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) man euvers between supine and 70 degrees standing at intervals of 60 s. Mean ce rebral blood flow velocity (FVMCA) was measured at the middle cerebral arte ry (MCA) with transcranial Doppler ultrasonography. Mean arterial blood pre ssure (ABP) was measured via the radial artery and adjusted at the level of the MCA (ABP(MCA)). Cerebral critical closing pressure (P-CC) was estimate d from the systolic-diastolic relationship between FVMCA and ABP(MCA). ABP( MCA) minus P-CC was considered the cerebral perfusion pressure (CPP). The t ilt maneuvers produced stepwise changes in both CPP and FVMCA. The FVMCA re sponse to SUT was well characterized by a linear second-order model. Howeve r, that to SDT presented a biphasic behavior that was described significant ly better (P< 0.05) by the addition of a slowly responding component to the second-order model. This difference may reflect both different cardiovascu lar responses to SUT or SDT and different cerebrovascular autoregulatory be haviors in response to decreases or increases in CPP.