Principles of Anesthesia in Veterinary Medicine

PHYSICAL RESTRAINT: for the safety of the animal and the handler. Usually, these agents are chosen for speed of action, potency, and predictability, rather than safety. These drug combinations for 'chemical restraint' should be reserved for unmanageable cases.PHYSICAL EXAMINATION: for certain diagnostic tests, such as radiology and ultrasound. The use of sedatives, especially in the case of ECGs, may manipulate the accuracy of results; however, a stressful and drug-free animal may as well.PERMIT SURGERY: the handling of sensitive tissues requires the use of anesthesia, both legally and ethically.INTENSIVE CARE: for extremely painful or critically animals that require long-term carePAIN MANAGEMENT: some anesthetics, such as opioids and benzodiazepines, also have analgesic properties
SEIZURE CONTROL: pentobarbitone and diazepam are used for the immediate and long-term control of seizures (status epilepticus)EUTHANASIA: commercial agents include concentrated doses of barbiturate, +/- an anticonvulsant, +/- a local anesthetic, along with a dye. Unconsciousness must be produced before any lethal effect.

The elimination of sensation by the controlled, reversible suppression of nervous function with drugs

Peripheral nerve endings are stimulated via touch, temperature, pressure, and pain. This stimuli travels via afferent pathways to the spine and then to the higher cerebral centers (the cerebrocortical projection areas).

Anywhere on the pain pathway between the peripheral nerves and the projection and sense areas of the brain

LOCAL REGIONAL: or analgesia; blocks sensation in the peripheral nerves or spinal cord (epidural). Does not cause loss of consciousness.GENERAL: disrupts the projection and sense areas of the brain, along with other areas to create unconsciousness, muscle relaxation, and a lowered sensitivity to otherwise painful sensation. General anesthetics reduce activity in the Articular Reticular Formations (ARF) in the spine and brainstem; however, they also suppress the processing of information related to blood pressure, plasma oxygen levels, carbon dioxide, and blood temperature. Deeper anesthesia causes more suppression of these vital centers, which predisposes the animal to hypotension, hypoventilation, and possibly hypothermia,

ANALGESIA: a reduced state of pain - more accurately the interpretation of pain. A noxious stimuli, such as tissue traction, still stimulates nerve endings, but the higher centers are unable to interpret the information. The unconscious body may still respond to this stimuli.An animal's reaction to pain depends on the level of the central nervous systems activated, and can be used to assess 'deepness' of anesthesia. Spinal reflexes are exemplified via the 'toe-pinch' reflex. At higher levels, such as the medulla, noxious stimuli causes and increased heart rate, blood pressure, and respiratory rate. At the even higher level of the hypothalamus, increased heart rate and blood pressure is also produced, along with piloerection and sweating, via catecholamine release. Additionally, ACTH, TSH, ADH, and PRL are released in response to pain, and can be measured as indicators of poor welfare conditions. Finally, at the level of the cortex, complex responses, like biting and attempting to flee, are possible - although not in the anesthetized patient.

GENERAL - CORTEX:alpha2-agonistsopioids
LOCAL - SPINAL, PROXIMAL PERIPHERAL:alpha2-agonistsopioids,benzodiazepinesNMDA antagonists
LOCAL - DISTAL PERIPHERAL:corticosteroidsnon-steroidals (NSAIDs)

State of drug-induced insensitivity, which allows the surgery to be performed comfortably for the staff and animals.
In addition to including unconsciousness, adequate analgesia is necessary, along with muscle relaxation to prevent reflex stimulation in response to noxious stimuli. Muscle relaxation is especially indicated in ocular and brain surgery, when movement is undesirable.

Through the use of three drug classes (anesthetics, analgesics, and neuromuscular blocking agents) the needs of surgical anesthesia are achieved. The anesthetic agent is not needed in high doses to produce analgesia and neuromuscular blockade, so vital center activity is preserved. This is ideal in patients that have diminished cardiac reserve, i.e. the very ill or the very old.
Downsides include the requirement for controlled ventilation and possible adverse drug interactions between agents.
Additionally, the meaning of balanced anesthesia can be expanded to include arreflexia, e.g. elimination of all reflexes during surgery, and the maintenance of O2 delivery to vital tissue.

(1) Patient specific factors: anatomical (e.g. ET in cattle), physiological (eg regurgitation in ruminants), pharmacological(2) Patient individual factors: size (risk of hypothermia), temperament, breed, health status, age, concurrent medication, repro status, function(3) Surgical procedure: duration, invasiveness, location(4) Other factors: cost, availability, personal experience