Unfavorable outcome in patients with primary electrical disease who survive ventricular fibrillation.

ischemia is an incomplete gauge of the myocardial 02 demand present at the time. Perhaps the apparent changes in "ischemic threshold" during the day described by Tzivoni and colleagues3 may be explained in part by fluctuations in the baseline heart rate or the duration of heart rate increases at different times of the day rather than solely to changes in coronary tone, as the investigators suggest. The results of our study underscore the complexity of the pathophysiological mechanisms leading to ischemia during daily activities and suggest that future studies investigating pathophysiology based on heart rate changes should be refined by incorporating the variables of baseline heart rate, magnitude of heart rate increase, and duration of heart rate increases before implicating changes in coronary tone.