Rapid Resolution of Hyperkinesis After Exercise: Discussion
The major finding of this study was that maximal systolic wall thickening occurred within 2 min after exercise, then returned to baseline, corresponding to changes in hemodynamics.
Temporal Variability in Thickening
Exercise two-dimensional echocardiography is used to identify wall motion abnormalities that develop during ischemia in patients with coronary artery disease. When myocardial perfusion is inadequate during the increased oxygen consumption associated with exercise, wall motion is reduced. In contrast, normal myocardium, after exercise, is hyperkinetic, with increased wall thickening, decreased systolic cavity size, and essentially normal diastolic dimensions. However, there are few studies on the time course of normal systolic function after exercise. In an M-mode echocardiographic study of normal subjects, Berberich et al demonstrated a trend toward baseline function by 3 min after exercise; by 4 to 5 minutes, function had returned to baseline. In their study, cavity dimensions, not wall thicknesses, were measured.
In our study, the return of systolic wall thickening to baseline occurred with the return of hemodynamics toward normal. This reflects the influence of circulating catecholamines on both contractility (specifically, wall thickening) and heart rate. Since systolic wall thickening returned to baseline at 2 to 4 min after exercise, our study provides objective data in support of the recommendations of other authors that imaging should be completed within 2 min of peak exercise 13.16.20 jn order to assess maximal wall thickening.
Regional Heterogeneity of Wall Thickening
Our findings of regional variability in wall thickening at baseline are similar to previous studies using M-mode echocardiography. Corya et al demonstrated that the normal septum thickened by 36 ± 12 percent (range, 14 to 57 percent) compared with 47 ± 16 percent (range, 21 to 92 percent) in the normal posterior wall. A similar difference was also suggested in a study by Mason et al of normal subjects at rest and after exercise. Septal thickening increased from 56 ±3 (SEM) percent to 77 ±7 percent and posterior wall thickening increased from 89 ±9 percent to 115 ±8 percent with exercise. While these studies suggested a trend toward increased thickening in the posterior wall compared with the septum at rest, as well as with exercise, the statistical significance of the differences was not tested.
Similar to the M-mode study by Corya et al, we calculated wall thickness change in addition to percentage wall thickening. Thickness change is useful since it may be readily applied to computer-based analysis systems of two-dimensional echocardiograms that have calibrated reference bars for the measurement of relative changes in endocardial motion. In the study by Corya et al, wall thickeness change was 3± 1.2 mm (range, 1 to 5 mm) in the septum and 5± 1.88 mm (range, 3 to 11 mm) in the posterior wall. These measurements, obtained at rest, are similar to our results.
The mechanism for the regional heterogeneity in systolic function between the septum and posterior wall is uncertain. Possible explanations include regional variation in the response to loading conditions or increased longitudinal shortening of the left ventricular free wall compared with the ventricular septurn. These differences in regional loading conditions and longitudinal shortening probably explain the regional variation in recovery of normal function after exercise.