The effect of hypovolemia due to hemorrhage on the minimum alveolar concentration of isoflurane in the dog.
OBJECTIVE
To determine the effect of hypovolemia on the minimum alveolar concentration (MAC) of isoflurane in the dog.


METHODS
Randomized, cross-over trial.


METHODS
Six healthy intact mixed breed female dogs weighing 18.2-29.0 kg.


METHODS
Dogs were randomly assigned to determine the MAC of isoflurane in a normovolemic or hypovolemic state with a minimum of 18 days between trials. On both occasions, anesthesia was initially induced and maintained for 40 minutes with isoflurane delivered in oxygen while vascular catheters were placed in the cephalic vein and dorsal metatarsal artery. In dogs assigned to the hypovolemic group, 30 mL kg(-1) of blood was removed at 1 mL kg(-1) minute(-1) from the arterial catheter. All dogs were allowed to recover from anesthesia. Thirty minutes after the discontinuation of isoflurane, anesthesia was re-induced with isoflurane in oxygen delivered by face mask. The tracheas were intubated, and connected to an anesthetic machine with a Bain anesthetic circuit. Mechanical ventilation was instituted at a rate of 10 breaths minute(-1) with the tidal volume set to deliver 10-15 mL kg(-1). Airway gases were monitored continuously and tidal volume was adjusted to maintain an end-tidal carbon dioxide level of 35-40 mmHg (4.67-5.33 kPa). Body temperature was maintained at 37-38 degrees C (98.6-100.4 degrees F). The MAC determination was performed using an electrical stimulus applied to the toe web and MAC was defined as the mean value of end-tidal isoflurane between the concentrations at which a purposeful movement did and did not occur in response to the electrical stimulus. The MAC values were compared between groups using a Student's t-test.


RESULTS
The MAC of isoflurane was significantly less in hypovolemic dogs (0.97 +/- 0.03%) compared with normovolemic dogs (1.15 +/- 0.02%) (p < 0.0079).


CONCLUSIONS
The MAC of isoflurane is reduced in dogs with hypovolemia resulting from hemorrhage. Veterinarians should be prepared to deliver a lower percentage of isoflurane to maintain anesthesia in hypovolemic dogs during diagnostic and therapeutic procedures.
Mattson
Sima F
SF
Department of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada.
Kerr
Carolyn L
CL
Dyson
Doris H
DH
Mirakhur
Kuldip K
KK
D000769:Anesthesia, Inhalation / Q000662:veterinary; D018685:Anesthetics, Inhalation / Q000008:administration & dosage / Q000493:pharmacokinetics*; D000818:Animals; D018592:Cross-Over Studies; D004285:Dogs / Q000502:physiology*; D005260:Female; D006470:Hemorrhage / Q000378:metabolism*; D020896:Hypovolemia / Q000378:metabolism*; D007530:Isoflurane / Q000008:administration & dosage / Q000493:pharmacokinetics*; D010313:Partial Pressure; D011650:Pulmonary Alveoli
D018685:Anesthetics, Inhalation; D007530:Isoflurane