(Chest. 2001;120:1003-1013.)
© 2001
American College of Chest Physicians
Clinical and Exercise Test Predictors of All-Cause Mortality*
Results From > 6,000 Consecutive Referred Male Patients
Manish Prakash, MD;
Jonathan Myers, PhD;
Victor F. Froelicher, MD;
Rachel Marcus, MD;
Dat Do, MD;
Damayanthi Kalisetti, MD and
J. Edwin Atwood, MD
*
From the Division of Cardiovascular Medicine, Stanford University Medical Center and the University of California Irvine, Veterans Affairs Palo Alto Health Care System, Palo Alto, CA.
Correspondence to: Victor Froelicher, MD, Cardiology Division (111C), Veterans Affairs Palo Alto Health Care System, 3801 Miranda Ave, Palo Alto, CA 94304; e-mail: vicmd{at}aol.com
 |
Abstract
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Objective: To report the prevalence of abnormal
treadmill test responses and their association with mortality in a
large consecutive series of patients referred for standard exercise
tests, with testing performed and reported in a standardized
fashion.
Background: Exercise testing is widely
performed, but few databases exist of large numbers of consecutive
tests performed on patients referred for routine clinical purposes
using standardized methods. Even fewer of the available databases have
information regarding all-cause mortality as an outcome.
Methods: All patients referred for evaluation at two
university-affiliated Veterans Affairs medical centers who underwent
exercise treadmill testing for clinical indications between 1987 and
2000 were determined to be dead or alive using the Social Security
death index after a mean 6.2 years (median, 7 years) of follow-up.
Clinical and exercise test variables were collected prospectively
according to standard definitions; testing and data management were
performed in a standardized fashion using a computer-assisted protocol.
All-cause mortality was utilized as the end point for follow-up.
Standard survival analysis was performed, including Kaplan-Meier curves
and a Cox hazard model.
Results: There were 6,213 male
patients (mean ± SD age, 59 ± 11 years) who underwent standard
exercise ECG treadmill testing over the study period with a mean
follow-up duration of 6.2 ± 3.7 years. There were no complications
of testing in this clinically referred population, 78% of whom were
referred for chest pain, or risk factors or signs or symptoms of
ischemic heart disease. Overlapping thirds had typical angina or
history of myocardial infarction (MI). Five hundred seventy-nine
patients had prior coronary artery bypass surgery, and 522 patients had
a history of congestive heart failure (CHF). Indications for testing
were in accordance with published guidelines. Twenty percent died over
the follow-up period, for an average annual mortality rate of 2.6%.
Cox hazard function chose the following variables in rank order as
independently and significantly associated with time to death: exercise
capacity (metabolic equivalents < 5, age > 65 years, history of
CHF, and history of MI. A score based on these variables
(summing up the four variables [if yes = 1 point]) classified
patients into low-risk, medium-risk, and high-risk groups. The
high-risk group (score 
3) has a hazard ratio of 5.0 (95%
confidence interval, 4.7 to 5.3) and a 5-year mortality rate of
31%.
Conclusion: This comprehensive analysis provides
rates of various abnormal responses that can be expected in patients
referred for exercise testing at a typical medical center. Four simple
variables combined as a score powerfully stratified patients according
to prognosis.
Key Words: coronary artery disease • exercise testing • prognosis
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Introduction
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Exercise
testing is widely used clinically, and its many applications are the
subject of a number of national1
2
and international
guidelines.3
4
All of the guidelines relative to ischemic
heart disease recommend the standard exercise test as the first choice
for the evaluation of the patient who presents with chest pain and does
not have resting ECG abnormalities that affect the interpretation of
the exercise ECG response.5
6
Because the literature lacks
a broad picture of treadmill test utilization, it is important to
present a large clinical series of consecutive patients presenting for
a standard exercise test. The results can be used to provide the
prevalence of unusual responses such as ST-segment elevation,
exertional hypotension, bundle branch block, and ventricular
tachycardia. Multicenter studies have presented treadmill results
from large samples of patients. However, the patients are highly
selected to fit the study protocol (Coronary Artery Surgical
Study,7
Quantitative Exercise Testing and
Angiography Study Group 8
), and series from institutions
(Duke,9
Mayo,10
and Cleveland
Clinics11
) consist of target populations to answer
specific research questions. Other large databases of treadmill tests
with follow-up consist of asymptomatic individuals (US Air
Force,12
Cooper Clinic,13
Israeli Air
Force14
) rather than clinical patients. Analysis of a
large, clinically referred, consecutive population also provides an
opportunity to compare our indications and methods to exercise
laboratories throughout the Veterans Affairs (VA) medical system. The
latter is possible since we have assessed the behavior of VA exercise
laboratories using questionnaires.15
Moreover, the easy
availability of Social Security death index data now readily available
on the World Wide Web makes it possible to evaluate predictors of
all-cause mortality. The purpose of the present study was to
report the statistical distribution of treadmill test responses and
their association with mortality in a large consecutive series of
patients referred for standard exercise tests, with testing performed
and reported in a standardized fashion.
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Materials and Methods
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Population
The population consisted of consecutive patients referred to two
clinical exercise laboratories (Long Beach VA, from 1987 to 1991; Palo
Alto VA, 1992 to 2000) directed in consistent fashion by two of the
authors (V.F.F. and J.M.). Patients who were subjects in research
protocols were not considered in the analyses.
Data Collection
No imaging modality was performed in conjunction with the tests;
however, expired gases were measured in approximately one fifth of
patients. Both laboratories had affiliations with universities and had
academic medical staffs with rotating house officers and fellows. All
tests were supervised directly by these individuals or by nurse
practitioners; all tests were overread by two of the investigators
(V.F.F. and J.M.). A thorough clinical history, listing of medications,
and risk factors were recorded prospectively at the time of exercise
treadmill testing using computerized forms beginning in
1987.16
17
The forms included standard definitions of
clinical conditions and exercise responses.
Exercise Testing
Patients underwent symptom-limited treadmill testing using the
US Air Force School of Aerospace Medicine protocol18
or an
individualized ramp treadmill protocol.19
Before ramp
testing, the patients answered a questionnaire to estimate their
exercise capacity; this allowed most patients to reach maximal exercise
within the recommended range of 8 to 12 min.20
Patients
were encouraged to only use the handrails for balance. Heart rate
targets were not used as an end point or to judge the adequacy of the
test. Patients did not perform a cool-down walk but were placed in a
supine position as soon as possible after exercise.21
Treatment with medications was neither changed nor stopped prior to
testing. After careful skin preparation, a standard 12-lead ECG was
obtained before and continuously during exercise and for at least 5 min
into recovery.22
Visual ST-segment depression was measured at the J junction and
corrected for preexercise ST-segment depression while standing;
ST-segment slope was measured over the following 60 ms and classified
as upsloping, horizontal, or downsloping. The ST-segment response
considered was the most horizontal or downsloping ST-segment depression
in any lead except aVR during exercise or recovery. An abnormal
response was defined as 1 mm of horizontal or downsloping
ST-segment depression. Ventricular tachycardia was defined as a run of
three or more consecutive premature ventricular contractions (PVCs), as
previously described.23
BP was measured manually, and
metabolic equivalents (METs) were estimated from treadmill speed and
grade.19
Exertional hypotension was coded as either a
10-mm Hg drop in systolic BP (SBP) after a rise, or a drop of 10 mm Hg
below standing pretest. An exercise SBP code was considered in which
0 = increase > 40 mm Hg, 1 = 31 to 40 mm Hg, 2 = 21 to 30 mm
Hg, 3 = 11 to 20 mm Hg, 4 = 0 to 10 mm Hg, and 5 = drop below
standing pretest as previously defined.24
s and
products were not considered in the analyses. All ECG signals were
digitized and stored on compact disks after being recorded (Mortara
E-scribe; Milwaukee, WI, or QUEST; Burdick/Spacelabs; Milton WI).
No test result was classified as indeterminate.25
The exercise tests were performed, analyzed, and reported per standard
protocol and utilizing a computerized database (EXTRA; Mosby
Publishers; Chicago, IL).26
The textual report was
automatically downloaded into the VA centralized computer
database for distribution.27
Follow-up
The Social Security death index was used to match all
of the patients using name and social security number. The index is
updated weekly, and current information was used. Death status was
determined as of July 2000.
Statistical Methods
Number Crunching System Software (NCSS; Salt Lake City, UT) was
used for all statistical analyses after transferring the data from a
database (ACCESS; Microsoft; Redmond, WA). Total (all-cause)
mortality was used as the end point for follow-up for survival
analysis. Censoring was not performed since data regarding subsequent
interventions were not available for all patients. Survival analysis
was performed using Kaplan-Meier curves to compare variables and
cut-points, and the Cox hazard function was used to demonstrate which
variables were independently and significantly associated with time to
death. Automatic selection of variables was performed with a
Z value cutoff of 2 iterations and 20 iterations. Hazard
ratios were calculated along with their 95% confidence intervals.
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Results
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Population Demographics
This male study population had a mean (± SD) height of
69 ± 4 inches, a mean weight of 191 ± 34 lb, and a mean body mass
index (BMI) of 28 ± 9 kg/m2. Patients
who died were significantly older and had a lower BMI than survivors.
Overall, 74% were white, 9% were Hispanic, and 12% were black. There
was no significant difference in the survivorship among the different
ethnicities. Average resting heart rate was 78 ± 24 beats/min, with
a corresponding mean SBP of 133 ± 22 mm Hg. No significant
differences in these parameters were noted between those who survived
and those who died. Other relevant variables for the entire population,
for those who survived, and for the 1,256 who died over the mean
6.2-year follow-up period (median, 6 years) are presented with
significance levels in Table 1
. There was an average annual mortality of 2.6%. Table 2
provides the risk factor data in these groups. Risk factors for
coronary disease were not clustered among those who died; in fact, risk
factors trended to be clustered in those who survived. Table 3
provides data on the resting ECG. All of the major ECG abnormalities
were significantly more prevalent in those who died. Medications for
congestive heart failure (CHF), ischemia, and hypertension were more
frequently prescribed for those who died. All of the major medical
history items were significantly more prevalent in those who died,
including typical angina. The prevalence of prior coronary artery
bypass surgery was twice the rate in those who died compared to those
who survived.
Exercise Test Responses
No major complications were encountered during testing.
Results for the entire population, specifically for those who survived
and the 1,256 patients who died, along with significance levels for
differences are presented in Table 4
. While angina occurred more frequently during the exercise test among
those who died, there was only a trend for it to be more common as the
reason for stopping the exercise test. Abnormal
exercise-induced ST-segment depression, the most common ECG
abnormality, occurred in one fourth of our population, and two thirds
of the time it was silent or asymptomatic. Both symptomatic and
asymptomatic ischemia (ie, ST-segment depression) were
significantly more common in those who died. The next most prevalent
ECG abnormality was frequent PVCs and/or ventricular tachycardia (three
beats in a row or more), and this combined response was significantly
more prevalent in those who died. ST-segment elevation was rare but
more common over Q waves. ST-segment elevation was more frequent in the
patients with Q waves who died, but there was no difference in those
without Q waves. Exercise-induced ventricular conduction abnormalities
were also rare but more common in those who died.
There were no major differences in resting heart rate or BP, but
hemodynamic measurements during exercise were significantly lower in
those who died. Failure to reach the age-predicted heart rate occurred
in half the population and was significantly more prevalent in those
who died. Exercise-induced hypotension (EIH) was relatively rare but
significantly more prevalent in those who died. Both groups gave a
similar effort as reflected in the mean Borg scale rating of "very
hard" (Borg score of 17). The results for univariate Kaplan-Meier
survival curves for selected variables are presented in Figures 1
2
3
and Table 5
.
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Figure 2.. Kaplan-Meier survival curves for ECG exercise test
responses. Top, A: exercise-induced
bundle branch block. Middle, B:
exercise-induced ST-segment depression. Bottom,
C: exercise-induced frequent PVCs or ventricular
tachycardia (VT). See Table 4
for definition of abbreviation.
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Reasons for Testing
The reasons for testing are provided in Table 6
. Seventy-eight percent of patients were tested for evalutation of chest
pain, or evaluation of signs or symptoms possibly due to coronary
disease, or for elevated risk factors. Those who survived were
more likely to be tested for "softer" indications, while those who
died were more likely to be tested for symptoms suggestive of an
abnormality (ie, arrhythmia, angina).
Prognostic Score
Using stepwise selection, the proportional hazards model was
allowed to build on each variable group. Table 7
lists the chosen clinical variables, and Table 8
lists the exercise variables.
Among clinical variables, age, CHF, history of MI, and pulmonary
disease were significant predictors of death followed by the following
ECG variables: left ventricular hypertrophy (LVH), resting ST-segment
depression, resting left bundle branch block (LBBB), and resting
right bundle branch block (RBBB). The most powerful exercise variables
were METs and maximal heart rate. The only BP measurement chosen was an
exertional hypotension coding but not a drop below standing pretest.
Clinical variables alone were entered first with subsequent additions
of exercise variables to arrive at the final model consisting of the
following: exercise capacity (METs), age, history of MI and/or Q wave
on the ECG, and history of CHF. Notably, exercise-induced ECG
abnormalities were not chosen as associated with time until death. The
continuous variables (age, METs) were dichotomized using cut-points of
5 for METs and 65 years for age. When only these four variables were
entered in the model, the coefficients from the Cox model were 0.67,
0.73, 0.67, and 0.50, respectively, enabling construction of the
following score: METs < 5 (1 = yes, 0 = no), plus age > 65
years (1 = yes, 0 = no) plus history of CHF (1 = yes, 0 = no)
plus history of MI or Q wave on ECG (1 = yes, 0 = no), as shown in
Table 9 . The population was then coded as to how many of these variables each
subject had, and univariate survival statistics were performed using
Kaplan-Meier survival curves for 0, 1, 2, and 3 or more, since so few
patients had all four characteristics.
The hazard ratios, confidence intervals, and p values for score
values are shown in Table 10
, and the Kaplan-Meier survival curves are shown in Figure 4
. The score enabled the identification of a low-risk group (42% of the
cohort) with an annual mortality rate of < 1.5%, two
intermediate-risk groups (51% of the cohort) with an annual
mortality rate from 3 to < 5%, and a high-risk group (7% of
the cohort) with an annual mortality rate > 6%.
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Figure 4.. Kaplan-Meier survival curves for the score (the
score is calculated by summing up the four variables [METs < 5, age
> 65 years, history of CHF, and history of MI] with if yes = 1 for
each).
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Discussion
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The present results further define the clinical utility of the
exercise test and its diagnostic and prognostic applications. The
strengths of this study include an unusually long follow-up period
(median, 7 years), the inclusion of all consecutive clinical referrals
to two similar academically aligned VA medical centers, and the
application of standardized methodology. This study is one of many to
document the safety of "symptom and sign"-limited maximal exercise
testing. It is important to note that in this relatively high-risk
population, trained physicians or nurses were always in direct
attendance of the test and one of the senior authors was always
available for consultation.
Our clinical indications for the test were consistent with the American
College of Cardiology/American Heart Association (ACC/AHA)
guidelines2
and were similar to exercise laboratories
throughout the VA system.15
The highest percentage of
patients who underwent exercise testing in our study were tested for
the diagnosis of chest pain or evaluation of possible coronary disease
(78%; Table 6
) fitting the ACC/AHA class 1 indication; ie,
there is a general consensus that the tests were justified. Class 1
indications are defined as those assisting with the diagnosis of
coronary artery disease, assessing exercise capacity, or estimating
prognosis among patients with known heart disease, including
predischarge and postdischarge testing after an MI or
postrevascularization (percutaneous transluminal coronary
angioplasty/stenting and coronary artery bypass surgery). However, a
great deal of variation existed in terms of criteria for abnormal
results and whether physician presence is required during testing at
other VA medical centers.13
We used the same ST-segment
response criteria as most other VA medical centers, but only 28% of
respondents to our survey used some type of treadmill score, possibly
because of the lack of a score as simple as ours.
Our study provides additional data regarding the prevalence of a wide
range of exercise test responses and their prognostic impact, and
represents the findings among all consecutive clinically referred
patients rather than a selected group or a research population. Table 11
provides a comparison with prior studies in regard to the frequency of
the reported exercise test responses and mortality.28
Similarities of test responses such as hemodynamics and rates of
abnormal responses demonstrate that concerns about the VA not being
representative of other populations are unfounded. The higher rates in
prior studies for some of the variables are due to the fact that
most of their populations were clinically selected to undergo cardiac
catheterization (ie, had considerable workup
bias).28
The rates of abnormal responses, including
ventricular tachycardia, ST-segment elevation, and exertional
hypotension emphasize the need for proper training and prioritization
of issues for certification of individuals performing exercise testing.
In addition, the need for physician supervision needs to be better
defined.
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Table 11.. Prevalence of Major Exercise Test Responses From
Prior Follow-up Studies for Comparison With Our Study*
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While many variables are univariately associated with risk for death,
it is somewhat surprising that four simple variables provide the
majority of the important prognostic information. Table 12
lists the number of times the major prognostic variables were chosen as
significantly and independently predictive of time to death out of the
times they were considered in the published prognostic
studies.24
29
30
31
32
33
34
35
36
The four most prognostic variables in
the present study (age, METs, history of MI, and history of CHF) were
well represented in this tabulation.
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Table 12.. Frequency of Clinical and Exercise Test Variables
Chosen as Significantly and Independently Associated With Time Until
Death in Nine Previous Prognostic Studies
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Other exercise variables not chosen in the final multivariate model but
found to be univariately significant included maximum heart rate,
exercise-induced ST-segment depression, arrhythmias during exercise,
and EIH. EIH has been demonstrated in most studies to be associated
with either a poor prognosis or a high risk of angiographically
documented coronary disease.37
EIH has been associated
with cardiac complications during exercise testing, making measurement
of BP during testing of utmost importance. In our study, EIH had a
relative risk of 1.25 for all-cause mortality. Exercise-induced
frequent PVCs (> 6/min) had a relative risk of 1.8, and
exercise-induced ST-segment depression had a relative risk of 1.8 for
all-cause mortality during the median follow-up period of 7 years. The
failure of ventricular tachycardia to be independently predictive is
consistent with our previous study.23
The relative unimportance of the ischemic variables may be due to our
inability to censor on interventions for ischemia (ie,
removal of intervened patients from observation when the intervention
occurs during follow-up) and the consideration of all-cause mortality
instead of cardiovascular mortality. This may also explain why the
ischemic variables included in the Duke score that clearly had
diagnostic power38
did not predict all-cause mortality.
While all-cause mortality has advantages over cardiovascular mortality
as an end point,39
the Duke score was generated using the
end points of infarction and cardiovascular death. In addition,
interventions such as bypass surgery or catheter procedures were
censored in the Duke study (that is, subjects were removed from the
survival analysis when interventions occurred). Such censoring should
increase the association of ischemic variables with outcome by removing
patients whose disease has been alleviated, and thereby would not be as
likely to experience the outcome. We did not censor patients based on
whether they had a cardiovascular procedure during follow-up because we
do not have that information. From a previous study using a similar VA
patient population with an annual all-cause mortality of 3%, our group
found that 75% of deaths were cardiovascular, and that 6% of patients
were censored in follow-up due to bypass surgery.40
If the
proportions are similar in our current population, it would not be
unreasonable to expect a bias against the predictive power of these
variables. The contradictory results could also be due to the more
effective methods of treatment currently available for coronary
disease.
The use of interventions as end points falsely strengthens the
association of ischemic variables with end points since the ischemic
responses clinically result in the intervention being performed. While
some investigators have justified their use by requiring a time period
to expire after the test before using the intervention/procedure as an
end point, this still influences the associations between test
responses and end points. Another problem has been that variables
predicting infarction can be different than those predicting death, so
that variables are working against themselves when predicting infarct
or death (ie, infarct-free survival).
What do these findings mean to the clinician? First, it should be noted
that all studies have population-specific attributes that may be
difficult to define. Nevertheless, if the aim is to predict
infarct-free survival, the Duke treadmill score may be preferred to
ours since censoring was performed and infarct-free survival was
predicted. If diagnosis is the issue, either the Duke score or other
treadmill diagnostic scores are indicated.41
If diagnosis
is known, prognostication using our score can help direct therapy. If
diagnosis is not known and prognosis is guarded, then further
diagnostic efforts may be indicated. If diagnosis is not determined and
a patient is high risk by the score, then risk is likely to be improved
by an exercise program and risk factor modification. If prognosis is
favorable by our score, perhaps diagnosis is not as important as
alleviating symptoms. Our findings strengthen the importance of
exercise capacity, a reflection of the integrity of the cardiopulmonary
system and a marker of a physically active lifestyle, as an important
predictor of survival hopefully because of, but possibly at times in
spite of, modern medical treatment.
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Summary
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This comparatively long follow-up of a large number of consecutive
patients with minimal workup bias referred for exercise testing at two
VA medical centers provides rates of abnormal responses that can be
expected in clinical exercise laboratories adhering to the ACC/AHA
indications for testing. The study has demonstrated the prognostic
power of four simple pieces of information: exercise capacity, age, and
history and signs of MI and CHF. These variables can be used in a
simple additive score to powerfully stratify the expected risk of death
after modern medical treatment. Our simple score stratified 42% of our
population at low risk (< 1.5% annual mortality), 51% at
intermediate risk (3 to 5% annual mortality), and 7% at high risk
(> 6% annual mortality). Although ischemic ECG variables and other
hemodynamic variables were univariately associated with death, they
were not chosen in the hazard model as independently and significantly
predictive of time until death. It may well be that modern therapies
for these responses and the associated conditions are so good that they
are not chosen in the hazard model. While these data are unique in
terms of the size of the sample and consistency of methodology, the
major limitations of only having all-cause mortality, neither data on
MI as an end point nor interventional procedures for censoring, and the
lack of women must be kept in mind. Nevertheless, to our knowledge,
these data represent the largest series of consecutive male patients
referred for a standard exercise test, all tested according to
consistent and standardized methodology, and provide further evidence
of the importance of exercise capacity as a determinant of prognosis.
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Footnotes
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Abbreviations:
ACC/AHA = American College of Cardiology/American Heart Association;
BMI = body mass index; CHF = congestive heart failure;
EIH = exercise-induced hypotension; LBBB = left bundle branch
block; LVH = left ventricular hypertrophy; MET = metabolic
equivalent; MI = myocardial infarction; PVC = premature ventricular
contraction; RBBB = right bundle branch block; SBP = systolic BP;
VA = Veterans Affairs
Received for publication December 1, 2000.
Accepted for publication February 27, 2001.
|
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TOP
Abstract
Introduction
5 METs.
Hx = history.