Static autoregulation in humans: a review and reanalysis

Introduction: Cerebral autoregulation (CA) is a theoretical construct characterized by the relationship between mean arterial pressure (MAP) and cerebral blood flow (CBF). We performed a comprehensive literature search to provide an up-to-date review on the static relationship between MAP and CBF. Methods: The results are based on 40 studies (49 individual experimental protocols) in healthy subjects between 18 and 65 years. Exclusion criteria were: a Delta MAP <5%, hypoxia/hyperoxia or hypo/hypercapnia, and unstable levels (<2 min stages). The partial pressure of arterial CO2 (PaCO2) was measured in a subset of the included studies (n=28); therefore, CBF was also adjusted to account for small changes in PaCO2. Results: The linear regression coefficient between MAP and CBF (or velocity) of 0.82 +/- 0.77%Delta CBF/%Delta MAP during decreases in MAP (n=23 experiments) was significantly different than the relationship of 0.21 +/- 0.47%Delta CBF/%Delta MAP during increases (n=26 experiments; p<0.001). After correction for increases/decreases in PaCO2, the slopes were not significantly different: 0.64 +/- 1.16%Delta CBF/%Delta MAP (n=16) and 0.39 +/- 0.30%Delta CBF/%Delta MAP (n=12) for increased vs. decreased MAP changes, respectively (p=0.60). Conclusion: The autoregulatory ability of the cerebral circulation appears to be more active in buffering increases in MAP as compared to reductions in MAP. However, the statistical finding of hysteresis is lost following an attempt to correct for PaCO2. (C) 2014 Published by Elsevier Ltd on behalf of IPEM.