David B. Brunson, DVM, MS, DACVA
Multiple factors must be considered when deciding on a chemical immobilization/anesthetic technique. Novel situations and rare species require technique and drug dosage extrapolation. Critical factors for evaluating published techniques are: individual characteristics of the animal such as age, body composition, level of excitation, and health status. Published dosages must also be interpreted prior to use. This includes determining if the published dosages represent an average of widely variable or precise drug effects. Route of administration, site of injection and the associated time-line are necessary for evaluation of the published technique. Use of validated sources and multiple references coupled with knowledge of the species will result in safe and effective immobilization techniques.
The challenge to find, evaluate and modify immobilization/anesthesia techniques is frequently encountered in zoological medicine. In many cases, finding a reference that matches the proposed capture situation or target species can be difficult. If found, the reference usually is missing pertinent information making it necessary to interpret the procedures used. Additionally, immobilization/anesthesia information will often not be the focus of the article and thus the technique presented will constitute a minor part of the citation. As such, immobilization procedures may not be reviewed for their clarity or content and may even be limited to anecdotal comments. The authors can frequently provide personal experiences that are helpful, providing they can be contacted for advice. These limitations often result in a decision dilemma to either modify a published technique or develop a new immobilization procedure (including drugs and dosages).
The goals of immobilization are consistent across species, institutions, clinicians and situations. To reach these goals, the correct drug, dosage, route, and delivery technique must be chosen. An understanding of the species and knowledge of the individual animal is important. Currently it is estimated that there are 9–10 million species of animals on the earth of which under 2 million have been characterized. Familial, breed, geographic and genetic differences further increase the variability among animals. When evaluating a recommended immobilization technique, the following factors should be used: individual animal variation, effect of emotional status, dosage recommendation and calculations, conditions of administration, site of injection and pharmacokinetics. This paper will discuss these issues and present approaches to interpretation of immobilization/anesthesia references.
Animal Variation: Physiologic Factors That Affect Animal Response to Anesthetic Drugs
Published anesthetic procedures should include information on age, environment, physical condition, and excitement level. The response to anesthetic drugs varies with age. Younger animals, in general, require higher dosages. A number of physiologic factors have been associated with this observation. Young animals have a higher metabolic rate. Thus, they absorb, redistribute, excrete, and eliminate drugs in a shorter time period. Establishing and maintaining an effective blood concentration requires higher dosages and more frequent redosing.
Multiple factors result in older animals requiring lower dosages of anesthetics when compared to young animals. These include lower metabolic activity, a higher percentage of body weight as fat and a lower capacity to distribute drugs out of the CNS and blood. Adipose tissue has a low blood supply in comparison to muscle. Thus, anesthetics are slow to redistribute to this tissue resulting in more drug remaining in the blood. In addition, drugs have a slower elimination from fat storage causing the recovery phase to be prolonged in the older animal. Also keep in mind that very young (newborn) and geriatric animals require lower dosages than other age groups.
The season of the year will affect the dosage of the immobilization drug. This is related to the physical activity of the animal, nutritional status, and reproductive activities. Wild animals have seasonal variations in the amount of exercise. Spring or fall migrations are associated with marked increases in energy utilization and muscle activity. Nutritional status changes in relationship to food availability and food type. Physical condition has also been shown to affect animals’ ability to handle stress. In one study, sedentary sheep were less tolerant to heat stress than were physically fit animals. Many captive and non-predatory species are not highly fit. The presence of infections or infestations will also lower animal tolerance to handling or capture. Bears with heavy layers of fat prior to hibernation must be treated differently than animals emerging from their den in the spring. Reproductive activities affect the mental status as well as the nutritional plane of the animal. Breeding and sparring in some species such as moose, leave the dominant bull nutritionally drained by the end of the rut. Appropriate immobilization at this time will require greater precision in selecting the correct dosage and providing necessary support than would be required in the late summer. Evaluate immobilization sources to identify factors such as the hair coat thickness, body fat depth to determine the appropriate dart needle length and site of injection.
Environmental factors can also play an important role in the clinically observed effect of CNS depressants. Ambient temperature and weather conditions can have an important effect on drug dosing. Low environmental temperatures will have an additive effect with the immobilization drugs. Hypothermia has been extensively studied in conjunction with general anesthesia. A slight drop in core body temperature (2°C) markedly depresses the CNS and enhances the effects of anesthetic drugs. Recovery will also be slowed if the animal has a body temperature lower than normal. Hyperthermia on the other hand will increase metabolic rate and decrease the effectiveness of CNS depressants. As the body temperature increases the demand for oxygen by the tissues increases and the risk for tissue hypoxia also increases. Drug dosages reported from hot climates may be excessive when used on the same animal that is hypothermic.
The activity level of the animal will change based on their normal diurnal behaviors. Animals that are active at night have an increased sensitivity to anesthetics when administered during the day. Studies have demonstrated that the effectiveness of opioids appears to vary according to time of day. This may be partially due to variations in the intensity of pain at different times of the day. Drug dosages should be selected that have been validated for the time of day when the immobilization has been scheduled.
Published techniques must provide information on the emotional status of the animals for which the dosages are recommended. Tame or calm animals require lower dosages. These animals have usually been raised by and have daily contact with people. Close proximity allows for less traumatic drug administration and lower catecholamine levels. If the animal to be immobilized has been conditioned to people, little to no excitement will occur during the induction phase. Minimal or no flight distance is associated with this group and hand injection or pole syringes can often be used. Animals that are accustomed to people but will not allow physical contact are considered semi-tame. They will have higher dosage requirements than fully tamed animals. The flight distance will be greater and will require remote delivery equipment. The risk of physically restraining these animals is much greater than with tame animals. Familiarity with people can be used to minimize the stress on the animal and improves the predictability of the immobilization procedures.
Wild animals consider people to be a threat. Detection of a person results in an autonomic response with high levels of fear and anxiety. Because of the animal’s apprehension, there will be larger flight distances. Unless immobilizing drugs can be administered without activating an alarm reaction, the dosage will be much higher than the tame or semi-tame animals. Dosages are frequently doubled if the animal is excited. Tranquilizers and mild sedatives will not immobilize animals which fall into this category.
Making Sense of Published Dosages: What Do the Numbers Mean?
Reported dosage ranges can provide insight into how the information was collected. If the dosage range is narrow this usually means that accurate body weights were known and a specific dose was selected for the immobilization procedure. Typically, the dosage will have a 1.5- to 2-fold range. If the reported range is quite wide (5- to 10-fold), it indicates that inaccurate body weight estimates were used or a single dose was used for all animals and the dose per kilogram was calculated after immobilization when a body weight could be obtained.
Dosages are often reported as a single dose/kg and at a level well beyond the usable accuracy of clinical practice. This usually indicates a calculated dosage based on multiple immobilizations and published as an average dose per kilogram. Examples of this would be a dose of 0.123 mg/kg. If standard error of the mean or standard deviations are published a range can be estimated for clinical use. Accuracy of drug administration is limited by the syringes we use, the drug concentration, the remote delivery equipment and the accuracy of the animal’s body weight estimate. Extrapolation of the dose from published sources should be rounded to a usable decimal place and a range established from which a dose can be selected. Well established dosages are almost always rounded to a whole number.
Conversion of working dosages from mg/lb to mg/kg is usually apparent. Dosages of 2.2 mg/kg were actually converted from 1 mg/lb. Errors in weight conversions should be considered when evaluating these dosages. One common practice is to estimate the animal’s weight in pounds and then divide by 2 for the estimated weight in kilograms. This process adds a 10% overestimation of the weight in the calculation and thus an increased dose of drug. Evaluation of the recommended dose should be judged with these inaccuracies in mind. Errors in estimation of animal body weight are probably the largest single concern. Knowledge of normal weights for the species, gender, age and the geographic area are important in minimizing the estimation error. In situations where estimation of animal size in not possible prior to dosage calculation, a good policy is to prepare three different doses for a range of animal sizes. Prepare a high dose, medium dose and a low dose, then when the animal is observed the dose closest to the animal’s ideal dose can be used.
Additional information should be determined from the information source. The conditions for administration can critically affect the efficacy of the dosage. Was the animal excited, was it chased, what was the site of injection, what type of remote delivery system was used? Site of administration will determine several aspects of the drug effectiveness. Injection into muscle is recommended because absorption is predictable while injections into fat will be slowly absorbed to the degree that effective blood levels may not even be reached. Inadvertent intravenous injections can occur from remote injections procedures, rapid deep anesthesia will occur if this happens. Ventilatory and circulatory support and administration of reversal agents should be provided if needed.
The basic question is, how similar is the published immobilization procedure(s) to the immobilization which is being planned?
Evaluation of the suitability of the published procedures also requires information on the time-line associated with the immobilization technique. Most times this will be an average of a number of immobilizations and will provide an indication of the variability to be expected with the technique. The times from successful drug administration to the loss of mobility and the loss of consciousness are important. Detailed information on the signs of drug onset will help to stage the immobilization and aid in the decision to supplement with additional drug. The duration of full immobilization enables evaluation of the applicability to the upcoming procedure. Evaluation of the technique should include analysis of all the drugs administered to determine if recovery was dependent on metabolism and excretion or if the recovery was controlled by administration of specific receptor antagonists. Each situation requires preparation for animal recovery. In controlled environments, slow recoveries may be acceptable whereas in wildlife situations, full recovery will be important as soon as feasible.
Where to Find Anesthesia and Immobilization Data and Dosages
Whenever possible finding published techniques for immobilizing the species you are planning to anesthetize is ideal. Many sources are now available including books, periodicals, and meeting proceedings.
Additional sources include collections of techniques at facilities such as zoological gardens, animal sanctuaries, universities, animal parks, game farms, etc. These are more difficult to access unless a contact person is known and available to search the records. Finding the “expert” on a given species is often the most satisfying method of species-specific immobilization data. However, at this time, a list of recognized species experts is not available. The International Species Information System (ISIS) maintains an inventory of animals at nearly 500 zoological institution members, from 54 countries, (http://www.worldzoo.org)(VIN editor: The original link was not accessible as of 3-4-21.). Accessing a list of locations for species in captivity may be a pointing device for finding a knowledgeable individual.
In the future, data libraries will be available to assist with communication between experts and individuals needing species-specific immobilization/anesthetic techniques. MedArks and the Animal Capture and Anesthesia Database (ACAD 1.0) are examples of databases that provide comprehensive information compiled from experts. Increasing access to these sources is needed to improve immobilization procedures and avoid repeating techniques that fail to meet the goals of safe and effective anesthesia.
What to Do When “Validatable” Reliable Species-Specific Data Are Not Available
If specific techniques are not available, extrapolation from related species is the usual approach. Species cross-references can be obtained through taxonomy sources. All of the considerations presented in the first part of this article must be included in the development of new species techniques. Temperament of the individual and the species will be one of the most critical considerations. Since the reference species may be either less or more excitable, adjustments in the immobilization procedures should be considered based on the observed excitement level of the animal to be immobilized.
Summary and Conclusions
A single source for species-specific immobilization information is not always available. In most cases, multiple sources are needed to provide all the necessary information to develop an immobilization technique that can be used with confidence. Critical consideration of the variations within the individual such as age, health status, time of year, and emotional status coupled with the ability to interpret published dosages and conditions of drug delivery are essential in developing a safe and rational approach to zoological animal anesthesia and immobilization. Integrating the published sources of information with key references, experts, and personal experience provide the optimal approach to novel species chemical restraint.