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Crocodilian anaesthesia
16
Reptile anaesthesia
Crocodilian anaesthesia
238
INTRODUCTION This section covers small crocodilians that may be kept by the reptile hobbyist. Local or national legislation may have specific requirements for the maintenance of these species in captivity, for example the Dangerous Wild Animals Act (1976) in the UK. All crocodilians should be considered dangerous, and due care taken when handling these species. The three families within the Crocodilia order are Gavialidae (gharials and false gharials), Alligatoridae (alligators and caimans) and Crocodylidae (crocodiles). Although the larger representatives are unlikely to be presented to the general veterinary practitioner, smaller species, such as the dwarf caiman (Paleosuchus palpebrosus), may be seen.
crocodilians are commonly inactive and use anaerobic metabolism, allowing them to tolerate prolonged dives without re-breathing (Lang, 1987).
Respiratory system The nostrils at the rostro-dorsal tip of the snout can be closed during submergence, and connect to the nasal passages. The gular fold or basihyoid plate (Fig. 16.1) prevents water entering the respiratory passages when the mouth opens underwater, for example when grabbing prey. The lungs are well developed, non-lobulated and highly vascular. The glottal valve retains air in the lungs for prolonged submergence (Lane, 2006). Crocodilians have a muscular pseudo-diaphragm.
ANATOMY AND PHYSIOLOGY Temperature As with other reptiles, environmental temperature is important in crocodilians, affecting behaviour and metabolism. Inadequate temperatures, below 25°C, will reduce appetite, digestion, and growth, and cause immunosuppression (Lane, 2006).
Cardiovascular system The four-chambered crocodilian heart lies in the ventral midline, caudal to the forelimbs by approximately eleven scale rows (Hernandez-Divers, 2006). A small foramen, the foramen of Panizza, in the ventricular septum allows mixing of left and right ventricular blood, and the flow of blood to the respiratory system is reduced when the animal is submerged. When the animal re-surfaces, normal breathing causes a reduction in pulmonary pressures (and thence the right ventricle) that closes the foramen. Submerged
Oropharynx
Velum palati (soft palate fold) Basihyal valve (basihyoid plate)
Tongue
Figure 16.1 • Open-mouth view demonstrating the gular fold or basihyoid valve in crocodilia.
Crocodilian anaesthesia
Digestive system This group are carnivorous, eating both live prey and carrion in the wild. They will eat at temperatures between 25°C and 35°C (Lane, 2006).
Integumentary system
• Low temperatures produce immune suppression.
Intubation After induction, the jaws are held open and the gular fold (see Fig. 16.1) deflected to visualise the glottis for intubation.
• The crocodilian heart has four chambers (unlike other reptiles that have three). • The gular fold allows breathing through the nose while the mouth is open under water. • Crocodilians have a muscular pseudo-diaphragm. • Bony plates dorsally and thickened skin make injections difficult.
TECHNIQUES Restraint Initial restraint should protect handlers from bites and scratches. The snout should be taped shut; gloves can be worn and towels used during restraint to reduce trauma from sharp claws. Several personnel will be required to restrain larger specimens.
Routes of administration These sites are similar to those used in lizards. The main difference is the use of larger and longer needles to penetrate the skin for injections.
Oral Owing to staff safety issues, the oral route is used to administer medication only in very small or severely debilitated animals. Medications can be added to feed. In very small animals, gavage dosing is similar to lizards, except with the mouth taped shut once a gag has been placed. The dosing tube must pass over the basihyal valve in the pharynx (Mitchell, 2006).
Subcutaneous
INDUCTION AND MAINTENANCE OF ANAESTHESIA Induction Owing to the demeanour of these species, most anaesthetic protocols are based on injectable agents. There is wide variation in species response to different agents. Muscle relaxants are commonly used to assist immobilisation, but do not provide analgesia. Induction with intravenous propofol into the ventral coccygeal vein may be possible in small specimens. Species variations in response to anaesthetic agents exist. For example, alligators respond well to etorphine, which is not very effective in other species. Gallamine, on the other hand, appears to be unsafe in alligators at doses greater than 1 mg/kg (Page, 1993). Ketamine may be used to sedate or anaesthetise crocodilians; recovery times are dose-dependent and may be prolonged. The intramuscular or subcutaneous routes are usually used to induce anaesthesia in crocodilian species. Addition of hyaluronidase to the anaesthetic agent accelerates absorption (Lloyd, 1999).
Neuromuscular blockers Crocodilians are a species in which neuromuscular blockers are commonly used. The agent produces immobilisation, enabling safer handling and induction of anaesthesia for painful procedures. These drugs do not produce analgesia. Intermittent positive pressure ventilation (IPPV) should be performed in animals that have received these agents.
This is rarely used in crocodilians due to their thick dermal scales.
Anaesthetic maintenance
Intravenous
If possible, the patient is intubated and maintained on volatile agents. Some injectable regimes will produce anaesthesia, but oxygen should still be supplemented if available.
The easiest site for venous access is the ventral coccygeal vein in small or medium animals and the supravertebral
Reptile anaesthesia
The skin has separate scales joined by connective tissue. Crocodilians have dorsal cornifications over bony plates, which precludes injections in this region (Lane, 2006).
vein in medium or large animals. The ventral coccygeal vein is accessed as for lizards. The supravertebral vein is accessed in the dorsal midline just caudal to the occiput, with the needle perpendicular to the skin. Slow advancement will avoid spinal trauma (Hernandez-Divers, 2006). In an emergency, the heart can be injected, but cannot be stabilised (Fudge, 2000).
239
Table 16.1: Anaesthetic agents used in crocodilians (see also Table 12.6) DRUG
DOSE (mg/kg)
ROUTE
COMMENT
Alpha-2-agonists: Medetomidine
0.159
IM
Sedation (incomplete immobilisation)
1–25,7
IM
Preanaesthetic (study in Nile crocodiles)
See tiletamine combination below
–
–
1 mg per 20 mg zolazepam5
IM, IV
Reversal of zolazepam
SC, IM, ICe
Induction ⬍30–60 min, recovery in hours to days
Reptile anaesthesia
Xylazine
240
Benzodiazepine: Zolazepam Reversal agent: Flumazenil
Bradycardia and bradypnoea seen
Dissociative agent: Ketamine5 20–40
Sedation
40–80
Anaesthesia
Dissociative agent combinations: Ketamine ⫹ medetomidine
5–15 ⫹ 0.1–0.252
IM
Tiletamine/zolazepam
5–151,8
IM
Anaesthesia (in American alligators [Alligator mississippiensis]) Lower doses required for adults compared to juveniles Reverse medetomidine with atipamezole Immobilisation for restraint and intubation Recovery may be prolonged
Neuromuscular blockers:9
Can be used alone or with ketamine or benzodiazepines to provide immobilisation No analgesia Use lower dose in larger animals
Gallamine
0.5–23
IM
–
Succinylcholine
0.25–1.23
IM
Variable induction and recovery periods with succinylcholine
Reversal agent: Neostigmine
0.03–0.063
IM
Antidote to gallamine (side effects include emesis and lacrimation, so often allow recovery without reversal) Can combine with hyaluronidase (75 mg) to enhance effects6
Opioids: Etorphine HCl8
0.30–2.754
IM
Crocodilians require high doses Poor relaxation Legal restrictions due to safety issues
Propofol
10–155
IV
Anaesthesia 30–90 min
Hyaluronidase
25 IU/dose5
SC
Combine with other drugs to accelerate SC absorption
Key: ICe ⫽ intracoelomic, IM ⫽ intramuscular, IV ⫽ intravenous, SC ⫽ subcutaneous 1 (Clyde et al., 1994); 2 (Heaton-Jones et al., 2002); 3 (Lane, 2006); 4 (Lawton, 1992); 5 (Lloyd, 1999); 6 (Lloyd et al., 1994); 7 (Page, 1993); 8 (Schumacher and Yelen, 2006); 9 (Smith et al., 1998)
Crocodilian anaesthesia
REFERENCES
Reptile anaesthesia
Clyde, V. C., P. Cardeilhac, and E. Jacobson. 1994. Chemical restraint of American alligators (Alligator mississippiensis) with atracurium or tiletamine-zolazepam. J Zoo Wildl Med 25: 525–530. Fudge, A. M. 2000. Laboratory Medicine Avian and Exotic Pets. WB Saunders, Philadelphia. Heaton-Jones, T. G., J. C. H. Ko, and D. L. Heaton-Jones. 2002. Evaluation of medtomidine-ketamine anesthesia with atipamezole reversal in American alligators (Alligator mississippiensis). J Zoo Wildl Med 33: 36–44. Hernandez-Divers, S. J. 2006. Diagnostic techniques. In: D. R. Mader (ed.) Reptile Medicine and Surgery. 2nd ed. pp. 490–532. Saunders Elsevier, St Louis, Missouri. Lane, T. 2006. Crocodilians. In: D. R. Mader (ed.) Reptile Medicine and Surgery. 2nd edn. pp. 100–117. Saunders Elsevier. Lang, J. W. 1987. Crocodilian behaviour: implications for management. In: G. J. W. Webb, S. C. Manolis and P. J. Whitehead (eds.) Wildlife Management: Crocodiles and Alligators. Surrey Beaty and Sons Printing in Association with the Conservation Commission of the Northern Territory, Chipping Norton, Australia.
Lawton, M. P. C. 1992. Anaesthesia. In: P. H. Beynon, M. P. C. Lawton and J. E. Cooper (eds.) Manual of Reptiles. pp. 170–183. Iowa State University, Ames, IA. Lloyd, M. L. 1999. Crocodilian anesthesia. In: M. E. Fowler and R. E. Miller (eds.) Zoo and Wild Animal Medicine: Current Therapy 4. pp. 205–216. WB Saunders, Philadelphia. Lloyd, M. L., T. Reichard, and R. A. Odum. 1994. Gallamine reversal in Cuban crocodiles (Crocodilus rhombifer) using neostigmine alone versus neostigmine with hyaluronidase. Proc Assoc Reptil Amph Vet Am Assoc Zoo Vet: 117–120. Mitchell, M. A. 2006. Therapeutics. In: D. R. Mader (ed.) Reptile Medicine and Surgery. 2nd edn. pp. 631–664. Saunders Elsevier, St Louis, Missouri. Page, C. D. 1993. Current reptilian anesthesia procedures. In: M. E. Fowler (ed.) Zoo and Wild Animal Medicine: Current Therapy 3. pp. 140–143. WB Saunders, Philadelphia. Schumacher, J., and T. Yelen. 2006. Anesthesia and analgesia. In: D. R. Mader (ed.) Reptile Medicine and Surgery. 2nd edn. pp. 442–452. Saunders Elsevier, St Louis, Missouri. Smith, J. A., N. C. McGuire, and M. A. Mitchell. 1998. Cardiopulmonary physiology and anesthesia in crocodilians. Proc Assoc Reptil Amphib Vet: 17–21.
241
Reptile anaesthesia
Crocodilian anaesthesia
238
INTRODUCTION This section covers small crocodilians that may be kept by the reptile hobbyist. Local or national legislation may have specific requirements for the maintenance of these species in captivity, for example the Dangerous Wild Animals Act (1976) in the UK. All crocodilians should be considered dangerous, and due care taken when handling these species. The three families within the Crocodilia order are Gavialidae (gharials and false gharials), Alligatoridae (alligators and caimans) and Crocodylidae (crocodiles). Although the larger representatives are unlikely to be presented to the general veterinary practitioner, smaller species, such as the dwarf caiman (Paleosuchus palpebrosus), may be seen.
crocodilians are commonly inactive and use anaerobic metabolism, allowing them to tolerate prolonged dives without re-breathing (Lang, 1987).
Respiratory system The nostrils at the rostro-dorsal tip of the snout can be closed during submergence, and connect to the nasal passages. The gular fold or basihyoid plate (Fig. 16.1) prevents water entering the respiratory passages when the mouth opens underwater, for example when grabbing prey. The lungs are well developed, non-lobulated and highly vascular. The glottal valve retains air in the lungs for prolonged submergence (Lane, 2006). Crocodilians have a muscular pseudo-diaphragm.
ANATOMY AND PHYSIOLOGY Temperature As with other reptiles, environmental temperature is important in crocodilians, affecting behaviour and metabolism. Inadequate temperatures, below 25°C, will reduce appetite, digestion, and growth, and cause immunosuppression (Lane, 2006).
Cardiovascular system The four-chambered crocodilian heart lies in the ventral midline, caudal to the forelimbs by approximately eleven scale rows (Hernandez-Divers, 2006). A small foramen, the foramen of Panizza, in the ventricular septum allows mixing of left and right ventricular blood, and the flow of blood to the respiratory system is reduced when the animal is submerged. When the animal re-surfaces, normal breathing causes a reduction in pulmonary pressures (and thence the right ventricle) that closes the foramen. Submerged
Oropharynx
Velum palati (soft palate fold) Basihyal valve (basihyoid plate)
Tongue
Figure 16.1 • Open-mouth view demonstrating the gular fold or basihyoid valve in crocodilia.
Crocodilian anaesthesia
Digestive system This group are carnivorous, eating both live prey and carrion in the wild. They will eat at temperatures between 25°C and 35°C (Lane, 2006).
Integumentary system
• Low temperatures produce immune suppression.
Intubation After induction, the jaws are held open and the gular fold (see Fig. 16.1) deflected to visualise the glottis for intubation.
• The crocodilian heart has four chambers (unlike other reptiles that have three). • The gular fold allows breathing through the nose while the mouth is open under water. • Crocodilians have a muscular pseudo-diaphragm. • Bony plates dorsally and thickened skin make injections difficult.
TECHNIQUES Restraint Initial restraint should protect handlers from bites and scratches. The snout should be taped shut; gloves can be worn and towels used during restraint to reduce trauma from sharp claws. Several personnel will be required to restrain larger specimens.
Routes of administration These sites are similar to those used in lizards. The main difference is the use of larger and longer needles to penetrate the skin for injections.
Oral Owing to staff safety issues, the oral route is used to administer medication only in very small or severely debilitated animals. Medications can be added to feed. In very small animals, gavage dosing is similar to lizards, except with the mouth taped shut once a gag has been placed. The dosing tube must pass over the basihyal valve in the pharynx (Mitchell, 2006).
Subcutaneous
INDUCTION AND MAINTENANCE OF ANAESTHESIA Induction Owing to the demeanour of these species, most anaesthetic protocols are based on injectable agents. There is wide variation in species response to different agents. Muscle relaxants are commonly used to assist immobilisation, but do not provide analgesia. Induction with intravenous propofol into the ventral coccygeal vein may be possible in small specimens. Species variations in response to anaesthetic agents exist. For example, alligators respond well to etorphine, which is not very effective in other species. Gallamine, on the other hand, appears to be unsafe in alligators at doses greater than 1 mg/kg (Page, 1993). Ketamine may be used to sedate or anaesthetise crocodilians; recovery times are dose-dependent and may be prolonged. The intramuscular or subcutaneous routes are usually used to induce anaesthesia in crocodilian species. Addition of hyaluronidase to the anaesthetic agent accelerates absorption (Lloyd, 1999).
Neuromuscular blockers Crocodilians are a species in which neuromuscular blockers are commonly used. The agent produces immobilisation, enabling safer handling and induction of anaesthesia for painful procedures. These drugs do not produce analgesia. Intermittent positive pressure ventilation (IPPV) should be performed in animals that have received these agents.
This is rarely used in crocodilians due to their thick dermal scales.
Anaesthetic maintenance
Intravenous
If possible, the patient is intubated and maintained on volatile agents. Some injectable regimes will produce anaesthesia, but oxygen should still be supplemented if available.
The easiest site for venous access is the ventral coccygeal vein in small or medium animals and the supravertebral
Reptile anaesthesia
The skin has separate scales joined by connective tissue. Crocodilians have dorsal cornifications over bony plates, which precludes injections in this region (Lane, 2006).
vein in medium or large animals. The ventral coccygeal vein is accessed as for lizards. The supravertebral vein is accessed in the dorsal midline just caudal to the occiput, with the needle perpendicular to the skin. Slow advancement will avoid spinal trauma (Hernandez-Divers, 2006). In an emergency, the heart can be injected, but cannot be stabilised (Fudge, 2000).
239
Table 16.1: Anaesthetic agents used in crocodilians (see also Table 12.6) DRUG
DOSE (mg/kg)
ROUTE
COMMENT
Alpha-2-agonists: Medetomidine
0.159
IM
Sedation (incomplete immobilisation)
1–25,7
IM
Preanaesthetic (study in Nile crocodiles)
See tiletamine combination below
–
–
1 mg per 20 mg zolazepam5
IM, IV
Reversal of zolazepam
SC, IM, ICe
Induction ⬍30–60 min, recovery in hours to days
Reptile anaesthesia
Xylazine
240
Benzodiazepine: Zolazepam Reversal agent: Flumazenil
Bradycardia and bradypnoea seen
Dissociative agent: Ketamine5 20–40
Sedation
40–80
Anaesthesia
Dissociative agent combinations: Ketamine ⫹ medetomidine
5–15 ⫹ 0.1–0.252
IM
Tiletamine/zolazepam
5–151,8
IM
Anaesthesia (in American alligators [Alligator mississippiensis]) Lower doses required for adults compared to juveniles Reverse medetomidine with atipamezole Immobilisation for restraint and intubation Recovery may be prolonged
Neuromuscular blockers:9
Can be used alone or with ketamine or benzodiazepines to provide immobilisation No analgesia Use lower dose in larger animals
Gallamine
0.5–23
IM
–
Succinylcholine
0.25–1.23
IM
Variable induction and recovery periods with succinylcholine
Reversal agent: Neostigmine
0.03–0.063
IM
Antidote to gallamine (side effects include emesis and lacrimation, so often allow recovery without reversal) Can combine with hyaluronidase (75 mg) to enhance effects6
Opioids: Etorphine HCl8
0.30–2.754
IM
Crocodilians require high doses Poor relaxation Legal restrictions due to safety issues
Propofol
10–155
IV
Anaesthesia 30–90 min
Hyaluronidase
25 IU/dose5
SC
Combine with other drugs to accelerate SC absorption
Key: ICe ⫽ intracoelomic, IM ⫽ intramuscular, IV ⫽ intravenous, SC ⫽ subcutaneous 1 (Clyde et al., 1994); 2 (Heaton-Jones et al., 2002); 3 (Lane, 2006); 4 (Lawton, 1992); 5 (Lloyd, 1999); 6 (Lloyd et al., 1994); 7 (Page, 1993); 8 (Schumacher and Yelen, 2006); 9 (Smith et al., 1998)
Crocodilian anaesthesia
REFERENCES
Reptile anaesthesia
Clyde, V. C., P. Cardeilhac, and E. Jacobson. 1994. Chemical restraint of American alligators (Alligator mississippiensis) with atracurium or tiletamine-zolazepam. J Zoo Wildl Med 25: 525–530. Fudge, A. M. 2000. Laboratory Medicine Avian and Exotic Pets. WB Saunders, Philadelphia. Heaton-Jones, T. G., J. C. H. Ko, and D. L. Heaton-Jones. 2002. Evaluation of medtomidine-ketamine anesthesia with atipamezole reversal in American alligators (Alligator mississippiensis). J Zoo Wildl Med 33: 36–44. Hernandez-Divers, S. J. 2006. Diagnostic techniques. In: D. R. Mader (ed.) Reptile Medicine and Surgery. 2nd ed. pp. 490–532. Saunders Elsevier, St Louis, Missouri. Lane, T. 2006. Crocodilians. In: D. R. Mader (ed.) Reptile Medicine and Surgery. 2nd edn. pp. 100–117. Saunders Elsevier. Lang, J. W. 1987. Crocodilian behaviour: implications for management. In: G. J. W. Webb, S. C. Manolis and P. J. Whitehead (eds.) Wildlife Management: Crocodiles and Alligators. Surrey Beaty and Sons Printing in Association with the Conservation Commission of the Northern Territory, Chipping Norton, Australia.
Lawton, M. P. C. 1992. Anaesthesia. In: P. H. Beynon, M. P. C. Lawton and J. E. Cooper (eds.) Manual of Reptiles. pp. 170–183. Iowa State University, Ames, IA. Lloyd, M. L. 1999. Crocodilian anesthesia. In: M. E. Fowler and R. E. Miller (eds.) Zoo and Wild Animal Medicine: Current Therapy 4. pp. 205–216. WB Saunders, Philadelphia. Lloyd, M. L., T. Reichard, and R. A. Odum. 1994. Gallamine reversal in Cuban crocodiles (Crocodilus rhombifer) using neostigmine alone versus neostigmine with hyaluronidase. Proc Assoc Reptil Amph Vet Am Assoc Zoo Vet: 117–120. Mitchell, M. A. 2006. Therapeutics. In: D. R. Mader (ed.) Reptile Medicine and Surgery. 2nd edn. pp. 631–664. Saunders Elsevier, St Louis, Missouri. Page, C. D. 1993. Current reptilian anesthesia procedures. In: M. E. Fowler (ed.) Zoo and Wild Animal Medicine: Current Therapy 3. pp. 140–143. WB Saunders, Philadelphia. Schumacher, J., and T. Yelen. 2006. Anesthesia and analgesia. In: D. R. Mader (ed.) Reptile Medicine and Surgery. 2nd edn. pp. 442–452. Saunders Elsevier, St Louis, Missouri. Smith, J. A., N. C. McGuire, and M. A. Mitchell. 1998. Cardiopulmonary physiology and anesthesia in crocodilians. Proc Assoc Reptil Amphib Vet: 17–21.
241