Vertical shift in cerebral autoregulation curve: a graded head-up tilt study.

According to the classical theory of cerebral autoregulation, cerebral blood flow (CBF) will be maintained at a near-constant level for cerebral perfusion pressure (CPP) within the range of 60-150 mmHg. In recent years, however, studies with contradicting results have demonstrated a change in CBF when the level of CPP had not changed significantly. It is suggested that a shift in the autoregulation curve may have taken place along the horizontal axis or along the vertical axis. This paper describes previously unreported findings of a graded head-up tilt (HUT) study which was designed as a preliminary experiment to test the protocol for possible use on astronauts upon returning from space-flight. Data from this study tend to support the findings of previous studies regarding possible shifts in the cerebral autoregulation curve. Five female and four male healthy volunteers were exposed to HUT for 5 minutes at each angle of 30 degrees, 60 degrees, and 90 degrees. Throughout the test, cerebrovascular and cardiovascular responses were evaluated by use of continuous acquisition of mean flow velocity (MFV) from the right middle cerebral artery with transcranial Doppler sonograhy, mean arterial blood pressure (MABP, Finapres) and heart rate (HR, ECG). Based on the last 60 seconds of data at each tilt angle and baseline, the percentage changes in MFV from baseline were found to be similar to those in MABP at the level of the brain (MABPbrain), and both were significant (p<0.05) at the 60 degrees (-9.1 +/- 7% for MABP brain, -9.8 +/- 5% for MFV) and 90 degrees (-13 degrees +/- 8%, -12.0 +/- 6%) positions. Heart rate (HR) increased significantly (p<0.05) from the baseline at 30 degrees (6.3 +/- 5%) through 90 degrees (23.3 +/- 8%). The trend toward decreasing MFV in normal subjects, even while MABP brain remained within the normal limits of cerebral autoregulation, may suggest a downward shift of the cerebral blood flow plateau in the classic cerebral autoregulation curve.