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Intracranial Pressure in Traumatic Brain Injury: From Thresholds to Heuristics.

Intracranial pressure (ICP) remains the most central monitoring variable in severe traumatic brain injury (TBI). Explanations for ICP’s hegemony include the following: a) refractory intracranial hypertension (IHT) is a precursor of brain herniation and cerebral circulatory arrest; b) IHT has been consistently associated with mortality in observational cohorts; c) measuring and monitoring ICP is relatively uncomplicated when compared with detecting other mechanisms leading to secondary brain injury (SBI); and d) provides the means for estimating cerebral perfusion pressure (CPP). Using ICP/CPP as targets is the recommended strategy by both the Brain Trauma Foundation (BTF) guidelines (1), and expert consensus (2), and a leading way of practicing (at least in highincome countries) (3). Inherent to these recommendations are the tactics of escalating tiers of management according to threshold values of ICP and CPP (4). The BTF recommends treating ICP above 22 mm Hg because epidemiologic values above this level have been associated with increased mortality (5) (level IIB recommendation) and to include clinical and radiographic data in management decisions (level III). This generic-threshold treatment paradigm is criticized for a number of reasons including methodologic issues in deriving thresholds, pathophysiologic reasons, and lack of high-level clinical outcome evidence (Table 1 provides an exegesis of criticisms). Notwithstanding objections, one should keep in mind that implementation of the BTF guidelines has been associated with a reduction in mortality after TBI (6) (remains unknown if this is a protocol effect vs the contribution of individual components); this fact will become further salient below in relation to a “heuristics” understanding. In view of the limitations, what alternative approaches could one entertain? One approach is to abandon ICP monitoring, and with it, ICP threshold–guided management. Such a strategy has been proposed as the Imaging and Clinical Examination protocol (ICE) and has been derived from and tested in the Benchmark Evidence from South American Trials: Treatment of Intracranial Pressure trial (BEST-TRIP) trial against an ICP-monitored control group (7, 8). A detailed exposition is out-of-scope; however, it is helpful to highlight two takeaways. First, the trial was negative in terms of primary clinical outcome, and second, the ICE group required more intensive brain-specific treatment. Another approach, on the opposite spectrum and inspired by multimodality neuromonitoring (MMM), aims to assess cerebrovascular pressure reactivity, brain tissue oxygenation, cerebral metabolism, and energy dynamics (9). The motivation is to remedy a number of the criticisms leveled against the fixed treatment threshold, including management individualization and determinations of optimal CPP (10), or patient-specific critical ICP (11).