I
would like to re-iterate the importance of the descriptive part of
CPET interpretation. At the very least consider it to be a checklist
that should always be reviewed even when you think you know what the
final interpretation is going to be.
After
ventilation, the next step in the flow of gases is gas exchange. The
descriptive elements for assessing gas exchange are:
What
was the maximum oxygen consumption (VO2)?
The maximum oxygen consumption
is the prime indicator of exercise capacity. Predicted values should
be based on patient height, age, weight and gender.
Note:
There is actually a surprising limited number of reference equations
for maximum VO2. The only one I’ve found that takes weight into
consideration in a realistic manner is Wasserman’s algorithm. Some
test systems do not offer this reference equation but I feel it is
worthwhile for it to
be calculated and used regardless. See appendix for the algorithm.
Note:
The maximum VO2 does not
necessarily occur at peak exercise (i.e.
test termination).
This can happen in various types of cardiac and vascular diseases
but also because the patient may decrease
the level of their exercise before
the test is terminated.
- Maximum VO2 > 120% of predicted = Elevated
- Maximum VO2 = 80% to 119% of predicted = Normal
- Maximum VO2 = 60% to 79% of predicted = Mild impairment
- Maximum VO2 = 40% to 59% of predicted = Moderate impairment
- Maximum VO2 < 40% of predicted = Severe impairment
Example:
The maximum VO2 was X.XX LPM { which is {mildly | moderately |
severely } decreased | within normal limits | elevated}.
What
was the maximum oxygen consumption in ml/kg/min?
This value is affected by a
patient’s BMI and for this reason may have a limited value in
assessing a patient’s exercise capacity but despite this it has
been used extensively in disability and pre-operative assessments.
Underweight patients may have an elevated VO2 in ml/kg/min and
overweight patients may have a reduced VO2 in ml/kg/min even when VO2
expressed a LPM is normal.
- VO2 < 10 ml/kg/min: Surgery is contraindicated.
- VO2 = 10 to 15 ml/kg/min: Unable to perform most jobs; at increased risk for invasive surgical procedures.
- VO2 = 15 to 25 ml/kg/min: Able to work at jobs that do not require extended work above 40% of maximum VO2; surgical risk is low.
- VO2 > 25 ml/kg/min: Able to perform most jobs.
Example:
The maximum VO2 was XX
ml/kg/min.
Was
there a VO2 plateau?
A VO2 plateau is defined as a
stable, unchanging VO2 for > 1 minute, particularly while the work
rate is still increasing. When present it suggests the patient has
achieved their maximum cardiac output. It can also occur in patients
being exercised at very low work loads.
Note:
When looking at raw test data,
a VO2 plateau should be accompanied by an increase in Minute
Ventilation (Ve) and CO2 production (VCO2). If these do not continue
to increase then the VO2 plateau is more likely due to the patient
slowing down and exercising less than
it is to a true VO2 plateau.
Example:
{There was no VO2 plateau. |
There was a VO2 plateau occurring during the
final XX seconds of testing.}
What
was the Oxygen Saturation (SaO2) at rest and at maximum exercise?
A
resting SaO2 < 95% is abnormal. A decrease in SaO2 during
exercise ≧
3% is abnormal.
Note:
Oximeter
readings can be inaccurate at high heart rates, from
motion
artifact or poor peripheral circulation.
Note:
In
an adequate test, a
significant
decrease
in SaO2 indicates that the primary exercise limitation is pulmonary
(either ventilatory
or vascular) while
a
normal
SaO2 indicates that the primary limitation is cardiac. This
is discussed in greater detail in the previous posting DLCO/Qc,
SaO2 and CPETs.
Example:
The baseline SaO2 was {reduced
| normal}. There was {no} significant desaturation {to
XX%} during exercise.
What
was the maximum PetCO2?
End-tidal
CO2 (PetCO2) is an indirect measurement of ventilatory efficiency and
is the product of PACO2 and ventilation. Although PetCO2 at rest is
approximately 2 mm Hg below PaCO2 the relationship between PetCO2 and
PaCO2 is much less exact during exercise and will be overridden by an
exaggerated ventilatory response to exercise. PetCO2 should rise
during exercise and peak near AT, then decrease until maximum
exercise is attained with a further drop following exercise.
Note:
PetCO2 during exercise was
discussed in greater detail in the previous posting PetCO2
during exercise, a quick diagnostic indicator.
An
arbitrary grading scale for maximum PetCO2 is:
- ≧ 35 = normal
- 30 to 35 = mildly reduced
- 25 to 30 = moderately reduced
- <25 = severely reduced
Example:
The maximum PetCO2 was XX mmHg
which is {normal | {mildly
| moderately | severely reduced}}.
What
was the RER pattern during exercise?
The
Respiratory Exchange Ratio (RER) is VCO2/VO2. A normal resting value
is 0.70 to 0.90 and is influenced by diet (lower with a high protein
diet, higher with a high carbohydrate diet). RER usually falls below
the baseline value at the beginning of exercise (due to a delay in
VCO2 kinetics) and then increases thereafter.
An RER of 1.10 or higher at peak
exercise is an indicator of an adequate exercise test effort.
Note:
Patients exercising on a
treadmill have a greater difficulty in achieving an RER of 1.10 and
for this reason an RER of 1.05 is likely an
indication of an adequate test.
Note:
When
elevated at rest (>1.00) this often means nothing more than
patient anxiety. If the RER
is elevated at rest and does not decrease below 1.0 during the first
level of exercise this suggests a metabolic disorder or continued
hyperventilation.
Example:
The RER was X.XX at peak exercise {which indicates an adequate
exercise test effort}.
What
was the Ve/VCO2 at Anaerobic Threshold (AT)?
Ve/VCO2
is the amount of
ventilation
required per unit of CO2 production. High values suggest inefficient
ventilation either
due to ventilatory
or gas exchange causes.
Ve/VCO2
tends to be elevated
at rest (40-50),
decreases to minimum value at or shortly after AT, and can then
increase or remain flat to peak exercise.
A
Ve/VCO2 > 35 at AT is likely abnormal and raises the question of
pulmonary vascular disease or hyperventilation.
Note:
A
diagnosis of pulmonary vascular disease needs a significant decrease
in SaO2 and/or a reduced DLCO for confirmation.
Example:
The Ve/VCO2 at AT was XX which
is {within
normal limits | elevated}.
What
was the lowest observed Ve/VCO2?
Anaerobic
threshold is a somewhat arbitrary point for the assessment of
Ve/VCO2. This is because the nadir in Ve/VCO2 usually occurs
following the nadir in Ve/VO2 which is a
primary marker
for
the AT. For the reason the lowest observed Ve/VCO2 may be a more
accurate reflection of ventilatory efficiency.
A lowest observed Ve/VCO2 >
35 is likely abnormal.
Example:
The lowest
observed Ve/VCO2 was XX
which is {within
normal limits | elevated}.
What
was the Ve-VCO2 slope to AT?
The
Ve-VCO2 slope is determined using
linear regression when Ve is plotted versus VCO2 and like the Ve/VCO2
is a way to measure the efficiency of gas exchange. The Ve-VCO2
slope is usually calculated from rest to AT, but peak exercise values
can be substituted when a patient is unable to reach AT or AT is
indeterminate. This is discussed in more detail in the previous
posting Ve-VCO2
slope: Just to AT or all the way to peak?
Note:
My
personal opinion is that the Ve-VCO2 slope is a more accurate
reflection of ventilatory efficiency than Ve/VCO2 and this is because
it is based on a majority of the VCO2 and Ve measurements rather than
just a few. However, many test systems do not calculate Ve-VCO2
slope and if needed it must be calculated manually (spreadsheet
actually, with
manual
entry).
A
Ve-VCO2 slope to
AT >
34 is abnormal and
carries an increased risk for surgical procedures.
Example:
The Ve-VCO2 slope to AT was XX
which is {within normal limits | elevated}.
What
was the Ve-VCO2 slope to peak exercise?
The AHA recommends calculating
the Ve-VCO2 slope from rest to peak exercise, rather than just to AT.
A Ve-VCO2 slope to peak exercise
< 30 is considered normal. A Ve-VCO2 slope to peak exercise >
40 is considered abnormal.
Note:
The
Ve/VCO2 slope to peak exercise is highly dependent on how far a
patient is willing
to push themselves. An early termination will reduce the peak
Ve-VCO2 slope. This
is discussed in more detail in the previous posting Ve-VCO2
slope: Just to AT or all the way to peak?
Example:
The Ve-VCO2 slope to peak
exercise was XX which is
{within normal limits | elevated}.
After having gone through this checklist it should be apparent whether the gas exchange response to exercise was normal or abnormal, and as importantly, specifically which element was normal or abnormal.
A gas exchange defect will be indicated by a reduced maximum oxygen consumption, inefficient ventilation and, in more severe cases, a significant decrease in SaO2. These findings are not specific to obstructive (COPD, not asthma) or restrictive diseases. Even patients with normal lung mechanics can have limitations in gas exchange, a prime example of this being a pulmonary emboli. Elevations in Ve/VCO2 and Ve-VCO2 slope are also frequently seen in cardiac disease
It should also be apparent that it is necessary to review all of the raw test data, not just that presented for baseline, AT and peak
exercise. Summary reports automatically generated by test systems often have inaccuracies in how test data is selected and averaged and these reported results should always be verified.
As always however, attention should be paid to test quality as suboptimal testing can skew results.
Next: CPET Interpretation part 3: Cardiovascular response to exercise
Previous:
CPET
interpretation part 1: Ventilatory response to exercise
Appendix:
Wasserman’s VO2 calculation algorithm
Male:
Cycle factor = 50.72 – (3.72 x
age)
Predicted
weight (kg) = (0.79
x height (cm)) – 60.7
If actual weight = predicted
weight then:
VO2 (ml/min) = Actual weight x cycle factor
If actual weight < predicted
weight then:
VO2 (ml/min) = ((Predicted weight + actual weight) / 2) x cycle factor
If actual weight > predicted
weight then:
VO2 (ml/min) = (predicted weight x cycle factor) + (6 x (actual weight – predicted weight))
If treadmill is used rather than
a bicycle ergometer then multiply VO2 by 1.11
Female:
Cycle factor: 22.78 x (0.17 x
age)
Predicted weight (kg) = (0.65 x
height (cm)) – 42.8
If actual weight = predicted
weight then:
VO2 (ml/min) = (actual weight (kg) + 43) x cycle factor
If actual weight < predicted
weight then:
VO2 (ml/min) = ((predicted weight + actual weight + 86) / 2) x cycle factor
If actual weight > predicted
weight then:
VO2 (ml/min) = ((predicted weight + 43) x cycle factor) + (6 x (actual weight – predicted weight))
If treadmill is used rather than
a bicycle ergometer then multiply VO2 by 1.11
Note:
Wasserman et al does not
specify over what range an actual weight should be considered the
same as the predicted weight (particularly since they are very rarely
if ever
identical). As a rule of thumb I use +/- 10% as an acceptable
“normal” range.
References:
Balady
GJ; et al. Clinician’s guide to cardiopulmonary exercise testing in
adults: A scientific statement from the American Heart Association.
Circulation 2010; 122: 191-225.
Wasserman K, Hansen JE, Sue DY,
Stringer WW, Whipp BJ. Principles of exercise testing and
interpretation, 4th edition, page 166. Lippincott,
Williams and Wilkins, publisher.
PFT
Blog by Richard
Johnston
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