IMPAIRED BAROREFLEX SENSITIVITY AND SYMPATHOVAGAL BALANCE IN SYNDROME-X

Alterations of autonomic nervous control of cardiac function have been described in syndrome X. The characteristics, however, of the autonom ic control of the cardiovascular system in patients with syndrome X ha ve not been adequately studied thus, the aim of the present study was to investigate the role of baroreceptor sensitivity and sympathovagal balance in syndrome X. The study group included 12 patients with syndr ome X, 12 age- and sex-matched control patients with coronary artery d isease, and 12 age- and sex-marched controls with no evidence of heart disease. Baroreceptor sensitivity was evaluated by calculating the re gression line relating phenylephrine-induced increases in systolic blo od pressure to the attendant changes in the RR interval. Sympathovagal balance was assessed by using heart rate variability in the time and frequency domain and measuring plasma norepinephrine at rest and durin g incremental bicycle exercise. Baroreceptor sensitivity was significa ntly reduced in syndrome X compared with that in control normal subjec ts (7.4 +/- 1.2 vs 16.8 +/- 2.3 ms/mm Hg; p <0.02). This was associate d with a significantly lower percentage of adjacent normal RR interval s that differ by >50 ms, lower root-mean-square of the difference of a djacent RR intervals, and lower logarithmic value of the high-frequenc y component in patients with syndrome X compared with normal subjects. A nonsignificant trend toward lower baroreceptor sensitivity was foun d in patients with syndrome X compared with control ischemic patients (7.4 +/- 2 vs 12.2 +/- 1.3 ms/mm Hg). A nonsignificant trend toward a higher value of the low- to high frequency ratio was also observed in patients with syndrome X than in both control groups, No difference wa s detected in norepinephrine levels either at rest or during exercise or in the exercise-induced norepinephrine increase between the 3 group s. No difference was also observed between ischemic patients and norma l subjects in either baroreceptor sensitivity or heart rate variabilit y measurements. A significant correlation (r = 0.80, p <0.01) was foun d between baroreceptor sensitivity and the high-frequency component in normal controls but not for other measurements of autonomic function in the 3 groups. In conclusion, patients with syndrome X have an alter ed autonomic control of the cardiovascular system characterized by imp aired baroreceptor sensitivity and reduced heart rate variability. Abn ormal autonomic regulation of the cardiovascular system may be of path ophysiologic importance in syndrome X. (C)1998 by Excerpta Medica, Inc .