Gelinas JC, Marsden KR, Tzeng YC, Smirl JD, Smith KJ, Willie CK, Lewis NC, Binsted G, Bailey DM, Bakker A, Day TA, Ainslie PN. Influence of posture on the regulation of cerebral perfusion. Aviat Space Environ Med 2012; 83:751–7. Background: Posture has a major influence on cerebral blood flow (CBF). Unlike head-up tilt (HUT), less is known about how CBF is regulated during head-down tilt (HDT). We hypothesized that CBF would be elevated during HDT and decreased during HUT. Methods: In 21 healthy young adults, while controlling for end-tidal Pco₂, we combined concurrent measurements of middle cerebral artery velocity and posterior cerebral artery velocity (MCAv and PCAv, respectively), blood pressure (BP), and heart rate (HR). Measures were made at rest and, in a randomized order, during −90° HDT and +90° HUT. Dynamic cerebral autoregulation was quantified using transfer function analysis. In a subgroup, volumetric blood flow recordings were obtained in the common carotid artery (CCA; N = 11), internal and external carotid arteries (ICA; N = 8 and ECA; N = 6), and vertebral artery (VA; N = 4). Results: End-tidal Pco₂, CCA, ICA, VA, MCAv_mean and PCAv_mean remained unchanged during −90° HDT and +90° HUT compared to supine. During −90° HDT, mean BP (+22 mmHg) and cerebral vascular resistance (CVR) in both the MCA and PCA were elevated relative to supine, whereas HR remained unchanged. During +90° HUT, when compared to supine, HR increased (+18 bpm), and mean arterial pressure (MAP) total power and low frequency (LF) power in the MCA and PCA increased. In both the very low frequency (VLF) and LF ranges, coherence during +90° HUT increased (P < 0.05 vs. supine) in both the MCA and PCA. In contrast, coherence was reduced during −90° HDT. Discussion: Despite marked changes in perfusion pressure with HUT or HDT, our findings indicate that cerebral perfusion is well maintained during acute severe changes in posture.