Adequate training is absolutely vital to performing anesthesia. You need to have a good working knowledge of the pharmacology of anesthetic agents and the normal animal physiology. Information provided herein provides the most rudimentary information. At UAF, you will be required to demonstrate competency before being approved by the IACUC to conduct anesthesia by yourself. We strongly encourage you to spend some time reading books, journal articles, and visiting websites prior to performing anesthesia. Research protocols are quickly suspended if mistakes are made and there is little room for error with anesthesia and surgery.
Anesthesia is controllable and reversible loss of consciousness induced by intoxication of the central nervous system. Lowered sensitivity to stimuli from the environment and diminished motor responses are major features of anesthesia.
Review the definitions again. Anesthesia may be administered at many different levels for very different purposes. This may range from the mild "numbing" of a localized are for a simple biopsy to deep, general anesthesia for surgery.
Local and regional anesthesia is conducted by using local anesthetics to eliminate the sensation to a specific area allowing you to perform minor procedures. The animal remains fully awake and conscious.
For purposes of this section we are referring primarily to surgical anesthesia which is: general anesthesia with relaxation and analgesia sufficient to enable the safe performance of surgery.
Chemical capture of wildlife is often referred to as immobilization. However, you are practicing anesthesia. The level of anesthesia is solely dependent upon the drugs employed and the dose administered. Therefore, wildlife immobilization often covers anything from sedation through neuroleptanalgesia to surgical anesthesia.
Injectable anesthesia requires the inoculation of one or more anesthetic agents via intramuscular, intravenous, or, in the case of rodents, intraperitoneal injection. Some agents have reversal agents but, once injected you generally have to wait for the animal to detoxify the agent(s). Remember - these agents ate toxins!
Gas anesthesia offers better control, the ability to rapidly change level of anesthesia, and the ability to take over breathing for the animal.
As a biologist or biology student, you may be faced with a variety of anesthetic situations, from a highly controlled/monitored surgical anesthetic setting to a field immobilization. Everyone would like to have ideal conditions while performing procedures that require anesthetic, but for the field biologist, this may not be an option. Environmental obstacles, lack of supportive equipment, weather conditions, predators and injection site are all factors that must be taken into consideration when anesthetizing a wild animal.
At a minimum, heart rate/pulse strength, respiratory rate, temperature and depth of anesthesia should be monitored closely. This will give you basic information on how the cardiovascular, respiratory and nervous systems are performing in your subject. If you are going to be performing surgical procedures yourself, it is highly recommended that you have a person dedicated to anesthetic monitoring. It is difficult for one person to concentrate on two things at once and if your subject does not survive, the surgical procedure was not worth it!
The following section aims at educating you on the various stages of anesthetic depth, prepares you to recognize a problem before it becomes life-threatening and provides recommendations on what to do if you do have an anesthetic emergency.
During induction of general anesthesia the patient goes through certain stages of progressive depression of reflexes and vital functions:
- Analgesia and amnesia
- Loss of motor coordination and loss of consciousness
- Reduction of protective reflexes
- Blockage of afferent stimuli
- Muscle relaxation
- Respiratory and cardiovascular depression
- Apnea
- Cardiovascular standstill
- Death
So, we have nine stages here: the first five we like, the last four we don’t. The key is to catch a problem before it progresses to these last four stages.
Anesthesia can be divided into four progressive phases. The signs relating to a certain phase are based upon the presence or absence of certain reflexes which are progressively lost as the amount of administered anesthetic increases. The time when reflexes are lost varies with the drug used.
Phase I
This is the stage of voluntary excitement marked by struggling and breath holding if induction is slow and by inhalation. Respiratory rate and pulse rate are increased and the pupils are dilated. Urination and defecation frequently occur. These are signs of the fear response.
Phase II
In this stage, loss of consciousness occurs. Ventilation is irregular and involuntary excitement may occur.
Phase III
The stage of surgical anesthesia. Ventilation is regular. This stage can be loosely divided into three planes of progressive depression of cardiopulmonary function and responsiveness to stimuli.
In plane one painful stimuli may cause an increase in heart rate, blood pressure and respiratory rate. The palpebral reflex is present (the palpebral reflex is stimulated by tapping the skin at the medial canthus of the eye or by running the finger along the eyelashes. The reflex disappears in light to medium plane of surgical anesthesia in small animals). Lacrimation is not depressed so the cornea still looks "shiny". In dogs and cats the eyeball is centrally positioned.
In plane two, the laryngeal reflexes are lost. There is further depression of the cardiopulmonary system. The eyeball rotates down in cats and dogs under halothane, isoflurane, barbiturate and propofol anesthesia. The palpebral reflex is lost, and this plane is adequate for nearly all surgical procedures.
In plane three, respiration becomes more depressed and the breathing pattern becomes diaphragmatic in cats and dogs. Cardiac contractility and blood pressure is further reduced. In cats and dogs the eyeball becomes central and there is no palpebral reflex. Lacrimation also ceases, and the cornea looks dry. This plane is a warning that the animal is receiving too much anesthetic and should be `lightened'!
Phase IV
Overdose. Paralysis of the intercostal respiratory muscles occurs and weak diaphragmatic movements are made. With further deepening these also disappear. The abdomen looks as if it is bulging out and the thorax caving in during inspiration. The pulse becomes weaker. The pupils start to dilate and the cornea looks very dry. If these warning signs go unheeded there is complete paralysis of the medulla of the brain, and cardiopulmonary collapse. If the anesthetic is withdrawn (inhalational agents) and artificial respiration instituted before profound cardiovascular depression occurs, then recovery is possible. With parenteral anesthetic agent overdose, reversal agents can be used, but not all parenteral drugs have reversal agents.
There are two ways to monitor anesthesia in an animal. The first involves actual physical contact with the animal and monitoring of reflexes and the second involves machines, most commonly used during inhalation anesthesia:
Physical monitoring methods
Please note: these reflexes can and DO vary from species to species and with drug type/dose. If you are unfamiliar with the reflex in the species or drug you are working with, do not depend upon it as a reliable indictor of anesthetic depth. Generally, the reflexes I find most reliable/useful across the board include palpebral, swallowing, laryngeal, anal reflexes and muscle relaxation. From experience, I have found the corneal reflex and pupillary light response to be extremely variable among species and drug administered, and thus of less value in assessing your patient’s status.
The reflex responses used to determine the depth of anesthesia are:
Other important physical factors that MUST be monitored during anesthesia:
mucous membrane color and capillary refill time
Should be less than 2 seconds. Some pallor occurs with peripheral vasoconstriction and/or decreased perfusion. Cyanosis (blue tinged mucous membranes) indicates hypoxia. Brick red mucous membranes indicate hypercapnia. Do not rely on CRT alone since it is possible to see a (sluggish) CRT in an animal which has recently suffered a cardiac arrest.
pulse rate and quality
Femoral pulses are palpable at the base of the hind leg on the ventral part of the body. A pulse can be taken from the anterior tibial and digital arteries, as well. Also, in larger animals you can get a lingual pulse (from under the tongue), but again, DON’T GET BIT!
respiratory rate and quality
Watch the animal’s chest as well as the rebreathing bag. Please note that this does not reveal adequacy of distribution or gaseous exchange in the lungs.
temperature (!!!)
Mechanical monitoring devices
Please see section on mechanical anesthetic monitoring devices. As was mentioned previously, these methods are most commonly employed during inhalant anesthesia.
So, you have monitored your animal closely, administered the correct drugs in the correct concentrations over the correct time span, but your subject still arrests (cardiac or respiratory).
What now???
Assess your situation. If you are at the University, send someone to contact the veterinarian on duty for help. Is the anesthetic agent you are using reversible? If so, consider reversing anesthesia in order to bring your patient back to a more normal physiologic state. If inhalant anesthesia is being used, as a general rule it is best to discontinue administration (and note the time). We all are familiar with the ABC’s of human CPR, and what I recommend here is not too far off from these rules. Just a few minor alterations are present, mostly involving safety- these rules were developed for dogs and cats, not wild animals that may be harboring infectious diseases or be more prone to bite. Always keep in mind the variables you are dealing with, then proceed with the ABC’s:
A. Airway
The first step in animal CPR, after determining non-responsiveness, is to obtain a patent airway. You should not continue on, until this step has been achieved.
B. Breathing
After achieving a patent airway, one must determine whether the animal is breathing, and whether this breathing is effective:
C. Circulation
This is the final step of CPR and should only be initiated after the airway and breathing steps have been completed:
D,E and F
These stand for Drugs, ECG and Fluids. Most actions related to these categories should be attempted only by a veterinarian, with the exception of reversal agents (which dose you should be familiar with) and sublingual Dopram, a respiratory stimulant in some species, that has a wide margin of error making it somewhat safer to use than most emergency drugs.