Reliability of the transient hyperemic response test in detecting changes in cerebral autoregulation induced by the graded variations in end-tidal carbon dioxide

The transient hyperemic response (THR) in the middle cerebral artery (MCA) after the release of brief compression of the ipsilateral common carotid artery has been used to study cerebral autoregulation. We conducted the present study to evaluate the reliability of THR to detect changes in cerebral autoregulation induced by graded variations in PETCO2. Seven healthy adult volunteers were recruited. Fifteen THR tests were performed on every volunteer: three at baseline PETCO2, three each at PETCO2 of 7.5 mm Hg and 15 mm Hg above the baseline, and then three each at PETCO2 Of 7.5 mm Hg and 15 mm Hg below the baseline. Transient hyperemic response ratio (THRR) and strength of autoregulation (SA) were calculated using established formulae. Both THRR and SA were highly sensitive (96%) in detecting the changes in cerebral autoregulation induced by graded changes in PETCO2. The within-individual variability of SA was significantly smaller than that of THRR at all levels of PETCO2. Implications: This study demonstrates the reliability of the THR test, when used for repetitive measurements, in detecting changes in cerebral autoregulation induced by graded changes in PETCO2. This test may provide a simple and noninvasive method of evaluating changes in cerebral autoregulation within an individual.