Central hemodynamic response to activation of the vestibular sympathetic reflex with head-down rotation in healthy young adults

Abstract

The purpose of this study was to assess the effect of vestibular sympathetic activation on estimated measures of central (aortic) hemodynamic load in young adults. Thirty-one participants (n = 14 females and 17 males) underwent cardiovascular measures in the prone position with the head in a neutral position and during 10 min of head-down rotation (HDR), as a means of activating the vestibular sympathetic reflex. Radial pressure waveforms were acquired via applanation tonometry and, with the use of a generalized transfer function, used to synthesize an aortic pressure waveform. Popliteal vascular conductance was derived from diameter and flow velocity measured via Doppler-ultrasound. Subjective orthostatic intolerance was assessed using a 10-item orthostatic hypotension questionnaire. There was a reduction in brachial systolic blood pressure (BP) during HDR (111 ± 10 vs. 109 ± 9 mmHg, P < 0.05), but there was no change in aortic systolic BP (100 ± 9 vs. 99 ± 8 mmHg, P > 0.05). Aortic augmentation index (-5 ± 11 vs. -12 ± 12%, P < 0.05) and reservoir pressure (28 ± 8 vs. 26 ± 8 mmHg, P < 0.05) were reduced with a concomitant reduction in popliteal conductance (5.6 ± 0.7 vs. 4.5 ± 0.7 mL/min mmHg, P < 0.05). Change in aortic systolic BP was associated with subjective orthostatic intolerance score (r = -0.39, P < 0.05). Activation of the vestibular sympathetic reflex via HDR resulted in slight reductions in brachial BP concomitant with preservation of aortic BP. Despite peripheral vascular constriction during HDR, there was a reduction in pressure from wave reflections and reservoir pressure. Finally, there was an association between change in aortic systolic BP during HDR and orthostatic intolerance score, suggesting that individuals who cannot defend against drops in aortic BP during vestibular sympathetic reflex activation may be more likely to experience higher subjective symptomatology of orthostatic intolerance.