The aim of this work was to explore the relationship between intracranial pressure (ICP)-derived indices of cerebrovascular reactivity and the lower limit of autoregulation (LLA) during arterial
hypotension. We retrospectively reviewed recorded physiological data from piglets that underwent
controlled hypotension.
Hypotension was induced by inflation of a balloon
catheter in the inferior vena cava. ICP, cortical
laser Doppler flowmetry (LDF), and arterial blood pressure (ABP) monitoring was conducted. ICP-derived indices were calculated: pressure reactivity index (PRx; correlation between ICP and mean arterial pressure [MAP]); pulse amplitude index (PAx; correlation between pulse amplitude of ICP [
AMP] and MAP); and RAC (correlation between
AMP and cerebral perfusion pressure [
CPP]). LLA was estimated by piece-wise linear regression of
CPP versus LDF. We produced error bar plots for PRx, PAx, and RAC against 5-mm Hg bins of
CPP, displaying the relationship with the LLA. We compared
CPP values at clinically relevant thresholds of PRx, PAx, and RAC to
CPP measured at the LLA. Receiver operating curve (ROC) analysis was performed for each index across the LLA using 5-mm Hg bins for
CPP. Mean LLA was 36.2 ± 10.5 mm Hg. Error bar plots demonstrated that PRx, PAx, and RAC increased, with
CPP decreasing below the LLA.
CPP at clinically relevant thresholds for PRx, PAx, and RAC displayed weak associations with the LLA, indicating that thresholds defined in TBI may not apply to a model of arterial
hypotension. ROC analysis indicated that PRx, PAx, and RAC predicted the LLA, with AUCs of: 0.806 (95% confidence interval [CI], 0.750-0.863; p < 0.0001), 0.726 (95% CI, 0.664-0.789; p < 0.0001), and 0.710 (95% CI, 0.646-0.775; p < 0.0001), respectively. Three ICP-derived continuous indices of cerebrovascular reactivity, PRx, PAx, and RAC, were validated against the LLA within this experimental model of arterial
hypotension, with PRx being superior.