Regional changes in cerebral blood flow between the upright and supine posture and over 3 days of bed rest

A reduction in cerebral blood flow (CBF) has been observed during spaceflight and bed rest. We aimed to examine the magnitude and regional heterogeneity of the decrease in CBF during bed rest compared to posture changes on Earth. Seventeen participants (age, 29 ± 9 years, 7 females) were studied in the upright and supine posture and over 3 days of bed rest. We assessed blood flow via duplex ultrasonography in the internal carotid (ICA) and vertebral arteries (VA), and via transcranial Doppler of the middle cerebral artery (MCAv). Mean arterial pressure (MAP) and end-tidal CO₂ ( $E{T}_{C{O}_2}$ ) were assessed at all time points. By day 3, total CBF (1078 ± 302 to 853 ± 245 mL min^-1, P < 0.0001) and MCAv (61 ± 15 to 49 ± 12 mL min^-1, P < 0.0001) were decreased compared to the supine posture. CBF values did not fall below the upright posture (all P > 0.05) but were lower than a calculated 24-h mean baseline (P = 0.0132). MAP remained stable (P = 0.971), as did $E{T}_{C{O}_2}$ (P = 0.0803), while VA blood flow decreased after 24 h and again after 72 h (P = 0.0024). These findings indicate that CBF decreases during short-term bed rest, but not below values observed in the upright posture.