Introduction

• Anesthesia means 'without feeling'; includes without pain.
• Is used to describe the loss of sensation to the entire or any part of the body.
• Reversible, controlled process of intoxication of the central nervous system (CNS).
• Aims of anesthesia are to prevent awareness of, and response to pain; to provide restraint, immobility and muscle relaxation without creating a risk to the patient's safety.

• Emphasize some special risks, eg brachycephalic breeds Anesthesia: breed considerations, individual hypersensitivity; intercurrent or pre-existing conditions.
• Conditions which relate to prognosis, eg diabetes, cardiac dysfunction, renal or hepatic insufficiency merit special mention.

Indications

• Restraint, eg for diagnostic procedures.
• To allow surgery that would otherwise cause pain.
• Legal requirement, eg experimental animals.
• Capture, handling of wild animals.

Mechanisms

• Volatile agents: largely unknown.
• Some anesthetics act at receptors, enhancing inhibitory pathways (ie propofol Propofol and alfaxalone Alfaxalone).
• Ketamine Ketamine:  multiple binding sites, including NMDA-receptors (antagonist).
• Some probably disrupt membranes, affecting ion channels/receptors (volatile agents).

Measurement of anesthesia

• Depth of anesthesia (developed by John Snow) describes the signs and stages of ether anesthesia, based on observations of the patient (viz ocular reflexes, nature of breathing). Correlation of changing physical signs with anesthetic depth progression.

Stages of ether anesthesia (now historic), based on original observations by Guedel

• I: induction or voluntary excitement stage.
• II: involuntary excitement.
• III: surgical anesthesia: divided into planes 1-3 (or 4) of light to deep surgical anesthesia.
• IV: overdose → excessive central nervous system and cardiovascular function.
• Assumes linear effect of anesthesia, all anesthetics have same action.
• Different agents have different effects, eg greater respiratory depression with halothane than ether.
• Anesthesia is a dynamic state, depends on all inputs/overall result on brain.
• Modern techniques include various drugs affecting autonomic nervous system (balanced anesthesia), making Guedel's classification less reliable.

Modern methods of measurement

• Absence of spontaneous movements. jaw tone.
• Ocular: reflexes and position - presence of palpebral and corneal reflexes, nystagmus. (Do not check corneal reflexes as this could result in trauma to the cornea.)
• Cardiovascular: heart rate and rhythm, blood pressure, cardiac output, etc. Most inhalation agents produce dose dependent depression of cardiovascular function Anesthetic monitoring: overview.
• Respiratory: rate and tidal volume - minute ventilation, arterial carbon dioxide and oxygen tensions, and hemoglobin saturation. Most inhalation agents produce dose dependent depression of respiratory function.
• End-tidal inhalation agent concentration indicates relative depth for inhalation agent of known MAC (minimum alveolar concentration).
• Plasma: concentrations of intravenous agents of known pharmacokinetics.
• Electroencephalogram: brain electrical activity with surface electrodes. Depth of anesthesia/anesthetic agents used produce characteristic patterns of electrical activity.

Balanced anesthesia

Adverse effects of anesthesia

Methods for general anesthesia

Anesthetic records and patient safety checklist

Further Reading

Publications

Refereed papers

• Recent references from PubMed and VetMedResource.
• Brodbelt D C, Blissitt K J, Hammond R A et al (2008) The risk of death: the confidential enquiry into perioperative small animal fatalities. Vet Anaesth Analg 35 (5), 365-373 PubMed.
• Hall L W & Weaver B M Q (1954) Some notes on balanced anesthesia for the dog and cat. Vet Rec 66, 289-293 VetMedResource.

Other sources of information

• Duke-Novakovski T, de Vries M, Seymour C (2016) BSAVA Manual of Canine and Feline Anaesthesia and Analgesia 3^rd edn. BSAVA Quedgeley, UK.
• Grimm K A, Lamont L A, Tranquilli W J, Greene S A & Robertson S A (2015) Lumb and Jones' Veterinary Anesthesia and Analgesia 5^th edn. Wiley Blackwell, Ames, IA, USA.
• Clarke K W, Trim C W, Hall L W (2014) Veterinary Anaesthesia. 11^th edn. Saunders Elsevier, London, UK.