Reproducibility and variability of dynamic cerebral autoregulation during passive cyclic leg raising

Dynamic cerebral autoregulation (dCA) estimates require mean arterial blood pressure (MABP) fluctuations of sufficient amplitude. Current methods to induce fluctuations are not easily implemented or require patient cooperation. In search of an alternative method, we evaluated if MABP fluctuations could be increased by passive cyclic leg raising (LR) and tested if reproducibility and variability of dCA parameters could be improved. Middle cerebral artery cerebral blood flow velocity (CBFV), MABP and end tidal CO2 (PetCO2) were obtained at rest and during LR at 0.1 Hz in 16 healthy subjects. The MABP-CBFV phase difference and gain were determined at 0.1 Hz and in the low frequency (LF) range (0.06-0.14 Hz). In addition the autoregulation index (ARI) was calculated. The LR maneuver increased the power of MABP fluctuations at 0.1 Hz and across the LF range. Despite a clear correlation between both phase and gain reproducibility and MABP variability in the rest condition, only the reproducibility of gain increased significantly with the maneuver. During the maneuver patients were breathing faster and more irregularly, accompanied by increased PetCO2 fluctuations and increased coherence between PetCO2 and CBFV. Multiple regression analysis showed that these concomitant changes were negatively correlated with the MABP-CBFV phase difference at 0.1 Hz Variability was not reduced by LR for any of the dCA parameters. The clinical utility of cyclic passive leg raising is limited because of the concomitant changes in PetCO2. This limits reproducibility of the most important dCA parameters. Future research on reproducibility and variability of dCA parameters should incorporate PetCO2 variability or find methods to keep PetCO2 levels constant.