Administration of anaesthesia

The use of injectable agents requires very little specific equipment and the administration techniques are no different from those of any other injection (see Ch. 1). The use of inhalational agents (i.e. gases or volatile liquids) requires the use of complicated and expensive anaesthetic machines and monitoring equipment and it is vital for the veterinary surgeon, who bears ultimate responsibility for the survival and recovery of the patient, to understand how the system works and is maintained.

It is not the responsibility of this book to describe the range and pharmacology of the anaesthetic agents used in practice; the reader is recommended to consult a dedicated textbook on anaesthesia.

Preparation for anaesthesia

Procedure: Patient preparation for anaesthesia

Patients admitted for anaesthesia may fall into one of four groups:

• Normal healthy animals for elective routine procedures (e.g. neutering).

• Apparently normal animals to undergo diagnostic procedures.

• Sick animals for treatment – may present an increased anaesthetic risk.

• Emergency cases – pre-anaesthetic care and assessment may be minimal as they will be admitted quickly, but support and monitoring are vital.

1. Action: The patient should be starved for 12 hours prior to surgery and owners should be advised of this when they book their appointment.

Rationale: Animals that are not fasted may vomit as a result of the anaesthetic drugs and may inhale their vomit. During anaesthesia the swallow reflex is lost and the airway is thus unprotected.

NB This does not apply to rabbits as they are unable to vomit and if starved may suffer a fatal hypoglycaemia. Encouraging them to eat to within an hour of the anaesthetic may prevent postoperative anorexia, which may lead to gut stasis.

2. Action: If possible all patients booked in for elective procedures should be encouraged to urinate and defaecate before being brought to the surgery. Owners should be asked to do this.

Rationale: This is not easy but is more pleasant for everyone involved in looking after the animal during the day.

3. Action: Weigh the patient – should be done for all patients.

Rationale: This is essential for the accurate administration of premedicant and induction drugs.

4. Action: Take the client and patient into a consulting room and explain to the client exactly what will happen to their animal during the surgical procedure. You should also give an estimate of cost and this should be written down on the case records.

Rationale: You may have already discussed cost and the details of the procedure, in which case it may be part of the practice’s protocol for a veterinary nurse to admit the animal. Recording your estimate may prevent disputes later on.

5. Action: Ask the client to sign a consent form.

Rationale: This may be done by a veterinary nurse. The client, or an agent of the client, must read, understand and sign the consent form – and must be over 18 years of age.

6. Action: At some time you should have taken a full history and carried out a detailed clinical examination (see Ch. 2).

Rationale: If the patient is booked in to undergo a routine procedure (e.g. spaying) you may not have seen the animal for some time. It is important to check that the animal is healthy before proceeding. If the animal is sick you will have carried out a detailed clinical examination prior to deciding to admit the animal.

7. Action: Collect a blood sample for a pre-anaesthetic blood test.

Rationale: This should be a routine procedure in any practice, but many practices will not do the test in young animals having routine operations such as neutering. The test is used to assess whether there are any underlying problems that may increase the anaesthetic risk. Parameters making up the pre-anaesthetic blood test are shown in Table 8.1.

Table 8.1

Biochemical and blood parameters that may be measured prior to anaesthesia

8. Action: Administer your chosen premedicant drugs. The aim of the drugs is to calm the animal, enabling easy handling, and to allow for a smooth induction using a small dose of induction agent.

Rationale: Choice of drugs depends on practice protocols and the condition of the patient. Routine predication may be administered by your veterinary nurse.

9. Action: The patient should then be placed in a warm, stress free kennel to await the procedure.

Rationale: A calm, happy patient is less of an anaesthetic risk than a nervous, worried one.

The anaesthetic machine

There are many designs of anaesthetic machine from simple small ones to large cumbersome pieces of equipment that are very often older or bought from hospitals. Whatever their shape and size, they are all designed to deliver accurate volumes of carrier gas and vaporized volatile liquids to the patient to produce anaesthesia. Figure 8.1 shows a generic form of a machine. Nowadays many practices have done away with oxygen cylinders attached to the machine and use piped oxygen from large cylinders kept outside the building. These are more convenient and more economical, although there may still be a small cylinder attached to the machine in case of emergency. There are also machines that extract oxygen from room air. Table 8.2 describes the parts and functions of the anaesthetic machine.

Table 8.2

Parts of the anaesthetic machine

Component	Description	Function
Gas supply	Oxygen cylinders are black with a white collar. Nitrous oxide cylinders are blue. Smaller cylinders, such as E and F, can be incorporated into the anaesthetic machine. Larger cylinders, J and G, may be kept separate and gas supply to the anaesthetic machine is through pipes in the wall	Supplies fresh oxygen and nitrous oxide to the patient. An E-sized cylinder contains enough oxygen to supply a 20 kg dog for 3 hours
Pressure-reducing valve or regulator	Usually incorporated into the yoke and is therefore impossible to identify	Reduces the pressure of the gas leaving the cylinder to ensure a constant flow to the anaesthetic machine regardless of the pressure changes within the cylinder. It provides a safe operating pressure for the machine
Pressure gauge	Usually attached to the anaesthetic machine. As the pressure in the cylinder falls so does the pressure reading, indicating the amount of gas remaining in the tank. When using nitrous oxide, no pressure change will be seen until the cylinder is almost empty because the pressure does not fall until all the liquid has evaporated. This is not a reliable method for measuring the cylinder content of nitrous oxide – the cylinder should be weighed instead	Indicates the pressure of gas being delivered. It will read zero when the tank is empty or when the tank is switched off and the gas in the pipe evacuated
Flowmeters or rotameters	Consist of a tapered, glass tube with the flow rate written on it and either a glass ball or bobbin in the tube. Provided that the ball or bobbin can rotate freely within the tube, it will give an accurate reading of the gas flow rate. A bobbin must be read from the top, a ball from the middle. Flowmeters are gas specific and control knobs are usually colour coded: they should never be overtightened as the valve seat is easily damaged	Control and measure the flow of gas in litres per minute
Vaporizers: uncalibrated	The Boyle’s bottle (sometimes seen on very old machines) is an example of an uncalibrated vaporizer: whilst output can be varied, it fluctuates with temperature and gas flow changes	Deliver concentrations of volatile anaesthetic, in a vapour form, to the patient
Vaporizers: calibrated	The Tec, Penlon and Plenum vaporizers (Fig. 8.1) are calibrated. They remain accurate despite temperature and gas flow changes. They are agent specific. High and low flow rates may cause problems with accuracy	Deliver a known concentration of anaesthetic vapour to the patient
Back bar	Flowmeters and vaporizers can be attached to the back bar in series. This allows more than one volatile agent to be available. The ‘Selectatec’ manifold allows swift attachment or removal of Tec 3 and 4 vaporizers	Supports the flowmeters and vaporizers
Common gas outlet	Location varies between anaesthetic machines	Connects anaesthetic circuits, ventilators or oxygen supply devices to the machine
Oxygen flush valve	Oxygen reaches this valve swiftly, bypassing the vaporizer. High flow rates are produced. The valve may be locked open in some cases, by rotating it 90°	Provides oxygen in emergency situations and purges anaesthetic from the circuit before disconnection to minimize pollution
Low oxygen alarm	An alarm sounds or, in some cases, a light flashes when oxygen levels become dangerously low	Warns the anaesthetist of low oxygen levels