Pro-Con Debate: The Clinical (Ir)relevance of the Lower Limit of Cerebral Autoregulation for Anesthesiologists

Abstract

In this Pro-Con commentary article, we discuss whether the lower limit of cerebral autoregulation is clinically relevant for anesthesiologists. The central question regarding this issue is whether mean arterial blood pressure below the lower limit of autoregulation is detrimental for the brain. The Pro side argues that continuous monitoring of cerebral autoregulation has revealed an association between going below the lower limit and mortality in the critically ill patient. Conversely, the Con side argues that cerebral autoregulation is only one of various defense mechanisms of the brain that protect against cerebral hypoperfusion, and that cerebral autoregulation may be more important to protect against intracranial hypertension.

Dynamic cerebral autoregulation during step-wise increases in blood pressure during anaesthesia

“…Secondly, the dogmatic view that the lower limit of CA is the inflection point under which cerebral hypoperfusion starts to occur is challenged. In fact, this is in accordance with earlier work, 45–47 and the question remains whether the lower limit of CA has any relevance at all for anaesthesiologists 3 . The results of this study support the view that CA is not an on-off mechanism and suggest that the lower limit is somewhat arbitrary, as dynamic CA continues to improve as MAP increases in the autoregulatory range.…”

“…For the brain, this finding is consistent with the dogmatic physiological concept that cerebral (pressure) autoregulation (CA) maintains constancy of cerebral blood flow (CBF), by adjusting cerebrovascular tone to changes in MAP down to a lower limit (LLCA) of approximately 60 mmHg 2 . In reality, this idea of ‘static’ CA is much more nuanced but remains a widespread model that serves as a basis for clinicians to make clinical decisions 3 . Conversely, dynamic CA describes how quickly cerebrovascular tone is adjusted to a sudden change in MAP 4 …”

“…2 In reality, this idea of 'static' CA is much more nuanced but remains a widespread model that serves as a basis for clinicians to make clinical decisions. 3 Conversely, dynamic CA describes how quickly cerebrovascular tone is adjusted to a sudden change in MAP. 4 Factors that influence cerebrovascular tone may interfere with CA.…”

The Clinical (Ir)relevance of the Lower Limit of Cerebral Autoregulation for Anesthesiologists

“…[10][11][12][13] Perhaps, we have to conclude according to the McNamara fallacy: "Measure what is important, don't make important what you can measure." 14 For this Open Mind Paper, 15 authors from 2 centers (Amsterdam and Cambridge) were invited for a procon debate to discuss whether a blood pressure below the lower limit of cerebral autoregulation impairs cerebral perfusion and whether this affects clinical outcome. Both physiological arguments and the results of clinical studies will be reviewed to support or reject this hypothesis.…”