The cerebrovascular response to lower-body negative pressure vs. head-up tilt

Lower-body negative pressure has the potential to serve as a MRI-compatible surrogate of orthostatic stress, but a comparison with actual orthostasis was lacking. This study showed that the pronounced reduction in end-tidal Pco2 partial pressure together with gravitational effects on the brain circulation lead to a larger decline in cerebral blood flow velocity in response to head-up tilt than during lower-body negative pressure. This should be taken into account when employing lower-body negative pressure as an MRI-compatible alternative to orthostatic stress.