Is Flumazenil a Good Choice for Reversing Anesthesia? B y
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B a s i l l i o
There is little doubt that benzodiazepines are valuable co-induction agents for maintaining a balanced anesthesia regimen in horses. The drugs modulate and potentiate the actions of the neurotransmitter gamma-aminobutyric acid (GABA) at central GABAA receptors, which results in muscle relaxation, coupled with a reduction in anesthetic drug requirements and an extended anesthetic duration. Plus, they tend not to compromise cardiovascular function. However, benzodiazepines are long-acting—the biological half-life of diazepam is anywhere from 7 to 21 hours, and the median terminal half-life for midazolam ranges from 3.6 to about 7 hours. Drug effects could last even longer as a result of altered hepatic blood flow. Horses are susceptible to compromised gas exchange during and following administration of general anesthesia due to pulmonary atelectasis and ventilation perfusion mismatch. Coupled with the long half-lives of benzodiazepines, respiratory depression and ataxia may last well after anesthesia and compromise recovery. Enter flumazenil. The GABA receptor antagonist competitively blocks the benzodiazepine binding site on the GABAA receptor complex. In humans and companion animals, flumazenil offers an improvement in respiratory depression caused by benzodiazepines and enhances recovery from general anesthesia. However, the literature falls short of direct examinations of the use and effects of the drug in the horse. To narrow the knowledge gap, Hope Douglas, VMD, DACVS (Large Animal), a resident in anesthesia at New Bolton Center in Kennett Square, Pa., Klaus Hopster, DVM, PhD, DECVAA, Bernd Driessen, DVM, PhD, DACVAA, DECVPT, and their colleagues recently published a study in the Equine Veterinary Journal that investigated the effects of flumazenil in horses induced with midazolam-ketamine and maintained with isoflurane and oxygen. The study involved 6 healthy horses between the age of 2 and 18 years (average age, 11 years) who had not undergone a prior general anesthesia event. Breeds included 3 Standardbreds, 1 Thoroughbred-
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