Influence of upper body position on middle cerebral artery blood velocity during continuous positive airway pressure breathing

Continuous positive airway pressure (CPAP) is a treatment modality for pulmonary oxygenation difficulties. CPAP impairs venous return to the heart and, in turn, affects cerebral blood flow (CBF) and augments cerebral blood volume (CBV). We considered that during CPAP, elevation of the upper body would prevent a rise in CBV, while orthostasis would challenge CBF. To determine the body position least affecting indices of CBF and CBV, the middle cerebral artery mean blood velocity (MCA V _mean) and the near-infrared spectroscopy determined frontal cerebral hemoglobin content (cHbT) were evaluated in 11 healthy subjects during CPAP at different body positions (15° head-down tilt, supine, 15°, 30° and 45° upper body elevation). In the supine position, 10 cmH₂O of CPAP reduced MCA V _mean by 9 ± 3% and increased cHbT by 4 ± 2 μmol/L (mean ± SEM); (P < 0.05). In the head-down position, CPAP increased cHbT to 13 ± 2 μmol/L but left MCA V _mean unchanged. Upper body elevation by 15° attenuated the CPAP associated reduction in MCA V _mean (−7 ± 2%), while cHbT returned to baseline (1 ± 2 μmol/L). With larger elevation of the upper body MCA V _mean decreased progressively to −17 ± 3%, while cHbT remained unchanged from baseline. These results suggest that upper body elevation by ∼15° during 10 cmH₂O CPAP prevents an increase in cerebral blood volume with minimal effect on cerebral blood flow.