Ketamine Infusions: Pharmacokinetics and Clinical Effects

The clinical effects and pharmacokinetics of ketamine, administered as an i.v. infusion, were studied in 31 patients. Anaesthesia was induced with ketamine 2 mg kg−1 i.v. and maintained using an i.v. infusion of ketamine, supplemented by nitrous oxide. The plasma concentrations of ketamine, nor-ketamine and dehydro-nor-ketamine were analysed using gas-liquid chromatography. The average maintenance dose of ketamine was 41 ± 21 μg kg−1 min−1, but there was an obvious decrease in the dose required as anaesthesia progressed. This dose gave a stable plasma concentration of ketamine of 9.3 ± 0.8 μmol litre−1. Patients recovered at 2.7 ± 0.9 μmol litre−1. Plasma half-life of ketamine was 79 ±8 min. Maximum concentration of nor-ketamine was 4.7 ± 2.4 μmol litre−1 and of dehydro-nor-ketamine 3.2 ± 1.9 μmol litre−1. There were transient increases (15–30% of pre anaesthetic values) in arterial pressure, heart rate and cardiac output during operation. No postoperative respiratory depression was seen.