Abstract

Previous reports have suggested that elephants immobilized with etorphine may be at risk for developing rapid and severe hypoxemia.^4,8,7 In the elephant, moderate to severe hypoxemia may be defined as a partial pressure of oxygen in arterial blood (PaO₂) < 50 mm Hg and a corresponding oxygen-hemoglobin saturation (SpO₂) < 90%. It is at this point on the elephant oxygen-hemoglobin saturation curve that desaturation occurs rapidly with relatively small changes in PaO₂.² Hemoglobin saturation values indicative of near hypoxia (SpO₂< 75-80%) have been demonstrated by pulse oximetry in immobilized elephants lying in lateral recumbency.^7,8 Unanesthetized elephants lying in lateral recumbency maintain PaO₂ values above 80 mm Hg and corresponding SpO₂ values > 95%.⁵ Taken together, these data suggest that hypoxemia in immobilized elephants is directly related to the effects of etorphine.

Several studies have documented the positive effects of oxygen supplementation in spontaneously-breathing anesthetized elephants.^3,4 We sought to go one step further by developing a portable method of delivering intermittent positive pressure ventilation (IPPV) that would allow for control of both oxygenation and ventilation. Intermittent positive pressure ventilation has been shown to prevent hypoxemia in laterally recumbent horses, and to slow the onset of hypoxemia in horses in dorsal recumbency.¹ To date, the effect of IPPV on oxygenation and ventilation in laterally recumbent elephants has not been studied. We designed a very simple oxygen delivery system for this purpose, one that can be used even in remote field situations.

This study was performed as part of a cooperative elephant conservation effort between the North Carolina Zoological Park and the World Wildlife Fund for Nature-Cameroon. The aim of the conservation study is to characterize elephant migratory habits in populated areas of northern Cameroon. Surveys indicate that nearly 30% of the human population of northern Cameroon has been impacted in some negative way by migrating elephants.⁹ As part of the study, satellite tracking collars were placed on four adult females and one young adult male dispersed among Waza, Benoue, and Bouba Ndjida National Parks.

Elephants were darted either from a vehicle or on foot at a distance of 40-60 m using an extra long range (powder) projector (Palmer Cap Chur Equipment, 3545 King Dr., Douglasville, Georgia 30135). Darts with 60 X 5 mm collared needles were used for intramuscular drug delivery. A total dose of 18 mg (four elephants) or 12 mg (one smaller female) of etorphine hydrochloride (Wildlife Pharmaceuticals, Fort Collins, CO 80524 USA) was administered either in the cranial thigh or hip. (The high dose requirement likely reflects heat degradation of etorphine, which was stored for over 1 yr.) Once immobilized, elephants were rolled into lateral recumbency with their trunks extended. Elephants were intubated with a 35-mm ID cuffed endotracheal tube (Bivona, 5700 West 23rd Ave., Gary, IN 46406 USA). Intermittent positive pressure ventilation was administered at a rate of 4-5 breaths/min via two Elder demand valves (Model 034-100, Matrix Medical, 145 Country Dr., Orchard Park, NY 14127 USA) attached in parallel to a standard oxygen H (6700 L) or aluminum MM cylinder (3500 L, American Home Products, 3420 Tar Heel Dr., Raleigh, NC 27606 USA) with a regulator setting of 60 psi. The demand valves were inserted into one side of a PVC Y-piece (Ohmeda, Madison, WI 53706 USA) that was connected to the endotracheal tube. The other side of the Y-piece acted as a port for a rubber stopper that was inserted manually on inspiration and removed on expiration. This device allowed oxygen delivery at a combined flow rate of 300 L/min (5 L/sec). Five-second inflations were used to achieve adequate chest expansion (approximately 8-10 ml/kg tidal volume). A single MM cylinder weighing approximately 40 pounds provided sufficient oxygen for 30-40 min of ventilation, which was typically more than enough time to put a collar in place.

Indirect cardiopulmonary monitoring was performed throughout the anesthetic period. Heart rate (HR) and mean arterial blood pressure (MAP) were monitored oscillometrically by placing a 12 x 30 cm (small adult) sized cuff on the base of the tail (Vet/BP 6000, Heska, 1613 Prospect Parkway, Fort Collins, CO 80525 USA). Oxygen-hemoglobin saturation (SpO₂) was measured indirectly by pulse oximetry using a C-probe at the very tip of the tongue (Model 3303, SurgiVet/Anesco, N7 W22025 Johnson Rd., Waukesha, WI 53186 USA). End-tidal CO₂ (ETCO₂) and respiratory rate (RR) were monitored by sidestream capnography (Vet/Cap 7100, Heska). Arterial blood was drawn from an auricular artery and placed on ice for blood gas and pH determination. Samples were taken prior to intubation, and at 15 and 30 min post-intubation. Samples were analyzed with a hand-held pH and blood gas analyzer (I-Stat with EG 7+ cartridges, Heska Corp.). The I-Stat and cartridges were maintained at 18-20°C in a 12 V thermoelectric cooler (Coleman, 1900 18th St., Spirit Lake, IA 51360 USA). All blood gas values were corrected to body temperatures, which ranged from 39-41°C.

Three elephants displayed similar cardiopulmonary responses to etorphine immobilization. Heart rate ranged from 40-60 bpm, and MAP from 70-130 mm Hg. Oxygen-hemoglobin saturation ranged from 89-99%. End-tidal CO₂ was maintained between 35-55 mm Hg whether the elephant was spontaneously breathing or being ventilated. (Elephant number three could not be intubated because of extreme jaw rigidity most likely caused by an excessive dose of etorphine.) Initial PaO₂ values were similar between these three elephants, ranging from 60-80 mm Hg. The PaO₂ values increased dramatically to 300-400 mm Hg following IPPV in elephants one and two. The PaO₂ values decreased to 55-60 mm Hg with spontaneous ventilation in elephant number three. The HR of the fourth elephant ranged from 75-80 bpm, and MAP from 55-70 mm Hg. Oxygen-hemoglobin saturation ranged between 83-87%. Only one of the Elder demand valves was working while ventilating this elephant, which was insufficient to generate an adequate tidal volume. The PaO₂ never exceeded 60 mm Hg. The fifth elephant appeared to be in cardiopulmonary distress when first approached, with a HR of 35 bpm, MAP of 100 mm Hg, and SpO₂ of 75% (despite taking deep breaths-8 breaths/min). Blood gas analysis revealed a PaO₂ value of 40 mm Hg, indicating severe hypoxemia. Following intubation and IPPV, the SpO₂ increased to 95%, and PaO₂ values increased to 97 and 366 mm Hg at 15 and 30 min, respectively. At the end of each procedure, all five elephants were reversed with naltrexone (Wildlife Pharmaceuticals B 100X etorphine dose) and recovered without incident.

Discussion and Conclusions

Hypoxemia in mammals may result from one or a combination of the following abnormalities: hypoventilation, alveolar-capillary diffusion impairment, ventilation-perfusion mismatch, intrapulmonary shunting, and/or a decrease in cardiac output.^6,10 The cause of hypoxemia in etorphine immobilized elephants is not known, but likely results from a combination of centrally mediated respiratory depression, decreased thoracic wall compliance (due to opioid-induced intercostal muscle rigidity), and positioning-imposed redistribution of blood flow within the lungs. With the exception of a significant irreversible pulmonary shunt (> 30%), all of the underlying causes of hypoxemia listed above will respond to an increase in the inspired fraction of oxygen.⁶ Our results indicate that etorphine-immobilized elephants do not develop transient irreversible shunts, and respond remarkably well to IPPV despite their enormous size. Given the potential adverse effects of hypoxemia, such as further respiratory depression, cardiovascular depression, arrhythmias, lactic acidosis, myopathy, and abortion, we feel strongly that some means of IPPV should be available for elephants undergoing similar types of field studies.

Acknowledgments

Supported in part by a grant from the NCSU Office of Internationalization, Raleigh, NC 27606 USA.

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