TRANSIENT AUTONOMIC DYSFUNCTION PRECEDES ST-SEGMENT DEPRESSION IN PATIENTS WITH SYNDROME-X

Increased sympathetic drive has been suggested to play a role in the p athogenesis of syndrome X (angina pectoris, positive exercise testing, and angiographically normal coronary arteries). Heart rate variabilit y (HRV) studies have shown that patients with syndrome X have an imbal ance in autonomic nervous system activity (sympathetic predominance). However, it is not known if transient ST-segment depression which occu rs in syndrome X during daily activities is related to this autonomic nervous system dysfunction. This study investigates the relation betwe en the response of the autonomic nervous system, as assessed by HRV an alysis, and the occurrence of transient ST-segment depression during 2 4-hour ambulatory electrocardiographic monitoring in 23 patients (4 me n and 19 women, mean age 55 +/- 6 years) with syndrome X. The frequenc y-domain variables of HRV low-frequency (0.04 to 0.15 Hr) and high-fre quency (0.15 to 0.40 Hz) power were measured at 6-minute intervals dur ing the 30 minutes preceding the onset of transient ST-segment depress ion. Fourteen patients (61%) had greater than or equal to 1 episode of ST-segment depression in the 24 hours, whereas the remaining 9 patien ts (39%) had no significant ST-segment change. HRV measures differed a ccording to whether or not ST-segment depression was associated with i ncreased heart rate. Episodes of ST-segment depression associated with increased heart rate were preceded by a reduction of high-frequency p ower and on increase in the low-frequency-high-frequency ratio, wherea s episodes of ST-segment depression not associated with increased hear t rate showed no significant HPV changes. Low-frequency power remained unchanged irrespective of heart rate. Thus, in patients with syndrome X, a sympathovagal imbalance (sympathetic predominance due to vagal t one withdrawal) precedes episodes of ST-segment depression that are as sociated with an increased heart rate.