Importance of Postexercise ECG

Jari Viik

Ragnar Granit Institute, Tampere University of Technology, Tampere, Finland

Correspondence: Jari Viik, Ragnar Granit Institute, Tampere University of Technology, P.O. Box 692, FIN-33101 Tampere, Finland.
E-mail: jari.viik@tut.fi, phone +358 3 3115 2158, fax +358 3 3115 2162

1.  Introduction

At the beginning of the era of exercise testing, ECG measurements were made only before and after stress. With technical development of ECG apparatus and signal analysis facilitating ECG measurements during stress, the main interest is currently focused on ECG changes during the exercise phase. However, recently investigators have been suggested that the diagnostic accuracy of the exercise test in the detection of coronary artery disease (CAD) can be improved by considering also ST-segment changes during recovery^1-3. Furthermore, several studies have demonstrated a good competence of the attenuated heart rate (HR) recovery after exercise in a prediction of mortality^4-6.

2.  ECG Responses to Exercise

2.1. Normal ECG responses

Besides the increase in HR, exercise-induced electrical changes can be seen in ECG waveforms. During exercise P-wave magnitude increases and the P-axis becomes more vertical. The T-wave magnitude decreases during early exercise and after exercise, but at maximum exercise increases. Changes in Q-wave are usually very small, but it may become slightly larger at maximal exercise. The R-wave amplitude is observed to decrease near maximal effort and the S-wave increases. The obvious response to the increase in HR is shortening of the PR, QRS and QT intervals. These changes occur in normal subjects and are usually related to a normal HR response⁷.

2.2. Exercise ECG in CAD detection

The most prominent abnormal response in ECG during the exercise test is an ST-segment deviation, mostly depression caused by subendocardial ischemia. ST-segment elevation is less common and has been associated with reciprocal changes for the ST depression, transmural or epicardial injury, and also coronary spasm. In addition to ST-segment deviation a deep T-wave inversion, an increase in R-wave, Q-waves, QRS changes and QT interval are considered to be sensitive in the detection of CAD⁸. However, there are also several studies yielding discrepant results, which would suggest that ST-segment deviation is still the most accurate exercise ECG variable for CAD detection⁸.

3.  Recovery Phase

It is generally assumed that early onset of ST-segment depression and its prolonged recovery after exercise signify more severe CAD⁹. Observation of the time course of ST depression during and after exercise was found to add significantly to the information gained during exercise testing. Other researchers have also stressed the importance of relating ST-T changes to the time of their occurrence during and after exercise. In addition ischemic ST changes developing during recovery has shown to have similar prognostic significance than changes appearing during exercise¹⁰.

3.1. ST-Segment Heart Rate Loop

Although the diagram of the ST-segment depression against the HR during the postexercise recovery phase was alleged by Bruce and McDonough¹¹ to be different for normal patients and for patients with ischemic heart disease as far as back 1969, this observation was only quantitatively proved in 1989 by Cornell group¹² with the HR recovery loop.  This dichotomous diagnostic variable records whether the ST depression at 1 minute of recovery is less or greater than that at matched HR during exercise.

The further development to the ST/HR loop has been focused to the comparison of ST values in exercise and recovery phases. The continuous variable ST/HR hysteresis^{1, 2} has been demonstrated to be more accurate in the detection of CAD than conventional parameters. ST/HR hysteresis represents the average difference in ST depressions between the exercise and recovery phases at an identical HR up to three minutes of recovery. This method requires computerized analysis and thus its implementation has been tardy.

3.2. Heart Rate Recovery

Recent studies have manifested the prognostic value of an attenuated HR recovery after exercise test^3-5. It has been shown to be an independent predictor of all-cause mortality. This phenomenon has though to be a marker of reduced parasympathetic activity.

4.  Conclusion

Despite the exercise ECG has been studied over 50 years in the detection of CAD and in prognosis, it is not at all completely explored. Recent studies have shown that improved detection of CAD and more reliable prognosis can be achieved using sophisticated method focusing to the recovery phase of exercise test.

References

1. Lehtinen R, Sievänen H, Viik J, Turjanmaa V, Niemelä K and Malmivuo J. Accurate detection of coronary artery disease by integrated analysis of the ST-segment depression/heart rate patterns during the exercise and recovery phases of the exercise electrocardiography test. Am J Cardiol 78:1002-1006, 1996.

2. Viik J, Lehtinen R, Väinö Turjanmaa V, Niemelä K and Malmivuo J. The effect of lead selection on traditional and heart rate-adjusted ST-segment analysis in the detection of coronary artery disease during exercise testing. Am Heart J 134:488-494, 1997.

3. Suurk¨¨la M, Arvidsson A, Fagerberg B, Bjurö T and Wikstrand J. A new method to quantify postexercise ST-deviation--the ST-deficit. A study in men at high and low-risk for coronary heart disease. Clin Physiol 21:541-55, 2001.

4. Cole, C, Blackstone, E, Pashkow, F, Snader, C and Lauer, M. Heart-rate recovery immediately after exercise as a predictor of mortality. N Engl J Med 341:1351-7, 1999.

5. Nishime EO, Cole CR, Blackstone EH, Pashkow FJ and Lauer MS. Heart rate recovery and treadmill exercise score as predictors of mortality in patients referred for exercise ECG. JAMA. 284(11):1392-1398, 2000.

6. Shetler K, Marcus R, Froelicher VF, et al. Heart rate recovery: validation and methodologic issues. J Am Coll Cardiol. 38(7):1980-1987, 2001.

7. Froelicher, VF and Myers, JN. Exercise and the Heart. 4th ed. Philadelphia, Pennsylvania. W.B. Saunders Company. 2000.

8. Viik, J. Diagnostic Properties of Exercise Electrocardiographic Leads and Variables in the Detection of Coronary Artery Disease. Doctoral dissertation, p. 4-5. Tampere, Finland. Tampere University of Technology, 2000.

9. Ellestad, MH, Thomas, L, Ong, R and Loh, J. The predictive value of the time course of ST segment depression during exercise testing in patients referred for coronary angiograms. Am Heart J 123:904-8, 1992.

10. Rywik, TM, Zink, RC, Gittings, NS, Khan, AA, Wright, JG, O'Connor, FC and Fleg, JL. Independent prognostic significance of ischemic ST-segment response limited to recovery from treadmill exercise in asymptomatic subjects. Circulation 97:2117-22, 1998.

11. Bruce, RA and McDonough, JR. Stress testing in screening for cardiovascular disease. Bull N Y Acad Med 45:1288-305, 1969.

12. Okin, PM, Ameisen, O and Kligfield, P. Recovery-phase patterns of ST segment depression in the heart rate domain. Identification of coronary artery disease by the rate-recovery loop. Circulation 80:533-41, 1989.