Contributors: John Dodam, Marieke de Vries
Species: Canine | Classification: Miscellaneous
- 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.
- Restraint, eg for diagnostic procedures.
- To allow surgery that would otherwise cause pain.
- Legal requirement, eg experimental animals.
- Capture, handling of wild animals.
- 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.
- Four components of balanced anesthesia:
- Muscle relaxation.
- Alteration of autonomic reflexes.
- Each component can be produced on its own.
- Balanced anesthesia refers to the use of all four components, each to the degree required for the procedure.
- Because combined, smaller doses of drugs can be used, minimizing potential side-effects.
- Drug-induced state of unconsciousness from which the patient cannot be easily aroused.
- Analgesia Analgesia: overview.
- Pain - conscious (cortical) perception of unpleasant stimuli.
- Nociception - unconscious perception of a noxious stimulus. It does not require consciousness and can continue unabated during general anesthesia in absence of sufficient anti-nociceptive techniques.
- Reflex suppression - suppression of autonomic reflexes induced by nociceptive stimuli transmitted to noncortical areas of the brain.
- Analgesic component of balanced anesthesia will prevent pain in conscious, prevent autonomic reflexes in unconscious animals.
- Prevents autonomic tachycardia, hypertension, tachypnea, lacrimation, sweating and reflex muscle activity.
- Reduces catabolic endocrine and metabolic response of surgery, ie the stress response.
- Reduces the dose of anesthetic agents required to produce and maintain unconsciousness and therefore their potential side-effects.
- Narcosis can produce muscle relaxation via CNS depression causing reduced activity of the ventral horn activity of the spinal cord, but often at the expense of significant cardiopulmonary depression.
- Can be produced independent of generalized central depression.
- Spinal depression at the internuncial neurones of the spinal cord.
- Peripheral nerve blocks Anesthesia: peripheral nerve block - pelvic limb Anesthesia: peripheral nerve block - thoracic limb can produce regional muscle relaxation.
- Neuromuscular block Anesthesia: non-depolarizing neuromuscular blockade with agents that target the neuromuscular junction produces general muscle relaxation Muscle relaxant: overview.
Function of geneal muscle relaxation
- Part of balanced anesthesia.
- Facilitation of ocular, abdominal and thoracic surgery.
- Smooth IPPV.
- Facilitation of esophageal foreign body removal.
Adverse effects of anesthesia
- Ideal anesthetic produces anesthesia without affecting other body systems:
- Does not exist.
- Generally depression of cardiovascular, respiratory, and other systems.
- 1 in 600 dogs die due to anesthesia related complications (overall risk) Anesthesia: complications - overview.
- Dose dependent.
- Myocardial depression, reduces cardiac output.
- Reduction in peripheral vascular resistance.
- Hypotension and compromised peripheral perfusion can result.
- Dose dependent.
- Hypoventilation via central depression of respiratory center and peripheral impedance to lung inflation (chest wall/diaphragm movement restriction):
- Central depression reduces the sensitivity of the respiratory center to carbon dioxide.
- Hypercapnia Hypercapnia results - reduced alveolar ventilation.
- Hypoxemia Hypoxemia - ventilation/perfusion mismatch Ventilation/perfusion mismatching and reduced ventilation.
- Anesthetics depress hypoxic drive to respiration.
- Adequacy of ventilation Anesthetic ventilators: overview linked to degree of intercostal muscle activity.
- Increase dead space/tidal volume ratio via reduction in tidal volume and increased dead space.
- Reduced functional residual capacity due to anteriorly displaced end-expiratory diaphragm and reduced serratus ventralis muscle tone.
- Anesthesia produces a transient decrease in renal blood flow and glomerular filtration rate (GFR) Glomerular filtration rate.
- Hypoperfusion/hypotension during anesthesia causes a reduction in GFR for the duration of the hypotensive episode.
- Renal blood flow can compensate to some degree via renal autoregulation which is effective over range of mean arterial pressure of 60-150 mmHg.
- Ability to compensate can be compromised by drugs administered.
- NSAIDs Analgesia: NSAID prevent renal autoregulation by inhibiting postaglandin release Analgesia: overview.
- Renal failure can result.
- Some agents can be hepatotoxic, eg halothane Halothane. Although reported in humans, this is extremely rare in dogs and cats.
- Hepatic vasomotor control is minimal and a decrease in hepatic blood flow may be due to a decrease in portal vein or hepatic artery pressure. This in turn can be due to reduced systemic blood pressure, increased hepatic venous pressure, or increased splanchnic resistance.
- Hypoperfusion could exacerbate hepatic injury.
- Stress response caused by any insult, including general anesthesia.
- Normally provokes fight or flight reaction.
- Sympathetic stimulation - cadiovascular stimulation. Initially lifesaving, later can lead to shock if prolonged.
- Endocrine response - increase catabolic hormones, eg cortisol.
- Metabolic response - mobilizes energy stores: hyperglycemia, lactic acidemia, lipid store mobilization. May lead to serious protein loss.
- Anesthesia impairs central thermoregulatory control mechanisms.
- Alters heat loss/gain and the ability of the body to compensate for changes.
- Hypothermia Hypothermia is common.
Methods for general anesthesia
- General anesthesia can be achieved by intramuscular, subcutaneous, intravenous, intraperitoneal and inhalational routes.
- Method is animal and procedure dependent.
- Independent of metabolism for termination of action and elimination.
- Allows rapid induction, quick alteration in depth, and rapid recovery.
- Analgesic actions.
- Does not depress cardiopulmonary function.
- Not irritant to any tissue.
- Inexpensive, stable, non-inflammable, non-explosive.
- Requires no special equipment for administration.
- No ideal agent presently available.
Considerations for type of anesthetic
- Animal species, breed, age.
- Physical status.
- Time required for surgery, its type and severity, surgeon's skill.
- Familiarity with anesthetic technique.
- Equipment and personnel available.
- Smooth induction of anesthesia.
- Infusion for maintenance.
- Non-cumulative agents required to avoid prolonged recovery.
- Advantages of potentially less cardiovascular depression, reduced stress response, no atmospheric pollution, and reduced equipment requirements.
- Requires IV catheter placement and more difficult to alter anesthetic depth rapidly. Often significant respiratory depression.
- Often significant respiratory depression.
- Renal and hepatic compromise can significantly prolong duration of action.
- Mainstay of general anesthesia.
- Can be used for induction anesthesia Anesthetic induction: overview - dependent on animal and agent used.
- Ease of use - rapidly able to alter depth of anesthesia.
- Rapid recovery.
- Dose dependent cardiorespiratory depression.
- Atmospheric pollution and occupational hazard to be considered.
- Some agents cause hepatic and renal toxicity.
- Route dependent on animal.
- Intraperitoneal for small animals.
- Intramuscular for wild animals/uncooperative animals.
Anesthetic records and patient safety checklist
- The Association of Veterinary Anaesthetists (AVA http://www.ava.eu.com/) has produced patient safety checklists and general anesthesia recording charts for use during the peri-anesthetic period. The RCVS Practice Standards Scheme recommends these resources.
- Two versions are available:
- Low ASA status:
- Suitable for shorter procedures and lower risk patients.
- Also incorporates the AVA safety checklist.
- High ASA status:
- Suitable for higher risk patients and more advanced monitoring.
- Low ASA status:
- The general anesthesia recording charts and patient safety checklists can be downloaded here:
- Low ASA status monitoring form - UK
- Low ASA status monitoring form - European
- High ASA status monitoring form
- AVA safety checklist - UK
- AVA safety checklist - European
- AVA checklist booklet
The AVA strongly advocates using a well designed GA monitoring record for every patient, as it not only forms part of the legal patient record but is also essential to ensure accurate handover of patient information and provides prompts to avoid missing critical steps in the anesthetic process.