Since the introduction of the lipid emulsion formulation in 1986, propofol has become established for induction as well as for maintenance of anaesthesia in veterinary practice1 including cats2;3-8. Propofol is rapidly metabolized by hepatic glucuronidation in most species and it has also been shown to undergo extrahepatic metabolism9-13, so that total body clearance may exceed liver blood flow in certain species. Because of their highly carniverous diet, cats are little exposed to antiherbivory compounds so that they have become deficient in UGP-glucuronosyltransferase (UGT)14. Consequently, a number of drugs are eliminated slowly15;16, often giving rise to prolonged half-lives of the parent drugs. Cats are therefore sensitive to the adverse effects of many drugs and toxins that are normally glucuronidated before elimination. It is therefore likely that the disposition of propofol may differ markedly from that of humans and other animal species17. Adam et al18 reported that for the cremophor propofol formulation in cats, volumes of distribution were smaller and elimination halflives were longer than those of pigs, rats and rabbits. In addition, pulmonary uptake has been demonstrated to occur in cats,19 however propofol’s pharmacokinetics have not been studied formally. The purpose of this study was to determine the pharmacokinetic behaviour of propofol after single intravenous injections. In comparison with man, the apparent central volume of distribution in domestic cats is small (0.56L.kg-1 body weight vs. 0.228L.kg-1) for the human pharmacokinetic parameter set of Marsh et al20 and the clearance (0.0086 L.kg-1.min-1 vs. 0.027 L.kg- 1.min-1) is approximately 2½ times slower in cats when compared with humans. Slow clearance should not influence recovery from anaesthesia following standard induction doses, because the early decreases in blood concentrations are predominantly due to redistribution of drug to various tissues (similar to the disposition of thiopentone which exhibits a slow total body clearance21. However it is possible that drug may accumulate within the body after prolonged infusions, resulting in delayed recovery times. This phenomenon is best described by calculating “context-sensitive” decrement-times by computer simulation22-24. Computer software♣ were used to calculate the 20%, 50% and 80% context-sensitive decrement times for the cat pharmacokinetic model. For comparative purposes, similar calculations were performed for an adult human male (weight 70 kg) using the pharmacokinetic parameter-set of Marsh et al20. Assuming that recovery from anaesthesia occurs after a 50% decrease in blood concentrations has taken place, it is apparent from the 50% context-senstive decrement-time graph that for infusions lasting up to 20 minutes (during which concentrations are kept constant), recovery can be expected to be rapid and predictable. However if infusions are administered for longer than 20 minutes, the recovery times of the “average” cat increase rapidly, reaching a plateau of 36 minutes, while recovery times of the human remain short, albeit increasing slowly. Awakening times become dramatically prolonged and unpredictable in both cats and humans if propofol concentrations are required to decrease by 80% for recovery to occur. Under these circumstances the 80% decrement time after a two-hour infusion is approximately two hours in cats and 45 minutes in humans. On the other hand, if dosing is conservative, so that blood concentrations need to decrease by only 20% for awakening to occur, then recovery times are short and predictable, being only a few minutes, regardless of the duration of the preceding infusion. These findings are in accordance with those of Pascoe et al25 who reported that cats took longer to recover after a short (30 min) infusion than after a long (150 min) infusion. In their crossover study, the propofol infusion rates were adjusted so that the cats were maintained at a light level of anaesthesia at which they responded sluggishly to pedal stimulation. It is therefore likely that propofol concentrations were kept steady and were similar during the 30-minute as well as during the 150-minute infusions. Delayed recovery has also been reported when propofol was administered to cats on consecutive days26. Conclusions and clinical relevance: We recommend that propofol infusions be administered to cats only for fairly short procedures and that for prolonged surgery, maintenance of anaesthesia should be accomplished using other drugs. In order to decrease the propofol dose, premedication and analgesic supplements should be co-administered to provide “balanced” anaesthesia. ♣ TIVA Trainer version 8, author Frank Engbers, Leiden University Medical Centre Copyright
Dissertation (MMedVet)--University of Pretoria, 2010.