Abstract

Cetacean sedation and anesthesia technique development has lagged behind that of terrestrial mammals over the last 40 years. This has been due in part to a number of different factors including misinformation on cetacean respiratory physiology and its response to sedatives and anesthetic agents, popular myths and fear of the potential impact of “voluntary” breathing capability, improper use of some anesthetic agents, and inadequate monitoring and support of sedated and anesthetized animals. In addition, there have been only a limited number of institutions that displayed these animals in any significant numbers, or that employed full-time veterinary staff that were more likely to investigate proper sedation and anesthetic protocols.

While Navy veterinary personnel pioneered anesthesia in the 1960s and 70s,¹ most other facilities did not begin to commonly use sedation and anesthesia techniques until the 1980s. Clinical presentations requiring sedation or anesthetic procedures in cetaceans are no different than terrestrial species and the need for safe and effective drug protocols is just as important. Initial sedation attempts at Sea World Orlando were based on the recognition that few veterinarians or curators felt that these animals could be safely sedated. As a result, the dosage levels were started low and increased over numerous applications. The drug requirements for initial use of sedatives included the potential for reversibility, a wide safety margin, a history of use in numerous species, low cardiac or respiratory compromise, and preliminary low-dose trials, which developed to clinically useful levels. Drugs which fit these requirements included diazepam (Hospira, Inc., Lake Forest, IL), meperidine (Demerol; Abbott Laboratories, Chicago, IL), midazolam (Versed; Roche Laboratories, Inc, Nutly, NJ) and lately, butorphanol (Torbugesic-SA; Fort Dodge Animal Health, Fort Dodge, IA).

Diazepam has been used orally as well as parenterally. Oral diazepam dosages range from 0.1 mg/kg of body weight for anti-anxiety or appetite stimulation to 0.2 mg/kg for sedation for transport. The onset of effect can range from less than one hour to four hours, partially depending on the amount of food present in the stomach at the time of ingestion. Parenteral diazepam has been used intravenously (IV) as well as intramuscularly (IM). The IV dosage has ranged from 0.05 to 0.1 mg/kg for diazepam. The drug can be somewhat irritating and has in one case caused perivascular necrosis in the ventral fluke vein area. The IM dosage has ranged from 0.5 to 0.15 mg/kg.

The introduction of midazolam to the market, with its increased reliability for predictable IM sedation effects, can replace the use of diazepam except in those cases where the clinician is uncomfortable with substitution or when seizure activity is anticipated. It was hoped that midazolam would also be beneficial for cetacean use if it produced amnesia similar to that seen in humans. Intramuscular midazolam sedation dosages have ranged from 0.045 to 0.1 mg/kg of body weight. It has not been used IV at this point. One of the disadvantages of the benzodiazepines is the ability of a fractious or excited animal to override the sedation effect. In a case where an animal is overly excited the clinician should be cautious about redosing since overdosing is possible. In general, the cetacean that is given midazolam or diazepam IM should be closely monitored. Acceptable peak effects are reached by approximately 25 minutes, during which time the animal is left in the water and observed for changes in respiratory behavior, loss of equilibrium, capability to surface, and ability to recognize and avoid environmental obstacles. When given for short procedures, midazolam and diazepam are generally reversed with flumazenil (0.5 mg/ml; Bedford Labs, Bedford, OH) administered in a volume equal to that of the sedative.

Meperidine was initially introduced in cetaceans in combination with diazepam or midazolam for procedures that were likely to be painful such as tooth extraction or local surgical approaches. Used in combination with midazolam at 0.06 to 0.075 mg/kg, it was initially introduced at 0.025 mg/kg and eventually increased to 0.05 to 1.0 mg/kg. Some clinicians have used it up to 2.0 mg/kg IM alone. Meperidine is often not reversed unless the patient is showing signs of extended depression or incoordination when used alone or after the reversal of other anesthetic or sedative drugs.

The use of butorphanol in cetaceans has increased the potential list of sedative options in these animals. To date it has been given at dosages of 0.05 to 0.15 mg/kg IM in three different species including killer whales (Orcinus orca), false killer whales (Pseudorca crassidens), and bottlenose dolphins (Tursiops truncatus). The time from administration to peak effect is similar to midazolam at 25 minutes. Its use has some advantages over midazolam in that it appears to be more difficult for the animal to override the sedation level with an increase in excitement. One male Tursiops was reported to have increased activity and excitability shortly after administration of butorphanol. This was believed to be due to an adverse drug interaction with aminophylline with which the animal was being treated for pneumonia. The effect was immediately reduced then eliminated within 10 to 15 minutes after the reversal agent, naltrexone, was given at 0.01 mg/kg IM.

Parameters monitored in sedated cetaceans have included respiration rate and depth, blood gases at the start of a procedure, and 15 minutes later, electrocardiogram and heart rate by palpation. Many animals have also been supplemented with oxygen delivered on inspiration by a demand valve held six to eight inches from the nares (blow).

General anesthetic procedures are not as common in cetaceans but three have been performed at Sea World Orlando in bottlenose dolphins for pyometra, a postpartum ruptured uterus, and a kidney biopsy. Induction has varied from the use of IV ketamine (Bioneche Pharma Ltd., London, ON, Canada), IV propofol (Bedford Laboratories, Bedford, OH), or mask induction prior to maintenance gas anesthesia. Mask induction is not recommended because of the potential for complications with blood gas imbalance. Intubation has been performed through the oral cavity with manual repositioning of the larynx and placement of the endotracheal tube through the glottis. The dolphins were then provided isoflurane and respirations were assisted with mechanical ventilation. Blood gas analysis showed that each dolphin developed hypercapnia, which likely contributed to delayed arousal at the end of the procedures in two of the animals. Changes in respiratory support, including increasing respiratory rate and pressure, were able to reverse this anesthesia-induced complication.

Acknowledgments

The authors would like to thank the Sea World Animal Care training, veterinary, and Discovery Cove staff for their assistance with the animals involved. In addition, we would like to thank those individuals that have pioneered improvements in the cetacean medicine field.

Literature Cited

1.  Ridgeway SH. Homeostasis in the Aquatic Environment. In: Ridgeway SH, ed. Mammals of the Sea, Biology and Medicine. Springfield, IL: Charles C. Thomas; 1972:714–721.