Analysis of high-frequency P_btO₂ measures in traumatic brain injury: Insights into the treatment threshold

OBJECTIVE Brain tissue hypoxia is common after traumatic brain injury (TBI). Technology now exists that can detect brain hypoxia and guide corrective therapy. Current guidelines for the management of severe TBI recommend maintaining partial pressure of brain tissue oxygen (P_btO₂) > 15–20 mm Hg; however, uncertainty persists as to the optimal treatment threshold. The object of this study was to better inform the relationship between P_btO₂ values and outcome for patients with TBI. METHODS P_btO₂ measurements were prospectively and automatically collected every minute from consecutive patients admitted to the San Francisco General Hospital neurological ICU during a 6-year period. Mean P_btO₂ values in TBI patients as well as the proportion of P_btO₂ values below each of 75 thresholds between 0 mm Hg and 75 mm Hg over various epochs up to 30 days from the time of admission were analyzed. Patient outcomes were determined using the Glasgow Outcome Scale. The authors explored putative treatment thresholds by generating 675 separate receiver operating characteristic curves and 675 generalized linear models to examine each 1–mm Hg threshold for various epochs. RESULTS A total of 1,380,841 P_btO₂ values were recorded in 190 TBI patients. A high proportion of P_btO₂ measures were below 20 mm Hg irrespective of the examined epoch. Time below treatment thresholds was more strongly associated with outcome than mean P_btO₂. A treatment window was suggested: a threshold of 19 mm Hg most robustly distinguished patients by outcome, especially from days 3–5; however, benefit was suggested from maintaining values at least as high as 33 mm Hg. CONCLUSIONS This analysis of high-frequency physiological data substantially informs the relationship between P_btO₂ values and outcome. The results suggest a therapeutic window for P_btO₂ in TBI patients along with minimum and preferred P_btO₂ treatment thresholds, which may be examined in future studies. Traditional treatment thresholds that have the strongest association with outcome may not be optimal.