Advertisement

Effects of a muscarinic type-2 antagonist on cardiorespiratory function and intestinal transit in horses anesthetized with halothane and xylazine

Francisco J. Teixeira NetoDepartment of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada, N1G 2W1.
Present address is the Department of Veterinary Surgery and Anesthesiology, Faculdade de Medicina Veterinária e Zootecnia, Unesp, Botucatu, SP, Brazil, 14870-000.

Search for other papers by Francisco J. Teixeira Neto in
Current site
Google Scholar
PubMed
Close
 MV, PhD
,
Wayne N. McDonellDepartment of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada, N1G 2W1.

Search for other papers by Wayne N. McDonell in
Current site
Google Scholar
PubMed
Close
 DVM, PhD
,
William D. BlackDepartment of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada, N1G 2W1.

Search for other papers by William D. Black in
Current site
Google Scholar
PubMed
Close
 DVM, PhD
,
Aury N. MoraesDepartment of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada, N1G 2W1.

Search for other papers by Aury N. Moraes in
Current site
Google Scholar
PubMed
Close
 MV, DVSc
, and
Sumit DuronghphongtornDepartment of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada, N1G 2W1.

Search for other papers by Sumit Duronghphongtorn in
Current site
Google Scholar
PubMed
Close
 DVM

Abstract

Objective—To evaluate the cardiorespiratory and intestinal effects of the muscarinic type-2 (M2) antagonist, methoctramine, in anesthetized horses.

Animals—6 horses.

Procedure—Horses were allocated to 2 treatments in a randomized complete block design. Anesthesia was maintained with halothane (1% end-tidal concentration) combined with a constant-rate infusion of xylazine hydrochloride (1 mg/kg/h, IV) and mechanical ventilation. Hemodynamic variables were monitored after induction of anesthesia and for 120 minutes after administration of methoctramine or saline (0.9% NaCl) solution (control treatment). Methoctramine was given at 10-minute intervals (10 µg/kg, IV) until heart rate (HR) increased at least 30% above baseline values or until a maximum cumulative dose of 30 µg/kg had been administered. Recovery characteristics, intestinal auscultation scores, and intestinal transit determined by use of chromium oxide were assessed during the postanesthetic period.

Results—Methoctramine was given at a total cumulative dose of 30 µg/kg to 4 horses, whereas 2 horses received 10 µg/kg. Administration of methoctramine resulted in increases in HR, cardiac output, arterial blood pressure, and tissue oxygen delivery. Intestinal auscultation scores and intestinal transit time (interval to first and last detection of chromium oxide in the feces) did not differ between treatment groups.

Conclusions and Clinical Relevance—Methoctramine improved hemodynamic function in horses anesthetized by use of halothane and xylazine without causing a clinically detectable delay in the return to normal intestinal motility during the postanesthetic period. Because of their selective positive chronotropic effects, M2 antagonists may represent a safe alternative for treatment of horses with intraoperative bradycardia. (Am J Vet Res 2004;65:464–472)

Abstract

Objective—To evaluate the cardiorespiratory and intestinal effects of the muscarinic type-2 (M2) antagonist, methoctramine, in anesthetized horses.

Animals—6 horses.

Procedure—Horses were allocated to 2 treatments in a randomized complete block design. Anesthesia was maintained with halothane (1% end-tidal concentration) combined with a constant-rate infusion of xylazine hydrochloride (1 mg/kg/h, IV) and mechanical ventilation. Hemodynamic variables were monitored after induction of anesthesia and for 120 minutes after administration of methoctramine or saline (0.9% NaCl) solution (control treatment). Methoctramine was given at 10-minute intervals (10 µg/kg, IV) until heart rate (HR) increased at least 30% above baseline values or until a maximum cumulative dose of 30 µg/kg had been administered. Recovery characteristics, intestinal auscultation scores, and intestinal transit determined by use of chromium oxide were assessed during the postanesthetic period.

Results—Methoctramine was given at a total cumulative dose of 30 µg/kg to 4 horses, whereas 2 horses received 10 µg/kg. Administration of methoctramine resulted in increases in HR, cardiac output, arterial blood pressure, and tissue oxygen delivery. Intestinal auscultation scores and intestinal transit time (interval to first and last detection of chromium oxide in the feces) did not differ between treatment groups.

Conclusions and Clinical Relevance—Methoctramine improved hemodynamic function in horses anesthetized by use of halothane and xylazine without causing a clinically detectable delay in the return to normal intestinal motility during the postanesthetic period. Because of their selective positive chronotropic effects, M2 antagonists may represent a safe alternative for treatment of horses with intraoperative bradycardia. (Am J Vet Res 2004;65:464–472)