The cardiopulmonary bypass apparatus must temporarily substitute for the cardiac and pulmonary function of the patient undergoing heart surgery. In order to meet the metabolic needs of the patient and the technical demands of the surgeon, within the limits of engineering technology, a number of major alterations are made in normal physiology. The patient is typically cooled to 27 degrees C and perfused with a non-pulsatile flow of blood which has been diluted with saline to a haematocrit in the mid-20s. Blood flow and pressure are often considerably less than normal. Blood coagulation is prevented by administration of a massive dose of heparin. Central redistribution of blood flow, elaboration of stress-reactant hormones, and fluid and electrolyte shifts occur in response to these changes. In the postoperative period, these alterations are reversed, and normal physiology is restored. Effects upon the pharmacokinetics of drugs are anticipated. The clearance of many drugs may be reduced. Protein binding is diminished by haemodilution, but may rise above normal in the postoperative period for basic drugs which bind to alpha 1-acid glycoprotein. Changes in volume of distribution depend upon the opposing influences of protein binding and reduced peripheral perfusion. Previous studies on the pharmacokinetics of drugs during and after cardiopulmonary bypass illustrate many of these effects. The clearance of digoxin, fentanyl, and the cephalosporins is reduced after cardiopulmonary bypass, and the volume of distribution of cefazolin is increased during cardiopulmonary bypass. Studies of digitoxin and propranolol are also reviewed. Many of the investigations in this area of study have been limited by logistical and methodological factors. Thus, the effects of cardiopulmonary bypass on the pharmacokinetics of drugs are incompletely understood, and the subject merits further attention.

We have investigated the use of constant-rate delivery of fentanyl by i.v. and transdermal routes for the treatment of pain after major surgery. Forty-five males, ASA I-III, received in a double-blinded fashion either placebo (n = 6) or fentanyl (n = 39) i.v. at one of four dose rates (25, 50, 100 or 125 micrograms h-1). Stable serum concentrations of fentanyl were produced by the end of surgery and were maintained for a total of 24 h. Calculated clearance of fentanyl was 1.05 +/- 0.38 litre min-1 and was not related to weight or age. Both the 100- and 125-micrograms h-1 dose rates produced significant analgesic efficacy as assessed by postoperative morphine requirements. Mean serum concentrations of fentanyl in these groups were 1.42 +/- 0.14 (SD) and 1.90 +/- 0.30 ng ml-1, respectively. One of 10 patients receiving fentanyl 100 micrograms h-1 and three of nine patients receiving 125 micrograms h-1 had evidence of respiratory depression. Eight additional patients were treated with a transdermal drug delivery system containing fentanyl (TTS-fentanyl). Steady-state serum concentrations in this group were 2.15 +/- 0.92 (SD) ng ml-1. Post-operative morphine requirements were minimal (less than 0.5 mg h-1) and PaCO2 remained acceptable in all patients. Serum concentrations of fentanyl increased slowly (15 h to plateau) and decreased slowly (apparent half-life, 21 h). We conclude that delivery of analgesic doses of fentanyl is feasible by the transdermal route.

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Haloperidol kinetics were determined after oral and intravenous drug doses in 15 men. Mean elimination t1/2 for the subjects was 17.9 +/- 6.4 (SD) hr. After 0.125 mg/kg IV, mean distribution t1/2s in six subjects were 0.19 +/- 0.07 and 2 +/- 1 hr, and in 12 subjects mean clearance was 11.8 +/- 2.9 ml/kg/min and mean steady-state volume of distribution was 17.8 +/- 6.5 l/kg. After 0.50-mg/kg oral doses in eight subjects, mean lag time before drug absorption was 0.82 +/- 0.25 hr. Mean absorption t1/2 was 0.37 +/- 0.18 hr and mean distribution t1/2 was 0.96 +/- 0.20 hr. Bioavailability was 0.65 +/- 0.14 after oral doses. In 14 kinetic studies in nine subjects, data was analyzed by both model-dependent (open two- and three-compartment models using nonlinear regression) and model-independent (AUC and first moment curve) approaches. Results of the two were found to be in close agreement. The long elimination t1/2 of haloperidol is explained by the drug's extensive tissue distribution.