Head rotation during upright tilt increases cardiovagal baroreflex sensitivity.

INTRODUCTION

Vestibular activation of semicircular canals has been linked to increased activity of parasympathetic neurons and has been proposed as a contributing factor to orthostatic intolerance. However, the functional consequences of head yaw rotation during orthostasis on human autonomic function have not been documented. The purpose of this study was to determine the effects of voluntary head rotation on autonomic function during upright tilt.

METHODS

ECG, beat-by-beat finger arterial pressure, respiratory rate, PetCO2, and cerebral blood flow velocity were recorded in 10 healthy subjects. Subjects were studied supine and during two separate 5-min periods of upright tilt to 70 degrees with and without continuous head yaw rotation (180 degrees at 0.25 Hz). Breathing frequency was controlled strictly at 0.25 Hz during each trial and data were analyzed in both time and frequency domains. Cross-spectral techniques (transfer functions) were used to assess cardiovagal baroreflex sensitivity and dynamic cerebral autoregulation.

RESULTS

Head rotation during upright tilt increased transfer function magnitude between systolic pressure and R-R interval at the low frequency (0.04-0.15 Hz) from 8.1 +/- 2.3 to 10.4 +/- 1.6 ms x mmHg(-1) (p = 0.04), but did not affect estimates of vagal-cardiac efferent activity or cerebral blood flow velocity, hemodynamic oscillations, or dynamic cerebral autoregulation.

CONCLUSIONS

Increased cardiovagal baroreflex sensitivity in conjunction with preserved cerebral autoregulation suggests that head rotation during upright tilt is unlikely to trigger orthostatic instability in normal, healthy humans.