The role of pharmacokinetics in anaesthesia: application to intravenous infusions

Isoflurane minimum alveolar concentration sparing effects of fentanyl in the dog

OBJECTIVE

To characterize the isoflurane-sparing effects of a high and a low dose of fentanyl in dogs, and its effects on mean arterial pressure (MAP) and heart rate (HR).

STUDY DESIGN

Prospective, randomized crossover trial.

ANIMALS

Eight healthy male Beagle dogs weighing 12.1 ± 1.6 kg [mean ± standard deviation (SD)] and approximate age 1 year.

METHODS

Dogs were anesthetized using isoflurane and minimum alveolar concentration (MAC) was determined in duplicate by the bracketing method using an electrical stimulus on the tarsus. Animals were administered fentanyl: low dose (33 μg kg-1 loading dose, 0.2 μg kg-1 minute-1) or high dose (102 μg kg-1 loading dose, 0.8 μg kg-1 minute-1) and MAC was re-determined (MACISO-F). Blood was collected for analysis of plasma fentanyl concentrations before administration and after MACISO-F determination. All values are presented as mean ± SD.

RESULTS

Isoflurane MAC (MACISO) was 1.30 ± 0.23% in the low dose treatment, which significantly decreased to 0.75 ± 0.22% (average MAC reduction 42.3 ± 9.4%). MACISO was 1.30 ± 0.18% in the high dose treatment, which significantly decreased to 0.30 ± 0.11% (average MAC reduction 76.9 ± 7.4%). Mean fentanyl plasma concentrations were 6.2 and 29.5 ng mL-1 for low and high dose treatments, respectively. MAP increased significantly only in the high dose treatment (from 81 ± 8 to 92 ± 9 mmHg). HR decreased significantly in both treatments from 108 ± 25 to 61 ± 14 beats minute-1 with the low dose and from 95 ± 14 to 42 ± 4 beats minute-1 with the high dose.

CONCLUSIONS AND CLINICAL RELEVANCE

Fentanyl administration resulted in a dose-dependent isoflurane MAC-sparing effect with bradycardia at both doses and an increase in MAP only at high dose. Further evaluation is needed to determine the effects of fentanyl on the overall cardiovascular function.

VARIABLE RATE IV INFUSION OF MORPHINE AND HAEMODYNAMIC RESPONSE TO LARYNGOSCOPY AND TRACHEAL INTUBATION

Haemodynamic response to laryngoscopy and tracheal intubation was studied in two groups of 45 patients each. Group I patients received variable rate infusion of morphine for 30 minutes followed by intravenous (IV) bolus dose of diazepam. Group II patients received intramuscular morphine as preanaesthetic premedication and preinduction IV bolus dose of diazepam. The method of induction of GA was same in both the groups. Systolic and diastolic blood pressure (SBP and DBP) mean arterial pressure and pulse-rate were recorded at intubation and at different time intervals upto 10 minutes after intubation. In group I patients, there was no statistically significant increase in pulse-rate and SBP following intubation but transient significant increase in DBP. In group II patients, there was statistically significant increase in pulse-rate, SBP and DBP upto 3 minutes post-intubation. Thus IV infusion of morphine in combination with IV bolus dose of diazepam proved more effective in attenuating haemodynamic response to tracheal intubation.

Infusions of propofol to supplement nitrous oxide-oxygen for the maintenance of anaesthesia. A comparison with halothane

The peri-operative and postoperative effects of propofol given by infusion were compared with halothane as a supplement to nitrous oxide-oxygen anaesthesia for body surface surgery in patients who breathed spontaneously. Anaesthesia was induced after opioid premedication, with either propofol 2.5 mg/kg or thiopentone 4-5 mg/kg which were followed respectively by an infusion of propofol 12 mg/kg/hour for 10 minutes and at a variable rate thereafter, or by halothane at a mean inspired concentration of 1.2%. Maintenance of anaesthesia required a median rate of infusion of propofol of 149.4 micrograms/kg/minute. The cardiovascular effects during induction and maintenance of anaesthesia were similar in the two groups. The overall incidence of side effects was low but immediate recovery was significantly faster in patients who received propofol.