Anesthesia is a state of unconsciousness induced in an animal. The three components of anesthesia are analgesia (pain relief), amnesia (loss of memory) and immobilization. The drugs used to achieve anesthesia usually have varying effects in each of these areas. Some drugs may be used individually to achieve all three. Others have only analgesic or sedative properties and may be used individually for these purposes or in combination with other drugs to achieve full anesthesia.
Curariform skeletal muscle relaxants or neuromuscular blockers (e.g. succinylcholine, decamethonium, curare, gallamine, pancuronium) are not anesthetics and have no analgesic effects. They may only be used in conjunction with general anesthetics. Normally, artificial respiration must be provided. Physiologic monitoring methods must also be used to assess anesthetic depth, as normal reflex methods will not be reliable.
It is important to realize that anesthesia is not a simple thing. It has profound effects on an animal's physiology because of the generalized central nervous system effects as well as specific effects on all other body systems. Thus, while anesthesia is necessary to prevent pain or distress in research animals, it must not be ventured into lightly. It is important to learn about the drugs you will be using and about the physiology of the animal you will be monitoring. Specific anesthetic drugs and their use are detailed below. All drug dosages are listed in RAR's formulary.
Stages of Anesthesia
These stages occur (when using inhalation anesthesia alone; other drugs added will modify these stages.
Stage 1 (Induction, aka voluntary excitement). Excitement and struggling are common. Usually accompanied by ephinephrine release with associated rise in respiratory rate and heartrate.
Stage 2 (delirium, involuntary excitement). Voluntary centers and loss of consciousness begin. Exaggerated reflexive responses to stimuli are common, as is vomiting (in species that can vomit). Breath holding may occur. Common hazard: self-injury.
Stage 3 General Anesthesia
- Plane 1—Light anesthesia. Most reflexes (pedal, corneal, palpebral) are still present.
- Plane 2 – Medium anesthesia. Most surgeries are conducted at this level. Muscles are relaxed. Most reflexes (pedal, palpebral, corneal) are absent.
- Plane 3—Deep anesthesia. Intercostal muscles are relaxed; ability to maintain respiration is endangered. Pupillary light reflex may be slow or absent.
- Plane 4—Too Deep. All muscles, including diaphragm & intercostal muscles, are paralyzed.
Stage 4 Irreversible Anesthesia—respiratory arrest, followed by circulatory collapse. Death within 1-5 minutes.
Stages of Anesthetic Recovery (including comments about monitoring)
Recovery Stage 4- Animal is unconscious or semi-conscious and in lateral recumbency. Some reflexes are still diminished or absent. In RAR’s Post-Operative Care program, the animal is monitored closely until it has passed through these recovery stages. For animals in Stage 4, it is standard procedure to assess body temperature, heart rate & rhythm, pulse, respiratory rate and character, capillary refill time and state of hydration at least every two hours. The condition of the surgical site is monitored, and analgesics are administered when the animal becomes semi-conscious.
Recovery Stage 3- Animal is conscious and all reflexes are present, but may not be able to control its body position. The swallow (gag) reflex is present, and the endotracheal tubes (if used) can be removed. In Post-Op, the animal is still being closely monitored. The parameters listed above are assessed, but less frequently- approximately every 8-12 hours. Analgesics are continued.
Recovery Stage 2- Animal can either maintain itself in a sternal position, or can stand and move about, but may still show some sedation, ataxia, hypothermia or dehydration. In Post-Op, the parameters listed above, as well as attitude, activity, food and water consumption, are assessed at least every 12 hours.
Recovery Stage 1- All functions are normal, unless altered directly by the experimental procedure. In Post-Op, the animal is monitored every 12 hours for the parameters listed above.. When there are no clinical problems, or signs of pain, discomfort or distress, the animal can be discharged from RAR’s Post-Operative Care program.
Intra-operative and Anesthesia Records
The Guide for the Care and Use of Laboratory Animals and the Institutional Animal Care and Use Committee Guidebook require that animals under anesthesia be carefully monitored to insure adequate depth of anesthesia, animal homeostasis, timely attention to problems, and support during anesthetic recovery. Monitoring includes, but is not limited to, checking anesthetic depth and physiological parameters (minimum: heart rate and respiratory rate) on a regular basis (minimum every 10 minutes).
Record keeping is an essential component of peri-operative care. For major surgical procedures on non-rodent mammals, an intra-operative anesthetic record must be kept and included with the surgeon�s report as part of the animal�s records. In addition to the above requirements, the record should include all drugs administered to the animal, noting the dose, time, and route of administration. These records should be available to RAR and any other personnel providing post-operative care. Although it is not required, RAR strongly encourages the use of an intra-operative rodent anesthetic record during surgical procedures.
The required monitoring will vary according to the species and the complexity of the procedure, but should include:
- A pre-surgical assessment;
- Adequate monitoring of anesthetic depth and homeostasis
- Support such as fluid supplementation, external heat, or ventilation
- Monitoring and support during anesthetic recovery
- Post-operative monitoring (provided by RAR for all large animals)
The following are suggestions from the American College of Veterinary Anesthesiology for monitoring anesthetized animals:
- Circulation: to ensure that blood flow to the tissues is adequate.
- Methods: Heart rate, Palpation of peripheral pulses, ECG, auscultation of heartbeat, non-invasive or invasive blood pressure monitoring.
- Oxygenation: to ensure adequate oxygen concentration in the animal�s arterial blood.
- Methods: observation of mucous membranes color and CRT, pulse oximetry, blood gas analysis
- Ventilation: to ensure that the animal�s ventilation is adequately maintained.
- Methods: respiratory rate, observation of thoracic wall movement or breathing bag movement if animal is spontaneously breathing, ascultation of breath sounds, respiratory monitor, capnography, blood gas monitoring.
What is Analgesia?
Analgesia is the relief of pain. Pain is normally defined as an unpleasant sensory and emotional experience associated with potential or actual tissue damage. Pain is difficult to assess in animlas because of the inability to communicate directly about what the animal is experiencing. Instead, indirect signs of pain are often used. Because of the difficulty of determining when an animal is in pain, animal welfare regulations require that analgesia be provided whenever a procedure is being performed or a condition is present that is likely to cause pain. In the absence of evidence to the contrary, it is assumed that something that is painful in a human will also be painful in an animal. It is best if analgesia can be provided to animals preemptively, or prior to the painful procedure, rather than waiting until after clinical signs of pain are observed. Analgesia is normally provided using one of several types of pharmaceutical preparations.