Differences in the dynamic cerebrovascular response between stepwise up tilt and down tilt in humans.

We studied dynamic cerebrovascular responses in eight healthy humans during repetitive stepwise upward tilt (SUT) and stepwise downward tilt (SDT) maneuvers between supine and 70 degrees standing at intervals of 60 s. Mean cerebral blood flow velocity (FV(MCA)) was measured at the middle cerebral artery (MCA) with transcranial Doppler ultrasonography. Mean arterial blood pressure (ABP) was measured via the radial artery and adjusted at the level of the MCA (ABP(MCA)). Cerebral critical closing pressure (P(CC)) was estimated from the systolic-diastolic relationship between FV(MCA) and ABP(MCA). ABP(MCA) minus P(CC) was considered the cerebral perfusion pressure (CPP). The tilt maneuvers produced stepwise changes in both CPP and FV(MCA). The FV(MCA) response to SUT was well characterized by a linear second-order model. However, that to SDT presented a biphasic behavior that was described significantly better (P < 0.05) by the addition of a slowly responding component to the second-order model. This difference may reflect both different cardiovascular responses to SUT or SDT and different cerebrovascular autoregulatory behaviors in response to decreases or increases in CPP.