BACKGROUND: The onset of acute myocardial infarction and sudden cardiac death has a circadian variation, with the peak occurrence between 6 AM and 12 noon. OBJECTIVES: To determine if a circadian variation exists for transient myocardial ischemia in patients admitted to the coronary care unit with unstable coronary syndromes. METHODS: The sample was selected from patients enrolled in a prospective clinical trial who had had ST-segment monitoring for at least 24 hours and had had at least one episode of transient ischemia. The 24-hour day was divided into 6-hour periods, and comparisons were made between the 4 periods. RESULTS: In 99 patients, 61 with acute myocardial infarction and 38 with unstable angina, a total of 264 (mean +/- SD, 3 +/- 2) ischemic events occurred. Patients were more likely to have ischemic events between 6 AM and noon than at other times. A greater proportion of patients complained of chest pain between 6 AM and noon than during the other 3 periods. However, more than half the patients never complained of chest pain during ischemia between 6 AM and noon. CONCLUSION: Transient ischemia occurs throughout the 24-hour day; however, ischemia occurs more often between 6 AM and noon. An important nursing intervention for detecting ischemia is continuous electrocardiographic monitoring of the ST segment, even during routine nursing care activities, which are often at a peak during the vulnerable morning hours.

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