Arterial to end-tidal carbon dioxide difference during craniotomy in severely head-injured patients

Clinical data suggest that cerebral blood flow (CBF) can be abnormally low within the first four to eight hours after severe head injury (SHI). An aggressive hyperventilation can additionally worsen CBF and provoke cerebral ischemia. Therefore an accurate PCO2 monitoring in SHI patients (pts) is necessary. PetCO2 failed to reflect PaCO2 in SHI pts treated in neurosurgical ICU. Up to now, the validity of PetCO2 monitoring in estimating PaCO2 during an acute posttraumatic craniotomy has not been studied. Forty five adult SHI pts operated on because of an acute intracranial posttraumatic haematoma within 8 hours after head trauma entered the study. The standard anaesthetic protocol included N2O/O2, fentanyl and pancuronium bromide anaesthesia, and mechanical ventilation with respiratory rate 10 divided by 12 bpm and tidal volume in mL = body weight (kg) x 10 - 100. After obtaining a stable PetCO2 arterial blood sample was taken for PaCO2 measurement and P(a-et)CO2 = PaCO2 - PetCO2 was calculated. P(a-et)CO2 ranged -9 divided by 20 mm Hg (5 +/- 6; mean +/- SD). P(a-et)CO2 between 2 mm Hg and 6 mm Hg was found in 17 (38%) patients only. A negative P(a-et)CO2 was stated in 20% of patients. No relationships between P(a-et)CO2 and pts age and mean arterial pressure were found. P(a-et)CO2 was higher in normocapneic pts than in hyperventilated ones and tended to decrease with an increase in heart rate. We can conclude that during an acute craniotomy in SHI pts, PetCO2 does not reflect accurately PaCO2 and the monitoring of adequacy of ventilation should be based on repeated or continuous measurements of an arterial PCO2.