Alexandrine Gagnon, Mathieu Laroche, David Williamson, Marc Giroux, Jean-François Giguère, and Francis Bernard
After craniectomy, although intracranial pressure (ICP) is controlled, episodes of brain hypoxia might still occur. Cerebral hypoxia is an indicator of poor outcome independently of ICP and cerebral perfusion pressure. No study has systematically evaluated the incidence and characteristics of brain hypoxia after craniectomy. The authors’ objective was to describe the incidence and characteristics of brain hypoxia after craniectomy.
The authors included 25 consecutive patients who underwent a craniectomy after traumatic brain injury or intracerebral hemorrhage and who were monitored afterward with a brain tissue oxygen pressure monitor.
The frequency of hypoxic values after surgery was 14.6% despite ICP being controlled. Patients had a mean of 18 ± 23 hypoxic episodes. Endotracheal (ET) secretions (17.4%), low cerebral perfusion pressure (10.3%), and mobilizing the patient (8.6%) were the most common causes identified. Elevated ICP was rarely identified as the cause of hypoxia (4%). No cause of cerebral hypoxia could be determined 31.2% of the time. Effective treatments that were mainly used included sedation/analgesia (20.8%), ET secretion suctioning (15.4%), and increase in fraction of inspired oxygen or positive end-expiratory pressure (14.1%).
Cerebral hypoxia is common after craniectomy, despite ICP being controlled. ET secretion and patient mobilization are common causes that are easily treatable and often not identified by standard monitoring. These results suggest that monitoring should be pursued even if ICP is controlled. The authors’ findings might provide a hypothesis to explain the poor functional outcome in the recent randomized controlled trials on craniectomy after traumatic brain injury where in which brain tissue oxygen pressure was not measured.
Laura Dellazizzo, Simon-Pierre Demers, Emmanuel Charbonney, Virginie Williams, Karim Serri, Martin Albert, Jean-François Giguère, Mathieu Laroche, David Williamson, and Francis Bernard
Avoiding decreases in brain tissue oxygenation (PbtO2) after traumatic brain injury (TBI) is important. How best to adjust PbtO2 remains unclear. The authors investigated the association between partial pressure of oxygen (PaO2) and PbtO2 to determine the minimal PaO2 required to maintain PbtO2 above the hypoxic threshold (> 20 mm Hg), accounting for other determinants of PbtO2 and repeated measurements in the same patient. They also explored the clinical utility of a novel concept, the brain oxygenation ratio (BOx ratio = PbtO2/PaO2) to detect overtreatment with the fraction of inspired oxygen (FiO2).
A retrospective cohort study at an academic level 1 trauma center included 38 TBI patients who required the insertion of a monitor to measure PbtO2. Various determinants of PbtO2 were collected simultaneously whenever a routine arterial blood gas was drawn. A PbtO2/PaO2 ratio was calculated for each blood gas and plotted over time for each patient. All patients were managed according to a standardized clinical protocol. A mixed effects model was used to account for repeated measurements in the same patient.
A total of 1006 data points were collected. The lowest mean PaO2 observed to maintain PbtO2 above the ischemic threshold was 94 mm Hg. Only PaO2 and cerebral perfusion pressure were predictive of PbtO2 in multivariate analysis. The PbtO2/PaO2 ratio was below 0.15 in 41.7% of all measures and normal PbtO2 values present despite an abnormal ratio in 27.1% of measurements.
The authors’ results suggest that the minimal PaO2 target to ensure adequate cerebral oxygenation during the first few days after TBI should be higher than that suggested in the Brain Trauma Foundation guidelines. The use of a PbtO2/PaO2 ratio (BOx ratio) may be clinically useful and identifies abnormal O2 delivery mechanisms (cerebral blood flow, diffusion, and cerebral metabolic rate of oxygen) despite normal PbtO2.