Stability of the arterial to end-tidal carbon dioxide difference during anaesthesia for prolonged neurosurgical procedures.

This study was undertaken to examine the variation of the arterial to end-tidal PCO2 (Pa-PETCO2) difference during prolonged neurosurgical anaesthesia. Hyperventilation is often used to reduce intracranial pressure in neurosurgical patients. Continuous end-tidal CO2 monitoring is used as a guide between arterial CO2 measurements. We examined the stability of the Pa-PETCO2 difference in 21 patients undergoing elective craniotomies lasting greater than four hours. A balanced neuroanaesthetic technique was used with the ventilation variables at the discretion of the attending anaesthetist. Once patients were positioned for surgery, simultaneous samples of arterial PCO2 through an arterial catheter, and end-tidal PCO2 via a mass spectrometer were obtained. The Pa-PETCO2 differences of each patient were plotted against time and a slope was derived with simple linear regression. The mean slope for all patients was then computed. There were no changes in the Pa-PETCO2 difference with time (P > 0.05) suggesting a constant relationship between the arterial and end-tidal PCO2 measurements over time. We conclude that end-tidal PCO2 can be used as a reliable guide to estimate arterial PCO2 during neurosurgical procedures of greater than four hours duration once the Pa-PETCO2 difference has been established.