Postural effects on cerebral blood flow and autoregulation

Cerebral autoregulation ( CA ) is thought to maintain relatively constant cerebral blood flow ( CBF ) across normal blood pressures. To determine if postural changes alter CA , we measured cerebral blood flow velocity ( CBF v) in the middle cerebral arteries, mean arterial blood pressure ( MABP ), cardiac output (Q), and end‐tidal carbon dioxide ( PETCO 2 ) in 18 healthy individuals (11 female and seven male; 26 ± 9 years) during repeated periods of supine and seated rest. Multiple regression was used to evaluate the influence of PETCO 2 , MABP , Q, and hydrostatic pressure on CBF v. Static CA was assessed by evaluating absolute changes in steady‐state CBF v. Dynamic CA was assessed by transfer function analysis of the CBF v response to spontaneous oscillations in MABP . In the seated versus supine posture, MABP (67.2 ± 7.2 vs. 84.2 ± 12.1 mmHg; P  < 0.001), CBF v (55.2 ± 9.1 vs. 63.6 ± 10.6 cm/sec; P  < 0.001) and PETCO 2 (29.1 ± 2.6 vs. 30.9 ± 2.3 mmHg; P  < 0.001) were reduced. Changes in CBF v were not explained by variance in PETCO 2 , MABP , Q, or hydrostatic pressure. A reduction in MABP to CBF v transfer function gain while seated ( P  < 0.01) was explained by changes in the power spectrum of MABP , not CBF v. Our findings suggest that changes in steady‐state cerebral hemodynamics between postures do not appear to have a large functional consequence on the dynamic regulation of CBF .