Comparison of the Ejection Time-Heart Rate Relationships in Normal and Ischemic Subjects

Transcription

1 Comparison of the Ejection Time-Heart Rate Relationships in Normal and Ischemic Subjects Clarence M. AGRESS, M.D., Stanley WEGNER, Shigeo NAKAKURA, M. D., Eugene H. LEHMAN, Jr., Ph. D., and Leonardo CHAIT, M. D. The ejection time-heart rate relationship was examined in athletes, sedentary normal, and coronary insufficient subjects undergoing treadmill, bicycle ergometer and 2-step exercise tests. Linear regression analyses were performed relating ejection time to heart rate. Changes in ejection time to changes in heart rate from rest to exercise were also examined. No differences of significance were demonstrated between the subject groups or the type of exercise performed even though the coronary insufficient patients manifested ischemic S-T segment changes and angina. It was concluded that measurement of ejection time during exercise at any heart rate level is not useful for the detection of ischemic heart disease. HE hope that the duration of cardiac phases would provide an index of heart function was first expressed by Katz1) in 1921: git may be found that the duration of cardiac phases in the diseased heart as compared to the normal, or the comparative changes induced in these periods by measured strains would give a method of determining the functional integrity of the myocardium h. Of all the phases, the ejection period has commanded the greatest attention, in part because of its ease of measurement. The inverse relationship between ejection time and heart rate was first noted by Garrod2) in 1871, who devised a formula relating these two parameters. Katz and Feil,3) using a similar formula found that in atrial fibrillation and in congestive heart failure, ejection time was shortened relative to heart rate. Most recently, Weissler et al.4) examined this same relationship and found again that ejection time, relative to heart rate, was shorter in subjects in failure than in normals. This finding was attributed to a low stroke volume in the patients with failure. It was the purpose of this investigation to compare the effect of exercise on the heart rate-ejection time relationship in normal subjects and subjects with ischemic heart disease in order to determine whether the ejection period can be used to detect impaired myocardial function. From the Institute for Medical Research, Cedars of Lebanon Division Center, 4751 Fountain Avenue, Los Angeles, California , Cedars-Sinai Medical 497

2 498 AGRESS, ET AL. Jap. Heart J. N ovember, 1965 METHOD Fifty-nine subjects were used in this study. Nineteen of these were highly competitive cyclists who were in peak physical condition; 21 were normal subjects without training; and 19 were patients with coronary disease manifested by angina or ordinary activity (Class III, New York Heart Association). The age range in the athlete group was 17 to 32 years (mean=26). In the sedentary normal groups it was 33 to 59 years (mean=51), and in the ischemic group it was 35 to 75 years (mean=53). All but one of the patients with coronary insufficiency had electrocardiographic proof of previous myocardial infarction and all had recurrent angina which interfered with their usual activities. During exercise testing, 17 out of 19 of the ischemic patients exhibited significant S-T segment depression;5) 9 had angina and several others had atypical chest discomfort. Two patients were unable to complete the test because of pain and dyspnea. The vibrocardiogram was used to obtain the measurement of ejection time in these experiments. This technique, which makes use of a displacement-sensitive transducer placed in the left parasternal area, has been described in previous reports.6) The reliability of this method for the measurement of cardiac events in animals has been established,7),8) and in the human its advantages over other techniques have been enumerated.9) In order to test further the validity of this technique for the measurement of ejection time, comparisons were made in 10 subjects between the values of ejection time as obtained by the vibrocardiogram and those measured from a carotid artery tracing. In each subject, comparisons between these values were made at rest and during two levels of exercise. Three methods of exercise testing were employed: the bicycle ergometer, the treadmill and the double Master 2-step test. Electrocardiograms, phonocardiograms (40-400cps.) and vibrocardiograms were recorded simultaneously. The recording instrument for the treadmill and ergometer studies was an Electronics for Medicine photographic recorder with a paper speed of 200mm./sec.; for the 2-step test, a Minneapolis Honeywell photographic recorder with a 250mm./sec. paper speed was used. Measurements of the vibrocardiographic intervals were made directly from the oscillographic charts. Heart rate was calculated from the R-R interval of the ECG of the cycle preceding the one used for the ejection time measurement. The bicycle ergometer test, used only in the athlete group, consisted of pedalling at a uniform rate (20Km. H) for 3 minute intervals at incremental loads of 0, 1.5, 2, and 3 kiloponds.* Recordings of vibrocardiograms were obtained sitting at each of these levels. The treadmill test was used in 10 normal and 10 ischemic subjects. This test which has been described by Bruce10) consists of walking at a speed of 1.73 mph. on a 10% grade for 10 min. Recordings were obtained standing, at rest and at the fourth and tenth minutes of exercise. The double 2-step test was used in the remaining 10 normal and 10 ischemic subjects and supine records were obtained at rest and immediately upon completion of the test. Linear regression analyses11) were performed relating ejection time (ET) to *1 Kilopond=the force acting on the mass of 1Kg. at normal acceleration of gravity.

3 Vol. 6 No. 6 EJECTION TIME-HEART RATE RELATIONSHIPS 499 heart rate (HR) both for the type of exercise and classification of subject. Regression analyses of the change in ejection time (delta ET) on the changes in heart rate (delta HR) from rest to exercise in each individual were also performed. RESULTS A record of the VbCg with the carotid trace and phonocardiogram illustrating the methods for measuring ejection time is presented in Fig. 1. The comparison of ET measurements obtained by these two methods is shown in Fig. 2 and the data given in Table I. The high correlation between these Table I

4 500 AGRESS, ET AL. Jap. Heart J. N ovember, 1965 Fig. 1. Methods by which ejection time measurements were obtained from the vibrocardiogram and the carotid pulse. Fig. 2. Ejection time measurements obtained by the vibrocardiogram compared to those obtained by the carotid trace.

5 Vol. 6 No. 6 EJECTION TIME-HEART RATE RELATIONSHIPS 501 Table II. Normal Master Test Data Table III. Ischemic Master Test Data

6 502 AGRESS, ET AL. Jap. Heart J. N ovember, 1965 Table IV. Normal Treadmill Data Table V. Ischemic Treadmill Data

7 Vol. 6 EJECTION TIME-HEART RATE RELATIONSHIPS 503 No. 6 Table VI. Athlete Ergometer Data

8 504 AGRESS, ET AL. Jap. Heart J. N ovember, 1965 two measurements (r=0.98), substantiates the use of the vibrocardiographic interval for the determination of ET. The data from the exercise tests are presented in Tables II through VI for the step test, treadmill and ergometer respectively. Figs. 8 and 9 summarize the HR-ET and delta HR-ET regression lines, and Fig. 10 compares the estimates of the slopes. The step test data for the normal and ischemic subjects are presented in Figs. 3 and 4. The regression equations for these two groups were not significantly different and the correlation coefficients were identical (r=0.87). The delta ET ~delta HR data showed much higher variability with r values of 0.70 for the normal and 0.46 for the ischemic subjects. The slope of the delta regression line showed greater differences between the groups (Fig. 10), than with the ET-HR slopes, but were still within one standard deviation of the estimate of the slope. Figs. 5 and 6 present the treadmill exercise data. A broader heart rate range was achieved with this form of exercise (50-120) than with the step test, and the correlation coefficients were higher (0.93 and 0.90). The regression slopes, however, were within one standard deviation. The delta ETdelta HR regression line showed a greater correlation than with the step test Fig. 3. Regression data obtained in the sedentary normal subjects. values are given in msec. and abscissa in beats per min. Ordinate

9 Vol. 6 EJECTION TIME-HEART RATE RELATIONSHIPS 505 No. 6 Fig. 4. Regression data for the ischemic double 2-step test. Note the greater difference between the ET-HR and delta ET-delta HR regression lines. Fig. 5. Regression data for the normal subjects undergoing treadmill exercise.

10 506 AGRESS, ET AL. Jap. Heart J. N ovember, 1965 Fig. 6. Regression data for the ischemic subject treadmill exercise. Note the similarity of the ET-HR regression lines to the other subjects and tests. Fig. 7. Regression data obtained from the athletes undergoing bicycle ergometer exercise.

11 Vol. 6 EJECTION TIME-HEART RATE RELATIONSHIPS 507 No. 6 Fig. 8. A summary of the ET-HR regression equations obtained in this study. Note that the standard deviations preclude any separation of the groups or types of tests. Fig. 9. A summary of the delta ET-delta HR regression data. The ischemic subjects tend to illustrate lower slopes, but the standard deviations do not permit definite separation.

12 508 AGRESS, ET AL. Jap. Heart J. November, 1965 Fig. 10. A comparison of HR-ET slopes. NM and IM=Normal and Ischemic Master Test, NT and IT=Normal and Ischemic Treadmill, A=Athletes Ergometer Test. (r=0.83, r=0.62). The slope differences were greater between normal and ischemic subjects, but were still within one standard deviation separation. The athlete bicycle ergometer data (Fig. 7) afforded the greatest HR range in this study (42-180). The correlation coefficients were 0.92 for ET- HR and 0.79 for delta ET-delta HR regression lines. In this group, the slope of the delta regression line was nearly the same as the ET-HR line. The range of the delta values was greater by a factor of 2 than with the other subjects. The regression data for all groups of subjects are summarized in Figs. 8, 9 and 10. The ET-HR lines (Fig. 8) were not significantly different either for the subject classification or the type of exercise employed. The delta ET -delta HR regression lines (Fig. 9) showed greater differences between groups, but the variation within groups far overshadowed this difference. The variation of the estimate of the slope (Fig. 10) further illustrates the overlap of the data in both the type of test and classification of subject. DISCUSSION The validity of the vibrocardiographic measurement of ejection time is well established by its high degree of correlation with ET determination obtained from the carotid pulse. The relationship between carotid and central aortic ET was established by Weissler et al.4) who found that the ET duration obtained from the carotid trace exceeded slightly that of the central aortic pulse. This was indirectly confirmed by this study, as the VbCg ET was consistently shorter than carotid ET (VbCg ET=0.91 carotid ET). The vibrocardiographic technique has advantages over the carotid trace method in that the ejection period is well defined and easy to measure even during exercise. The carotid trace is most difficult to record during exertion,

13 Vol. 6 EJECTION TIME-HEART RATE RELATIONSHIPS 509 No. 6 and further, a poorly defined incisura (Fig. 1) can introduce large measurement errors unless a simultaneous phonocardiogram is recorded. The regression data relating HR and ET compare favorably with other studies. Jones and Foster 12) examined the effects of HR, stroke volume, aortic diastolic pressure and ET and found that HR was the major factor in the determination of ET. Their data showed slightly lesser average slopes (-1.11) than were obtained in this study but were within the standard deviation range of our data. Weissler et al.4) demonstrated consistently higher slopes in a study of normal subjects under resting conditions (-2.1). He later reported a slope of -1.6 which was based on a more extensive investigation. 13) The coronary insufficient subjects in this study, despite the development of ischemic S-T segment changes and angina, demonstrated no significant deviations from the normal regression data. This finding indicates that the measurement of ET at any exercise HR level will not discriminate between the response of an athlete and that of a patient with advanced ischemic heart disease. Examination of the delta HR-delta ET data reveal differences between these groups due to the fact that the ischemic subjects had a lesser change in ET for a given incremental change in HR. However, the variation within groups was such that the significance of this observation is questionable. Since the coronary insufficient patients manifested ischemic ECG changes as well as anginal pain, it appears that any differences in the cardiodynamic factors which are involved in ET as measured during exercise are not revealed by this measurement. SUMMARY The relationship of ejection time to heart rate was examined in 19 athletes, 21 sedentary normal subjects and 19 patients with anginal ischemic heart disease. Exercise tests were performed using the step test, the bicycle ergometer and the treadmill. Regression analysis was applied to the data by regressing ET on HR and delta ET on delta HR. No significant differences were demonstrated between these groups even though the coronary insufficient patients manifested ischemic S-T segment changes and angina. Slight differences existed between the normal and ischemic groups when the delta ET-delta HR relationship was examined, but the variance of the data challenged the significance. The data indicated that at any level of heart rate induced by exercise,

14 510 AGRESS, ET AL. Jap. Heart J. November, 1965 the measurement of ejection time will not distinguish between trained athletes, normal sedentary subjects or coronary insufficient patients. REFERENCES 1. Katz, L. N.: J. Lab. Clin. Med. 6: 291, Garrod, A. H.: J. Anat. Physiol. 5: 17, Katz, L. N. and Feil, H. S.: Arch. Int. Med. 32: 672, Weissler, A. M., Peeler, R. G., and Roehll, W. H.: Am. Heart J. 62: 367, Mattingly, L. W.: The electrocardiogram in angina pectoris, coronary heart disease. Likoff and Moyer: Grune and Stratton, p , Agress, C. M., Fields, L. G., Wegner, S., Wilburne, M., Shickman, M. D., and Muller, R. M.: Am. J. Cardiol. 88: 22, Agress, C. M., Wegner, S., Bleifer, D.J., Estrin, H. M., Schroyer, K., and Labins, G.: Am. J. Cardiol. 13: 340, Agress, C. M., Wegner, S., and Nakakura, S.: Jap. Heart J. 5: 414, Agress, C. M., Wegner, S., Bleifer, D., Lindsey, A., Van Houten, J. Schreyer, K., and Estrin, H. M.: Am. J. Cardiol. 13: 226, Bruce, R. A.: Modern Concepts of Cardiovas. Dis. 25: 321, Snedecor, G. W.: Statistical methods applied to experiments in agriculture and biology. ed. 5, Ames Iowa, Iowa State College Press, Jones, W. B. and Foster, G. L.: J. Appl. Physiol. 19: 279, Weissler, A. M., Harris, L. C., and White, G. D.: J. Appl. Physiol. 18: 919, 1963.