CIRCADIAN VARIATION OF ISCHEMIC THRESHOLD IN SYNDROME-X

To evaluate whether the ischemic threshold has a circadian rhythm in p atients with syndrome X, we analyzed 90 episodes of Si depression dete cted on 24-hour Halter recordings of 12 such patients. Ischemic thresh old was considered as heart rate (HR) at 1 mm ST depression. To correc t for differences in basal HR among patients, however, the ischemic th reshold was also calculated as a normalized index of HR at 1 mm ST dep ression: [(HR at 1 mm ST - 24-hour modal HR)/24-hour modal HR]. 100. M ean hourly values of both absolute and normalized HRs at 1 mm ST depre ssion were obtained by grouping and averaging respective values of all episodes detected in every hour of the day in all patients. Chronobio logic analysis was performed by single cosinor method. A significant c ircadian rhythm was found for HR (mesor 76 beats/min, amplitude 10 bea ts/min, acrophase at 2:16 P.M., p <0.001), number of episodes of ST de pression (mesor 3.75, amplitude 2.9, acrophase at 2:45 P.M., p <0.001) and cumulative time of ischemia, with a high correlation of distribut ions. Episodes of ST depression showed a double peak initially in the morning, and again in the afternoon. Both raw and normalized values of HR at 1 mm ST depression also had a significant circadian variation i n ischemic threshold, which was lower in the night and early morning h ours, progressively increased until the first afternoon hours, and sub sequently decreased in the evening. Normalized HR had a mesor of +20.5 %, amplitude 14.5%, and acrophase at 2:55 P.M. (p <0.01). Thus, our da ta demonstrate that ischemic episodes and ischemic threshold follow a significant circadian variation in patients with syndrome X. The simil arity between the circadian rhythms of the episodes of ST depression a nd HR suggests a significant role of the increase in myocardial oxygen demand in the induction of ischemia. The significant circadian rhythm in ischemic threshold, however, indicates that variations in vasomoto r tone have a remarkable role in the pathophysiologic mechanisms of tr ansient ST-segment depression in these patients.