Changes in cardiac autonomic activities in patients with syndrome X: A study of spectral analysis of heart rate variability

The present study was designed to assess cardiac autonomic activities, coronary microvascular function, and their relationship in patients with syndrome X. Control of coronary blood flow is complex, and impaired coronary flow reserve has been attributed as the cause of myocardial ischemia in patients with syndrome X. It is unknown whether cardiac autonomic activities are altered in the presence of coronary microvascular dysfunction in patients with syndrome X. Eighteen patients with syndrome X were studied. Great cardiac vein flow was measured by the thermodilution method and the coronary flow reserve was determined by intravenous dipyridamole (0.56 mg/kg) infusion. Twenty-four-hour ambulatory electrocardiograms were obtained in a drug-free state. Another 14 age- and sex-matched normal subjects served as a control group. The amplitude (in ms) of ultralow (ULF), very-low (VLF), low (LF), and high (HF) frequency bands and total spectra of heart rate variability were measured for twenty-four-hour and every four-hour interval of the day. The mean coronary flow reserve was 2.12 ± 0.57 (range 1.33- 2.94) in 15 syndrome X patients. Compared with those in control subjects, mean LF and HF oscillations were lower in patients with syndrome X, especially in the morning and evening hours and the twenty-four-hour analysis. Circadian HF and LF activities were blunted. The LF/HF ratio tended to be higher. Among patients with syndrome X, ULF, VLF, HF bands, and total spectra were significantly lower in the morning and/or evening hours in the subgroup presenting a higher mean daily heart rate (n=9) than in the other subgroup with a lower rate (n=9). There was a positive correlation between coronary flow reserve and ULF activities in the morning (r=0.47-0.48, P=0.07- 0.08) and evening hours (r=0.67, P=0.006). The LF/HF ratio negatively correlated with coronary flow reserve in hours 1200-1600 (r=-0.55, P=0.03). Coronary microvascular function and cardiac autonomic activities were impaired in patients with syndrome X. The predominant parasympathetic withdrawal suggests loss of cholinergic vasodilation in coronary microvasculatures, especially as sympathetic tone was activated. Abnormal cardiac autonomic activities could either play a role in the pathogenesis or just be a mark of coronary microvascular dysfunction in these patients.