Brain tissue oxygen tension and its response to physiological manipulations: influence of distance from injury site in a swine model of traumatic brain injury

Abstract

OBJECTIVE The optimal site for placement of tissue oxygen probes following traumatic brain injury (TBI) remains unresolved. The authors used a previously described swine model of focal TBI and studied brain tissue oxygen tension (P_btO₂) at the sites of contusion, proximal and distal to contusion, and in the contralateral hemisphere to determine the effect of probe location on P_btO₂ and to assess the effects of physiological interventions on P_btO₂ at these different sites. METHODS A controlled cortical impact device was used to generate a focal lesion in the right frontal lobe in 12 anesthetized swine. P_btO₂ was measured using Licox brain tissue oxygen probes placed at the site of contusion, in pericontusional tissue (proximal probe), in the right parietal region (distal probe), and in the contralateral hemisphere. P_btO₂ was measured during normoxia, hyperoxia, hypoventilation, and hyperventilation. RESULTS Physiological interventions led to expected changes, including a large increase in partial pressure of oxygen in arterial blood with hyperoxia, increased intracranial pressure (ICP) with hypoventilation, and decreased ICP with hyperventilation. Importantly, P_btO₂ decreased substantially with proximity to the focal injury (contusion and proximal probes), and this difference was maintained at different levels of fraction of inspired oxygen and partial pressure of carbon dioxide in arterial blood. In the distal and contralateral probes, hypoventilation and hyperventilation were associated with expected increased and decreased P_btO₂ values, respectively. However, in the contusion and proximal probes, these effects were diminished, consistent with loss of cerebrovascular CO₂ reactivity at and near the injury site. Similarly, hyperoxia led to the expected rise in P_btO₂ only in the distal and contralateral probes, with little or no effect in the proximal and contusion probes, respectively. CONCLUSIONS P_btO₂ measurements are strongly influenced by the distance from the site of focal injury. Physiological alterations, including hyperoxia, hyperventilation, and hypoventilation substantially affect P_btO₂ values distal to the site of injury but have little effect in and around the site of contusion. Clinical interpretations of brain tissue oxygen measurements should take into account the spatial relation of probe position to the site of injury. The decision of where to place a brain tissue oxygen probe in TBI patients should also take these factors into consideration.

Keywords: ABG = arterial blood gas; CCI = controlled cortical impact; CPP = cerebral perfusion pressure; FiO2 = fraction of inspired oxygen; GLM = general linear model; ICP = intracranial pressure; LMM = linear mixed model; MAP = mean arterial pressure; PaCO2 = partial pressure of carbon dioxide in arterial blood; PaO2 = partial pressure of oxygen in arterial blood; PbtO2 = brain tissue oxygen tension; REML = restricted maximum likelihood modeling; TBI = traumatic brain injury; in vivo; licox; optimal location; oxygen monitor; traumatic brain injury.