Principles of Anaesthesia

Introduction

Anaesthesia can be defined as states of unconsciousness produced by controlled reversible drug induced intoxication of the central nervous system in which the patient neither perceives nor recalls noxious stimuli. The word is derived from the Greek for ‘without feeling’. It can be classified into two major groups – General or Local/Regional.

Indication for Use

Anaesthesia is indicated for a number of reasons and protocols should take into consideration the procedure requirements, patient needs and any current medical conditions that may affect the anaesthetic.

The most common indication is when a painful procedure such as surgery or a diagnostic test is to be performed, to reduce suffering by the patient but also reduces the risk to the person performing the procedure. However, it can also be used for the control of convulsions where it is used to allow stabilisation of the patient and gives time for treatments to take effects. It can also be used for euthanasia.

Protocol Selection

When designing an anaesthetic protocol, it is important to look at the requirements of both patient and operator. Within these there are a number of considerations:

• Mode of action
• Duration of action
• Preoperative medical conditions
• Recovery duration and quality
• Available equipment

Preoperative Period

Preparation

During the preoperative period, a full physical examination of the patient should be performed. This is to allow the anaesthetist to choose the correct agents to induce and maintain anaesthesia throughout the procedure. It is also important to decide whether the patient is stable enough to undergo an anaesthetic or whether they require any treatments to correct any imbalances or dehydration that may be present that may affect the anaesthetic.

Most animals are starved for at least 12 hours before an anaesthetic. However, in some species this is not ideal and so they are starved for less time or not at all, e.g. rabbits and small furries.

Preoperative preparation may also include clipping, cleaning out the mouth and administration of anti-tetanus and/or antibiotics.

Premedication

Premedication is often given as it helps to calm and control the animal before induction and reduces the amount of induction agent given. However, sometimes if the patient is that unwell it may be that it is not necessary to premedicate it. The premedication often includes an analgesic and/or low dose sedative e.g. opioid and alpha 2 agonist. Other benefits of premedications include pain relief and reduction in side effects of other agents.

Induction

• Induction of anaesthesia can be via either an intravenous agent or inhalation depending on the species and status of the patient.
• It is important to have a calm, quiet environment and the appropriate monitoring equipment ready as well as a readily available oxygen source.
• Commonly after the patient has been induced, placement of an endotracheal tube occurs so that the patient can be maintained on inhalation agents.

Maintenance

• Depending on the procedure, anaesthesia can be maintained either with an inhalation agent or an intravenous anaesthetic via a Total Intravenous Anaesthesia (TIVA).
  • Maintainance on inhalation agents requires the placement of an ET tube as discussed.
• During an anaesthetic it is important to monitor the patient to assess depth of anaesthesia as well as cardiovascular and respiratory status as well as temperature among others.
• It may be beneficial to have a patient on fluids during a procedure e.g. during a long procedure or in geriatric patients. Common fluids used are Hartmanns or 0.9% Soduim Chloride.
  • If there is a concern about hypothermia in a patient that is receiving fluids then wrapping the giving set around a "warm hand" will warm the fluids prior to reaching the patient thereby aiding prevention of hypothermia.

Post Operative Considerations

Monitoring of the patient should continue, even after the anaesthetic agent has been switched off. If the patient has had an ET tube placed then it is vital that the animal is observed until removal of the tube. When to remove the tube varies between species, but it should not be removed until the animal is breathing spontaneously and unaided. Dogs tolerate the placement of an ET tube well and it can often be left in until after the dog is swallowing and in some cases once the dog is in sternal recumbany. Cats however are less tolerant of ET tubes and so it is important to remove the tube before the cat swallows as it can cause trauma to the larynx. Horses have the tube removed as soon as possible, but a nasal tube is placed to help with any drainage.

Recovery after an anaesthetic should be in a quiet and calm environment. This allows the patient to full recover slowly allowing for a smoother recovery period. If the animal has undergone a timely procedure or the patient is a small and so likely to have a degree of hypothermia then it may be appropriate for them to be placed into an incubator (if available) or on heatpads or warm hands. To try and prevent such a drop in body temperature during the surgery heatpads or warm hands can be used.

Another important post-operative consideration is analgesia. Many procedures are painful and so it is important to aid recovery and make the animal patient that analgesia is continued after the procedure has been completed. It is important to remember that some analgesics may have a sedative effect also so patients should be continually monitored.

Assessment of pain should be carried out at regular intervals to adjust analgesia appropriately. Factors to observe include :-

• Attitude and behaviour.
• Degree of vocalisation.
• Response to stroking and gentle examination and manipulation of wounds and surrounding area.