Brain tissue oxygen monitoring in traumatic brain injury - part II : isolated and combined insults in relation to outcome

Background: The primary aim was to explore the concept of isolated and combined threshold-insults for brain tissue oxygenation (pbtO(2)) in relation to outcome in traumatic brain injury (TBI). Methods: A total of 239 TBI patients with data on clinical outcome (GOS) and intracranial pressure (ICP) and pbtO(2) 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. Outcome was dichotomised into favourable/unfavourable (GOS 4-5/1-3) and survival/mortality (GOS 2-5/1). PbtO(2) was studied over the entire monitoring period. Thresholds were analysed in relation to outcome based on median and mean values, percentage of time and dose per hour below critical values and visualised as the combined insult intensity and duration. Results: Median pbtO(2) was slightly, but not significantly, associated with outcome. A pbtO(2) threshold at 25 and 20 mmHg, respectively, yielded the highest x(2) when dichotomised for favourable/unfavourable outcome and mortality/survival in chi-square analyses. A higher dose and higher percentage of time spent with pbtO(2) below 25 mmHg as well as lower thresholds were associated with unfavourable outcome, but not mortality. In a combined insult intensity and duration analysis, there was a transition from favourable towards unfavourable outcome when pbtO(2) went below 25-30 mmHg for 30 min and similar transitions occurred for shorter durations when the intensity was higher. Although these insults were rare, pbtO(2) under 15 mmHg was more strongly associated with unfavourable outcome if, concurrently, ICP was above 20 mmHg, cerebral perfusion pressure below 60 mmHg, or pressure reactivity index above 0.30 than if these variables were not deranged. In a multiple logistic regression, a higher percentage of monitoring time with pbtO(2) < 15 mmHg was associated with a higher rate of unfavourable outcome. Conclusions: Low pbtO(2), under 25 mmHg and particularly below 15 mmHg, for longer durations and in combination with disturbances in global cerebral physiological variables were associated with poor outcome and may indicate detrimental ischaemic hypoxia. Prospective trials are needed to determine if pbtO(2)-directed therapy is beneficial, at what individualised pbtO(2) threshold therapies are warranted, and how this may depend on the presence/absence of concurrent cerebral physiological disturbances