Abstract
Propofol and isoflurane are commonly used in neuroanaesthesia. Some published data suggest that the use of these agents is associated with impaired cerebral blood flow/carbon dioxide (CO 2 ) reactivity. Cerebrovascular CO 2 reactivity was therefore measured in three cohorts of adult merino sheep: awake (n=6), anaesthetized with steady-state propofol (15 mg/min; n=6) and anaesthetized with 2% isoflurane (n=6). Changes in cerebral blood flow were measured continuously from changes in velocities of blood in the sagittal sinus via a Doppler probe. Alterations in the partial pressure of carbon dioxide in arterial blood (P a CO 2 ) over the range 18–63 mmHg were achieved by altering either the inspired CO 2 concentration or the rate of mechanical ventilation. Cerebral blood flow/CO 2 relationships were determined by linear regression analysis, with changes in cerebral blood flow expressed as a percentage of the value for a P a CO 2 of 35 mmHg. Propofol decreased cerebral blood flow by 55% relative to pre-anaesthesia values (P=0.0001), while isoflurane did not significantly alter cerebral blood flow (88.45% of baseline, P=0.39). Significant linear relationships between cerebral blood flow and CO 2 tension were determined in all individual studies (r 2 ranged from 0.72 to 0.99). The slopes of the lines were highly variable between individuals for the awake cohort (mean 4.73, 1.42–7.12, 95% CI). The slopes for the propofol (mean 2.67, 2.06–3.28, 95% CI) and isoflurane (mean 2.82, 2.19–3.45, 95% CI) cohorts were more predictable. However, there was no significant difference between these anaesthetic agents with respect to the CO 2 reactivity of cerebral blood flow.