Ca. Morillo et al., TIME AND FREQUENCY-DOMAIN ANALYSES OF HEART-RATE-VARIABILITY DURING ORTHOSTATIC - STRESS IN PATIENTS WITH NEURALLY-MEDIATED SYNCOPE, The American journal of cardiology, 74(12), 1994, pp. 1258-1262
The role of autonomic balance during upright tilt in patients with neu
rally mediated syncope is unclear. To assess the characteristics of au
tonomic 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 compar
ed with the following control groups: (1) 15 patients (mean age 33.5 y
ears) with greater than or equal to 2 episodes of recurrent syncope an
d a negative tilt response, and (2) 15 age- and sex-matched healthy vo
lunteers (mean age 34 years) with no previous history of presyncope or
syncope. Time domain measurements assessed were mean RR interval, sta
ndard deviation of normal RR intervals, and percentage of normal conse
cutive 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 calculat
ed in the supine position and during the first 5 minutes of upright ti
lt. No significant difference was observed in the time and frequency d
omain variables in the supine position between control groups with a n
egative 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% v
s 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 increas
ed 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 rat
io less than or equal to 6 during the first 5 minutes of tilt was high
ly predictive of syncope occurrence, subsequently noted at 15 +/- 6 mi
nutes of tilt (sensitivity 88%, specificity and positive predictive va
lue 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 parasympath
etic tone during upright tilt may play a role in determining susceptib
ility to recurrent neurally mediated syncope.