A Description of a New Continuous Physiological Index in Traumatic Brain Injury Using the Correlation between Pulse Amplitude of Intracranial Pressure and Cerebral Perfusion Pressure.

To describe a new continuous index of physiologic measurement in a traumatic brain injury (TBI) population, the moving correlation coefficient between intracranial pressure (ICP) pulse amplitude (AMP) and cerebral perfusion pressure (CPP), which we refer to as RAC. We use patient examples of sustained intracranial hypertension, systemic arterial hypotension, and plateau waves, as well as the retrospective analysis of 358 non-decompressive craniectomy (DC) TBI patients with high-frequency ICP and arterial blood pressure data, to explore the relationships of this new index, RAC, with AMP, ICP, CPP, RAP (correlation coefficient between AMP and ICP), pressure reactivity index (PRx), and pulse amplitude index (PAx). We compared the RAC-CPP relationship to that observed between CPP and both PRx and PAx. RAC displays temporal responsiveness to sustained increased ICP, arterial hypotension, and plateau waves, with positive values found during episodes of high ICP and low CPP. Analysis of AMP versus CPP and AMP versus ICP relationships in data from the entire non-DC cohort show lower breakpoints for AMP/CPP at CPP = 40 mm Hg and upper breakpoints for AMP/ICP at ICP = 50-60 mm Hg. RAC trends to positive values with increasing ICP, particularly with ICP values above the peak in AMP (ICP >50 mm Hg), though its interpretation requires concomitant interpretation of AMP, RAP, and PRx/PAx to determine contributions of compensatory reserve and cerebrovascular responsiveness to the changes observed in RAC. There is a parabolic relationship between RAC versus CPP, with trends toward positive RAC values near (and beyond) limits for low and high CPPs, suggesting that RAC may be used in the determination of optimum CPP. RAC appears to carry information regarding both cerebrovascular responsiveness and cerebral compensatory reserve. This contributes to RAC's uniqueness and complex interpretation. Further prospective, clinical studies of RAC in CPP optimum estimation and outcome prediction in TBI are required.