Brain tissue oxygen monitoring in traumatic brain injury : part I-To what extent does PbtO(2) reflect global cerebral physiology?

Background The primary aim was to explore the association of global cerebral physiological variables including intracranial pressure (ICP), cerebrovascular reactivity (PRx), cerebral perfusion pressure (CPP), and deviation from the PRx-based optimal CPP value ( increment CPPopt; actual CPP-CPPopt) in relation to brain tissue oxygenation (pbtO2) in traumatic brain injury (TBI). Methods A total of 425 TBI patients with ICP- and pbtO2 monitoring for at least 12 h, who had been treated at the neurocritical care unit, Addenbrooke's Hospital, Cambridge, UK, between 2002 and 2022 were included. Generalized additive models (GAMs) and linear mixed effect models were used to explore the association of ICP, PRx, CPP, and CPPopt in relation to pbtO(2.) PbtO(2) < 20 mmHg, ICP > 20 mmHg, PRx > 0.30, CPP < 60 mmHg, and increment CPPopt < - 5 mmHg were considered as cerebral insults. Results PbtO(2) < 20 mmHg occurred in median during 17% of the monitoring time and in less than 5% in combination with ICP > 20 mmHg, PRx > 0.30, CPP < 60 mmHg, or increment CPPopt < - 5 mmHg. In GAM analyses, pbtO(2 )remained around 25 mmHg over a large range of ICP ([0;50] mmHg) and PRx [- 1;1], but deteriorated below 20 mmHg for extremely low CPP below 30 mmHg and increment CPPopt below - 30 mmHg. In linear mixed effect models, ICP, CPP, PRx, and increment CPPopt were significantly associated with pbtO(2), but the fixed effects could only explain a very small extent of the pbtO(2) variation. Conclusions PbtO(2 )below 20 mmHg was relatively frequent and often occurred in the absence of disturbances in ICP, PRx, CPP, and increment CPPopt. There were significant, but weak associations between the global cerebral physiological variables and pbtO(2), suggesting that hypoxic pbtO2 is often a complex and independent pathophysiological event. Thus, other variables may be more crucial to explain pbtO(2) and, likewise, pbtO2 may not be a suitable outcome measure to determine whet