenever the FVC is below normal and at least moderate airway obstruction is also present or the patient is elderly (and given my current age I am not willing to indicate where the dividing line between middle-aged and elderly may be except to say it’s always going to be somewhere older than me).

Performing spirometry is part ability and part psychology. Many patients stop exhaling long before they reach 6 seconds not because they are not able to exhale longer but because they don’t “get it” or because they don’t like or even fear the sensation of a complete exhalation. (I can’t remember the number of times I’ve had patients stop exhaling after the initial blast oer and over despite my encouragement and even demonstrating the maneuver multiple times myself. Sometimes after enough tries you could see a light bulb go on over their head and they’d say “oh, you mean like this” and then proceed to perform the maneuver flawlessly.) To help patients learn to exhale longer I’ve frequently had them try a SVC maneuver instead. Showing a patient how much they could exhale when they weren’t worried about doing it quickly helped some of them perform the FVC better and even if it didn’t I often got a larger VC anyway.

Some patients, most often those with asthma or bronchiectasis, show a symmetrically reduced FVC and FEV1 during exacerbations of their disease. It has been estimated that up to 10% of all asthmatics show this pattern at one time or another and it is attributed to non-homogeneous distribution of airway bronchoconstriction or collapse. The flow-volume loops of these patients often show a restrictive pattern (normal peak flow with an elliptical contour). The fact is that they are obstructed not restricted and that a steady effort may sometimes enable them to exhale more air than a forced effort. I’d suggest that a patient who has previously shown evidence of airway obstruction (or comes with a diagnosis of asthma or bronchiectasis) and has a low vital capacity with a normal FEV1/FVC ratio should attempt a SVC maneuver.

There are two different ways to perform a SVC maneuver and the way in which a given testing system’s software analyzes spirometry breathing maneuvers may limit which SVC maneuver can be performed. The SVC can be performed by starting with a maximal inhalation to TLC and then followed by a maximal exhalation to RV or instead by starting with a maximal exhalation to RV and ending with a maximal inhalation to TLC. For physiological reasons patients are often able to produce their largest vital capacity when inhaling from RV to TLC but for many patients this is a conceptually more difficult maneuver to perform than exhaling from TLC to RV.

The order in which inspiration or expiration is performed does not matter in the SVC module in our lab testing software but in the spirometry module the software does not recognize or measure an inspiratory vital capacity when it is performed in the “wrong” order. This has limited us to performing SVC maneuvers only by inhalation to TLC and exhalation to RV. This is not necessarily problem except for those labs that have mandated that the SVC maneuver be performed by an exhalation to RV and inhalation to TLC. It may also only be a problem with our lab’s software. The ATS-ERS statement on spirometry says that when an IVC is performed as part of a FVC maneuver, it can be performed either before the forced exhalation or after it. Despite this our lab’s software will only measure an IVC when it is performed after the forced exhalation. A work-around to this problem would be to switch to the SVC testing module when performing the SVC test but I think this adds a layer of complexity on the patient spirometry testing session and the could make reporting and reviewing results confusing.

Since performing an SVC during spirometry needs to be done in only a minority of patients one problem is training staff to know when and when not to perform the SVC. My suggestion would be to perform an SVC as part of spirometry when:

• the FVC and FEV1/FVC ratio are below normal.
• the FVC is less than 6 seconds.
• the FVC and FEV1 are symmetrically reduced in a patient with a prior history of airway obstruction or a diagnosis of Asthma or Bronchiectasis.

There are a number of reasons why performing an SVC maneuver as part of spirometry in order to obtain a larger vital capacity is a good idea. It is something that should be done for quality patient testing and it can lead to a more accurate patient diagnosis and less overall testing for the patient. This is an observation on my part however, and not part of the ATS-ERS standards so determining whether or not this should be done is something each PFT lab will have to decide for itself.

References:

Brusasco V, Crapo R, Viegi G. ATS/ERS Task Force: Standardisation of Lung Function Testing: Standarisation of spirometry. Eur Respir J 2005; 26: 319-338.

Brusasco V, Crapo R, Viegi G. ATS/ERS Task Force: Standardisation of Lung Function Testing: Interpretive strategies for lung funciton tests. Eur Respir J 2005; 26:v948-968.

Hyatt RE, Cowl CT, Bjoraker JA, Scanlon PD. Conditions associated with an abnormal nonspecific pattern of pulmonary function tests. Chest 2009; 135: 419-424.

Swanney MP, Jensen RL, Crichton DA, Beckert LE, Cardino LA, Crapo RO. FEV6 is an acceptable surrogate for FVC in the spirometric diagnosis of airway obstruction. Am J Respir Crit Care Med 2000; 162: 917-919.

 

PFT Blog by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

My PFT Lab has recently been asked by several doctor’s offices and clinics to advise them on the purchase of an office spirometry system. I am not a fan of office spirometry because I think the test quality is often low. Office spirometry is usually performed by poorly trained office staff using poorly maintained equipment and under these conditions quality is going to suffer. Despite my misgivings the reality is that office spirometry is not going away and in fact its use is probably expanding.

There are several good reasons why this is happening. More testing of all kinds is being done at the point of care and there is an increased awareness of standards of care for COPD and Asthma. There is also revenue generation (the websites of several office spirometer manufacturers have downloadable documents showing return on investment and the proper codes to use (ICD9 and CPT) when billing).

I think that we ignore this trend at our own peril and that the proper response should be to reach out and offer assistance in selecting office spirometers and training office staff to perform spirometry instead. Although this will require extra effort with no immediately apparent payback I think this should be done not only because it is the right thing to do for the patient’s sake but also because it will pay dividends in the long run.

I actually do not think that office spirometry is a significant threat to pulmonary function labs. Yes, there may be a slight decrease in patients referred just for spirometry but most spirometry performed in my PFT Lab is performed for the pulmonary physician clinics and I suspect that to one degree or another this is true for most hospital-based PFT Labs. Office spirometry is actually more likely to generate additional referrals to pulmonary physicians and for more complete pulmonary function testing than not. When that happens if a doctor’s office or clinic has a choice about where to refer their patients then being part of the solution for them rather than part of the problem is going to make it more likely these patients end up with you than not.

One of the first concerns I have about physician offices and clinics selecting a spirometer without assistance is that the choice will often be made according to the up-front cost without thinking about long term costs and staff work flow.

My list of recommended office spirometer features includes:

• Accuracy
• Patient hygiene
• Reference equations
• Ability to configure reports
• Data storage

The last time I researched office spirometers was several years ago and when I started researching them again I was struck by the increase in features and the decrease in price. I have been unable to find any comparative reviews (other than those from the manufacturers themselves) so it is hard to determine what the current level of test quality and accuracy is like. Having said that, the same can also be said of the equipment from the major pulmonary function test equipment manufacturers. Most office spirometer manufacturers claim that their systems meet the ATS-ERS standards and in the absence of any evidence to the contrary I am willing to assume that this is more or less true. If an office spirometer does not make this claim then it should not be considered.

One thing I am adamant about for the sake of the patient’s health is that an office spirometer should either use a barrier filter or a disposable sensor. Spirometers that only use a plastic or cardboard mouthpiece or that are supposed to be cleaned between uses absolutely should not be considered. This increases the per-test costs (which offices and clinics often do not factor into their buying process) but patient hygiene is non-negotiable.

An office spirometer should be able to use the same reference equations as your own PFT Lab (and if you are not using NHANESIII or GLFI then why not?). I’ve noticed that several office spirometers either did not list which reference equations they used or had a very limited set of (out-) dated reference equations. The correct set of reference equations is of course important for the patient’s sake but you also don’t want to have to waste your time repeatedly explaining why the percent predicted values on your test results differ from those of the office spirometer when the results are the same.

I think I once counted all of the numerical values that can be obtained from a Forced Vital Capacity and came up with over two dozen of them. A report with all of these values looks very impressive but is mostly useless noise. There are at most a half-dozen test values that should be reported (and in my opinion that list is FVC, FEV1, FEV1/FVC ratio, Peak Flow, expiratory time, back-extrapolation). If an office spirometer report cannot be pared down to the essential values then that spirometer should not be considered.

If patient demographics (name, date of birth, height, etc) are not stored in some kind of a database then the office staff will have to re-enter it every time spirometry needs to be performed. If the patient’s test results are not stored (and if a trend page in the report is not available) then how will results from the current test be compared to previous tests? A test system that does not have a patient database should not be considered. I would also argue that if it is not able to trend results on a report it should not be included.

Other issues that have to do with staff work flow include:

• how results are physically reported
• portable versus stationary 

How are the test results are physically reported? LCD display? 4-inch wide thermal paper? 8-1/2 x 11 inch paper from a regular computer printer? This can have a significant bearing on staff work flow. If the results only appear on an LCD display then they will have to be manually written down or typed into the patient’s records (I would also suggest that there may also be an issue with insurer payments if the office is audited because a paper report of any kind is evidence the test was actually performed whereas results that only appear in notes may not be). A strip of thermal paper (or the equivalent) will likely have to be stapled onto a backing sheet if it is going to go into a patient’s chart and for this reason a report on 8-1/2 x 11 paper saves extra work.

There’s portable and then there’s portable. Some of the less-expensive but fully-featured office spirometry systems consist of just a flow sensor that is plugged into the USB port of an existing desktop or laptop computer. A laptop can be considered to be portable but the reality is that it needs to be placed on a work surface of some kind and the patient will need to be positioned nearby in order to use the spirometer attached to it. As an alternative a number of office spirometers consist of a portable, battery-powered sensor and a docking station. In these systems the sensor can be brought to wherever the patient is located and the tests performed there. When docked the results are automatically uploaded into the computer. Neither approach is necessarily better or worse than the other, but how they effect staff work flow does matter and needs to be considered.

One final technical consideration that I have is how are the results shared? The hospital I work for is the tertiary care facility for a network of hospitals, clinics and physician offices. If each clinic or office goes it alone and chooses office spirometry systems by themselves then most of these systems will not talk to each other or to the hospital information system. Patient results will then be spread across numerous systems and in the worst case patient results will either remain in paper format in a paper chart or the test results will be hand entered into the patient visit notes. At the moment I do not have a good answer to this question. Several solutions come to mind but which, if any, of them actually makes sense is unclear.

My PFT Lab has already developed a program for training office staff in spirometry and we already use it in a small number of local clinics. I’d like to say that we anticipated the trend in office spirometry but the fact is that we have a relationship with several physicians and clinics associated with, but not part of, the Pulmonary division and started this program a number of years ago at their request. Regardless of the reason this has left us in a position to support staff education programs in local offices and clinics with little additional effort (other than the actual training, of course). We also have the ability to support a regular (annual? biannual? quarterly?) program of visits to offices and clinics to perform calibration checks with a 3-liter syringe. I think this is important service to provide because it would reinforce the need for quality assurance and give us the opportunity to run a quick refresher course for the office staff.

My current recommendation to the lab’s medical director, manager and hospital administrator is to develop system-wide technical and procurement standards for office spirometers. In addition I also recommend that the Pulmonary Lab reach out to all offices and clinics associated with the hospital and offer staff training and quality assurance for their office spirometry.

A long term problem for hospitals, clinicians and patients is sharing patient test results. Eventually I think there is going to be a long term solution and it will probably be a standards-based web-enabled system but at the moment the closest we can come to this would be to approach the hospital’s Information Systems department and see if a more universal interface for patient test results can be developed.

So far there has been a positive response to these recommendations but it’s still too early in the process to see where this will go.

Office spirometry is not going away. Instead of considering it an ignorable problem think about doing what you can to improve it and treat it as the opportunity it really is.

PFT Blog by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

Patients that have problems with oxygenation at sea level are going to have even more problems at higher elevations where the barometric pressure and oxygen partial pressure are lower. During commercial aircraft travel the cabin pressure is required by U.S. Federal regulations to be pressurized to at least 565 mm Hg which is the equivalent of 8000 feet altitude. It has been reported that most airliners are pressurized to an equivalent altitude of between 5000 and 8000 feet but this will depend on both the airplane and the airline in question.

There is a general relationship between a patient’s PaO2 at sea level and their PaO2 at altitude and a variety of studies have developed equations to predict an individual’s PaO2 at altitude using ABG, spirometry, DLCO and exercise SpO2 results. These prediction equations however, have been shown to have poor accuracy when compared to a Hypoxia Altitude Simulation Test (HAST).

The concept behind the HAST is relatively simple and that is to have the patient in question breathe a gas mixture containing a concentration of 15.1% oxygen (which at sea level is the equivalent to an altitude of 8000 feet) while monitoring the patient’s arterial oxygen partial pressure or saturation. The HAST has been shown to give the same results as a hypobaric chamber. Although HAST has been performed by a number of different research groups the technique has not been standardized which may to some extent account for some of the differences in reported results.

There is general agreement about which patients will need a HAST in order to ensure safe airplane travel and these are patients with a resting SpO2 between 92% and 95%, particularly ones with a comorbidity such as cardiac or pulmonary disease. Patients with a resting SpO2 above 95% will most likely not require supplemental oxygen and do not require a HAST. Patients with a resting SpO2 below 92% will likely require supplemental oxygen during airplane travel and will not require a HAST although a HAST can be used to titrate their supplemental oxygen flow rate.

There is also general agreement about the length of the HAST (20 minutes) and which HAST results that indicate the need for supplemental oxygen:

• PaO2 below 55 mm Hg
• SpO2 below 85%

The lack of standardization is in the nuts and bolts of how to perform the HAST. Delivery of the 15.1% oxygen gas mixture is most often by use of a tight-fitting mask but a mouthpiece with nose clips, filling a body box with the gas mixture or loose-fitting masks of different types have also been proposed as delivery techniques. Several years ago when my PFT lab decided to start performing this test we evaluated these different approaches and used ease of testing and low expense as primary factors in deciding how to perform the test. Part of the low-cost factor includes making as many of the components disposable after use because sterilization is relatively high cost for us.

Although a breathing circuit with a mouthpiece and nose clip is probably the simplest approach, one key component of the HAST is the need to be able to titrate supplemental O2 if it turns out to be needed. Since supplemental O2 is most often given by nasal cannula it’s not possible to titrate O2 with a mouthpiece and this factor alone made this approach unfeasible.

It might be possible to use a body box but the plethysmographs we have do not particularly lend themselves to this. At the very least a hole would have had to have been drilled in the wall of the box so that a nipple could installed as an inlet for the HAST gas mixture. Most plethysmographs come with either a solenoid valve or low-pass filter to keep the interior of the box at atmospheric pressure so an additional opening for outflow would not necessarily be a problem and a small oxygen tank and regulator could be placed in the box to deliver and titrate supplement O2.

The biggest drawback of this technique is that the volume of the box would require a large amount of the HAST gas mixture to flush and fill the box properly. Most plethysmographs have a volume on the order of 600 liters but it would probably take at least two to three times that volume in order to flush any residual room air. This may not be a problem for those labs that are able to mix the HAST gas mixture with a blender from oxygen and nitrogen (at least in terms of expense) but a blender would have been an extra expense for my lab. Significant additional test time to fill and flush the body box would also be needed while the patient is inside (even with a gas flow rate of 100 LPM it would take somewhere between 10 and 18 minutes to do this and I have to wonder about the noise level inside the box during this time). Finally the oxygen concentration inside the box must be monitored reasonably accurately as well (another expense for my lab) and if the gas concentration is over or under the 15.1% O2 goal even more time would be needed to correct it. All these factors ruled out the use of a body box for us.

This left us with the mask approach and to use a mask the first question would be what kind of mask? At least one group of investigators used a 35% venturi mask driven with nitrogen instead of by oxygen. My prior experience with venturi masks showed me that the gas concentration is dependent to some extent on driving pressure and that individual masks can have minor differences in the concentrations they provide. Since HAST results are sensitive to oxygen concentration using a venturi mask would have required a precision oxygen analyzer to verify that the correct oxygen concentration was being generated and for my lab this would have been an additional expense.

It’s possible that a simple non-re-breathing oxygen mask could be used and it would be easy for the patient to wear a nasal cannula underneath it but since it is loose fitting it would need a high flow rate of the HAST gas mixture (+30 LPM?) to make sure the patient got the right oxygen concentration. This in itself is not necessarily a problem but the high flow rate of gas into the mask would likely disturb the supplemental O2 when it was delivered by the nasal cannula by flushing it away before it was inhaled. This meant we couldn’t rely on the accuracy of the supplemental O2 flow rate needed to maintain a normal SpO2 and this factor ruled out this approach for us.

A number of relatively inexpensive tight-fitting masks in a range of sizes with a wide, low-pressure seals that would permit a nasal cannula to be placed underneath are available so the next question is how the HAST gas mixture should be delivered. The most economical approach in terms of the amount of test gas used would be to have a reservoir bag and a two-way breathing valve that the mask would be attached to. The problem with this that when a patient breathes in, they cause a slight amount of negative pressure inside the mask and any leaks around the edge of the mask (facial hair, sunken cheeks, fit around the bridge of the nose) would cause room air to be drawn inside, raising the oxygen concentration. Even a well-designed two-way valve would add extra resistance which would increase this negative pressure and therefore increase the likelihood of this happening. In addition, masks tend to impose an extra dead space load on the patient and tend to retain exhaled CO2. All of these factors ruled out a reservoir bag and two-way valve and led us to a mask with a blow-by system (or to be more accurate in this case, a blow-at system).

We took an oxygen tee and fit the end opposite the oxygen nipple into the mask. This meant that the HAST gas mixture was blowing towards the mask and thereby maintaining a slight positive pressure inside the mask. If there was a leak around the edge of the mask for any reason the positive pressure would make it less likely for the oxygen concentration to be affected. Even a blow-by system needs some kind of reservoir in order to meet the patient’s inspiratory flow requirements. For this purpose we got a roll of 22 mm aerosol tubing and cut off a 6 foot segment (there’s roughly 10 cc per inch of this kind of tubing, so this makes a reservoir of about 700 cc) and attached it to the remaining limb of the tee. All parts of this breathing circuit are relatively low cost and all parts are disposable.

The final problems with the HAST test were procedural. Some investigators suggest that patients should have their ECG monitored during the HAST test. After some thought we decided this was overkill. The patients that are most likely to become hypoxic enough to have heart arrythmias are those with a low resting SpO2 and are already receiving supplemental O2. The only reason to perform a HAST on these patients is to titrate supplemental O2 and you’d start the test with their normal supplemental O2 flow rate. We also decided not to automatically wait the full 20 minutes before adding supplemental O2. When a patient’s SpO2 drops below 85% we start supplemental O2 at that time.

Finally, some investigators advocate the use of arterial blood gases as the primary way to assess the response to a HAST. We decided that it wasn’t practical to titrate supplemental O2 by ABG and that if we depended on an oximeter to assess titration then it was also accurate enough to assess hypoxia. We also didn’t see any value in performing a HAST just to see if a patient met the criteria for supplemental O2 without being able to immediately start adding supplemental O2 and titrating it as well. Performing an ABG would mean that we’d have to wait for the test results from the chemistry lab before going forward with titration. Patient time and technician time are both valuable and you can only bill for one type of HAST test on a given day so for all these reason we use an oximeter.

Other than the supplies for each test all we had to acquire was a regulator for the HAST gas cylinder and a high-flow flowmeter. This has meant that performing a HAST is relatively low cost for the lab and other than a place to keep the HAST gas cylinder, it requires almost no space. This is important because we do not perform a large number of these tests. Since we started to perform the HAST we have averaged between two and three dozen tests annually. To give some perspective the lab usually has between 6000 and 7000 patient visits annually so at best only around one half a percent of our patients has a HAST. Even though it’s a low volume test doesn’t mean that it isn’t a worthwhile test to perform. As mentioned previously it isn’t possible to accurately predict a patient’s possible hypoxia from a resting ABG with or without spirometry and DLCO results. This means that a HAST is the only way to determine if it is safe for patient with pulmonary disease to travel by airplane and that makes this an important and valuable test to provide to our patients.

References:

Bradi AC, Faughnan ME, Stanbrook MB, Deschenes-Leek E, Chapman KR. Predicting the need for supplemental oxygen during airline flight in patients with chronic pulmonary disease: a comparison of predictive equations and altitude simulation. Can Respir J 2009; 16(4): 119-124.

Dillard TA, Moores LK, Bilello KL, Phillips YY. The preflight evaluation. A comparison of the hypoxia inhalation test with hypobaric exposure. Chest 1995; 107: 352-357.

Dine JC, Kreider ME. Hypoxia altitude simulation test. Chest 2008; 133(4): 1002-1005.

Mohr LC. The hypoxia altitude simulation test. An increasingly performed test for the evaluation of patients prior to air travel. Chest 2008; 133(4): 839-842.

Robson AG, Hartung TK, Innes JA. Laboratory assessment of fitness to fly in patients with lung disease: a practical approach. Eur Respir J 2000; 16: 214-219.

Seccombe LM, Kelly PT, Wong CK, Rogers PG, Lim S, Peters MJ. Effect of simulated commercial flight on oxygenation in patients with interstitial lung disease and chronic pulmonary disease. Thorax 2004; 59: 966-970.

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