Cetacean Anesthesia: A Review of 34 General Anesthesia Events, Lessons Learned, and Future Plans
Abstract
Although applied infrequently, anesthesia of cetaceans has become an acceptable risk. Historical failures and limited experience led to many myths and misconceptions about anesthesia of cetaceans. As well, the relative difficulty with invasive support and monitoring had caused hesitation in performance of medically necessary invasive procedures, only increasing the probability of negative outcomes and further perpetuating these misconceptions. A coalition-of-the-willing was built at the IAAAM conference over the recent past, and IAAAM members have continued efforts to update surgical and anesthetic management of cetaceans.1-23 The following is evidence of their strong efforts.
We present here a series of anesthesia events involving bottlenose dolphins (Tursiops truncatus, Tursiops truncatus gilli, and Tursiops truncatus aduncus) and Pacific white-sided dolphins (Lagenorhynchus obliquidens). Currently, benzodiazepines alone (diazepam, midazolam), or in combination with opioids (butorphanol, meperidine) are used for preanesthetic sedation. Intravenous access via ultrasound-guided cannulation of the lateral subcutaneous caudal vein (LSCV) has become routine. The injectable anesthetic agent, propofol, is used for rapid sequence induction allowing manual orotracheal intubation by rostral luxation of the elongated epiglottal and cricoarytenoid cartilages or “goosebeak.” The inhalation anesthetic agents, isoflurane and sevoflurane, are used reliably for maintenance of anesthesia. Though statistical analysis cannot easily be applied, the median subject (22-year-old and 187.5 kg) minimally received either 0.25 mg/kg diazepam PO or 0.08 mg/kg midazolam IM for sedation, followed by median dosages of 2.93 mg/kg of propofol for induction, and an end-expired sevoflurane concentration of 1.0% to 2.8% for maintenance of anesthesia, for a total of 96 minutes. Conventional Controlled Mechanical Ventilation (CMV) was applied with variable success, as well as the alternative mode, Apneustic Anesthesia Ventilation (AAV), currently undergoing clinical trials. Anesthetic monitoring and vascular access are complicated by anatomic specializations; however, direct arterial blood pressure monitoring was feasible via cannulation of the median artery of the pectoral flipper. The mean arterial blood pressure (MAP) of those measured was initially low (55 mm Hg), but appropriate support improved values to median highs of 84 mm Hg, intraoperatively. Such support involved reducing anesthetic where possible, balancing the anesthetic technique with opioids, volume expanding with IV crystalloid fluids, and use of inotropes (ephedrine, 0.05–0.1 mg bolus IV; dobutamine, 0.2–2.0 mcg/kg/min IV) to increase contractility, cardiac output, and blood pressure. All MAP improved upon discontinuation of anesthesia for recovery (131 mm Hg). Vasopressors (phenylephrine, 1–3 mcg/kg/min IV) were also used in certain instances along with the inotropes.
Upon entering recovery, the median body temperature was 35.5°C, and successful extubation required 61 minutes (including those requiring re-intubation and additional time), thus necessitating continued physiologic monitoring and support in recovery. Common concerns include endotracheal intubation and extubation, body positioning and padding, body temperature management, vascular access, blood pressure, variable response to opioids, pharmacokinetics of anesthetic agents, ventilation and acid-base derangements. New modes of ventilation, imaging techniques, and advanced non-invasive monitoring methods will advance the care of these species under general anesthesia, and lessons learned with plans for improving management will be presented here in brief.
Table 1
Thirty-four anesthesia events involving 27 different individual small cetaceans. Bottlenose dolphin Tt = Tursiops truncatus; Ttg = Tursiops truncatus gilli, and Tta = Tursiops truncatus aduncus; Common dolphin Dc = Delphinus capensis; Pacific white-sided dolphin Lo = Lagenorhynchus obliquidens; Cumulative dosages of the drugs: AT = Atropine; ATRA = Atracurium; B = Butorphanol; D = Diazepam; E = Edrophonium; F = Flumazenil; Iso = Isoflurane; Mp = Meperidine; T = Tramadol; M = Midazolam; Nalx = Naloxone; Nalt = Naltrexone; Propofol = P; Sevo = Sevoflurane. PO = per os; IM = intramuscular; IV = intravenous. FeAgent = fraction expired inhalation anesthetic. Anesthesia duration was documented as the time from injection of IV induction agent until the vaporizer was turned off and the anesthetic circuit was flushed with oxygen to minimize inhalation anesthetic for the recovery phase to begin. CMV = conventional Controlled Mechanical Ventilation; AAV = Apneustic Anesthesia Ventilation; PIP = Peak Inspiratory Pressure; Phigh = plateau pressure or pressure high; LSCV = Lateral subcutaneous caudal vein; CP = caudal peduncle vein; MAP = mean arterial blood pressure (note: parenthesis indicate MAP in semi-conscious and spontaneously ventilating subject during recovery phase with arterial catheter maintained until extubation). Time of detection of hypotension generally coincided with the time of placement of the arterial catheter. Reversal agent dosages were total dosages that include repeat dosing. Temp = Temperature as measured at end of anesthesia. Extubation = Time to extubation was measured from the point the anesthetic circuit was initially flushed with oxygen to minimize inspired inhalation anesthetic until extubation. (Reproduced with permission from Bailey J. Innovative Veterinary Medicine Inc.: 101 Marketside AVE, STE 404-402, Ponte Vedra, FL 32081-1541, 2021).
Figure 1
CMV vs AAV ventilation mode spirometry waveforms. Example of the change in airway pressure and lung volume during the Controlled Mechanical Ventilation mode (CMV; left column) and during the Apneustic Anesthesia Ventilation mode (AAV; right column) when applied to the same 190 kg adult bottlenose dolphin. In applying CMV the airway pressure increases from residual or ambient pressure to the peak pressure of 29 cm H2O (range: 20 to 30 cm H2O in anesthetized dolphins) to generate a 3.5-liter tidal volume. In contrast, when applying AAV the pressure is released from the selected upper level (Phigh) to lower level (Plow). In this instance, the Phigh is 26 cm H2O and the Plow is 2 cm H2O, generating the same 3.5 liter tidal volume. CMV increases up from the functional residual capacity (FRC) or lower over time, where AAV decreases from the selected elevated plateau value, down to a level slightly above FRC. Following the release time of AAV, the lung volume is rapidly re-established by reapplication of Phigh. (Reproduced with permission from Bailey J. Innovative Veterinary Medicine Inc.: 101 Marketside AVE, STE 404-402, Ponte Vedra, FL 32081-1541, 2021).
Acknowledgements
The institutions driving these recent advancements in anesthetic care of cetaceans are not only large, such as the U.S. Navy Marine Mammal Program, National Marine Mammal Foundation, SeaWorld USA, and Vancouver Aquarium, but also relatively smaller, as well as international facilities. Many individuals of the marine mammal community, and other interested personages, have contributed to the development of cetacean anesthetic techniques over the years—and so many more have worked to advance the methodology by contributing to the cases discussed here. Acknowledgment and gratitude go to all, but their names are far too numerous to list here. However, there is one individual who pioneered the methods of dolphin anesthesia we use today, and has mentored, guided, or in some way promoted all of our efforts. His name is Dr. Sam H. Ridgway—anchor and concatenator of the past, present, and future of cetacean anesthesia. Thank you, Sam.
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