Head position modifies cerebrovascular response to orthostatic stress

Previous experiments have shown that the vestibular system participates in cardiovascular control. However, the effects of vestibular activation on cerebrovascular regulation are not known. Therefore, the present experiment tested the hypothesis that specific vestibular activations may be beneficial to cerebral circulation during simulated orthostatic stress. Middle cerebral artery blood flow velocity (CBV; Doppler ultrasound) was measured to examine the effects of head-down neck flexion (HDNF) compared to head-down neck extension (HDNE) with and without lower body negative pressure (LBNP; -40 mmHg) (n=9). The change in CBV (ΔCBV) during HDNF and HDNE were not different during baseline conditions, however, during LBNP, ΔCBV was greater in HDNE compared to HDNF (-5.5±3.2 cm/s, -11±4.6%) vs. (-0.7±1.0 cm/s, -1.9±1.9%), respectively (P<0.05). Concomitantly, the change in cerebrovascular resistance (ΔCVR) between rest and LBNP was also greater during HDNE (0.48±0.08 mmHg/cm per s, 42.8±10.8%) compared with HDNF (0.26±0.05 mmHg/cm per s, 22±4.1%) (P<0.05). P_ETCO₂ was greater in HDNE (45±2 mmHg) compared to HDNF (42±2 mmHg; P<0.05) during LBNP. These results suggest that the vestibular system may affect cerebrovascular tone during simulated postural stress by either constriction or dilation, depending on the vestibular stimulus.