Effect of sedation with detomidine and butorphanol on pulmonary gas exchange in the horse.
BACKGROUND
Sedation with alpha2-agonists in the horse is reported to be accompanied by impairment of arterial oxygenation. The present study was undertaken to investigate pulmonary gas exchange using the Multiple Inert Gas Elimination Technique (MIGET), during sedation with the alpha2-agonist detomidine alone and in combination with the opioid butorphanol.


METHODS
Seven Standardbred trotter horses aged 3-7 years and weighing 380-520 kg, were studied. The protocol consisted of three consecutive measurements; in the unsedated horse, after intravenous administration of detomidine (0.02 mg/kg) and after subsequent butorphanol administration (0.025 mg/kg). Pulmonary function and haemodynamic effects were investigated. The distribution of ventilation-perfusion ratios (VA/Q) was estimated with MIGET.


RESULTS
During detomidine sedation, arterial oxygen tension (PaO2) decreased (12.8 +/- 0.7 to 10.8 +/- 1.2 kPa) and arterial carbon dioxide tension (PaCO2) increased (5.9 +/- 0.3 to 6.1 +/- 0.2 kPa) compared to measurements in the unsedated horse. Mismatch between ventilation and perfusion in the lungs was evident, but no increase in intrapulmonary shunt could be detected. Respiratory rate and minute ventilation did not change. Heart rate and cardiac output decreased, while pulmonary and systemic blood pressure and vascular resistance increased. Addition of butorphanol resulted in a significant decrease in ventilation and increase in PaCO2. Alveolar-arterial oxygen content difference P(A-a)O2 remained impaired after butorphanol administration, the VA/Q distribution improved as the decreased ventilation and persistent low blood flow was well matched. Also after subsequent butorphanol no increase in intrapulmonary shunt was evident.


CONCLUSIONS
The results of the present study suggest that both pulmonary and cardiovascular factors contribute to the impaired pulmonary gas exchange during detomidine and butorphanol sedation in the horse.
Department of Environment and Health, Faculty of Veterinary Medicine and Animal Science, Swedish University of Agricultural Sciences, Skara, Sweden. gorel.nyman@gmail.com
Görel
G
Nyman
Stina
S
Marntell
Anna
A
Edner
Pia
P
Funkquist
Karin
K
Morgan
Göran
G
Hedenstierna
D000316:Adrenergic alpha-Agonists / Q000008:administration & dosage; D000701:Analgesics, Opioid / Q000008:administration & dosage; D000818:Animals; D001784:Blood Gas Analysis / Q000662:veterinary; D002077:Butorphanol / Q000008:administration & dosage*; D016292:Procedural Sedation / Q000379:methods / Q000662:veterinary*; D018592:Cross-Over Studies; D004338:Drug Combinations; D005260:Female; D006439:Hemodynamics / Q000187:drug effects; D006736:Horses / Q000502:physiology*; D007093:Imidazoles / Q000008:administration & dosage*; D008297:Male; D010100:Oxygen / Q000097:blood; D010101:Oxygen Consumption / Q000187:drug effects / Q000502:physiology; D011659:Pulmonary Gas Exchange / Q000187:drug effects* / Q000502:physiology; D011897:Random Allocation; D012119:Respiration / Q000187:drug effects; D014692:Ventilation-Perfusion Ratio / Q000187:drug effects / Q000502:physiology
D000316:Adrenergic alpha-Agonists; D000701:Analgesics, Opioid; D004338:Drug Combinations; D007093:Imidazoles; C041255:detomidine; D002077:Butorphanol; D010100:Oxygen