libro 1

Anaesthesia and analgesia

Introduction

Physical restraint alone, although sometimes more economical and faster than anaesthesia, is often problematic. Many reptiles are capable of maiming or killing handlers. Persistent struggling will result in muscle contraction and consequent damage, possible hypercalaemia and lactic acidaemia. Although large reptiles that are bound appear immobile, they may still be contracting skeletal muscle isometrically.

The ideal anaesthetic agent should provide restraint, muscle relaxation and analgesia, with ease of reversal or recovery, and be safe both for the patient and for veterinary personnel. Unexpected recovery, which may occur with some anaesthetic protocols, may be extremely hazardous if working with venomous or otherwise dangerous species. The only anaesthetic agent currently licensed for reptiles in the UK is isoflurane. However, many other anaesthetic agents have been used successfully in a variety of reptile species and the choice of anaesthetic regimen, whilst paying due regard to current legislation, must also consider the needs of the patient and personnel.

Correct identification of the reptile to the species level is important, as species-specific susceptibilities to certain anaesthetic agents, even significant mortality, have been identified in some cases. Correct species identification will also allow the correct preferred body temperature for that species to be maintained during general anaesthesia.

Cardiopulmonary anatomy and physiology

Anatomy

Lung anatomy varies between species: lizards and snakes have simple sac-like lungs, whereas the chelonian lung has a more complex structure that increases surface area (Figure 11.1).

The glottis is positioned at the base of the tongue in lizards (Figure 11.2) and snakes (Figure 11.3), and more caudally in chelonians (Figure 11.4). The chelonian trachea bifurcates quite rostrally, approximately half way along the neck, a feature which must be taken into account when intubating these animals (Figure 11.5).

Blood shunting

The incompletely divided ventricle in non-crocodilian reptiles means that these animals can shunt blood across the heart to bypass the pulmonary circulation. This may occur in response to changes in the ratio of the pulmonary and systemic resistances or to adrenergic changes to the vascular resistance in either circulation. During the normal apnoeic phase of respiration, it is cholinergic contraction of the pulmonary artery that prevents a perfusion-ventilation mismatch.

Heart rate

Heart rate varies with species and is also dependent upon factors such as temperature and body size. In lizards an increase of bodytemperature of 10°C over the range of 20-40°C increases heart rate by a factor of approximately 2.

Respiration

Mouth breathing is possible in all reptiles, yet healthy reptiles usually breathe via the nostrils. Both inspiration and expiration are active processes. Chelonians lack intercostal musculature, so ventilation is achieved using the movement of the abdominal viscera, limbs, pelvic and pectoral girdles, and two groups of abdominal muscles that compress the abdominal viscera. In lizards and snakes the negative pressure to begin inspiration is provided by the intercostal, thoracic and abdominal musculature. There is also smooth muscle in the lung airways to assist ventilation, this occurs even if the coelom is opened.

Respiratory cycle

Reptiles breathe in one of two breathing patterns:

- Terrestrial animals tend to take single breaths followed by variable periods of breath-holding

- Aquatic species tend to have periods of breathing followed by extended periods, lasting minutes to hours, of no ventilation.

Reptiles may vary breathing frequency (respiratory rate) and depth (tidal volume) and also the duration of apnoea (non-ventilatory periods).

Control of respiration

Respiration is controlled by responses to arterial partial pressures of carbon dioxide and oxygen, acid-base balance and lung stretch receptors, with differences between species.

Oxygen pressure: The respiratory drive in reptiles is different to that in mammals, in that reptiles are more sensitive to a low partial pressure of oxygen (pO2) than to a high partial pressure of carbon dioxide (pC02). Reptiles are also "capable of prolonged anaerobic metabolism. These factors may contribute to a prolonged recovery time (relative to normal respiratory rates) from general anaesthesia when 100% oxygen is administered.

Studies on turtles have shown that the respiratory rate increases, as does the heart rate, if the animal becomes hypoxic. Right-to-left cardiac shunting also occurs. If the hypoxia continues for >60 minutes, heart rate may fall to 50% of original, and a right-to-left shunting of up to 80% of cardiac output occurs. These abnormalities are rapidly reversed when normoxic air is offered. Turtles have been shown to exhibit hypoxic pulmonary vasoconstriction at a lower threshold than mammals, and then to perform a rightto left intracardiac shunt to prevent a perfusion-ventilation mismatch. This has relevance to the use of gaseous general anaesthesia: despite apparent ventilation, the lack of perfusion together with cardiac shunting will prevent absorption of sufficient gas to maintain general anaesthesia.

Chemoreceptors and stretch receptors: Many snakes have both intrapulmonary chemoreceptors and stretch receptors, whereas many chelonians have only stretch receptors. In periods of high inspired carbon dioxide, snakes can over-ride the volume-related feedback. If snakes are breathing a gas with a high carbon dioxide level, respiration slows and a high tidal volume is seen. Hypercapnia leads to an increase in tidal volume by suppressing lung stretch receptors. Hypoxia increases the breathing frequency by reducing or eliminating the non-breathing periods. These effects are exacerbated at higher temperatures. In snakes studies have shown that respiration is primarily governed centrally, with less reliance on respiratory sys:em mechanics such as stretch receptors. The respiratory system compliance (elasticity and flexibility) of snakes is also higher when compared with 7ammals of similar bodyweight. This is important as snakes may assume many different postures that affect the mechanics of the lungs, and this mechanism allows the breathing to remain unchanged.

Tidal volume

The normal tidal volume of reptiles varies between species, e.g. 12.5 mg/kg in Boa spp., 45 ml/kg in Trachemys spp. Reptiles also commonly have ventilation-perfusion mismatch and right-to-left pulmonary shunts, and this may be exacerbated by voluminous coelomic contents such as eggs and ingesta. Such shunting may complicate the use of arterial blood gas measurements. Although occurring naturally, espe

cially in aquatic species, ventilation-perfusion mismatch may be exacerbated in anaesthetized reptiles that are placed into lateral or dorsal recumbency.

General considerations for anaesthesia

Many of the physiological systems of reptiles are affected by changes in temperature, sometimes quite dramatically. Therefore, it is recommended that reptiles are maintained at their preferred optimum body temperature during the perianaesthetic period. Parenteral (injectable) anaesthetics are removed from the body by biotransformation (metabolism) and/or renal excretion. Decreased body temperature, by decreasing enzyme activity and renal perfusion, will delay drug removal and prolong anaesthetic duration. The relative potency of inhalational anaesthetics is generally the same across most animal species, but at temperatures below a reptile's preferred optimum temperature, potency is lower. Additionally, the uptake and removal of inhalational anaesthetics is slower than in mammals and birds, due to a lower respiratory efficiency and slower circulation time. A further complication occurs in those reptiles that can shunt blood away from the lungs for variable time periods as these shunts can effectively block the uptake and/or removal of inhalational anaesthetics.

Hypothermia ('cold narcosis')

Hypothermia or the induction of `cold narcosis' is neither humane nor an appropriate substitute for reptile anaesthesia. Although it will produce immobility, it is questionable whether it induces analgesia. Furthermore, hypothermia impairs drug metabolism and depresses immune function. Necrotic changes to the brain of snakes and tortoises have been described following hypothermic episodes (Bennett, 1998). Rarely, hypothermia may be used in conjunction with anaesthetic techniques to facilitate cardiopulmonary surgery such as removal of pentastomids from the lung and intra-atrial thrombi (Bennett, 1998).

Pre-anaesthetic considerations

Clinical examination

A thorough clinical examination to ensure that the animal is free from clinical disease, especially with regard to respiratory and cardiovascular function, is essential prior to inducing general anaesthesia (see Chapter 5). Any abnormalities should preferably be corrected prior to general anaesthesia or at least permit stabilization of the patient in the perianaesthetic period. Any animal that is compromised, e.g. by dehydration, blood loss, cachexia, anorexia or infection, will pose a greater anaesthetic risk than a clinically normal animal. A complete pre-anaesthetic assessment and stabilization is therefore especially important. Food and water intake should be recorded preoperatively and used to assess the postoperative recovery.

Fasting

Starvation is rarely necessary prior to anaesthesia. Aspiration is unlikely, as regurgitation is rare with the possible exception of a snake that has recently fed. General anaesthesia should be avoided in snakes with a full stomach, as the mass may lead to cardiopulmonary disturbances if the lungs are compressed. Starvation of herbivorous reptiles is to be avoided, as many of these species have enteric flora that may be disrupted by prolonged starvation, leading to digestive disturbances.

In general, a period of time sufficient for digestion of the last meal to have been completed is adequate for reptiles. For non-herbivorous species, the starvation period may vary from 18 hours in chelonians and smaller lizards to 72-96 hours in larger carnivorous lizards and snakes. This time should be adequate to avoid the presence of live invertebrates in the gastrointestinal tract of insectivores.

Hydration

All reptiles should be adequately hydrated prior to surgery.

Immersion in water or in oral electrolyte replacement or rehydration fluids can be used several days prior to general anaesthesia. The reptile is placed in a shallow water bath at its preferred body temperature, to encourage drinking. Chelonians are also able to absorb water via the cloaca. The water should be deep enough to allow the head to be fully submerged when required, but shallow enough to allow the animal to easily keep its head above water. Sick, weak or debilitated animals should be constantly monitored to pret vent drowning.

Oral fluid therapy with electrolyte replacement or rehydration fluids may be given several days prior to general anaesthesia (Figure 11. 7)

For parenteral administration, equal volumes of 5% glucose in 0.9% sodium chloride, Ringer's solution and water may be given via the intravenous, intraosseous, intracoelomic or subcutaneous routes (see Chapter 10). All injections should be given aseptically to avoid introducing infection.

Preparation for medication

An intravenous or intraosseous catheter may be preplaced for peri-anaesthetic care. The patient should be weighed immediately before surgery to enable correct dosing with medications.

Handling and positioning

The animal should be handled correctly to minimize trauma and stress. Care should be taken when positioning or moving the patient during general anaesthesia. For example, elevating the head will increase the heart rate in lizards. The dorsal position of the lungs in chelonians means that positioning these animals in dorsal recumbency will lead to lung compression and respiratory embarrassment.

Safety

Reptiles are resilient animals and capable of surviving physiological disturbances that would rapidly kill a mammal or bird; therefore it must be remembered that the apparent safety of a particular anaesthetic regimen may be merely a reflection of this physiological resilience. Impaired blood flow (secondaryto marked hypotension and dehydration) and severe hypoxaemia may induce renal tubular necrosis that manifests as renal failure and visceral gout several days to weeks after an anaesthetic episode. Therefore, survival in the immediate post-anaesthetic period does not necessarily mean that the general anaesthetic regimen used was safe and efficacious.