Circadian variations in cardiovascular function and their relation to the occurrence and timing of cardiac events.

Abstract

Circadian patterns of risk for cardiac events and their implications for prevention and treatment of cardiovascular conditions are discussed. Sympathovagal tone, a major biological determinant of circadian variation in cardiovascular function, is modulated through circadian patterns of sleep-wake activity. The influence of neurohumoral activity on cardiovascular function is manifested by 24-hour variability in heart rate, blood pressure, and vasomotor tone. Platelet aggregation and plasminogen-activator inhibitor-1 activity peak around the time of awakening. Conversely, endogenous plasminogen activator exhibits a nadir around the time of awakening. Studies of patterns of occurrence of cardiac disorders such as acute myocardial infarction, sudden cardiac death, stroke, and ventricular arrhythmia show an increased occurrence during the period surrounding awakening. These patterns are consistent with observed circadian patterns in cardiovascular function. Diabetes, left ventricular dysfunction, and congestive heart failure may contribute to alterations in patterns of occurrence of cardiac events. Factors such as race, sex, and age may lead to alterations in circadian variation in cardiovascular function. Unusual physical exertion, stress, and anger may act as triggers of an event at any time of day. The circadian patterns of cardiac events follow the natural fluctuations in endogenous physiological processes, with a vulnerable period consistently observed in the early morning; dynamic assessment of markers of cardiovascular function may assist in determining the extent of disease progression and in selecting cardiovascular therapies.