Cerebral oxygen reactivity in the dog

Brain tissue oxygen reactivity is a measure of the increase in tissue oxygen pressure (PtO2) relative to an increase in arterial oxygen pressure (PaO2). Clinical studies show that PtO2 reactivity is increased after cerebral injury. However, the impact of patient ventilation on these measures is not known. We determined whether changes in end tidal carbon dioxide pressure (ETCO2) would affect PtO2 reactivity in dogs. After a craniotomy, a Neurotrend probe that measures PtO2 was inserted into the cerebral cortex of eight dogs. PtO2 reactivity was measured at five concentrations of inspired oxygen (room air, 40%, 60%, 80%, 95%) at three levels of ETCO2 (20 mmHg, 40 mmHg, 60 mmHg) in random order. PtO2 reactivity at ETCO2 of 20 mmHg was 0.2 and increased to 0.3 when ETCO2 was 40 mmHg was 0.4 when ETCO2 was 60 mmHg (p < 0.05). These results show that PtO2 reactivity increases from hypocapnia to normocapnia. It is important to consider the ventilation state of each patient when evaluating PtO2.