Role of pressure reactivity index in neurocritical care

Abstract

Direct arterial blood pressure (ABP) and intracranial pressure (ICP) are two fundamental variables most often selected for brain monitoring after severe traumatic brain injury (TBI). Otherwise, cerebral perfusion pressure (CPP=ABPICP) and ICP-oriented therapy is not efficient. Additional information can be derived from the pressure reactivity index (PRx) which is calculated from the continuous correlation between slow waves (20 s to 2 min periods) of ICP and ABP. Positive PRx indicates impaired cerebrovascular reactivity, whereas negative PRx reflects intact reactivity. PRx can be interpreted as a surrogate for continuous index of cerebral autoregulation. In such a paradigm, dynamic changes in ICP (slow waves) are evoked by changes in cerebral blood volume which are in turn modulated by changes in cerebral blood flow. Impaired cerebrovascular reactivity was observed in patients who died following TBI. PRx stays close to +1 during temporal episodes (plateau waves) or before/during refractory intracranial hypertension. Distribution of PRx along varying CPP values shows a distinctive U shape with minimum PRx (best cerebrovascular reactivity) indicating the optimal value for CPP. This is important as patients with CPP below optimal level have an increased mortality rate, while patients with CPP above optimal CPP are at a higher risk of severe disability. The highest percentage of favorable outcome can be seen in those patients for whom current CPP was close to optimal value of CPP. These findings await further analysis by prospective randomized trial.