Endotracheal Intubation
Endotrachael intubation after induction of anesthesia is relatively easy to perform in most reptile species and is recommended in all patients to maintain a patent airway, prevent aspiration of fluids (e.g., during oral surgery), and allow positive pressure ventilation during maintenance of anesthesia.
The glottis is located rostrally in snakes and in carnivorous lizards. Herbivorous lizards and chelonians have a fleshy tongue, and the glottis is located at the base. For visualization of the tracheal opening, adequate jaw relaxation should be present. A laryngoscope blade and a light source aid in the visualization of the glottis. Chelonians have a relatively short trachea, and care should be taken not to intubate one bronchus. An adequately sized uncuffed endotracheal tube is recommended for most reptiles to avoid damage to the tracheal mucosa that could result in ischemic injury. In oral surgical procedures, cuffed endotracheal tubes can be used, but care should be taken not to overinflate the cuff. Therefore, large-volume low-pressure cuffs are preferable over low volume high-pressure cuffs. For minimization of the potential for overinflation of the cuff, a small syringe should be used for inflation. In respiratory emergencies, such as obstructive processes in the oral cavity or the trachea (e.g., granulomas, foreign bodies), a tracheostomy can be performed to gain access to the trachea and secure an airway.
MAINTENANCE AND MONITORING
All anesthetic agents have cardiopulmonary depressant effects, and during maintenance of anesthesia, cardiopulmonary performance should be closely monitored. Minimally, rate and depth of respiration and heart rate should be recorded. Supportive care during maintenance of anesthesia includes adequate fluid therapy on the basis of laboratory findings. For correction of major fluid deficits and for maintenance fluid therapy, IV or 10 administration of fluids is most effective. In small reptile species, commercially available syringe pumps are mandatory to deliver accurate fluid volumes at a constant rate. The rate of fluid administration depends on the degree of dehydration. In critical cases, a venous blood sample can be collected during anesthesia to monitor success of fluid therapy and initiate corrective measures if indicated. For maintenance fluid requirements, 5 to 10 mL/kg/h of a balanced electrolyte solution is recommended. Also recommended is monitoring of blood parameters such as PCV, hemoglobin, total protein, glucose, and electrolytes during anesthesia and at regular intervals into the recovery period.
During the anesthetic event, the reptile should be maintained within the POTR. Supplemental heat can effectively be provided via heating blankets and heat lamps. During surgery, the reptile should be frequently assessed for effective analgesia. If signs of pain are present during anesthesia and surgery, such as movement or increase in heart and respiratory rate in response to a painful stimulus, the analgesic protocol should be reviewed and additional analgesic agents should be administered during surgery. Critical assessment of the following parameters is recommended for effective anesthetic monitoring of the reptilian patient.
Reflexes
In reptiles, muscular tone and reflexes are evaluated for assessment of anesthetic depth, and the presence or absence of reflexes should be recorded. During a surgical plane of anesthesia, the righting reflex is absent as is the palpebral reflex in chelonians and most lizard species. The corneal reflex should be present; its absence indicates a deep plane of anesthesia. In some lizard species and in all snakes, the palpebral and corneal reflexes cannot be evaluated because of the presence of the spectacle. Additional reflexes to be monitored include the tail, toe, and cloacal reflexes. If no response is found to a surgical stimulus, the anesthetic depth should be critically evaluated to ensure the patient's condition is not too deep. If the reptile is in a surgical plane of anesthesia, slight movement in response to a stimulus is normally not associated with the perception of pain.
Cardiovascular Performance
The most useful monitoring equipment is a Doppler flow device with the probe positioned at the level of the heart (snakes and lizards) or over the carotid artery (chelonians and lizards) to monitor heart rate and rhythm. The probe can also be placed over the coccygeal artery in lizards and snakes. In chelonians, a pencil probe should be placed at the level of the thoracic inlet, close to the heart and the major vessels.
Electrocardiography (ECG) is a useful monitoring tool for detection of changes in heart rate, such as tachycardia and bradycardia and arrhythmias; however, it does not determine mechanical performance of the heart. ECGs can be recorded with leads attached in a conventional manner. ECGs should especially be recorded in reptiles suspected of or diagnosed with cardiac disease or when arrhythmias are detected during routine monitoring.
Direct arterial blood pressure measurements are the most accurate tool for continuous assessment of arterial blood pressure. However, they are impractical in most reptile patients because of the limited access to a peripheral artery. In most cases, a cut-down procedure is necessary to gain access to the femoral or carotid artery. In those lizard and chelonian patients in which arterial catheterization is necessary, the left carotid artery is the most accessible artery.
Respiratory Performance
During a surgical plane of anesthesia, all reptiles exhibit respiratory depression characterized by bradypnea or even apnea. Consequently, all reptiles need assisted ventilation or intermittent positive pressure ventilation (IPPV) during anesthesia, either manually or via mechanical ventilators (Figures 27-5 and 27-6). Little work has been published on effective and safe ventilation in anesthetized reptiles. The general principles of IPPV should also be applied in reptilian patients. Manual ventilation can be effectively administered to reptiles, however, it is labor intensive and allows less control of tidal volume and peak airway pressure. Small animal ventilators, pressure driven or volume driven, are commercially available and can be used for reptiles. The tidal volume and respiratory rate determine minute ventilation and in most healthy mammals a tidal volume of 20 mL/kg at a respiratory rate of 10 breaths/minute is adequate to maintain a normal PaC02.
However, the tidal volume of reptiles is larger than that of mammals of comparable body mass, and the rate of IPPV is usually set between 4 and 8 breaths/minute. Peak airway pressure should not exceed 10 to
In human and domestic animal anesthesia, pulse oximetry is a useful tool for monitoring heart rate and trends in relative arterial oxygen saturation (SpO2) and detecting hypoxemia (Sp02 <>
Arterial blood gas analysis is impractical in most reptiles, and a cut-down procedure is necessary in most patients to gain arterial access. In addition, the size of the reptile is often the limiting factor in catheterization of a peripheral artery for arterial blood gas determination. Arterial blood gas analyzers directly measure PaO2 PaCO2, and pH, and these values can be interpreted as absolute numbers. Arterial blood oxygen saturation (SaO), however, is calculated on the basis of the human oxygen hemoglobin dissociation curve. Cardiac sampling for blood gas analysis is inaccurate in reptiles because of the mixture of arterial and venous blood within the ventricle. Venous blood gas analysis is of very limited value for assessment of pulmonary function in reptiles.
End-tidal PCOZ monitoring has become the standard in human anesthesia for determination of respiratory performance and estimation of PaCO2. Capnometry measures CO2 concentrations in the expired air for determination of adequate ventilation. Analyzers with high sampling rates (>100 mL/minute) are unsuitable for small reptiles; however, analyzers with low sampling rates of 50 mL /minute and less are available and are more suitable for most reptiles. End tidal CO2 monitoring in reptiles is limited by the fact that reptiles can develop cardiac shunts. A report in Green Iguanas concluded that no correlation exists between end-tidal CO2 concentrations and arterial PCO2 values. However, changes in end-tidal CO2 may give valuable information on existing complications. A decrease in end-tidal CO2 may indicate airway leaks, airway obstruction, disconnection of the reptile from the breathing system, or if IPPV is used, malfunction of the ventilator.
RECOVERY AND POSTOPERATIVE CARE
Recovery of the reptilian patient should be in a temperaturecontrolled and humidity-controlled environment that closely resembles the natural requirements of the species. Small animal incubators are ideal for this purpose, and most offer the ability to provide supplemental oxygen, if indicated (Figure 27-8). The reptile should only be extubated when oral and pharyngeal reflexes have returned and the animal is breathing spontaneously.
Throughout the recovery period, cardiopulmonary parameters, including heart rate and respiratory rate and pattern, should be frequently monitored and recorded. If indicated, respiratory support such as IPPV with room air should be administered. In reptiles, low oxygen concentrations are the stimulus to breath and high oxygen concentrations in the inspired air may prolong return to spontaneous respiration. In a study with Green Iguanas, patients were found to recover twice as quickly when breathing only room air as compared with those patients on supplemented with oxygen.
If supplemental oxygen is necessary, facemasks are most useful and the flow rate of oxygen should be 2 to
Throughout the recovery period, the absence or presence of reflexes, such as palpebral, corneal, foot, and tail withdrawal reflexes, should be recorded in regular intervals for assessment of the degree of recovery and return to a preanesthetic state. Adequate fluid therapy should be continued into the recovery period to ensure normovolemia of the patient. Balanced electrolyte solutions are recommended for most species, and if indicated, determination of hematologic and plasma biochemical parameters facilitates accurate assessment of effective fluid therapy. For reduction of recovery time, increase in environmental temperature during the recovery period above the POTR for the species is not recommended. Increases in environmental temperature result in increased metabolism and consequently an increased demand for oxygen by the tissues. During recovery, most reptiles have respiratory depression and may not be able to meet the increased oxygen demand.
Reptiles recovering from anesthesia should be monitored closely for any evidence of postoperative distress or pain. The analgesic regimen should be reevaluated in reptiles with signs of discomfort or pain. If indicated, additional analgesic agents should be administered in the postoperative period.
Only fully recovered animals should be returned to their enclosure, especially if they are housed in groups, to avoid potential injuries from cage mates. Aquatic species should only be returned back to their aquatic environment completely recovered to prevent accidental drowning.
ANALGESIA
Current knowledge of effective analgesic drugs and therapy in reptiles is scant (Table 27-2). Few studies have evaluated effective pain management in common reptile species. Unfortunately, lack of knowledge of effective drugs and dosage regimen often results in neglecting the management of pain in reptilian patients. A recent study concluded that provision of analgesia for the reptilian patient is uncommon. However, all vertebrates experience pain, and the major difference between reptiles and mammalian species may be different pain pathways and receptors. Reptiles posess an endogenous opioid system, and nociceptive neurns in crotaline snakes are similar to those identified in monkeys.40
In some cases, lack of recognition of pain in reptiles and unfamiliarity with analgesic agents may result in improper pain management. Conditions such as trauma, neoplasia, surgical procedures, and chronic disease processes commonly associated with pain in humans and mammals also cause pain and discomfort in reptiles.
Before treatment, familiarity with the reptile species is mandatory, including knowledge of normal behavior and signs that indicate discomfort and pain, such as restlessness, increased respiratory rate, anorexia, and aggressiveness. Assessment of pain and the required analgesic regimen is mandatory before treatment. The physiology and pathophysiology of pain have been described in detail elsewere.
Pain, stress, and discomfort are closely related, and effective pain management greatly reduces stress and discomfort of the reptile, thus reducing or eliminating the effects of acute and chronic pain on the animal's metabolism, such as compromised immune function, hematologic and biochemical imbalances, and metabolic changes. Although most veterinary practitioners are familiar with "normal" behavior in domestic animals such as dogs and cats, recognition of normal and abnormal behavior is often challenging in reptiles. Although the reptilian patient does not show obvious well-recognized signs of pain such as vocalization it does not mean the animal does not experience pain and discomfort. However, familiarity with normal behavior and normal body position may help in the diagnosis of pain. Abnormal body position in reptiles, such as hunched-up abdomen or resting in an abnormal position, reluctance to lie down in lizards and tortoises, and abnormal movement such as abnormal gait and restlessness may indicate discomfort. Additional signs in reptiles include anorexia, increased aggressiveness, depression, trembling, and increased respiratory rate as
Analgesic regimens in reptiles are often adjusted from the pharmacokinetic and pharmacodynamic principles known from domestic animals. For effective analgesia and comfort for the reptile, acute pain needs to be differentiated from chronic pain. Acute pain is the result of trauma, surgery, or an infectious event and is of relatively short duration; chronic pain persists beyond an acute injury and has severe effects on metabolic status. Chronic pain such as cancer pain and arthritis serves no biologic function and often results in severe impairment and distress of the animal.
Prevention of pain is the most effective method of pain management. Therefore, preemptive analgesic techniques are recommended in cases in which the animal undergoes elective surgical procedures. Similar to domestic animals, balanced analgesic techniques are most effective in the treatment of intraoperative and postoperative pain in reptiles. Often this includes administration of systemic analgesic agents (opioid agents) in combination with long-acting local anesthetic agents (e.g., bupivacaine).
Acute Pain Management
Opioid agents such as butorphanol and buprenorphine are most often used for the management of acute pain in reptiles. Indications for the treatment of acute pain include traumatic events such as shell fractures in chelonians; fractures of the long bones in lizards; bite wounds; thermal burns from default heating devices, especially in snakes and lizards; and surgical procedures such as coeliotomies. The latter are commonly performed in reptiles for removal of masses, such as granulomas and neoplasia; reproductive surgeries; and removal of bladder stones. Although effective drugs and dosage intervals are poorly understood in reptiles, the patient should be frequently assessed for evidence of pain, especially in the postoperative period. Recognition of signs of pain and, discomfort often facilitates effective analgesic therapy.
Local Anesthetic Agents
In reptiles, local anesthetics are often used for local procedures, such as surgical debridement of abscesses. For invasive procedures, administration of local anesthetic agents should be accompanied by concurrent administration of systemic analgesic agents such as opioid agents as part of a preemptive analgesic regimen. Both lidocaine and bupivacaine can be used in reptiles; the former has a fast onset of action, and the latter is more effective in controlling postoperative pain because of its long duration of action. Many techniques have described in domestic animals the use of local anesthetics to provide topical and regional anesthesia and local infiltration techniques and field blocks. These techniques are often directly applicable to reptiles. Local anesthetics can be directly applied to surgical wounds (e.g., abscess debridement) or injected into coeliotomy incisions in the reptiles. Although toxic doses of both drugs have not been determined in reptiles, one should not exceed 4 mg/kg bupivacaine and 10 mg/kg lidocaine in reptiles to avoid potential side effects such as arrhythmias and seizures.
Chronic Pain Management
Chronic pain management in reptiles is often neglected because of a poor understanding of the effects of drugs used for the management of chronic pain in domestic animals, especially nonsteroidal antiinflammatory agents (NSAIDs). Various metabolic bone diseases, gout, renal disease, and a variety of neoplastic diseases are a few examples of conditions associated with chronic pain in reptiles. NSAIDs can be used for the management of chronic pain in reptiles, although little information is available regarding effective treatment regimens and potential side effects in reptiles. NSAIDs offer the advantage of a long duration of action, and both ketoprofen and carprofen are useful analgesic agents for the reptilian patient diagnosed with chronic pain. Before administration of these agents, every effort should be made to determine the renal status of the patient because both drugs should not be used in patients with severe renal and gastrointestinal disease.
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