[Pharmacokinetics of the continuous infusion of propofol in the cirrhotic patient. Preliminary results]

Pharmacokinetic implications for the clinical use of propofol

Propofol, the recently marketed intravenous induction agent for anaesthesia, is chemically unrelated to earlier anaesthetic drugs. This highly lipophilic agent has a fast onset and short, predictable duration of action due to its rapid penetration of the blood-brain barrier and distribution to the CNS, followed by redistribution to inactive tissue depots such as muscle and fat. On the basis of pharmacokinetic-pharmacodynamic modelling, a mean blood-brain equilibration half-life of only 2.9 minutes has been calculated. In most studies, the blood concentration curve of propofol has been best fitted to a 3-compartment open model, although in some patients only 2 exponential phases can be defined. The first exponential phase half-life of 2 to 3 minutes mirrors the rapid onset of action, the second (34 to 56 minutes) that of the high metabolic clearance, whereas the long third exponential phase half-life of 184 to 480 minutes describes the slow elimination of a small proportion of the drug remaining in poorly perfused tissues. Thus, after both a single intravenous injection and a continuous intravenous infusion, the blood concentrations rapidly decrease below those necessary to maintain sleep (around 1 mg/L), based on both the rapid distribution, redistribution and metabolism during the first and second exponential phases (more than 70% of the drug is eliminated during these 2 phases). During long term intravenous infusions cumulative drug concentrations and effects might be expected, but even then the recovery times do not appear to be much delayed. The liver is probably the main eliminating organ, and renal clearance appears to play little part in the total clearance of propofol. On the other hand, because the total body clearance may exceed liver blood flow, an extrahepatic metabolism or extrarenal elimination (e.g. via the lungs) has been suggested. Approximately 60% of a radiolabelled dose of propofol is excreted in the urine as 1- and 4-glucuronide and 4-sulphate conjugates of 2.6-diisopropyl 1,4-quinol, and the remainder consists of the propofol glucuronide. Thus for hepatic and renal diseases, co-medication, surgical procedure, gender and obesity do not appear to cause clinically significant changes in the pharmacokinetic profile of propofol, but the decrease in the clearance value in the elderly might produce higher concentrations during a long term infusion, with an increased drug effect. In addition, the lower induction dose observed in relation to increased age might be partly explained by a smaller central volume of distribution.(ABSTRACT TRUNCATED AT 400 WORDS)

[The course of blood concentrations of propofol administered at a constant rate, combined with fentanyl]

The blood concentration of propofol was studied in 14 ASA 1 informed patients, who were to undergo orthopaedic or plastic surgery lasting at least 90 min. Anaesthesia was induced with a 2 mg.kg-1 bolus of propofol together with 0.86 microgram.kg-1 fentanyl. This was followed by a constant rate infusion of propofol and fentanyl, 5 mg.kg-1.h-1 and 3 micrograms.kg-1.h-1 respectively. The mean duration of propofol infusion was 153 +/- 63 min, with extremes of 90 and 315 min. Propofol concentration was measured using gas phase chromatography on total arterial blood; the lower limit of detection was 0.05 mg.l-1. During the infusion, blood concentrations were found between 2 and 4 mg.l-1. It was 2.25 mg.l-1 at the fifth min; this was 80% of the concentration found at the 120th min. There was in fact no statistically significant difference between the values found at the 90th, 120th and 150th min. On stopping the infusion, the concentrations fell rapidly during the first 5 min, and then more slowly. By the 30th min, it had reached a value 4.5 times less than that at the end of the infusion. However, individual variations were found, which could explain delayed recovery. The calculated pharmacokinetic parameters were: elimination half-life = 41.7 +/- 20 min, clearance = 2.14 +/- 0.55 l.min-1 and equilibrium distribution volume = 43.4 +/- 15.2 l. These results are discussed. It is therefore possible to give propofol continuously at a constant rate without having any accumulative effect.