Hyperoxemia and long-term outcome after traumatic brain injury

Abstract

Introduction: The relationship between hyperoxemia and outcome in patients with traumatic brain injury (TBI) is controversial. We sought to investigate the independent relationship between hyperoxemia and long-term mortality in patients with moderate-to-severe traumatic brain injury.

Methods: The Finnish Intensive Care Consortium database was screened for mechanically ventilated patients with a moderate-to-severe TBI. Patients were categorized, according to the highest measured alveolar-arterial O₂ gradient or the lowest measured PaO₂ value during the first 24 hours of ICU admission, to hypoxemia (<10.0 kPa), normoxemia (10.0 to 13.3 kPa) and hyperoxemia (>13.3 kPa). We adjusted for markers of illness severity to evaluate the independent relationship between hyperoxemia and 6-month mortality.

Results: A total of 1,116 patients were included in the study, of which 16% (n = 174) were hypoxemic, 51% (n = 567) normoxemic and 33% (n = 375) hyperoxemic. The total 6-month mortality was 39% (n = 435). A significant association between hyperoxemia and a decreased risk of mortality was found in univariate analysis (P = 0.012). However, after adjusting for markers of illness severity in a multivariate logistic regression model hyperoxemia showed no independent relationship with 6-month mortality (hyperoxemia vs. normoxemia OR 0.88, 95% CI 0. 63 to 1.22, P = 0.43; hyperoxemia vs. hypoxemia OR 0.97, 95% CI 0.63 to 1.50, P = 0.90).

Conclusion: Hyperoxemia in the first 24 hours of ICU admission after a moderate-to-severe TBI is not predictive of 6-month mortality.

Figure 1

Study population. TBI, traumatic brain injury; FICC, Finnish Intensive Care Consortium; GCS, Glasgow coma scale; ABG, arterial blood gas analysis.

Figure 2

Locally weighted scatterplot smoothing (lowess) curve showing the relationship between arterial oxygen value (PaO₂) and predicted 6-month mortality. Predicted risk of death showed good performance in predicting actual mortality with an area under the curve (AUC) of 0.869 and R_L² of 0.422. A clear association between increased risk of death and low (approximately <11 kPa) PaO₂ or values very high (approximately >42 kPa) PaO₂ values was noted. The predicted probability of death is calculated using the following variables: acute physiology and chronic health evaluation II (APACHE II) index independent of oxygenation (AP2no-ox), admission year (before or after 2007), emergency operation, intracranial pressure monitoring, controlled hypothermia and platelet count.

Figure 3

Observed and mean predicted in-hospital mortality differences between arterial oxygen tension (PaO₂) groups. The difference in mean predicted risk of death was significantly different among the groups (P <0.001), it being highest in the hypoxemia group and lowest in the hyperoxemia group. Predicted risk of death matched observed mortality very well within the quartiles with R_L² values between 0.097 and 0.746. The predicted probability of death was calculated using the following variables: acute physiology and chronic health evaluation II (APACHE II) index independent of oxygenation (AP2no-ox), admission year (before or after 2007), emergency operation, intracranial pressure monitoring, controlled hypothermia and platelet count.

Figure 4

Observed and mean predicted 6-month mortality differences between PaO₂ groups. The difference in mean predicted risk of death is significantly different among the groups (p < 0.001), being highest in the hypoxemia group and lowest in the hyperoxemia group. Predicted risk of death matched observed mortality very well within the quartiles with R_L² values between 0.519 and 0.603.

Figure 5

Observed and predicted 6-month mortality by deciles of arterial oxygen tension (PaO₂). PaO₂ was divided by deciles. Predicted risk of death matched observed mortality very well within deciles with R_L² values between 0.088 and 0.987. The predicted probability of death was calculated using the following variables: acute physiology and chronic health evaluation II (APACHE II) index independent on oxygenation (AP2no-ox), admission year (before or after 2007), emergency operation, intracranial pressure monitoring, controlled hypothermia and platelet count.