Measurements of CO2 reactivity and barbiturate reactivity in patients with severe head injury

In nine patients with severe head injury subjected to continuous hyperventilation and barbiturate coma treatment with pentobarbitone, the regional cerebral blood flow was measured as initial slope index (ISI) with a 32 channel Cerebrograph, and cerebral metabolic rate of oxygen (CMRO2) was calculated as the product of mean global CBF and the arterio-venous oxygen content difference. CBF was measured at strategic intervals either to follow the treatment (hyperventilation and/or pentobarbitone), or to determine whether these principles of treatment should be intensified or reduced. During the flow measurements the CO2 reactivity and the reactivity to a bolus injection of thiopentone 5 mg/kg were calculated globally and regionally. The global CO2 reactivity was calculated as relative (%change CBF/delta PaCO2 mmHg) and absolute (deltaCBF/deltaPaCO2 mmHg), and the reactivity to barbiturate was calculated globally as delta CMRO2, and regionally as %change rCBF. The absolute and relative global CO2 reactivities correlated positively with the mean CBF values before hyperventilation, and the global barbiturate reactivity was dependent on the CMRO2 value obtained before hyperventilation. However, at low levels of CMRO2 ranging between 1.0 and 1.1 ml O2 the barbiturate reactivity was abolished. The regional studies of CBF, CMRO2, CO2 reactivity and barbiturate reactivity gave important information, when decisions concerning therapeutic regimes with special reference to hyperventilation and sedation with pentobarbitone were necessary.