POWER SPECTRAL-ANALYSIS OF HEART-RATE-VARIABILITY DURING GRADED HEAD-UP TILTING IN PATIENTS WITH VASODEPRESSOR SYNCOPE

1. Two groups of age- and sex-matched subjects, eight healthy controls and 10 patients, suffering from recurrent vasodepressor syncope, part icipated in a study to examine autonomic function and sequential chang es in power distribution of heart rate (HR) variability during graded head-up tilt. 2. The following autonomic function tests were performed : valsalva ratio, HR responses to deep breathing and posture, BP respo nses to sustained handgrip and postural change. Each subject was tilte d at 15 degrees, 30 degrees, 45 degrees, 60 degrees and 80 degrees hea d-up, each for 15 min, or until symptoms occurred. The eight control s ubjects completed the tilt study without any symptoms, while all 10 pa tients developed presyncope and/or syncope at various tilt angles. 3. Resting blood pressure (BP) was lower in the patient group, while rest ing HR, autonomic function tests and resting HR variability components were similar in the two groups. 4. The control group showed a progres sive increase in low frequency power component (LF) from supine to end tilt (Delta LF 20.05 +/- 14.50%) and a progressive fall in high frequ ency (HF) component (Delta HF - 24.62 +/- 10.64%). In contrast, in the patient group, LF fell during tilt in the presyncope period (Delta LF - 10.57 +/- 12.93%, P < 0.01 vs control group). HF and HF:LF ratio re sponses did not differ significantly in the two groups. 5. At end tilt , the increase in plasma noradrenaline was significantly greater in th e control group than in the patient group (Delta NA 0.83 +/- 0.27 vs 0 .28 +/- 0.14 pmol/mL, P < 0.01). Plasma adrenaline: noradrenaline rati o at end tilt did not change in the control group, but increased signi ficantly in the patient group (supine 0.07 +/- 0.03, end tilt 0.16 +/- 0.09, P < 0.05). 6. It was concluded that sympathetic withdrawal play s a major role in the mechanism of vasodepressor syncope. Rey words: a utonomic function tests, blood pressure, high frequency power, low fre quency power.