Anaesthesia Notes

Anaesthesia

1. Introduction

General anaesthesia is controlled depression of the CNS to produce a lack of awareness of painful inputs (nociception). There should be minimal depression of hind-brain functions, such as the cardiovascular centre.

Local anaesthesia provides local analgesia. It may be given topically, infiltratively, conductively or as an epidural.

Anaesthesia allows pain free surgery and provides restraint and relaxation of patients, allowing better surgical access.

The triad of anaesthesia includes hypnosis (unconsciousness), anti-nociception (analgesia) and muscle relaxation.

The level of CNS depression can be assessed in anaesthetised patients by specific signs related to muscle relaxation, such as the position of the eye, which should be relaxed and rotated rather than centrally fixed. Respiratory rate, heart rate and blood pressure are signs related to brain stem depression.

Risks of anaesthesia include cardiovascular depression, reduced oxygen tissue delivery, cardiac dysrhythmias and cerebral hypoxia.

The ASA classification system can be used to assess risk to patients undergoing anaesthesia based on the presence of pre-existing disease. There are 6 classifications:

• I – normal healthy patient (elective procedures)

• II – pre-existing disease but no discernable systemic signs (skin tumour, fracture without shock)

• III – pre-existing disease with mild systemic signs (fever, dehydration, mild cachexia)

• IV – pre-existing disease with severe systemic signs (uraemia, toxaemia, emaciation)

• V – moribund patient not expected to survive with or without intervention (extreme shock, terminal malignancy)

• VI – a declared brain dead patient whose organs are being removed for donor purposes.

Risk can be minimised by good planning. Giving oxygen before the induction of anaesthesia can help, as well as keeping the patient warm during and after anaesthesia. Monitoring anaesthesia and recovery is also important. Anaesthesia record sheets are legal documents.

Dissociative anaesthesia is induced by drugs such as ketamine that dissociate the thalamocortic and limbic systems.

Normally, patients are assessed and a catheter placed. The patient is then induced using an injectable agent and intubated onto oxygen. They are then connected to an anaesthetic machine and supplied volatile anaesthetic carried by oxygen.

2. Premedication

Premedication will affect the characteristics of the ensuing anaesthetic and should always be tailored to the individual animal.

Premedication aims to provide sedation and axiolysis facilitating handling of the animal, reduce the amount of other anaesthetic agents required, provide a balances anaesthetic, provide analgesia and to contribute to a smooth, quiet recovery from anaesthesia.

Premedication drugs should have reliable sedation and anxiolysis, minimal effects on the cardiovascular system, cause minimal respiratory depression, provide anaesthesia and ideally be reversible.

1. Alpha-2 agonists

Alpha-2 receptors are located all over the body both pre- and post-synaptically. They are usually stimulated by norepinephrine. Alpha-2 agonists case sedation, bradycardia, low respiratory rates and fluctuation in blood pressure.

Medetomidine and dexmedetomidine are used in cats and dogs.

Xylazine, detomidine and romifidine are used in horses.

Xylazine and detomidine are used in cattle.

Xylazine is relatively unselective for the alpha-2 receptor, and therefore has reduced cardiovascular safety due to its agonistic effect at alpha-1 receptors in the heart.

Alpha-2 agonists provide good analgesia and profound, dose-related sedation. The duration of analgesia provided by a 10µg/kg dose of dexmedetomidine is approximately 1 hour.

The dose of induction and maintenance agents required after administration of alpha-2 agonists are dramatically reduced in small animals.

Alpha-2 agonists produce a biphasic effect on blood pressure – an initial increase followed be a return to normal or slightly below normal values. Heart rate is decreased throughout administration – in dogs it is 45-60bpm and in cats 100-120bpm. There are minimal effects on the respiratory system in healthy animals. Urine production is increased due to a reduction in vasopressin and renin secretion. Endogenous insulin secretion is also reduced, leading to a transient hyperglycaemia. Liver blood flow and rate of liver metabolism is reduced.

Sedation and anaesthesia of alpha-2 agonists is rapidly reversed by administration of atipamezole, a specific alpha-2 adrenergic receptor antagonist. This is best given intramuscularly for a smooth recovery.

Domosedan gel is an alpha-2 agonist gel given oro-mucosally for sedation.

2. Phenothiazines

Acepromazine is the only licensed phenothiazine.

The quality and reliability of sedation from acepromazine administration can be improved by also giving an opiod. This also provides analgesia – acepromazine alone does not provide analgesia.

Acepromazine is an antagonist of alpha-1 adrenoceptors. It can cause peripheral vasodilation and a fall in arterial blood pressure. This also results in heat loss.

It is long-lasting and non-reversible.

Acepromazine has anti-arrhythmic properties which are advantageous in anaesthesia.

Acepromazine is contraindicated in breeding stallions, as it can cause paralysis of the retractor penis muscle.

Acepromazine should also be used with caution in Boxer dogs, as they are uniquely sensitive to even small doses of acepromazine.

Acepromazine is also a dopamine antagonist.

Acepromazine is also available in gel (sedalin) and tablet (ACP) form.

3. Benzodiazepines

Midazolam or diazepam can be used as premedication.

Benzodiazepines produce minimal or no sedation in healthy cats and dogs, and may even cause excitation due to loss of learned ‘inhibitory’ behaviour. They are therefore given in combination with other sedatives.

In dogs, they are often combined with opiods.

In cats, they are often combined with ketamine.

Benzodiazepines have minor effects on the...

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