The Effect Of Posture On Cerebrovascular Reactivity To Acute Hypercapnic Gas

Cerebrovascular reactivity (CVR) represents the capacity of cerebral vessels to dilate and constrict in response to vasoactive stimuli, such as the partial pressure of carbon dioxide (PCO2) in arterial blood. CVR has been shown to be greater in the supine position compared to the seated position. Previous studies have typically evaluated this by having participants inhale hypercapnic gas for several minutes, to allow stabilization of end-tidal CO2 partial pressure (PETCO2). Recently, there has been an increase in studies assessing flow-dependent cerebrovascular dilation responses induced by shear stress (SR) through acute hypercapnic gas inhalation (~30 seconds). However, it remains unknown whether SR-induced cerebrovascular dilation responses are affected by posture. PURPOSE: To investigate SR-dependent vasodilation responses in the middle cerebral artery (MCA) during acute hypercapnic gas inhalation in both supine and seated postures. Methods: Twenty-six healthy subjects participated in this experiment, inhaling 5% CO2 for 30 seconds in both supine and seated postures. MCA velocity was assessed using transcranial Doppler ultrasound. To confirm shear-induced vasodilation, blood flow dynamics in the internal carotid artery (ICA) were measured using ultrasound vascular imaging. Results: The increases in MCA mean velocity due to inhaling 5% CO2 was significantly greater in the supine position (15.55 ± 4.56 cm/s) comparted to the seated position (11.82 ± 4.25 cm/s, P = 0.011), with a proportional relationship observed in intra-individual increases between the two postures (R = 0.63, P = 0.015). The SR-induced increase was confirmed by ICA reactivity. Although no significant difference was found in the increase in end-tidal PCO2 between postures (supine: 6.71 ± 1.06 mmHg, seated: 6.88 ± 0.95 mmHg, P = 0.570), no correlation was observed between postures due to individual variability in CO2 uptake. In addition, the peak MCA mean velocity in response to hypercapnia was reached earlier in the supine position than in the seated position. Conclusion: While no difference in SR-induced cerebral blood flow increase was noted due to acute 5% CO2 inhalation between postures, individual differences highlight possible variability in the underlying mechanisms. Supported by: 18H03166