Pathophysiology of Ischemia: Why are Some Episodes Silent?

Abstract

Myocardial ischemia occurring during daily life is characterized by a high frequency of silent ischemic episodes. Asymptomatic patients with severe coronary artery disease have only silent episodes while most symptomatic patients have both painful and silent episodes, with silent being prevalent. The extent and severity of coronary artery disease in asymptomatic patients is similar to that found in the symptomatic patients. In the symptomatic patients the silent episodes are accompanied by objective markers of ischemia that indicate that the silent episodes are as severe as the painful episodes. The silent episodes have a characteristic circadian variation with highest frequency between 6 AM and noon and lowest frequency between midnight and 6 AM. Silent episodes also occur at heart rate and activity levels lower than those achieved during exercise-induced angina, suggesting that myocardial oxygen supply is temporarily reduced. Recent data also suggests sympathetic activation and rise in heart rate occur during silent ischemic episodes. This would increase myocardial oxygen demand. All of these findings indicate that both supply- and demand-side mechanisms are important in the pathophysiology of silent ischemia.