Noninvasive autoregulation monitoring with and without intracranial pressure in the naive piglet brain

Abstract

Background: Cerebrovascular autoregulation monitoring is often desirable for critically ill patients in whom intracranial pressure (ICP) is not measured directly. Without ICP, arterial blood pressure (ABP) is a substitute for cerebral perfusion pressure (CPP) to gauge the constraint of cerebral blood flow across pressure changes. We compared the use of ABP versus CPP to measure autoregulation in a piglet model of arterial hypotension.

Methods: Our database of neonatal piglet (5-7 days old) experiments was queried for animals with naïve ICP that were made lethally hypotensive to determine the lower limit of autoregulation (LLA). Twenty-five piglets were identified, each with continuous recordings of ICP, regional cerebral oximetry (rSo2), and cortical red cell flux (laser Doppler). Autoregulation was assessed with the cerebral oximetry index (COx) in 2 ways: linear correlation between ABP and rSo2 (COx(ABP)) and between CPP and rSo2 (COx(CPP)). The lower limits of autoregulation were determined from plots of red cell flux versus ABP. Averaged values of COx(ABP) and COx(CPP) from 5 mm Hg ABP bins were used to show receiver operating characteristics for the 2 methods.

Results: COx(ABP) and COx(CPP) yielded identical receiver operating characteristic curve areas of 0.91 (95% confidence interval [CI], 0.88-0.95) for determining the LLA. However, the thresholds for the 2 methods differed: a threshold COx(ABP) of 0.5 was 89% sensitive (95% CI, 81%-94%) and 81% specific (95% CI, 73%-88%) for detecting ABP below the LLA. A threshold COx(CPP) of 0.42 gave the same 89% sensitivity (95% CI, 81%-94%) with 77% specificity (95% CI, 69%-84%).

Conclusions: The use of ABP instead of CPP for autoregulation monitoring in the naïve brain with COx results in a higher threshold value to discriminate ABP above from ABP below the LLA. However, accuracy was similar with the 2 methods. These findings support and refine the use of near-infrared spectroscopy to monitor autoregulation in patients without ICP monitors.

Figure 1

Cerebral oximetry index arterial blood pressure (COx_ABP) and cerebral oximetry index cerebral perfusion pressure (COx_CPP) for all 25 animals normalized to the lower limit of autoregulation (LLA). Box whisker plots show range, interquartile percentages, and median values for COx averaged in 5 mm Hg bins for each animal. Shaded boxes show values obtained at the LLA. Binning and averaging dynamic autoregulation metrics is a common display method and these figures allow a visual inspection of the ability of the 2 methods to discriminate adequate from inadequate arterial blood pressure.

Figure 2

Receiver operating characteristics for the cerebral oximetry index obtained without intracranial pressure monitoring (COx_ABP [left]) and with intracranial pressure monitoring (COx_CPP [right]) showing the similar ability of both COx methods to delineate arterial blood pressure (ABP) above and below the lower limit of autoregulation (LLA). AUC = area under the curve.

Figure 3

Scatter plots of the cerebral oximetry index obtained without intracranial pressure monitoring (COx_ABP [left]) and with intracranial pressure monitoring (COx_CPP [right]) showing data taken above and below the lower limit of autoregulation (LLA) with superimposed candidate threshold values of 0.5 for COx_ABP and 0.42 for COx_CPP.

Figure 4

Comparison of the cerebral oximetry index obtained with-out intracranial pressure monitoring (COx_ABP) and with intracranial pressure monitoring (COx_CPP) using linear regression (top panel) and the Bland-Altman method (bottom panel).