Exercise tolerance testing in a cardiac rehabilitation setting: an exploratory study of its safety and practicality for exercise prescription and outcome data collection

Abstract

An exercise test is a valuable tool that should be a part of every patient's assessment before beginning cardiac rehabilitation. We analyzed data from one exercise tolerance test used in a cardiac rehabilitation program among 103 subjects: 65 men with a mean age of 60.5 years and 38 women with a mean age of 62.4 years. Resultsindicated that, after cardiac rehabilitation, subjects had significantimprovementin maximum metabolic equivalents (an increase of 0.9, P < 0.0001), which indicates functional capacity, and an improvement in rate of perceived exertion (decrease of 1 point; not statistically significant), which indicates more tolerance at the same work level. In general, men showed more improvement than women on the various outcome measures. Further, the testing protocolwas shown to be safe. Blood pressure values did not exceed 188/86 mm Hg, and maximum heart rate did not exceed 165 beats per minute. The increased practice of exercise testing before and after cardiac rehabilitation may help expedite the development of a standardized exercise tolerance protocol to optimize patient rehabilitation and recovery and document outcomes for both individual patients and the rehabilitation program as a whole.

Cardiac rehabilitation helps patients recovering from a cardiac event to restore and maintain their optimal physiological, psychological, social, vocational, and emotional status and reduce their risk of future cardiovascular events (1). The process consists of three phases: inpatient education, a 6-to 12-week outpatient supervised exercise program, and a maintenance phase of rehabilitation to promote heart health. Exercise physiologists and nurses typically supervise the 6-to 12-week outpatient program and prescribe exercise.

In its guidelines, the American Association of Cardiovascular and Pulmonary Rehabilitation recommends the use of an exercise pretest and posttest and highly supports analysis of outcomes (2), yet no standardized test exists for contemporary cardiac rehabilitation programs.

Interestingly, cardiac rehabilitation facilities are somewhat of a blend between clinical and exercise training settings. In clinical settings, stress tests are often performed within the first few weeks after a myocardial infarction for diagnostic purposes (3). These stress tests, usually called graded exercise tests, commonly call for treadmill speed and incline to increase progressively throughout the stages of the test. The tests terminate at a specific heart rate designated by the physician—often 70% of age-predicted maximum heart rate or 150 beats per minute (4). When clinicians see an order for a graded exercise test, they often use standard protocols such as Balke/Ware or Bruce without considering the use of other tests that may be more appropriate for the individual patient. Typically, stress test protocols used in clinical settings require the presence of a doctor, are chargeable to insurance, and are used for diagnostic purposes.

Fitness professionals regularly use a fitness type of test, such as step tests or the Rockport 1.5-mile walk, to assess clients before they begin an exercise program (5). Although these tests differ from the graded exercise test, they are physiologically sound and provide valuable information for use in prescribing exercise and documenting outcomes. Fitness test protocols do not require the presence of a physician, are not chargeable to insurance, and are typically used to help clients set and monitor exercise training goals.

While there is a need for some sort of evaluation before cardiac rehabilitation, no current diagnostic graded exercise test or fitness test seems to be appropriate. While traditional stress tests could be used to guide exercise prescription in cardiac rehabilitation, insurance carriers often won't cover numerous tests over a period of time, and many patients have already had one or more stress tests before coming in for rehabilitation. Patient safety and physician coverage are additional issues that have made the development of a standardized test more difficult in the cardiac rehabilitation setting. One test, the 6-minute walk, can be done both before and after cardiac rehabilitation without insurance charges or the presence of a physician; however, it merely measures distance walked in 6 minutes, rather than heart rate, blood pressure, and other physiological parameters that can be used as indicators of aerobic capacity for exercise prescription.

In investigating assessments for cardiac rehabilitation, we learned that the Methodist DeBakey Heart Center in Houston, Texas, has proactively implemented an exercise tolerance test (ETT), so named so as to distinguish between routinely recognized graded exercise tests and fitness tests. The test is administered to patients at the beginning and end of rehabilitation. For this article, we analyzed ETT results to determine the test's safety, effectiveness, and ability to provide outcomes data. Before describing our methods and results, we elaborate on why an ETT is needed.

THE CASE FOR USING AN EXERCISE TOLERANCE TEST

Currently, exercise is often prescribed based on cardiac rehabilitation staff's subjective evaluation of the patient's capabilities. The prescription is usually conservative to ensure that patients do not work harder than their hearts can safely handle (6). Many clinicians prescribe exercise intensity using a rate of perceived exertion (RPE) range of 11 to 15 (2). Another common method for exercise prescription intensity at the beginning of cardiac rehabilitation is to work at a heart rate of resting plus 20 to 30 beats per minute if a graded exercise test is not performed. This method is used because heart rates of 30 to 40 beats per minute above standing have been shown in hospital data to be near maximal during graded exercise tests for many patients. Physiologically, patients on beta-blockers would respond similarly to those not on beta-blockers (7). However, this method is a general rule of thumb; thus, some should exercise at higher relative heart rates.

Individuals also have physiological differences that should be considered in the design of their exercise programs. Women have a lower exercise capacity than men when corrected for weight, possibly due to their lower blood volume, lower hemoglobin concentration, lower blood oxygen content, and smaller heart size compared with men (8). Additionally, age also appears to affect an individual's maximal aerobic capacity, which will influence the appropriate beginning exercise intensity (9). Maximum heart rate also decreases with age. This decrease may be due in part to the development of diastolic stiffness, which may slow myocardial blood flow (10).

The limitations of the generic conservative approach and of individuals’ physiologic differences can be overcome with appropriate patient testing. With ETTs, a patient's functional capacity can be defined and an individualized exercise program can be prescribed to help the patient recover quickly and safely and return to a normal lifestyle (3). An ETT can identify the metabolic equivalents (METs) that a patient is capable of achieving. While one test may not be appropriate for all patients due to various physical issues, a physiological evaluation can be conducted in some capacity, regardless of the patient's condition. In a group of elderly cardiac rehabilitation patients, Marchionni et al showed that appropriate tests should enable professionals to evaluate patients on various exercise modes to accommodate physical abilities and evaluate specific training effects produced during cardiac rehabilitation (11).

In addition to guiding the appropriate prescription of exercise, ETTs have many other benefits. They can give patients confidence that they can safely perform physical activity and avoid needless limitations. When posttests are added after the program, the ETTs serve as an outcomes tool for evaluating the quality and effectiveness of cardiac rehabilitation programs (12). The pretest and posttest ETTs allow patients to experience a sense of accomplishment and increase their confidence in their ability to be physically active. Additionally, they allow patients to compare themselves with others of the same age, sex, and diagnosis, which might be a motivation tool (13).

As mentioned, choosing a test to use in this context has its challenges. However, Ellestad (3) analyzed four popular treadmill protocols currently used in the clinical setting to determine the relative heart rate response and oxygen cost and found that the results of most protocols were very similar, despite some variation in the rate at which the workload increases. This may suggest that a standardized ETT for cardiac rehabilitation can be developed that is safe and effective and provides outcomes.

METHODS

ETT protocol

Patients were familiarized with the RPE scale and signs and symptoms of cardiac problems. They were familiarized with the treadmill and then performed the exercise protocol. The protocol was a 10-minute test with 1-minute stages. The patient began walking on a treadmill with 0% grade at 1.2 mph. Speed increased through minute 4, reaching 3 mph; then grade increased from 1% at minute 5 to 5% at minute 10. The testing was stopped upon patient request or if the patient achieved one of the following: a heart rate greater than resting plus 30 beats per minute, an RPE greater than 15, a blood pressure greater than 200 mm Hg systolic or 100 mm Hg diastolic, or signs and symptoms of cardiac stress. Patients who were not capable of performing the treadmill test were tested on a bike—again a 10-minute test with 1-minute stages, starting at 0.2 kilopond and increasing 0.2 kilopond every minute.

Study group

The exercise protocol study was conducted on patients with documented coronary artery disease. The subjects had recently undergone heart surgery, had a myocardial infarction, or had heart failure. A total of 148 assessments comprised results from tests of 65 men (age, 60.5 ± 11.28 years; weight, 202 ± 39.3 lbs; height, 69.8 ± 3.1 in) and 38 women (age, 62.4 ± 12.6 years; weight, 171 ± 41.8 lbs; height, 64.5 ± 3.6 in). The subjects were analyzed in two groups: those that dropped out of the study (dropout group) and those that completed the entire protocol (pretest, program, posttest;referred to as the full-protocol group). Pretest and posttest results among the full-protocol group were compared, along with results of men and women. The key measures analyzed included maximum heart rate, RPE, systolic and diastolic blood pressure, and maximum METs.

Statistical analysis

Given the exploratory nature of this study, we felt that as many between-group differences as possible should be examined. These differences were examined through analysis of variance (ANOVA). P < 0.05 was considered significant. However, we also analyzed directional results at P < 0.10 due to the exploratory nature of the study. A statistical software package (SPSS, version 13, Chicago, IL) was used for the statistical analyses.

RESULTS

As evident among the means for various measures in the Table, there was considerable variability in the data as a whole. All of the exercise protocol variables were within acceptable limits: mean exercise time, maximum heart rate, maximum METs, and RPE.

Table.

Mean exercise protocol study results by timing of test (pre vs post), sex, and group (dropout vs full protocol)^∗

Group	N	Time (min)	Max HR (beats/min)	Systolic BP (mm Hg)	Diastolic BP (mm Hg)	Max METs	RPE
All participants
All tests	148	6:35	100.1	134.1	65.9	4.1	13.5
Dropout group
Pretests	58	6:07	102.2	133	68.8	3.8	13.7
Women	18	4:10§	106.8	138.7§	68.8	3.0§	14.4†
Men	40	7:00	100.2	130.4	68.9	4.1	13.3
Full-protocol group
Pretests	45	5:50§	101.2	134.7	65.4	3.6§	13.8¶
Women	20	5:18	100.9	142.8†	69.9‡	3.5	14.4
Men	25	7:17	101.4	128.2	61.8	3.8	13.3
Posttests	45	7:56	99.4	134.9	62.8	4.5	12.9
Women	20	7:06†	94.5	135.8	64.7	4.2†	13.3
Men	25	8:36	103.1	134.2	61.2	4.8	12.6

∗ANOVA tests compared pretests and posttests for the full-protocol group. They also compared women and men through all phases of the study. Significance levels were as follows

†P < 0.05

‡P < 0.01

§P < 0.001

¶P < 0.10. Max indicates maximum; HR, heart rate; BP, blood pressure; MET, metabolic equivalent; RPE, rate of perceived exertion.

Comparison of results for men and women

Men had a much higher dropout rate than women (61.5% vs 47.3%). In cardiac rehabilitation, the patient dropout rate is 20% to 25% when patients start rehabilitation within 3 weeks of a cardiac event and 40% to 50% when patients start rehabilitation 6 to 12 months after an event (14). The sex comparison for the dropout group yielded the largest absolute difference in means for METs (women 3.0 vs men 4.1; P < 0.001). Similar results were found with RPE (women 14.4 vs men 13.3; P < 0.05). The exercise time and systolic blood pressures also showed significant differences between the sexes (P < 0.001 for both).

Among the full-protocol group, men had higher mean pretest scores for exercise time, maximum heart rate, and maximum METs;they also had lower RPE scores. Women had significantly higher systolic and diastolic blood pressures (P < 0.05 and P < 0.01, respectively). The posttest results for gender disparity again yielded more favorable results for men.

Comparison of pretest and posttest results

In the full-protocol group, two statistically significant outcome differences were noted: exercise time (P < 0.0001) and maximum METs (P < 0.0001). After cardiac rehabilitation, patients exercised a mean of 2 minutes and 8 seconds longer and increased their physiologic capacity by 0.9 METs. While they were also able to decrease their RPE rating by nearly 1 point (indicating more tolerance at the same work level), that change was not statistically significant. Greater changes in these measures would be key indicators of improved health and improved outcomes. When pretest and posttest results were compared only for women or only for men, the differences remained, suggesting that both sexes were able to benefit from cardiac rehabilitation.

DISCUSSION

This study investigated an ETT with emphasis on comparing results for men and women enrolled in cardiac rehabilitation to see if the test was safe and effective and provided outcomes data. The results identify some important, although exploratory, findings.

First, physiological response data showed that the test protocol was safe. Both systolic and diastolic blood pressures did not begin to approach the safe upper limit for exercise testing, 250/115 mm Hg (5). Further, maximum heart rates did not exceed 110 beats per minute, which is within the common range of resting heart rate plus 20 to 30 beats per minute given in exercise prescription.

Second, the test protocol provided outcomes. Maximum METs increased by 0.9 after cardiac rehabilitation, a difference that was statistically significant. The corresponding 0.9-point decrease in RPE in the full-protocol group was not statistically significant. Typically in cardiac rehabilitation, patients should increase their physiological capacity by 1 MET every 2 weeks (15). If we assume that patients in this study were prescribed such a progression, the test protocol needs to allow them to demonstrate their actual aerobic capabilities. Here, METs increased by only 0.9 after several weeks, suggesting that the protocol did not allow them to demonstrate their actual aerobic capabilities. Therefore, this test should be viewed as a minimal test, and a more challenging test within the confines of not requiring a charge to insurance or a doctor's presence should be pursued.

Some limitations to the study should be noted. First, the details of the testing protocol and its application to the subjects’ exercise prescription were not available; the pretest and post-test cardiac rehabilitation data were simply used to examine the safety and outcomes of the program. In addition, the significant difference between pretest and posttest values cannot be attributed completely to the cardiac rehabilitation program because of factors that cannot be controlled for, such as exercise outside of supervised cardiac rehabilitation.

Nevertheless, the examination of this test, which is safe, not charged to patients or insurance, and does not require the presence of a physician, provides evidence for the valuable use of an ETT in every cardiac rehabilitation program. Results can demonstrate both patients’ improvement and the efficacy of cardiac rehabilitation. While only one test protocol was examined, future research can develop a standard testing protocol that can be used before and after cardiac rehabilitation at all facilities nationally and globally. A standardized ETT can help change the way cardiac rehabilitation operates through safe and effective assessments of patients. The great diversity of methods used to conduct and investigate exercise protocols suggests that this is an area that needs further analysis.

Acknowledgments

Most sincere thanks to Cynthia Orticio, MA, ELS, for her help in the formulation of this manuscript.