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Gregory D. Curfman, M.D., and L. David Hillis, M.D.
Since the introduction of treadmill exercise testing by Robert Bruce five decades ago, graded exercise testing has been a cornerstone of diagnostic procedures for coronary artery disease. An article in this issue of the Journal from the Cleveland Clinic Foundation (pages 781�790), along with a previous Journal article from the same group,1 takes a new look at this frequently used test, focusing attention on the recovery period after the completion of exercise. Together, these studies show that careful observations during the period immediately after exercise provide important diagnostic and prognostic information; in so doing, they bring a new dimension to cardiac exercise testing.
Traditionally, interpretation of exercise testing has been based almost completely on observations made during exercise (see Table). For example, the maximal level of exercise achieved, usually expressed in metabolic equivalents (MET, which are multiples of the resting metabolic rate), has been shown to be a powerful predictor of prognosis. An article published recently in the Journal reported that an exercise capacity of less than 5 MET is associated with poor overall survival.2
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The electrocardiographic hallmark of exercise-induced myocardial ischemia is horizontal or down-sloping ST-segment depression of 1 mm or greater; marked ST-segment depression, particularly if it occurs at low levels of exercise, is often associated with severe coronary artery disease and a resultant poor prognosis. ST-segment elevation occurring in electrocardiographic leads lacking Q waves is also reflective of severe ischemia. Other important markers of exercise-induced myocardial ischemia include angina pectoris, ventricular arrhythmias,3 and inadequate response of blood pressure or heart rate to exercise (the latter is termed chronotropic incompetence). The Duke treadmill score, which incorporates into a single index the magnitude of ST-segment deviation, the presence or absence of angina, and the exercise capacity in MET, provides a useful estimate of prognosis.
In addition to observations made during exercise, those made during the recovery period (i.e., the several minutes after the termination of exercise) may provide important information. The prognostic implications of ST-segment depression that first appears during the recovery period are similar to those of ST-segment depression that develops during exercise. The data from the Cleveland Clinic further support the importance of continuing to monitor the patient and the electrocardiogram during the recovery period.
During graded exercise, the heart rate progressively increases, owing to an increase in activity of the sympathetic nervous system and a concomitant decrease in activity of the parasympathetic nervous system. On the termination of exercise, these changes in autonomic activity are reversed, and the heart rate decreases. The rate at which the heart rate decreases after exercise is a reflection of a person's level of physical fitness; as elite endurance athletes well know, the more rapid the decline, the higher the level of fitness. The data from the Cleveland Clinic show that a delay in the decrease in the heart rate after exercise, resulting from inadequate reactivation of vagal tone, predicts a poor outcome, with a quadrupling of the risk of death over the next six years. Thus, careful monitoring of the heart rate during the recovery period adds substantially to the value of exercise testing.
The new study from the Cleveland Clinic group builds further on these observations. The group found that the appearance of high-grade ventricular arrhythmias during the recovery period predicted subsequent mortality better than did the occurrence of ventricular arrhythmias during exercise. As with delayed slowing of the heart rate, the investigators suggest that the appearance of ventricular arrhythmias during recovery may be due to inadequate vagal reactivation, since vagal activity is known to suppress ventricular arrhythmias. Electrocardiographic tracings from one of their patients who had both delayed slowing of the heart rate and high-grade ventricular arrhythmia during recovery (see Figure) demonstrate the association of these two phenomena.
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At a time when radionuclide and echocardiographic imaging during exercise have become increasingly popular in cardiovascular diagnosis, exercise electrocardiography alone is still extremely useful. As the data from the investigators at the Cleveland Clinic clearly demonstrate, it is essential to monitor the electrocardiogram both during and after exercise
Source Information
From the Cardiovascular Division, Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas (L.D.H.).
References
- Cole CR, Blackstone EH, Pashkow FJ, Snader CE, Lauer MS.
Heart-rate recovery immediately after exercise as a predictor of mortality.
N Engl J Med 1999;341:1351-1357.
[Abstract/Full Text] - Myers J, Prakash M, Froelicher V, Do D, Partington S, Atwood JE.
Exercise capacity and mortality among men referred for exercise testing. N
Engl J Med 2002;346:793-801.
[Abstract/Full Text] - Jouven X, Zureik M, Desnos M, Courbon D, Ducimeti�re P.
Long-term outcome in asymptomatic men with exercise-induced premature
ventricular depolarizations. N Engl J Med 2000;343:826-833.
[Abstract/Full Text]