TIME AND FREQUENCY-DOMAIN ANALYSES OF HEART-RATE-VARIABILITY DURING ORTHOSTATIC - STRESS IN PATIENTS WITH NEURALLY-MEDIATED SYNCOPE

The role of autonomic balance during upright tilt in patients with neurally mediated syncope is unclear. To assess the characteristics of autonomic tone during orthostatic stress, 15 patients (mean age 32 years) with recurrent episodes of syncope (greater than or equal to 2) and a positive response to a 30-minute 60 degrees upright tilt were compared with the following control groups: (1) 15 patients (mean age 33.5 years) with greater than or equal to 2 episodes of recurrent syncope and a negative tilt response, and (2) 15 age- and sex-matched healthy volunteers (mean age 34 years) with no previous history of presyncope or syncope. Time domain measurements assessed were mean RR interval, standard deviation of normal RR intervals, and percentage of normal consecutive RR intervals differing by >50 ms. Frequency domain measurements of the low-frequency (LF) and high frequency (HF) bands were obtained, and the LF/HF ratio was also calculated. All variables were calculated in the supine position and during the first 5 minutes of upright tilt. No significant difference was observed in the time and frequency domain variables in the supine position between control groups with a negative head-up tilt response and the group with a positive response. The percentage of normal consecutive RR intervals differing by >50 ms during the first 5 minutes of head-up tilt was significantly higher in the group with positive tilt tests than in the controls (25 +/- 12% vs 7 +/- 4%, p <0.001). Patients with a positive tilt response did not experience equivalent increases in LF power with upright posture as in controls (40% vs 164%, p <0.0001). HF power was significantly increased in syncope patients with a positive tilt response during the first 5 minutes of head-up tilt (4.6 +/- 0.8 beats/min(2)/Hz) compared with the control group (1.1 +/- 0.5 beats/min(2)/Hz, p <0.01). An LF/HF ratio less than or equal to 6 during the first 5 minutes of tilt was highly predictive of syncope occurrence, subsequently noted at 15 +/- 6 minutes of tilt (sensitivity 88%, specificity and positive predictive value 100%). Sympathovagal balance during the first 5 minutes of head-up tilt was markedly abnormal in patients with neurally mediated syncope and a positive head-up tilt response. Failure to withdraw parasympathetic tone during upright tilt may play a role in determining susceptibility to recurrent neurally mediated syncope.