CURRE TECHNIQUES IN REPTILE ANESTHESIA AND SURGERY

R. Avery Bennett, DVM, MS, Diplomate ACVSSan Francisco Zoological Gardens, San Francisco, CA, USA

Anesthesia

The lungs of most reptiles are simple endothelium lined sacs attached to bronchi. The totallung volume is greater than that of mammalian lungs, but the surface area for gas exchangeis much smaller~ The left lung is absent or vestigial in most snakes. The lungs of reptilesare very fragile and care must be taken when positive pressure ventilation is performed toavoid rupture of the lung. The trachea of chelonians and crocodilians is composed ofcomplete rings while the tracheal rings of squamates are incomplete. Chelonians have avery short trachea so care must be taken to avoid intubation of a single lung.

The position of the glottis varies among reptiles being rostrally located in snakes, morecaudally located and partially obscured by the tongue in chelonians, and further obscuredby the well developed epiglottis in crocodilians. The glottis remains closed during states ofrest and a glottis dilator muscle opens the glottis to allow breathing. Intubation, however,is generally not difficult to accomplish.

Reptiles do not have a functional muscular diaphragm and, thus, have a pleuroperitoneumor coelomic cavity~ Reptiles generate negative pressure in the lungs by 2 methods. Mostreptiles use intercostal muscles aided by muscles of the trunk and abdomen to generatenegative pressure~ The wall of the lung also contains smooth muscle which contracts andrelaxes to move air~ These mechanisms allow reptiles to breathe even if there is a defect

the coelomic cavity as occurs with abdominal surgery. Chelonians are not capable ofintercostal movement but change intrapulmonary pressure by movement of the viscera,limbs, and pelvic girdle. Because the lungs lie dorsal to the viscera, positioning chelonians

dorsal recumbency will compress the lungs and reduce their tidal volume.

In addition to alveolar ventilation, reptiles employ other surfaces for gas exchange, such as,-,"U,l!vUAJi.'-""'.7Ua.:l' gas exchange is also a part of reptilian respiration.

Many reptiles are capable of converting to anaerobic metabolism when they ftbreath-hold".

If the patient's condition permits, pre-anesthetic fasting is recommended. Aspiration is notcommon in reptiles, but, because the tidal volume is affected by visceral volume, fasting mayallow for improved ventilation. It is somewhat difficult to monitor depth of anesthesia inreptiles because it can be difficult to visualize respiratory .and cardiac movements. The 3chambered heart of most reptiles does not produce readily auscultable sounds, makingstethoscopy difficult. An ECG monitor is very valuable in monitoring the anesthetized

patient. The QRS patterns of reptiles are generally inverted and slurred. As reptilesbecome anesthetized, relaxation progresses from cranial to caudal and during recovery,motor function returns in the opposite direction. The righting reflex is lost early duringanesthetic induction but is a useful indicator of recovery. Assisted ventilation may speed

36 1994 PROCEEDINGS ASSOCIATION OF REPTIUAN AND AMPHIBIAN VETERINARIANS

recovery but is not usually necessary. Failure to elevate the ribs when a finger is run downthe back or failure to move the tail when the vent or foot is squeezed indicate loss of spinalreflexes and a surgical plane of anesthesia. In chelonians, the head withdrawal reflex is alsouseful. Corneal reflex should be present at a surgical plane and when abolished the patientis excessively deep. Tongue withdrawal in snakes is present at a surgical plane and lost ifthe patient is too deep.

Recovery should occur in a quiet environment with the temperature and humidity at theupper end of the optimum range. Excessive warmth may be deleterious by excessivelyincreasing patient activity which increases tissue oxygen demand.

Tranquilizers

Acepromazine at 0.1-0.5 mgfkg 1M approximately 1 hr prior to induction of anesthesia hasbeen reported to lower the required dose of induction agent. This agent may also be usedas a tranquilizer.

Local anesthetics

Reptiles are very sensitive to skin stimulation but local anesthetics are effective.

Injectable anesthetic agents

These anesthetics require little equipment and may be more familiar to some veterinarians,but once given the effects cannot be reversed and the depth is difficult to control. Manyare best given by the IV route which may not be feasible with some reptile patients" Theeffects of injectable agents in reptiles are often unpredictable. The same dose given to 2different animals may yield no effect in one while producing profound anesthesia in theother. Reptiles also seem to require unusually high doses of some agents making their usevery costly. Narcotics are of little value in reptile anesthesia. It is unknown why reptilesare refractory to opiates.

Barbiturates have a long and unpredictable induction time and a very long recovery period.Size, condition, nutritional status, temperature, handling, and physiology of the individualanimal may influence the effects of barbiturates making their use in reptiles questionable.

Dissociative anesthetics such as phencyclidine, ketamine, and tiletamine have been used inreptiles. Ketamine has been successfully used in all orders of reptiles, though the responseis dose dependent and the effects vary with the species and individual. It is most useful forinduction of anesthesia for intubation, especially in "breath holding" species. A dose of 22­44 mgfkg 1M or SC has been recommended for sedation and 55-88 mglkg for surgicalanesthesia. Generally, at doses >110 mgfkg respiratory arrest and decreased heart ratemake ventilatory support necessary. Induction usually occurs in 10-30 min. and recoveryfrom 24-96 hrs. At a surgical plane, some animals will exhibit serpentine movement which

1994 PROCEEDINGS ASSOCIATION OF REPTIUAN AND AMPIDBIAN VETERINARIANS 37

can be controlled by taping the patient to the table. Ketamine may be the most satisfactoryinjectable anesthetic, but when used in debilitated patients, recovery may be dangerouslyprolonged (up to 6 days).

Tiletamine + Zolazepam (Telazol) - Tiletamine's action is similar to that of ketamine butit is 2-3 times more potent making the volume of administration smaller. Tiletamine alonecauses seizures but in combination with zolazepam with which it is synergistic, they produceanesthesia, analgesia, and muscle relaxation with anticonvulsant and antianxiety effects.Animals are very sensitive to stimulation while under the effects of Telazol resulting inexcessive movement. There is great species variation in response to this agent. Because ofthe rapid onset of effect, Telazol may be most useful as an induction agent or for sedationat 4-5 mg/kg 1M.

Neuromuscular Blocking Agents have primarily been used for restraint of large crocodiliansand chelonians. Succinylcholine is a depolarizing agent which has been used mostcommonly for this purpose. The dosage appears to be variable among species and sizes ofcrocodilians. A dose of 3-5 mg/kg used in American alligators weighing 2-5 kg resultedimmobilization in <4 min with recovery complete in 7-9 hr. The 1M administration ofsuccinylcholine is important because the drug may not be absorbed rapidly .enough to beeffective when administered SC. Other neuromuscular blockers have been used withvariable success.

Inhalant anesthetic agents

These agents offer several advantages over injectable agents. Most inhalant agents can beused without a precision vaporizer in reptile patients. Its use allows long term anesthesiato be maintained accurately.

A nonrebreathing system should be used on patients <5 kg with an oxygen flow rate oftwice the minute volume (300-500 ml/kg/min). The normal respiratory rate for most reptilesis 2-4/mine A circle system may be used for larger reptiles with 2-4 L/min oxygen forinduction and 1..2 L/min for maintenance.

Methoxyflurane (MOF) has a slow induction and recovery when compared with otherfluorinated hydrocarbon volatile anesthetics. The open drop technique can be used placing10 ml MOF in a 42,800 em3 box. A single exposure provides 10-30 min of surgicalanesthesia. If more time is needed the patient should be intubated and maintained usinga precision vaporizer.

Halothane may be used for induction using a precision vaporizer or the open drop method.With the vaporizer, induction may be achieved using 2-5.5% halothane in oxygen andmaintenance using 1.5-2.5%. For the open drop technique, 5 ml halothane are placed ina 2,840 em3 box. Induction with this technique occurs in 5-33 min and has a duration of 5­20 min following a single exposure. Reptiles generally go through an excitement phaseduring induction with halothane.

38 1994 PROCEEDINGS ASSOCIATION OF REPTIUAN AND AMPHIBIAN VETERINARIANS

Isoflurane is eliminated exclusively by the lungs and, therefore, causes minimal metaboliccompromise making it the agent of choice for use in debilitated patients. A concentrationof 4-5% isoflurane in 3-4 Umin oxygen has been used to induce anesthesia in 6-20 min.Maintenance at 1.5-4% isoflurane resulted in recovery in 30-60 min. Because it has a vaporpressure and maximum concentration similar to halothane, the open drop method ofinduction should be as effective for isoflurane as for halothane.

Surgery

Anatomy

The anatomy of reptiles varies among orders, families, and species. A knowledge of thebasic features of reptilian anatomy is therefore vital to surgeons.

Except

abdomen"

occurs..

most snakes, reptiles have a cecum" The stomach of crocodilians has 2rst is very muscular and frequently contains stones.. The second is

jl;.,JIl.'O..Ql.JII...JIl.~'lIo4A~r 1 reptiles a gall VJ!.U.'\,Au.~....,.Ii.

ck spots or streaks._......... '..............."',.".. masses

Urine passesdistal

3 coprodeum is the most cranial and receives'll'iII'lS"'A""~.nII'O'18'll111ri1"'l1l is middle receives ~VJlLBI..li.Ii>-~i,.4.A

wastes m ducts.. The caudal proctodeum acts as areservoir for fecal and urinary wastes before they are excreted. This is also the location ofthe openings of the musk glands..

The skin of reptiles is dry and virtually devoid of glands. Many lizards have femoral glands,which open on the medial aspects of the thighs. Crocodilians have a pair of scent glandsin the medial aspects of the lower jaw and another pair within the cloaca. In proposing sitesfor incisions, these glandular areas should be avoided. The skin of most reptiles is made upof scales and scutes. Soft shelled turtles and some lizards do not have scales but have aleathery, smooth skin. Crocodilians and some lizards have calcific plates, called osteoderms,located within the dermis designed for protection. Incisions can usually be made betweenosteoderms.. The shells of chelonians are composed of bony dermal plates covered with

1994 PROCEEDINGS ASSOCIATION OF REPTIUAN AND AMPHIBIAN VETERINARIANS 39

keratinized epidermal shields. The carapace contains 10 fused thoracic, lumbar, and sacralvertebrae as well as the ribs. The plastron and the carapace are joined at the so-calledbridge.

Histologically, the epidermis is composed of 3 layers. The outer stratum corneum is heavilykeratinized, acellular, and has a serrated surface. The middle intermediate zone iscomposed of daughter cells of the stratum gennina.tivum in various stages of differentiation.These 3 layers are present during the skin's resting phase. As ecdysis begins, the cells of thestratum germinativum undergo synchronous mitosis to form a new intermediate zone andstratum corneum under the old generation. The action of enzymes breaks down the cellsof the base of the old intermediate zone, and the subsequent influx of lymph causesseparation between old intermediate zone and the new stratum corneum. Blood vesselsand sinuses in the head become engorged and cause it to swell. The old skin splits andecdysis is completed by the animal rubbing off the old skin. In squamates, this processoccurs simultaneously over the entire body. In chelonians and crocodilians, proliferation andkeratinization are continuous and shedding occurs only at the flexible regions of the body.This produces growth rings between these scales as they grow and the previous, smallerlayers are not lost. The frequency of ecdysis is proportional to the growth and metabolicrates of the animal. Age, environmental temperature, availability of food, and space caninfluence the frequency. In squamates, the cells of the epidermis are mitotically active onlyduring ecdysis.

Patient preparation

Ideally, laboratory data should be obtained before induction of anesthesia. Because of thesmall size of many patients and the inaccessibility of most peripheral veins, blood samplesare often difficult to obtain. Environmental conditions, time of day, and laboratoryvariations can influence blood cell counts and biochemistry data making interpretationdifficult. If blood samples can be obtained repeatedly, trends provide valuable information.The hydration and nutritional status of patients is assessed as it would be for mammalianpatients. Balanced electrolyte solutions can be given IV or IP.

Reptiles are susceptible to a variety of microbial infections. It is imperative that aseptictechnique be used. Many cutaneous infections result in septicemia and lead to visceralgranuloma formation. Perioperative antibiotic therapy is more appropriate if intraoperativecontamination is anticipated. It has been suggested that amikacin be used in snakes at aloading dose of 5 mglkg followed by 2.5 mglkg every 72 hr. Gentamicin at 2.5 rnglkg every72 hr maintains adequate therapeutic plasma concentrations in gopher snakes and red earedslider turtles. In view of the long plasma half life of these antibiotics, one dose prior tosurgery should provide perioperative coverage.

Patient positioning is a challenge especially for legless and small reptiles. For snakes, asterile stockinette can be rolled over the surgically prepared patient. The snake can thenbe placed on a sterile drape providing an aseptic field. The dome shape of the carapace ofchelonians makes it difficult to position a patient in dorsal recumbency. A towel can be

40 1994 PROCEEDINGS ASSOCIATION OF REPTIUAN AND AMPHIBIAN VElERINARIANS

rolled into a ring such that the carapace will fit into the ring and prevent the patient fromrolling. Gear plastic adhesive drapes are very useful in reptiles. The entire patient remainsvisible under the sterile drape allowing for proper anesthetic monitoring. Sterile sprayadhesives can also be used to allow paper or cloth drapes to stick to the patient avoiding theuse of towel clamps.

Instrumentation

With a few exceptions, the instruments needed for surgery on reptiles are found in a generalsurgical pack. Most abscesses in reptiles contain caseous, inspissated pus. Dental curettesand cerumen loops help in removing this material. Eyelid retractors work well as abdominalretractors for small patients. With most chelonians, some type of saw or drill is needed toapproach the coelomic cavity and a restorative material should be available for repair ofshell defects and celiotomies.

The edges of incised reptilian skin have a tendency to invert. An everting suture pattern,such as a horizontal or vertical mattress, achieves accurate skin edge apposition. Therelatively tough reptile skin and scales help prevent sutures from tearing through. The breakdown of absorbable materials appears to be prolonged in reptiles and if used in the skin,removal is recommended after the incision has healed. Chromic catgut was still present ina rhinoceros viper 12 weeks after the material was used in the pleuroperitoneum and SQtissue. It appears to be best to use materials which are absorbed by hydrolysis rather thanproteolysis in reptiles. In squamates, suture removal should be performed after the ecdysissubsequent to surgery.. The shed skin usually sticks in the sutured area for several ecdysespostoperatively, but can be gently peeled away.

Postoperative care

Anesthetic recovery in reptiles can be prolonged and di cult to monitor. Increasing theenvironmental temperature to the upper end of the optimal range (30-36° C) will increasethe rate of metabolism of anesthetic agents. Once the patient is awake and responsive, itshould be placed a warm, dark, quiet place to complete its recovery. Oean paper shouldbe provided in the recovery area to prevent contamination. Hibernation should be delayedfor at least 6 mo as it delays healing. Swimming should be prevented for 7-14 days aftersurgery. Fluid therapy may be administered IV or IP as needed to maintain hydration.Many reptiles become anorectic after surgery. Force feeding or tube feeding might benecessary.

Skin wounds of reptiles undergo phases of healing similar to those obseIVed in mammals.Wounds strengthen slowly and skin sutures are generally not removed until at least 4-6 wk.Many factors influence wound healing in reptiles. Maintenance of the environmentaltemperature at the high end of the optimum range promotes healing. In snakes, cranial tocaudal wounds heal faster than dorsal to ventral wounds. Open wounds heal well by secondintention with a low incidence of infection.

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Celiotomy

Indications for celiotomy in reptiles include egg binding, egg peritonitis, gastrointestinalobstruction, ovariohysterectomy, colopexy for colon prolapse, cystotomy for calculi, andexploration for biopsy. The technique varies depending on the family to which the patientbelongs.

In snakes, abdominal incisions can be made at the lateral margin of the scntes or betweenthe first 2 rows of lateral scales. Incisions should be made between rather than throughscales if possible. The tips of the ribs should be avoided at the junction of the scntes andscales. The lateral approach is generally preferred over a ventral midline approach as it iseasier to keep clean" The suture line is not in direct contact with the substrate and is notstressed by rectilinear motion. Three layers are encountered: skin, muscle, andpleuroperitoneume When separate layers are not identifiable, a single-layer closure isadequate $

Paralumbar and midline incisions have been recommended for approaching the coelomiccavity of lizards and crocodilians. The ventral abdominal vein is a very large vein locatedinside the body wall on the ventral midline,. It should be avoided during celiotomy by usinga paramedian approach.

chelonians with a small plastron, the majority of abdominal structures can be approachedthrough an incision between the plastron and the femur in the flank region. In otherchelonians it is necessary to perform an osteotomy of the plastron. The pelvic bones should

avoided can identified using radiography. Usually the femoral and abdominalshields are osteotomized for the approach.. A high-speed burr or an orthopedic saw is usedto cut the plastron. Irrigation is used to dissipate heat and to remove dust. The bone iselevated from the underlying abdominal musculature using a periosteal elevator. Theincision into the abdominal wall can be performed using a flap technique or a ventralmidline incision. There are venous sinuses on each side of the midline approximatelymidway between the midline and the bridge.. These sinuses should be avoided but can beligated if necessary. The bone is replaced using restorative material as is described below.

Shell fracture

Small defects or cracks in the shell of chelonians can be maintained in reduction with wires,external bandages, or acrylic materials. Acrylic materials, such as those used for hoofreconstruction and dental repairs, can be used to hold fragments in apposition. The fractureshould be maintained in reduction for 3-7 days without exposure to water so that a seal canform~ The fixation should not be removed until there is radiographic evidence of union.Large defects should be repaired using prostheses~ Various restorative materials have beenused, including hoof or dental acrylics, boat or autobody fiberglass, and epoxy resin. Patchesof fiberglass cloth can be autoclaved. The fiberglass provides a matrix to enable the resinto bridge the defect. The patch should be large enough to extend beyond the margin of the

42 1994 PROCEEDINGS ASSOCIATION OF REPTIliAN AND AMPHIBIAN VETERINARIANS

defect. The shell should be cleaned with acetone, ether, or similar degreaser. Duringapplication, care must be taken to keep epoxy from the edge of the defect as its presencewill delay healing. The fiberglass patch is stretched over the defect and held in place,allowing the resin to penetrate the cloth and bond to the shell surface. When this layer hascured, a light coat of epoxy is applied to the fiberglass cloth over the defect. After this layerhas cured, several more thin layers of epoxy should be applied to strengthen and seal thedefect.

If a large fragment is to be replaced, as in the case of closing a celiotomy, the piece shouldbe bonded to the center of the cloth patch with epoxy before the patch is applied to thedefect. Healing of bone in reptiles takes at least 6-18 mo. In growing chelonians, the patchshould be removed from the growth rings after healing is complete to allow the shell tocontinue to grow. Epoxy dust can be toxic and carcinogenic to humans. Copious irrigationshould be used to prevent aerosolization, and a face mask should be worn.

Dystocia

Clinical signs of dystocia include anorexia, regurgitation, straining, cloacal discharge that isoften malodorous, paresis, respiratory distress, and edema of the cranial extremities.Noninvasive procedures should be attempted before surgical intervention. Intramuscularoxytocin at 1-10 IU/kg and 1M or SQ 1% calcium borogluconate at 10 ml/kg have beensuccessful to relieve dystocia when manipulation was not.. In species that produce soft andleathery eggs, percutaneous ovocentesis can collapse the eggs and allow them to pass moreeasily. Salpingotomy is indicated if noninvasive techniques fail or if there is radiographicevidence that natural passage is not possible.. In snakes, it might be necessary to make morethan one incision to access all eggs or fetuses. The incision in the salpinx and uterus shouldbe repaired with an inverting suture pattern of an absorbable material.Salpingohysterectomy should be considered if dystocia recurs, if the patient is not beingmaintained for breeding purposes, or if bacterial salpingitis is present. The ovaries of manyreptiles are not pedunculated making them difficult to remove. Removal of the ovaries maynot be necessary. During salpingohysterectomy the oviduct should be pulled free from theovary and the uterus should be ligated as close to the cloaca as possible.

The presence of egg yolk within the coelomic cavity produces severe inflammation. Fibrindeposition and serosal thickening are typical. Surgical removal of the yolk material andlavage are indicated, however, the prognosis in such cases is grave.

Cloacal organ prolapse

The cloaca has openings from the colon, uterus, urinary bladder, and reproductive tract.Ureteral prolapse has not been reported in reptiles.

Squamates have paired copulatory organs called hemipenes which lie inverted within the tail.Chelonians have a single penis which is everted during copulation. Although prolapse ofthe penis or hemipenes has been reported as a sequel to constipation and neurological

1994 PROCEEDINGS ASSOCIATION OF REPTIliAN AND AMPHIBIAN VEfERINARIANS 43

dysfunction, it is most frequently the result of infection, forced separation during copulation,or swelling secondary to probing for sex determination. The organ should be cleaned, gentlylubricated and replaced. A purse string suture is placed in the cloaca tight enough toprevent prolapse but to allow voiding. The suture should be left in place for 3-4 weeks. Ifthe prolapse cannot be reduced, the cloacal opening can be enlarged by incision. Surgeryis indicated in cases in which the organ is severely swollen and damaged. Amputation isperformed after mattress sutures are placed at the base of the organ to prevent hemorrhage.Snakes and lizards with one hemipenis are considered fertile.

Prolapse of the uterus is rare but does occur. Replacement should be attempted. Ifreduction is not possible, celiotomy and salpingohysterectomy should be considered.

Colon prolapse can result from straining because of constipation or bacterial or parasiticenteritis. Conservative management should be attempted before surgical therapy. Oftencolon prolapse is reducible and successfully managed by treating the primary cause whilemaintaining a purse-string suture in the cloaca. Frequently the venous return from theprolapsed colon is severely compromised and it becomes engorged and friable. Celiotomyand colopexy are recommended in such cases. An area of healthy colon should be selectedand sutured to the body wall. If the colon is severely compromised, it can be resected andanastomosis can be performed.

Gastrointestinal procedures

Principles of gastrointestinal surgery in reptiles are similar to those in mammals. Theintestines of most reptiles are thin walled, and the use of fine sutures and an atraumaticneedle is recommended. Such sutures as polydioxanone are strong and maintain theirtensile strength for several months in mammals which can be advantageous in slow healingreptiles. If the affected section cannot be adequately exteriorized, it should be well packedoff before enterotomy.. Copious coelomic lavage with saline should be performed beforeclosure.

Cystotomy

Cystic calculi can occur in those reptiles with a bladder but desert tortoises seem to havethe highest incidence. Qinical signs associated with cystic calculi are nonspecific and includeanorexia, lethargy, and depression. The urinary bladder is generally very mobile within thecoelomic cavity and is easily isolated during surgery. A 2 layer closure using an invertingpattern of absorbable suture is preferred.

44 1994 PROCEEDINGS ASSOCIATION OF REP11UAN AND AMPHIBIAN VETERINARIANS