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Comparison of the effects of morphine administered by constant-rate intravenous infusion or intermittent intramuscular injection in dogs

Anthony N. Lucas BVMS, MVS1,2, Ava M. Firth DVM, MVS, DACVECC3,4, Garry A. Anderson BAgrSc5, John H. Vine PhD6, and Glenn A. Edwards BVSc7
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  • 1 Veterinary Clinic and Hospital, School of Veterinary Science, University of Melbourne, Werribee, Victoria, 3030, Australia.
  • | 2 Present address is the Centre for Pharmaceutical Research, School of Pharmacy and Medical Sciences, University of South Australia, Adelaide, 5000, Australia.
  • | 3 Veterinary Clinic and Hospital, School of Veterinary Science, University of Melbourne, Werribee, Victoria, 3030, Australia.
  • | 4 Present address is Taloga White Veterinary Consulting, 11333 Polk St NE, Suite A, Minneapolis, MN 55434.
  • | 5 Veterinary Clinic and Hospital, School of Veterinary Science, University of Melbourne, Werribee, Victoria, 3030, Australia.
  • | 6 Racing Analytical Services Ltd, 400 Epsom Rd, Flemington, Victoria, 3031, Australia.
  • | 7 Veterinary Clinic and Hospital, School of Veterinary Science, University of Melbourne, Werribee, Victoria, 3030, Australia.

Abstract

Objective—To compare physiologic and analgesic effects of morphine when given by IV constant-rate infusion or by IM injection to dogs undergoing laparotomy and to determine pharmacokinetics of morphine in dogs following IV constant-rate infusion.

Design—Prospective randomized controlled trial.

Animals—20 dogs.

Procedure—Dogs undergoing laparotomy were treated with morphine beginning at the time of anesthetic induction. Morphine was administered by IV infusion (0.12 mg/kg/h [0.05 mg/lb/h] of body weight) or by IM injection (1 mg/kg [0.45 mg/lb]) at induction and extubation and every 4 hours thereafter. Treatments continued for 24 hours after extubation.

Results—Blood gas values did not indicate clinically significant respiratory depression in either group, and degree of analgesia (determined as the University of Melbourne Pain Scale score) and incidence of adverse effects (panting, vomiting, defecation, and dysphoria) were not significantly different between groups. Dogs in both groups had significant decreases in mean heart rate, rectal temperature, and serum sodium and potassium concentrations, compared with preoperative values. Mean ± SEM total body clearance of morphine was 68 ± 6 ml/min/kg (31 ± 3 ml/min/lb). Mean steady-state serum morphine concentration in dogs receiving morphine by constant-rate infusion was 30 ± 2 ng/ml.

Conclusions and Clinical Relevance—Results indicated that administration of morphine as a constantrate IV infusion at a dose of 0.12 mg/kg/h induced effects similar to those obtained with administration at a dose of 1 mg/kg, IM, every 4 hours in dogs undergoing laparotomy. Panting was attributed to an opioidinduced resetting of the hypothalamic temperature set point, rather than respiratory depression. (J Am Vet Med Assoc 2001;218:884–891)

Abstract

Objective—To compare physiologic and analgesic effects of morphine when given by IV constant-rate infusion or by IM injection to dogs undergoing laparotomy and to determine pharmacokinetics of morphine in dogs following IV constant-rate infusion.

Design—Prospective randomized controlled trial.

Animals—20 dogs.

Procedure—Dogs undergoing laparotomy were treated with morphine beginning at the time of anesthetic induction. Morphine was administered by IV infusion (0.12 mg/kg/h [0.05 mg/lb/h] of body weight) or by IM injection (1 mg/kg [0.45 mg/lb]) at induction and extubation and every 4 hours thereafter. Treatments continued for 24 hours after extubation.

Results—Blood gas values did not indicate clinically significant respiratory depression in either group, and degree of analgesia (determined as the University of Melbourne Pain Scale score) and incidence of adverse effects (panting, vomiting, defecation, and dysphoria) were not significantly different between groups. Dogs in both groups had significant decreases in mean heart rate, rectal temperature, and serum sodium and potassium concentrations, compared with preoperative values. Mean ± SEM total body clearance of morphine was 68 ± 6 ml/min/kg (31 ± 3 ml/min/lb). Mean steady-state serum morphine concentration in dogs receiving morphine by constant-rate infusion was 30 ± 2 ng/ml.

Conclusions and Clinical Relevance—Results indicated that administration of morphine as a constantrate IV infusion at a dose of 0.12 mg/kg/h induced effects similar to those obtained with administration at a dose of 1 mg/kg, IM, every 4 hours in dogs undergoing laparotomy. Panting was attributed to an opioidinduced resetting of the hypothalamic temperature set point, rather than respiratory depression. (J Am Vet Med Assoc 2001;218:884–891)