Cerebral physiological responses to bolus injection of racemic, S(+)- or R(-)-ketamine in the pig

Little is known about the influence of ketamine and its enantiomers on cerebral haemodynamics, and there are no direct comparison reports. This study was designed to evaluate cerebrovascular responses to bolus injections of racemic, S(+)- and R(-)-ketamine in an established experimental model.

Anaesthesia was induced with propofol in 14 pigs and maintained with fentanyl and vecuronium. The intra-arterial xenon clearance technique was used to calculate the cerebral blood flow (CBF). Eight pigs (part I) were given three consecutive 60-s intravenous (i.v.) bolus injections of 10 mg/kg of racemic ketamine (Ketalar, Pfizer), and cerebral and systemic physiological responses were studied for 30 min after each injection. Following the determination of equipotent doses of the racemate and its enantiomers by recumbency tests, bolus injections of racemic ketamine (10 mg/kg), S-ketamine (5 mg/kg) and R-ketamine (20 mg/kg) were given in randomized sequence to another six pigs (part II) and evaluated at 1, 5, 10, 15, 25 and 40 min.

No statistically significant acute tolerance in the CBF response to racemic ketamine was found in part I of the study. In part II, the decreases in the mean arterial pressure (MAP) and CBF by S-ketamine were significantly smaller than those by racemic and R-ketamine (both P < 0.001). No study drug had any significant effect on the cerebral arteriovenous oxygen content difference (C(av)O(2)) over time, but S-ketamine was associated with lower C(av)O(2) than racemic ketamine (P = 0.008) and R-ketamine (P = 0.016).

Bolus injection of S-ketamine was associated with less cerebral and systemic haemodynamic depression than racemic or R-ketamine in equipotent doses in this experimental model. These findings indicate possible advantages of S-ketamine over racemic ketamine.