I
would like to re-emphasize 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
gas exchange, the next step in the flow of gases is circulation. The
descriptive elements for assessing circulation are:
What
was the maximum heart rate?
The maximum predicted heart rate
is calculated from 220 – age.
A maximum heart rate above 85%
of predicted indicates that there has been an adequate exercise test
effort.
Example:
The maximum heart rate was XX%
of predicted {which indicates an adequate test effort}.
What
was the heart rate reserve?
The heart rate reserve is
(predicted heart rate – maximum heart rate). A heart rate reserve
that is greater than 20% of the (predicted heart rate – resting
heart rate) is elevated and may be an indication of either
chronotropic incompetence or an inadequate test effort.
Note:
A negative heart rate reserve
will occur whenever a patient exceeds their predicted heart rate.
Example:
The heart rate reserve is XX
BPM which is {within normal limits | elevated}.
What
was the heart rate recovery (HRR)?
The heart rate recovery is the
(maximum heart rate – heart rate at 1 minute post-exercise). A HRR
< 12 indicates the patient has a reduced vagal and parasympathetic
tone and carries a higher mortality risk.
Example:
The heart rate recovery was
XX BPM which is {within normal limits | reduced}.
What
was the chronotropic index?
The
chronotropic index relates the change in heart rate to the change in
oxygen consumption. A chronotropic index > 1.30 indicates a steep
heart rate response and can suggest an increased dependence on heart
rate to increase cardiac output secondary to a low stroke volume. A
chronotropic index < 0.8 can indicate a blunted heart rate
response (possible chronotropic incompetence) which in turn can
suggest a variety of cardiac dysfunctions but can also be caused by
medication (beta blockers). A low chronotropic index can also be
seen in exceptionally fit individuals (elevated
max VO2, elevated VO2 at AT),
and in these cases, is normal. In
an exceptionally fit individual a normal chronotropic index is likely
abnormal.
Note: Chronotropic index is calculated from:
and
is discussed in greater detail in the previous
posting Chronotropic
Index and O2 Pulse.
Example:
The Chronotropic Index was X.XX
which is {reduced | within normal limits | elevated|.
What
was the resting and maximum exercise blood pressure?
A
normal max systolic pressure during exercise is 160-220 mm Hg. A
normal increase in diastolic pressure during exercise is 10 mm Hg. A
diastolic pressure during exercise greater than 90 Hg is likely
abnormal and greater than 100 mm Hg is definitely abnormal and
suggests diastolic dysfunction. If systolic blood pressure did not
increase to greater than 130 mm Hg, or dropped >10 mm Hg during
exercise, consider left ventricular dysfunction, possible CAD
Note:
Guidelines for the
maximum allowable blood pressure during exercise remain
unclear. We use a diastolic pressure ≧
110 mm Hg or a systolic pressure ≧
250 mm Hg as an indication to stop testing but each institution will
have to develop their own guidelines.
Example:
The resting blood pressure was
XXX/XX which is {within normal limits | elevated}. Blood pressure
increased to XXX/XX during exercise which is {within normal limits |
elevated}.
What
was the systolic blood pressure 3 minutes post-exercise?
Blood
pressure should be measured at 3 minutes following exercise
termination. A
(3
minute post-exercise systolic blood pressure / peak exercise systolic
blood pressure)
ratio greater than 0.95 indicates a
blunted post-exercise decrease in blood pressure which is an
indicator for an increased mortality risk.
Example:
There was a {normal | blunted }
decrease in systolic blood pressure following exercise.
What
was the VO2 at Anaerobic Threshold?
A
low VO2 at AT indicates an abnormal cardiovascular limitation
(specifically a reduced O2 delivery to the exercising tissues). The
LLN threshold (in terms of percent of the predicted VO2) rises with
age:
Males:
-
Age:
20 LLN: 42% -
Age:
30 LLN: 43% -
Age:
40 LLN: 44% -
Age:
50 LLN: 45% -
Age:
60 LLN: 46% -
Age:
70 LLN: 47%
Females:
-
Age:
20 LLN: 41% -
Age:
30 LLN: 44% -
Age:
40 LLN: 47% -
Age:
50 LLN: 49% -
Age:
60 LLN: 52% -
Age:
70 LLN: 54%
Note:
The
Anaerobic Threshold generated automatically by test system software
should always be verified. Determining Anaerobic Threshold is
discussed in greater detail in the previous
posting There’s
more than one way to determine AT.
Example:
The VO2 at Anaerobic Threshold
was XX% of predicted which
is {within normal limits | reduced}.
What
was the Heart Rate – VO2 pattern?
The
Heart rate and VO2 should be plotted against each other in a graph.
Normally when this is done there should be a nearly straight line as
the relationship between heart rate and VO2 tends to be linear. An
upwards inflection in this line indicates that the heart rate is
increasing faster than the VO2 and is an indication of a rate-related
decrease in stroke volume which is often seen in valvular disease.
Note:
This is
discussed in greater detail in the previous
posting Chronotropic
Index and O2 Pulse.
Example:
{There was a normal Heart-Rate
VO2 pattern} | {There was an abnormal upwards inflection in the Heart
Rate-VO2 curve occurring at a heart rate of XXX BPM.}
What
was the O2 pulse pattern?
O2
Pulse is VO2/Heart rate (ml of oxygen consumed per heart beat). A
normal O2 Pulse response pattern shows an immediate upswing at the
beginning of exercise and a steady increase thereafter with a peak at
peak exercise. An early plateau can be abnormal. An O2 Pulse that
increases and then decreases is definitely abnormal and is usually
seen in conjunction with an abnormal HR/VO2 curve.
O2
Pulse is the product of the a-v O2 Content difference and Stroke
Volume. If there is no desaturation during exercise then O2 Pulse
can be an indicator of Stroke Volume.
O2
Pulse usually decreases when exercise stops. An increase in O2 Pulse
after the end of exercise is abnormal
an usually
seen in left
ventricular dysfunction
and other heart diseases.
Note:
This is
discussed in greater detail in the previous
posting Chronotropic
Index and O2 Pulse.
Example:
{The O2 pulse pattern was
normal.} | {There was an abnormal decrease in O2 pulse during
exercise} | {There was an abnormal increase in O2 pulse following
exercise.}
What
was the maximum O2 pulse?
The maximum predicted O2 pulse is calculated from:
(Predicted
Maximum VO2 (ml) / Predicted Maximum Heart Rate)
A
maximum O2 pulse < 80% of predicted is likely abnormal and can
indicate a stroke volume limitation or an inadequate exercise test
effort.
Example:
The maximum O2 pulse was XX.X ml/beat which is {within normal limits
| reduced}.
The
circulatory response to exercise primarily involves the delivery of
oxygen to the exercising muscles. After having gone through this
checklist it should be apparent whether the circulatory response to
exercise was normal or abnormal, and as importantly, specifically
which element was normal or abnormal.
A
significant reduction in cardiac output is primarilysignaled
by a reduced VO2 at AT and is usually due to either a decrease in
stroke volume or the inability to increase heart rate (chronotropic
incompetence). Depending on its severity a reduction in stroke
volume is usually indicated by a reduced maximum O2 pulse while
chronotropic incompetence is usually indicated by a reduced
Chronotropic Index. These findings are not mutually exclusive,
however and
some patients will have both indications.
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.
Previous: Interpreting CPET Tests, part 2: Gas Exchange
Next:
Interpreting CPET Tests, part
4: Interpretation and Summary
References:
Wasserman K, Hansen JE, Sue DY,
Stringer WW, Whipp BJ. Principles of exercise testing and
interpretation, 4th edition. Lippincott, Williams and
Wilkins, publisher.
PFT
Blog by Richard
Johnston
is
licensed under a Creative
Commons Attribution-NonCommercial 4.0 International License.
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