1. No category

Submaximal Exercise Testing in a Random Sample of an Elderly Population
C. P. RILEY, A. OBERMAN, T. D. LAMPTON and D. C. HURST
Circulation. 1970;42:43-52
doi: 10.1161/01.CIR.42.1.43
Circulation is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
Copyright © 1970 American Heart Association, Inc. All rights reserved.
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Submaximal Exercise Testing in a Random
Sample of an Elderly Population
By C. P. RiLEY, M.D., A. OBERMAN, M.D., T. D. LAMPTON, M.D.,
AND D. C. HURST, PH.D.
SUMMARY
To examine the electrocardiographic response to exercise in an older population, 280
participants in a longitudinal study of cerebrovascular disease were tested. Subjects
were randomly selected from the total 50 to 69-year-old population of Birmingham,
Alabama. Approximately 60% of the participants reached a heart rate more than 80%
of their predicted maximal heart rate.
Subjects were divided into three groups on clinical grounds: (I) vascular disease,
(II) risk factor(s) only, and (III) normal. Positive tests (at least 0.10 mv of ischemic
ST-segment depression) were most frequent in group I (24%), though not significantly
different from group II (19%) and group III (12%).
Hypercholesterolemia was significantly associated with a positive exercise test only
in those subjects with vascular disease. Nonspecific ST-T wave changes on the resting
electrocardiogram were associated with a positive test in all subjects, but with only a
borderline test in those subjects without vascular disease. No significant associations
were noted between abnormal exercise test and either cigarette smoking or hypertension.
Additional Indexing Words:
Cholesterol
Cigarette smoking
Blood pressure
Epidemiology
Coronary risk factors
Vascular disease
indicate that an abnormal electrocardiographic response to exercise infers an increased
risk of symptomatic coronary heart disease.'1-19 The prognostic and diagnostic
meaning of a positive exercise test in an older
population, especially in those individuals who
are otherwise clinically normal, is not known.
Controversy exists about virtually every
facet of exercise testing-the type of test
employed, criteria for a positive test, lead
selection, and types of recording devices.7
Most investigators would agree, however, that
the testing procedure and the interpretation of
results must be tailored to the population
being studied and the information being
sought. The purpose of this study is to
examine the electrocardiographic response to
exercise of an elderly population and to show
that the testing procedure employed is both
safe and feasible.
E XERCISE testing contributes to the
functional evaluation of patients with
cardiovascular diseases and facilitates the
diagnosis and management of patients with
ischemic heart disease.'-'0 Information on
exercise testing is largely limited to groups of
selected subjects, primarily male volunteers or
patients, and often to groups less than 50 years
old. Longitudinal studies of these groups
From the Departments of Medicine (Cardiology
Division), Public Health-Epidemiology, and Biostatisics, University of Alabama Medical Center, Birmingham, Alabama.
Presented in part at the American Heart Association's Conference on Cardiovascular Disease Epidemiology, New Orleans, March 3, 1969.
This study was supported in part by SRS Grant RD
1795-M.
Address for reprints: Dr. Albert Oberman, Spain
Rehabilitation Center, 1717-6th Avenue South,
Birmingham, Alabama 35233.
Received July 24, 1969; revision accepted for
publication March 18, 1970.
Circulation, Volume XLII, July 1970
Coronarv disease
Electrocardiography
43
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RILEY ET AL.
44
Subjects selected for exercise testing are
participating in the Birmingham Stroke Epidemiology Study, a prospective investigation
designed to identify the stroke-prone individual.20 21 Initially, the city of Birmingham,
Alabama (population 340,000), was divided
into seven subpopulations using the 1960
census tract data. Each subpopulation contained from six to 11 census tracts, grouped by
median age, race, median schooling, median
income, labor force (working and unemployed, male and female), and population per
household. Individual blocks were randomly
selected from a census tract, and everyone
between the ages of 50 and 69 in these blocks
was interviewed. In the city of Binningham
there were approximately 61,000 persons in
this age group. Eight thousand of these
subjects from selected blocks receive a layadministered health questionnaire; 2,000 were
done each year. The questionnaire is designed
specifically to elicit neurologic and cardiovascular symptoms, as well as provide an evaluation of general health. Of the 8,000 persons
being interviewed, a randomly selected sample of 3,000 receive a medical examination and
laboratory testing at a rate of 750 evaluations
per year. Each year of interviews and
examinations constitutes a complete subsample of the population.
Methods
Of the last 544 subjects examined, 280 persons
(51.5%) underwent submaximal exercise testing.
Table 1 shows the race, sex, and age distribution
of tested subjects. One hundred fifty-six subjects
(28.7%) were excluded for one or more medical
reasons. Physical deformity, acute illness, STsegment depression in lead V5, and dyspnea
comprised the majority of medical exclusions. One
Table
1
Race, Sex, and Age Distribution of Exercised
Subjects
Age
Race
White
White
Black
Black
Total
Sex
M
F
M
F
50-59 yr.
57
57
10
12
136
60-69 yr.
Total
47
69
18
10
144
104
126
28
22
280
hundred eight subjects (19.9%), although potentially testable, were not tested because of
unavailability of testing personnel or mechanical
difficulties; only three of these eligible subjects
refused to undergo the test.
Subjects report for examination in the fasting
state. Initial laboratory testing includes cholesterol, triglycerides, uric acid, protein-bound
iodine, VDRL (veneral disease research laboratory tests), glucose 2 hours after an oral load of
75 g of carbohydrate, urinalysis, vital capacity,
supraorbital thermography, resting electrocardiogram, and roentgenograms of the chest.
Submaximal exercise testing of subjects for
whom testing was considered safe or possible was
begun during the year 1967-68. Subjects selected
for this report were drawn from portions of two
sample years and do not constitute a complete
subsample of this population. A disproportionately large number of subjects from higher income
strata are included. This preliminary report
describes the exercise response of 280 (51.5%) of
the last 544 subjects examined.
Exercise testing was done on a motor-driven
treadmill by the method of Sheffield and
associates.6 All subjects had their oral carbohydrate load for blood glucose testing 1 hour before
resting and exercise electrocardiograms. Time and
personnel commitments required that an oral
glucose solution for subsequent (2-hour) blood
glucose testing be administered before resting and
exercise electrocardiograms were made. Smoking
was not permitted for the hour preceding the
electrocardiogram. Subjects were excluded from
testing if there was any suspicion of myocardial
infarction during the last year, moderately severe
dyspnea (inability to walk one level block or
climb one flight of stairs), pertinent physical
deformities or acute illness, or electrocardiographic abnormalities of left bundle-branch block,
second or third degree heart block, or ST-segment
depression in lead V5 (Minnesota code items 41,
42). All subjects taking digitalis preparations
were also excluded. Subjects with nonspecific STsegment or isolated T-wave changes (Minnesota
code items 43, 44, 51-53) were included in this
study.
Leads from an ECG cable were attached over
the clavicles at the midclavicular line and
anterior superior iliac spines and at V5 after
vigorous skin abrasion with acetone and a small
abrasive wheel. Recordings of V5 were obtained
continuously as an analog signal on magnetic tape
on a data acquisition cart as described by Caceres
and associates.22 23
The signal was simultaneously viewed on an
oscilloscope and recorded on paper for the last 6
of every 30 seconds during exercise and for 5
minutes after exercise. An attempt was made to
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SUBMAXIMAL EXERCISE TESTING
45
exercise each subject to 90% of his mean
predicted, maximal heart rate, as determined by
Lester and associates.24' 25 Exercise was begun at
1.7 mph and a 10% grade without any preceding
warmup period. The subject's target heart rate
was attained as quickly as possible by gradually
increasing both treadmill grade and speed and
was maintained for at least 2 minutes. The total
duration of exercise was approximately 5 minutes
for most subjects.
The test was terminated with the appearance
of a positive ST-segment response, rhythm or
conduction disturbance, chest pain, weakness,
undue fatigue, or dyspnea, or after 5 minutes of
exercise regardless of pulse rate attained. A test
was classified positive if ST-segment depression of
0.10 mv or greater occurred, beginning with Jpoint depression and continuing flat or downsloping for a duration at least equivalent to the
QRS duration before returning to the isoelectric
line, arbitrarily defined as the P-R (PQ) junction.
Similar ST-segment depression of 0.05 to 0.09 mv
was considered borderline. Tests showing frequent premature ventricular contractions or a
tachyarrhythmia during or after exercise, without
positive or borderline ST-segment depression,
were placed in a separate category. No distinction
was made between exercise and postexercise
criteria or electrocardiographic abnormality. STsegment depression was not adjusted for R-wave
voltage.
Using predetermined criteria, the same observer (C.P.R.) measured and graded all recordings.
To establish the validity of the electrocardiographic readings, Dr. L. T. Sheffield, University Hospital Heart Station, was asked to classify
30 randomly selected exercise electrocardiograms
as negative, borderline, or positive. Both observers
agreed perfectly on 24 tracings, differed
slightly on five tracings, and more on one tracing.
Of the six disagreements C.P.R. over-read two
and under-read four electrocardiograms in comparison with L.T.S. As a check on reliability the
observer randomly selected and re-read 10% of the
recordings. Variation between the two readings
small and random resulting in no change of
classification of any of the electrocardiograms.
Subjects tested were separated into three
groups: group I, subjects having clinical evidence
of coronary disease, peripheral vascular disease,
cerebrovascular disease, or electrocardiographic
evidence of myocardial infarction; group II,
subjects not in group I possessing one or more of
the following risk factors: (a) elevated blood
pressure (at least 160 mm Hg systolic or 95 mm
Hg diastolic, or both), (b) hypercholesterolemia
(serum cholesterol above 280 mg%), (c) current
cigarette smoking in any amount, (d) ST-T wave
abnormalities not meeting exclusion criteria, on
the resting electrocardiogram (Minnesota code
items 43, 44, 51-53)26; group III, subjects considered "normal" (that is, not in group I or II).
was
Results
One hundred eighty-nine subjects (67.5%)
had a negative test; 31 (11.1%) were borderline, and 53 (18.9%) were positive (table 2).
Seven subjects (2.5%) had a rhythm disturbance during exercise or within the first 2 minutes after exercise. All the rhythm disturbances were of short duration and terminated
spontaneously. No subject developed a myocardial infarction or sudden death shortly after
testing. No subject had symptomatic postural
hypotension, syncope, or prolonged chest
pain.
Figure 1 shows the relative sizes of two age
50 to 59 years old and 60 to 69 years
old, in each of the clinical groups. A larger
proportion of clinically normal subjects
(group III) was found in the younger age
group, and a larger proportion of subjects
with vascular disease (group I) was in the
older age group. In each of the clinical groups
groups,
Table 2
Distribution of Electrocardiographic Responses to Exercise by Clinical Groups
Negative
Clinical
groups
Vascular
disease
II. Risk
factor
III. "Normal"
Total
Exercise ECG response
Borderline
Positive
No.
No.
%
%
Arrhythmia
No.
%
Total
No.
%
28
59.6
6
12.8
12
24.5
1
2.2
47
122
66.7
78.0
67.5
22
3
31
12.0
6.0
11.1
35
6
53
19.1
12.0
18.9
4
2
2.2
4.0
2.5
183
50
280
I.
39
189
7
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46
RILEY ET AL.
loor
ELECTROCARDIOGRAPHIC
ABNORMALITIES N-91
801
PERCENT OCCURRING
m
6C
LI
I
lL&
I
0
%mIN GROLP
Only duringor immediately
AFTER EXERCISE(( 30Sec)
100_,C 80
_;
5 40
60
40 20
_tl
I;
B
I
-
POST- EXERCISE (C> 30 Seoc ).
_q 0_4
.z
20I 40 60 80 100
>
~,. -1 -I
- -- -
> ---
AGE GROUP
50-59
I-Vsas.orDioe
---
I
I
20
BORDERLINE
n=31
l
AGE GROUP
60-69
POSITIVE
n=42*
20.8% n=11
M-RIsk Facr M-Normai
100%
79.2'
.
Figure 1
Electrocardiographic exercise response in clinical
57%
ARRHYTHMIA
n=3
42.9%
groups by age.
2 OF THESE SUBJECTS WERE TRANSIENTLY
the relative percentage of positive tests
increased with age. Also more whites than
blacks and more men than women had
positive tests (fig. 2). These differences,
primarily in the 60 to 69-year-old age groups,
were not large enough to be significant at the
0.05 level. Further age-sex-race breakdowns
are indeterminable because of sample size.
Exercise electrocardiographic response varied in the three clinical groups (table 2). Of
B/ack Mole
Female
WhNte Male
White Femole
Black
30
M
F
20
Lz
w
F-
w
M
10
F
I
A
0
N
AGE
=
G9ROUPS
10
10
.-_.
57 57 18 12 69 47
50-59
50-5950-59 50-59
60-69
60-69 60-960-69
Figure 2
Percentage of positive electrocardiographic exercise
by race, age and sex among the 280
patients tested. N = number in each group tested.
Thus 10% of the 10 black males aged 50 to 59 years
tested had positive responses.
responses
POSITtVE DURING EXERCISE
Figure 3
Time of occurrence of exercise electrocardiographic
abnormalities.
the subjects tested 24.5% of those with vascular
disease, 19.1% of those in the risk factor group,
and 12.0% of the normals had positive tests.
If table 2 is set up as a contingency table
and a chi-square value computed, no statistically significant differences exist among the
three groups. The distribution patterns of the
three groups are very similar.
Figure 3 shows the time of occurrence of
exercise electrocardiographic abnormalities.
Bars to the left of the vertical line indicate the
percentage of subjects displaying a particular
abnormality who manifested this change only
during exercise or within the first 30 seconds
after exercise. These abnormalities would have
been missed without continuous monitoring of
the ECG signal. Bars to the right of the
vertical line indicate the percentage of subjects with abnormalities occurring in the
postexercise period. Two subjects who had
positive tests after exercise also had transiently positive tests during exercise. Figure 3 also
indicates the importance of continuous monitoring for the detection of serious rhythm
disturbances.
Figure 4 shows the cumulative percentage
of subjects attaining 50 to 90% of their
predicted maximal heart rates. The groups
having borderline and negative test responses
did not differ appreciably in percentage of
predicted maximal heart rates attained. Sixty
per cent of subjects with borderline and
negative tests achieved at least 80% of their
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SUBMAXIMAL EXERCISE TESTING
- Positive
47
l0(
Test
--Borderlirne & NegaiVe
8
7
.
j
80
Test
09
60
IA
o"
5
c
() 40-
II
0-St
003
2
II
20
11
Heart Rate
Percent predicted
maximal heartrte
88-92
106-110
50
60
123-128 140-147
158-166
Number of
0
Risk Factors
70
80
Number
in Group
90
0 a
-
40
119
2
3
4
0
83
18
4
59
114
2
3
4
82
21
4
% with
Figure 4
Vasc.Disorders 25
Cumulative percentage of subjects
heart rates attained.
versus
maximal
predicted maximal heart rate; these subjects
exercised sufficiently to produce a heart rate
ranging from 147 beats/min at age 50 to 140
beats/min at age 69. Fifty-eight per cent of
subjects with positive tests achieved similar
heart rates. Approximately 10% of subjects
with negative tests sustained heart rates of 160
beats/min or more without ill effects.
The number of risk factors present was
related to the prevalence of vascular disease in
both the tested and untested groups. While
more vascular disease exists in the untested
group, the patterns displayed are similar, an
increasing risk of vascular disease with an
increasing number of risk factors (fig. 5). The
distribution of risk factors in the tested
subjects was examined in detail to determine
whether a similar relationship existed between
the number of risk factors and the appearance
of a positive exercise test. Such a trend was
22.6 34.9 50.0 75.0 15.3
Figure 5
Number of risk factors versus
jects with vascular disease.
9.7 19.5 42.9 50.0
percentage of
suggested but was only just significant at the
0.05 level.
The relative risk is the ratio of the
prevalence of a positive or borderline exercise
ECG among those subjects with the risk factor
to the prevalence of the same exercise ECG
abnormality in those subjects without the risk
factor.27 Table 3 shows the relative risk of
different exercise electrocardiographic responses conferred by each of the four risk
factors measured. An elevated cholesterol was
associated with more positive tests in the
vascular disease group (P < 0.01). All of the
various combinations of risk factors were
examined in detail to see whether any
particular combination was more important.
No such relationship was apparent with the
number of cases available.
Table 3
Relative Risk of Positive and Borderline Exercise Tests for Selected Risk Factors
Clinical
group*
Risk
factor
Vascular disease
Smoker
Vascular disease
Chol. > 280
Vascular disease
BP > 160/95
Vascular disease
ST-Ta
No vascular disease Smoker
No vascular disease Chol. > 280
No vascular disease BP > 160/95
No vascular disease ST-TA
*Vascular disease
=
clinical group
Positive exercise test
Rel.
x2
P
risk
1.484
2.000
1.667
2.800
1.065
1.237
0.682
2.804
sub-
1.23
9.23
1.11
4.35
0.11
1.52
0.58
7.69
Borderline exercise test
Rel.
risk
NS
<0.01
NS
<0.05
NS
NS
NS
<0.01
I; no vascular disease
=
1.272
1.000
0.667
1.866
0.920
1.235
0.840
4.140
X2
0.23
0.00
0.19
0.73
0.11
0.99
0.09
14.89
clinical groups II and III.
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P
NS
NS
NS
NS
NS
NS
NS
<0.001
RILEY ET AL.
48
Because glucose loading by necessity preceded electrocardiography in the testing routine, the effects of this glucose ingestion on
resting and exercise electrocardiograms in this
population were determined in an independent and separate study to be reported in
detail later.28 Essentially, 35 subjects with
borderline or positive exercise electrocardiograms were retested on two separate days
being randomly assigned to either a glucose or
placebo solution 1 hour before the exercise.
Using the same method described in the
present study, prior glucose ingestion by
subjects resulted in the following changes in
the resting electrocardiogram: significant reduction in T-wave amplitude, an increase in
R-wave amplitude, an increase in heart rate
and further ST-segment depression of any
already present. Although maximal heart rates
attained during exercise after either glucose or
placebo were equivalent, positive and borderline tests occurred more frequently after
glucose. Exercise electrocardiographic classifications were significantly more abnormal after
glucose (P < 0.01). Computer measurements
of the S-T integral after exercise showed less
variability, but classifications based on computer analysis29 are not yet complete.
Discussion
Submaximal treadmill exercise testing of an
elderly population has been shown by this
study to be both safe and feasible. The lack of
complications may be attributed in part to
constant electrocardiographic monitoring with
immediate cessation of exercise after the
of rhythm disturbances or a
test.
Termination
of a test immediatepositive
ly with the onset of chest pain, undue fatigue,
appearance
dyspnea, or when the patient
complains of dizziness or appears hypotensive
also adds a large margin of safety. Up to 70%
of the examined population is considered
potentially testable. Most subjects excluded
had physical deformities, acute illness, dyspnea of at least moderate severity, or STsegment depression in lead V5 of the resting
electrocardiogram. A large number of subjects
were not tested at the beginning of this study
or
severe
because of initial mechanical difficulties or
unavailability of testing personnel. Our current rate of testing is approximately 60% of the
total group examined, or 86% of those eligible
for testing. Although many subjects failed to
attain the target (predicted) heart rates, the
amount of exercise they performed was
probably near-maximal for them because of
cardiovascular, pulmonary, and musculoskeletal limitation in this age group. Few subjects
had performed treadmill exercise before;
however, less than 1% of potentially testable
subjects refused the test.
Any test used in a prospective study of a
large population should be safe and feasible
and should provide a maximum of sensitivity
and specificity. Although the feasibility and
specificity of the standard Master's double
two-step test have been repeatedly demonstrated, the test lacks the sensitivity of
submaximal and maximal exercise testing.
Studies by Sheffield and associates6 provide
interesting data comparing the yield of
positive responses in subjects with ischemic
heart disease with graded submaximal exercise
(GXT) and the two-step exercise test. Overall, the GXT evoked a positive response in an
additional 24% of patients. An additional small
percentage of positive responses can be
obtained by pushing the patient to maximal
levels; however, safety, feasibility, and perhaps specificity suffer with maximal exercise
testing.
Estimation of the prevalence of ischemic
heart disease in a population is difficult
because of the large number of subjects with
clinically latent coronary disease.30 Longitudinal studies of subjects with an abnormal
exercise electrocardiogram have confirmed
that these subjects have an increased risk of
coronary heart disease morbidity and mortality. Robb and Marks17 found even mild
"ischemic" ST-segment depression (0.01 to
0.09 mv) to be associated with a coronary
heart disease mortality ratio of 2.5. In a group
of 81 manual workers 50 to 64 years old
studied by Astrand3' 19% subjected to "maximal" exercise testing had ischemic ST-segment
depression (0.05 mv or more). These subjects
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SUBMAXIMAL EXERCISE TESTING
attained a mean maximal heart rate of 157
beats/min. Berkson and associates32 demonstrated ischemic ST-segment depression (0.05
mv or more) in 20% of 49 men 40 to 59 years
of age. These subjects attained heart rates of
131 to 170 beats/min. In the study of
Blackburn and co-workers,10 less than 1% of
railwaymen (four of 570) between the ages of
45 and 64 years had ischemic type ST-segment
depression after a 3-minute treadmill walk at
3 mph on a 5% grade. Thirty per cent of our
280 subjects showed ST-segment depression
of 0.05 mv or more. Nineteen per cent demonstrated ST depression of 0.10 mv or more.
Assuming no reader discrepancies, two explanations for the differences between these
studies seem plausible, other than characteristics of the populations themselves; (1) the
differences are due to a higher prevalence
of latent ischemic heart disease in the population we studied; (2) prior glucose ingestion
by our subjects accentuated their ST-segment
response to exercise. The percentage of subjects showing 0.10 mv or more ST depression
in this study was essentially the same as the
percentage showing 0.05 mv or more ST depression in the studies by Astrand and Berkson. Glucose ingestion has been shown to accentuate ST depression,28' 335 and at least
some of the increased prevalence of positive
tests in this study might be explained on this
basis. The percentages of positive tests in these
three studies using different criteria are remarkably similar (19%, 19%, and 20%). The low
prevalence of abnormal exercise tests in the
study by Blackburn and associates'0 may be
due to the relatively low level of exercise performed.
Several longitudinal studies have identified
the following as coronary risk factors: cigarette smoking, elevated blood pressure, hypercholesterolemia, and nonspecific ST-T wave
changes on the resting electrocardiogram.3640
As expected, the more risk factors our subjects
had, the more prevalent vascular diseases
were. In some subjects minor ECG changes
may be the only manifestation of latent
coronary heart disease; however, the nonspecific and labile nature of these changes is well
49
recognized. We prefer to consider such
electrocardiographic changes as a risk factor
rather than evidence of the presence of
coronary heart disease.
Subjects without vascular disease in this
study but with ST-T wave changes on the
resting electrocardiogram had a significantly
(P <0.001) increased risk of a borderline
exercise test (0.05 to 0.09 mv). Both (the
vascular disease and the no vascular disease)
groups of subjects had an increased risk of a
positive exercise test in the presence of ST-T
wave changes on the resting ECG. Although
the relative risks of a positive test in the two
groups were equal (2.80), the relative risk in
the larger clinically normal group was more
significant. Such a response may have different
implications in the normal group from those in
the group with vascular disease. ST-T wave
changes are known to occur after ingestion of
a glucose solution.33-35 The electrocardiographic findings observed in subjects in this
study were definitely influenced by prior
glucose ingestion.28 Perhaps the borderline
and positive responses observed in normal
subjects were affected differently by glucose
than in subjects with vascular disease. Whether specificity of the exercise test was reduced
or sensitivity increased (or both or neither)
may be answered in the prospective portion of
our study.
An elevated serum cholesterol conferred a
significantly (P < 0.01) increased risk of a
positive exercise test in subjects with vascular
diseases but not in the remainder of subjects
tested. Berkson and co-workers32 and Bellet
and associates41 found an increased risk of an
abnormal exercise test in subjects with an
elevated cholesterol, but Keys' group42 did
not. Although findings in these studies are not
too dissimilar from ours, our results may have
other implications. Hypercholesterolemia increased the risk of a positive test (0.10 mv or
more), but not a borderline test (less than
0.10 mv), in those subjects who already had
clinically recognizable atherosclerosis. Assuming that a positive exercise test implies a
similar risk of coronary heart disease mortality
in both groups, an elevated cholesterol in
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RILEY ET AL.
so
subjects over 50 years old may or may not
have significance. In those subjects who have
somehow tolerated an elevated cholesterol
past the age of 50 without developing overt
atherosclerosis, an elevated cholesterol does
not confer an increased risk of coronary heart
disease mortality.43 4 Obviously, this is only
speculation, and prospective evaluation of
these subjects is necessary to clarify this
relationship.
Cigarette smoking in any amount did not
present a significantly increased relative risk
of a positive or a borderline exercise test. In
the study reported by Keys and associates,42 a
reversed trend was seen in smokers (that is,
fewer abnormal exercise tests among heavy
smokers than among other smokers or nonsmokers). Heavy smokers, however, have been
shown to have a poorer exercise performance
and a higher mean arterial pressure during
exercise.45 46 Because of elevated systolic
pressure and suspected alterations in myocardial metabolism, myocardial oxygen consumption per beat in heavy smokers is probably
higher. The stress to the coronary circulation
is correspondingly greater than in nonsmokers.
In this age group many heavy smokers have
pulmonary dysfunction which limits their
exercise performance. Attaining relatively
high heart rates (above 80% of predicted
maximal heart rate) is difficult. Determination
of the exact relationship of cigarette smoking
to the exercise electrocardiographic response
in this age group will be difficult.
Subjects with elevated blood pressure did
not have more positive or borderline tests.
Although this finding appears different from
that of Bellet and Roman,47 several differences
in these studies should be examined. Bellet
and Roman studied only hypertensive subjects. No attempt was made by them to
exclude those subjects who already had
ischemic ST-segment depression in lead V5 of
a resting electrocardiogram. We have excluded these subjects from exercise testing
because they already have evidence of ischemic heart disease (that is, subendocardial
ischemia as manifested by ST-segment depression in lead V5). These subjects usually show
further ST-segment depression during exercise, but this does not necessarily indicate
coronary heart disease. None of the subjects
we tested had systolic blood pressure of 200 or
more or diastolic blood pressure of 120 or
more. Most of the hypertensive subjects we
tested had blood pressures ranging from 165
to 180 systolic and 95 to 100 diastolic.
Analyses of larger numbers of our subjects
tested in the future will permit evaluation of
the exercise electrocardiographic response
versus different levels of blood pressure. Keys
and associates,42 showed a positive association
of abnormal exercise tests with extreme
obesity and the highest levels of blood
pressure only.
The increased prevalence of abnormal
exercise electrocardiograms with increasing
age confirms an earlier report.31 This finding is
consonant with existing knowledge of the
course of atherogenesis with advancing age.
The apparent lack of race or sex differences in
the distribution of exercise electrocardiographic responses is of interest, but numbers are too
small to draw conclusions.
The finding of ST-segment changes during
exercise in such a large proportion of clinically
normal subjects raises many questions about
the pathophysiology involved. Exercise testing
may indicate a pathophsiologic response but
does not reveal the underlying mechanism.
The abnormal ST-segment response during
exercise may be due to anxiety,48 hyperventilation,49 or vasoregulatory asthenia,j rather
than overt or latent coronary heart disease.
Near-maximal exercise in older, physically
unfit individuals may push the oxygen needs
of the heart to a point where even structurally
normal vessels may be unable to meet the
demands. This relative ischemia may lead to
alterations in myocardial metabolism resulting
in ST-segment changes. An altemative hypothesis is derived from the knowledge that even
younger individuals have at least slight to
moderate structural coronary disease.51* 52 In
the clinically normal group coronary flow
suffices for limited activity due to collateral
flow. During the stress of moderately severe
exercise collateral flow becomes insufficient
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SUBMAXIMAL EXERCISE TESTING
51
and myocardial ischemia results. Both morphologic and physiologic factors probably
play a role in the genesis of an abnormal
exercise test in this population.
Information from the prospective portion of
this study will resolve the question of whether
a positive ST-segment response infers a similar risk of death from coronary heart disease
in this older population as Robb and Marks17
have shown in middle-aged males.
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