Download The Treadmill Test—Where To Stop and What Does It Mean?*

The Treadmill Test—Where To Stop and
What Does It Mean?*
Gita Ramamurthy, MD;† Jamie E. Kerr, MD; David Harsha, MD; and
Morton E. Tavel, MD, FCCP
The prognostic utility of an exercise ECG test depends upon having an adequate workload to
stress the cardiac system. A negative stress test, in which there are no adverse clinical or ECG
findings, and in which an adequate workload is achieved, stratifies patients into a low-risk group.
The 1997 American Heart Association guidelines imply that any index of workload— heart rate,
rate-pressure product, or exercise duration in multiples of resting O2 consumption (METS)—
could be used to indicate that adequate stress was achieved. However, while there is considerable
evidence supporting the use of METS as a strong independent prognostic variable, there is less
support for the use of rate-pressure product or heart rate. Indeed, there is evidence that a high
heart rate at a low workload carries an adverse prognosis. Further research is needed to identify
the number of METS achieved that would define an adequate workload. In the meantime, a
review of the literature suggests that 7 to 10 METS is a reasonable ballpark figure of the
minimum workload in patients with a negative stress test that would imply a favorable outcome.
(CHEST 1999; 115:1166 –1169)
Key words: coronary artery disease; exercise test; heart rate; prognosis
Abbreviations: ETT 5 exercise treadmill test; METS 5 multiples of resting O2 consumption
exercise stress testing has been used as
H istorically,
a diagnostic tool to detect the presence of coronary artery disease. Exercise increases the requirement
for myocardial blood flow, and ischemia resulting from
this stress customarily has been detected through
changes in the ECG and other modalities. In recent
years, numerous studies have gone beyond this limited
use and examined the prognostic value of exercise
ECG. When an adequate workload is achieved and
when there is an absence of symptoms, arrhythmias, ST
abnormalities, or adverse hemodynamic changes, the
exercise treadmill test (ETT) is interpreted as negative
and stratifies patients into low risk groups. However,
with suboptimal workloads, the test provides less information.
The 1997 American Heart Association guidelines on
stress testing in men and women emphasize the importance of workload, describing maximum exercise capac*From the Departments of Medicine (Drs. Ramamurthy and
Tavel) and Sports Medicine (Dr. Harsha), St. Vincent Hospital,
Indiana University; Indiana Heart Institute (Dr. Tavel), Indianapolis, IN; and the Department of Medicine (Dr. Kerr),
Highland Hospital, University of Rochester School of Medicine
and Dentistry, Rochester, NY.
†Currently at the State University of New York Health Science
Center, Syracuse, NY.
Manuscript received August 27, 1998; revision accepted September 28, 1998.
Correspondence to: Gita Ramamurthy, MD, 211 Lafayette Road
#425, Syracuse, NY 13205
ity as “one of the strongest and most consistent prognostic markers” of event-free survival.1 Measures used
to assess exercise performance include peak heart rate,
rate-pressure product (peak heart rate 3 systolic BP),
and multiples of resting O2 consumption (METS)
achieved. The guidelines go on to state, “When interpreting the exercise test, it is important to take exercise
capacity into account; the specific variable used to
summarize this aspect of test performance is less
important.” Similarly, the 1996 American Heart Association guidelines on cardiovascular disease in women
state that, with atypical chest pain, a negative stress
ECG test in a female patient with adequate exercise
tolerance as measured by “workload (ie, METS) or
peak heart rate” [italics inserted] obviates the need for
further investigation.2
These guidelines imply that more than one exercise variable could be used to assess workload.
However, as we shall discuss below, all variables are
not created equal; as a measure of exercise capacity
for prognostic purposes, there is strong evidence for
using the maximum workload achieved, less evidence for the use of rate-pressure product, and little
support for the utility of maximal heart rate.
Maximal exercise capacity may be expressed in
terms of V̇o2 max—the maximum O2 consumed by
the entire body— or in METS, which correspond to
multiples of resting O2 consumption achieved during
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Opinions/Hypotheses
exercise. In order to circumvent the need for respiratory gas analysis with each test, data are available to
convert the stage of exercise achieved into the
approximate number of METS achieved.3
The rate-pressure product provides an indirect
measure of myocardial O2 requirements. Evidence
of ischemia at values below 14,000 mm Hg 3 beats/
min suggests significant limitation of coronary flow.
Presumably, the ability to reach higher rate-pressure
products is associated with more adequate coronary
perfusion and a better prognosis.
The heart rate response during exercise has been
demonstrated to reflect cardiac function. For example,
when the heart rate does not rise adequately in response to a given workload, the patient is either a
conditioned athlete whose exercise capacity is far above
the established norms, or the patient has “chronotropic
incompetence.” The pathophysiologic condition underlying an inadequate heart rate response to exercise is
not well understood; some factors contributing to this
include the following: autonomic dysfunction, sinus
node disease, and left ventricular dysfunction.4 Thus,
the presence of chronotropic incompetence increases
the likelihood of a cardiac disorder (such as coronary
artery disease) and a poor prognosis.5–7
Using high peak heart rate to identify patients in low
risk groups is problematic. When a high workload is
achieved, a high heart rate may further improve prognosis.6,8 However, in patients with known coronary
artery disease, there is evidence of a poor prognosis
when a high heart rate is achieved at low workloads.9,10
Tachycardia at low workloads is associated with deconditioning or inadequate stroke volume,3 the latter of
which is itself a strongly negative prognostic factor. This
suggests a possible bimodal relationship of heart rate to
prognosis in which either high heart rates at low
workloads or inappropriately low heart rates are associated with adverse prognoses.
If maximal heart rate achieved does not provide
adequate information regarding prognosis, a
clinician should reasonably ask, “what does?”
Several large trials on the predictive value of
exercise ECG have shown peak heart rate to be
correlated with prognosis in univariate analysis.6,11,12
However, these studies, all of which included at least
500 patients, subsequently examined heart rate in
the context of other variables such as exercise duration, ie, METS achieved. The multivariate analyses
rendered the prognostic importance of target heart
rate statistically insignificant. These studies also indicated that, unlike heart rate, exercise duration is a
key independent prognostic variable.
Important studies which examined prognosis and
exercise duration/METS achieved were done by
Weiner and colleagues,13 using the Coronary Artery
Surgery Study patient registry. They found a mortality rate of # 1%/yr in women and men reaching 10
METS or higher. Even patients with triple-vessel
disease, defined angiographically, who achieved 10
METS, had an 86% 4-year survival.14 Because other
end points, such as nonfatal cardiac events, were not
examined in this study, at this point, one cannot
conclusively state that reaching 10 METS signifies a
strong likelihood of event-free survival.
Interestingly, other groups have examined the
utility of multifactorial indexes predicting outcome,
such as the Duke treadmill score.15 This score was
derived from both men and women and has been the
most validated of the multivariate prognostic
scores.16,17 This score, shown below, includes exercise duration in minutes, depth of ST-segment
changes, and onset of chest discomfort (which may
or may not be exercise limiting).
Duke Score 5 exercise duration (equivalent to METS)
25 3 (ST-segment depression/measured in mm)
24 3 (angina index: 0 for
no angina, 1 for nonlimiting angina,
2 for exercise-limiting angina)
A score of five or higher identifies a low risk
group (99% 4-year survival). In patients with a
negative exercise test result, achieving 7 METS or
more would correspond to a score of at least five
(see Appendix for calculation). A score of seven
METS should serve as a useful ballpark figure to
stratify patients into a low risk category. An important caveat, however, is that the only study
outside Duke affiliated hospitals which validated
this formula did not include women and did not
examine nonfatal end points.17 Furthermore, in
two recent studies, the Duke score has not performed well.18,19
As mentioned, although a negative ECG stress test
with a high exercise capacity indicates an excellent
prognosis, it does not necessarily imply the absence
of coronary artery disease. The sensitivity of the ETT
for coronary artery disease has been reported in a
widely quoted meta-analysis of 147 studies20 to be
between 23 and 100%, depending on the definition
of a negative test, patient selection, and other factors.
Weiner and colleagues14 reported that among patients with no ST changes achieving at least 10
METS, 55% had significant coronary artery disease.
The subgroup of patients with coronary disease,
including those with triple-vessel disease, who also
reached $ 10 METS had excellent survival rates.14
The ETT can divide patients with similar coronary
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anatomy into different prognostic groups. It is well
known that the angiogram and the ETT measure
different aspects of cardiac function.5 The angiogram
defines coronary artery disease at rest, whereas the
stress ECG is a test of cardiac function during exercise
and may partly reflect coronary flow with activity.
In conclusion, exercise ECG can provide extremely useful information to clinicians if the test is
adequate. Chronotropic competence may be necessary but in itself is not sufficient for a test to be
adequate. Rather than using an arbitrary heart rate
as a target to stop the test, symptom-limited testing
makes it possible to assess maximum performance.
An important question: What number of METS
can be most effectively used to stratify patients
into low-risk categories for subsequent events and
mortality? Unfortunately, that cannot be answered
at this time with complete confidence given the
above described limitations of the Duke treadmill
score and the study by Weiner and colleagues.14 In
a literature review of exercise testing in men and
women, Morris and coworkers21 stated that an
exercise capacity of , 6 METS predicts higher
mortality, whereas an exercise capacity of . 10
METS indicates excellent survival. They concluded that the specific minimum number of
METS carrying an excellent prognosis remains a
matter of debate.
We look forward to further research in this area.
In the meantime, as a practical guide to clinicians,
the following four points are suggested:
1. Stopping the ETT at an arbitrary heart rate risks
underestimation of the exercise capacity as measured
by METS. As recommended by several authorities,
the ETT should be terminated on the basis of
symptoms or maximal effort. Under these circumstances, a reduced exercise tolerance is indicative of
a poor prognosis.
2. Inappropriate rise of heart rate relative to
workload indicates poor cardiac function or, at best,
deconditioning.
3. If the effort tolerance is poor, prognosis is apt to
be poor even in the absence of ECG changes.
4. Effort tolerance of 7 to 10 METS with appropriate increased heart rate in the absence of ECG or
unfavorable clinical findings implies good prognosis
regardless of coronary artery disease19 and warrants
less aggressive management.
Appendix
Calculation for Exercise Duration Needed to
Achieve Low-Risk Score of 5 on Duke Formula
Duke Score 5 exercise duration
(equivalent to METS) 2 5 3
(ST-segment depression/measured in mm)
24 3 (angina index: 0 for no angina,
1 for nonlimiting angina,
2 for exercise-limiting angina)
5 5 exercise duration (min on Bruce protocol)
2 5 3 0 2 4 3 0 exercise duration
5 5 min 2 roughly equivalent to 7 METS
Achieving a Duke score of 5 implies excellent
survival. In order to achieve a score of at least 5, a
patient with a negative stress test— 0 mm ST change
and no angina (angina index 5 0)—needs an exercise
duration of at least 5 min on the Bruce protocol,
which is roughly equivalent to a minimum of 7
METS.
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