Compensatory-Reserve-Weighted Intracranial Pressure and Its Association with Outcome After Traumatic Brain Injury

Abstract

Objective: We introduced 'compensatory-reserve-weighted intracranial pressure (ICP),' named 'weightedICP' for brevity, as a variable that may better describe changes leading to mortality after traumatic brain injury (TBI) over the standard mean ICP.

Methods: ICP was monitored prospectively in over 1023 sedated and ventilated patients. The RAP coefficient (R-correlation, A-amplitude, and P-pressure) was calculated as the running correlation coefficient between slow changes in the pulse amplitude of ICP and the mean ICP. RAP has a value of 0 on the linear part of the pressure-volume curve and a value of + 1 on the ascending exponential part. Then, RAP decreases towards zero or even becomes negative when ICP increases further-a phenomenon thought to be related to the critical closing of cerebral vessels. In this study, we investigated a derived variable called weightedICP, calculated as ICP*(1 - RAP).

Results: Mortality after TBI was associated with both elevated ICP and weightedICP. Analysis of variance showed higher values of test statistics for weightedICP (K = 93) than for ICP (K = 64) in outcome categorization. Additionally, receiver operator curve analysis indicated greater area under the curve for weightedICP (0.71) than for ICP (0.67) with respect to associated mortality; however, the difference was not statistically significant (p = 0.12). The best threshold (maximizing sensitivity and specificity) was 19.5 mm Hg for mean ICP, and 8 mm Hg for weightedICP. Mortality rate expressed as a function of mean ICP and weightedICP showed an ascending profile in both cases.

Conclusion: The proposed variable shows a significant association with mortality following head injury. It is sensitive to both the rising absolute ICP and to the critical deterioration of pressure-volume compensation.

Keywords: Brain trauma; Compensatory reserve; Intracranial pressure; Mortality.