Characteristics of the relationship between plasma ketamine concentration and its effect on the minimum alveolar concentration of isoflurane in dogs.
OBJECTIVE
To characterize the shape of the relationship between plasma ketamine concentration and minimum alveolar concentration (MAC) of isoflurane in dogs.


METHODS
Retrospective analysis of previous data.


METHODS
Four healthy adult dogs.


METHODS
The MAC of isoflurane was determined at five to six different plasma ketamine concentrations. Arterial blood samples were collected at the time of MAC determination for measurement of plasma ketamine concentration. Plasma concentration/effect data from each dog were fitted to a sigmoid inhibitory maximum effect model in which MAC(c)= MAC(0) - (MAC(0)-MAC(min)) x C(gamma)/EC(50)(gamma)+C(gamma), where C is the plasma ketamine concentration, MAC(c) is the MAC of isoflurane at plasma ketamine concentration C, MAC(0) is the MAC of isoflurane without ketamine, MAC(min) is the lowest MAC predicted during ketamine administration, EC(50) is the plasma ketamine concentration producing 50% of the maximal MAC reduction, and gamma is a sigmoidicity factor. Nonlinear regression was used to estimate MAC(min), EC(50), and gamma.


RESULTS
Mean +/- SEM MAC(min), EC(50) and gamma were estimated to be 0.11 +/- 0.01%, 2945 +/- 710 ng mL(-1) and 3.01 +/- 0.84, respectively. Mean +/- SEM maximal MAC reduction predicted by the model was 92.20 +/- 1.05%.


CONCLUSIONS
The relationship between plasma ketamine concentration and its effect on isoflurane MAC has a classical sigmoid shape. Maximal MAC reduction predicted by the model is less than 100%, implying that high plasma ketamine concentrations may not totally abolish gross purposeful movement in response to noxious stimulation in the absence of inhalant anesthetics.


CONCLUSIONS
The parameter estimates reported in this study will allow clinicians to predict the expected isoflurane MAC reduction from various plasma ketamine concentrations in an average dog.
Pypendop
Bruno H
BH
Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616, USA. bhpypendop@ucdavis.edu
Solano
Adrian
A
Boscan
Pedro
P
Ilkiw
Jan E
JE
D000778:Anesthetics, Dissociative / Q000097:blood / Q000378:metabolism / Q000494:pharmacology*; D018685:Anesthetics, Inhalation / Q000032:analysis* / Q000097:blood / Q000378:metabolism; D000818:Animals; D019540:Area Under Curve; D001784:Blood Gas Analysis; D001794:Blood Pressure; D004285:Dogs / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D007530:Isoflurane / Q000032:analysis* / Q000097:blood / Q000378:metabolism; D007649:Ketamine / Q000097:blood / Q000378:metabolism / Q000494:pharmacology*; D011650:Pulmonary Alveoli / Q000187:drug effects* / Q000378:metabolism; D012189:Retrospective Studies
D000778:Anesthetics, Dissociative; D018685:Anesthetics, Inhalation; D007649:Ketamine; D007530:Isoflurane