Basic Elements of General Anesthesia
Provides overall insensitivity and unconsciousness. Basic elements of general anesthesia include:
- amnesia (loss of memory of pain or distress)
- muscle relaxation
- diminished motor response to noxious stimuli
Two main delivery systems are used in veterinary medicine, inhalation and injection; a brief review of common drugs follows:
Are gaseous or volatile agents administered via the respiratory tract. Inhalation anesthetics, when properly administered allow one to control and regulate anesthetic depth. Disadvantages include need for specialized delivery equipment and potential toxicosis to personnel chronically exposed to anesthetic vapors. Endotracheal intubation facilitates effective and safe delivery of inhalation anesthetics. A waste gas scavenging system should be used to minimize exposure to personnel. Examples include: ether, halothane, methoxyflurane, and isoflurane.
Can serve as the sole anesthetic agent, be used to induce anesthesia before inhalation anesthesia, or supplement regional anesthesia. To minimize the chance of drug overdose and to reduce drug-related tissue damage, drugs used for smaller (<4 kg) laboratory animals may need to be diluted.
Drugs are widely used for inducing or managing general anesthesia. They induce a dose-dependent spectrum of CNS depression, from sleep to deep general anesthesia. Higher doses may cause medullary paralysis, respiratory arrest and death. Examples include: pentobarbital, sodium thiamylal, sodium thiopental and chloral hydrate.
- alpha 2-Adrenergic
Receptor agonists have tranquilizing, sedative, and potent analgesic properties. Examples include: xylazine (trade name Rompum).
- Dissociative Drugs
Induce anesthesia, have short duration of action, a wide safety margin, and cause minimal cardiopulmonary depression. To decrease undesirable actions such as muscle hypertonus and emergence delirium, they may be used in combination with hypnotics, tranquilizers, or alpha 2-adrenergic agonists. Examples include: ketamine and tiletamine which is often in combination with the tranquilizer, zolazepam.
- Muscle Relaxants
Are predominantly central or peripheral acting. The peripheral acting muscle relaxants are devoid of sedative and analgesic properties. These drugs, when used during surgery, must be administered only in conjunction with general anesthetics. The investigator must remain aware that the animal is unable to respond with purposeful movement to noxious stimulation and the animal must be closely monitored (e.g. heart rate, arterial blood pressure) to ensure adequate general anesthesia. Examples include: succinylcholine and dimethyl tubocurarine.
Local and Regional Anesthesia
Used most often in large animals, such as horses, cattle or sheep. Localized insensitivity in awake or mildly sedated animals can result from topical application or injection of appropriate anesthetics in the region of the surgical incision (local anesthesia); injection in proximity to nerve trunks (nerve block); or injection into the subarachnoid or epidural spaces (regional anesthesia).
Local and regional anesthesia advantages include:
- the cost of drugs and equipment is low
- the need for special post-anesthetic care is minimal
- avoids undesirable complications that accompany unconsciousness and recumbency:
- regurgitation and aspiration of gastric contents
- inefficient respiratory gas exchange
- traumatic complications associated with recovery
- myopathy (particularly in large, heavy animals)
Local and regional anesthesia disadvantages Include:
- some degree of animal cooperation is necessary
- the anesthetic period is relatively short
- tissue damage has been associated with injection of large volumes of anesthetic agents
- rapid absorption or inadvertent intravascular injections of large quantities of anesthetic agents can cause life-threatening reactions.
Used to reduce pain are initiated pre-emptively during the surgical period and continued well into the postoperative recovery period. It is often erroneously presumed that an animal is not in pain, when there is no obvious change in behavior. Pain can be difficult to detect because of individual and species variation. "Typical" signs of pain include guarding the painful area, vocalization, licking, biting, self mutilation, signs of depression, grinding of teeth, restlessness, lack of mobility, failure to groom, abnormal posture, changes in sleep patterns, and failure to eat or drink. Pain can be assumed to have been present if administration of analgesics causes these signs to disappear.
Consultation with the attending veterinarian is recommended since there is tremendous variation between species as to their response to analgesic drugs. Limited animal studies have been conducted and for many species there will be widely different published dosage schedules. Never extrapolate from human data, please consult with the veterinarian.