CO2 reactivity and brain oxygen pressure monitoring in severe head injury

Objective: To investigate the effect of hyperventilation on cerebral oxygen ation after severe head injury. Design:A prospective, observational study. Setting: Neurointensive care unit at a university hospital. Patients: A total of 90 patients with severe head injury (Glasgow Coma Scal e score less than or equal to 8), in whom continuous monitoring of brain ti ssue oxygen pressure (Pbro(2)) was performed as a measure of cerebral oxyge nation. Interventions: Arterial P-CO2 was decreased each day over a 5-day period fo r 15 mins by increasing minute volume on the ventilator setting to 20% abov e baseline. Arterial blood gas analysis was performed before and after chan ging ventilator settings. Multimodality monitoring, including Pbro(2), was performed in all patients. Absolute and relative Pbro(2)/Paco(2), reactivit y was calculated. Outcome at 6 months was evaluated according to the Glasgo w Outcome Scale. Measurements and Main Results: Effective hyperventilation, defined by a dec rease of Pace, greater than or equal to 2 torr (0.27 kPa), was obtained in 218 (84%) of 272 tests performed. Baseline Paco(2) averaged 32.3 +/- 4.5 to rr (4.31 +/- 0.60 kPa), Average reduction in Paco(2) was 3.8 +/- 1,7 torr ( 0.51 +/- 0.23 kPa) Pbro(2) decreased by 2.8 +/- 3.7 torr (0.37 +/- 0.49 kPa ; p < .001) from a baseline value of 26.5 +/- 11.6 torr (3.53 +/- 1.55 kPa) , Pbro(2)/Paco(2) reactivity was low on day 1 (0.8 +/- 2.3 torr [0.11 +/- 0 .31 kPa]), increasing on subsequent days to 6.1 +/- 4.4 torr (0.81 +/- 0.59 kPa) on day 5, Pbro(2)/Paco(2) reactivity on days 1 and 2 was not related to outcome, In later phases in patients with unfavorable outcome, relative reactivity was increased more markedly, reaching statistical significance o n day 5, Conclusions: Increased hyperventilation causes a significant reduction in P bro(2), providing further evidence for possible increased risk of secondary ischemic damage during hyperventilation, The low Pbro(2)/Paco(2) reactivit y on day 1 indicates the decreased responsiveness of cerebral microvascular vessels to Pace, changes, caused by generalized vascular narrowing, The in creasing Pbro(2)/Paco(2) reactivity from days 2 to 5 suggests that the risk of compromising cerebral oxygenation by hyperventilation may increase over time.