Why do we need to use a vaporiser?
Modern anaesthetic agents are very potent, so it is easy to inadvertently overdose an animal. To avoid this, and also to ensure that onset (induction) of anaesthesia is smooth and that an appropriate depth of anaesthesia is produced, an anaesthetic vaporiser should be used.
These devices are designed to deliver a precise concentration of anaesthetic, even when the temperature and rate of gas flow are changed. A carrier gas flows through the vaporiser, and this then delivers a set concentration of anaesthetic to the animal. As all anaesthetics depress respiration, it is best to use oxygen as the carrier gas. Anaesthetic vaporisers are usually mounted on an anaesthetic machine.
What is an anaesthetic machine?
Oxygen is supplied either from pressurised cylinders mounted on the anaesthetic machine or from hoses from larger, centrally located cylinders.
A guide to the anaesthetic machine
Checking an anaesthetic machine
Filling an anaesthetic chamber
Small rodents are often anaesthetising using an anaesthetic chamber. The animal is placed into the chamber and filled with anaesthetic vapour in oxygen from the anaesthetic machine.
With all anaesthetic agents, animals first become slightly ataxic (wobbly), may then go through a period of involuntary excitement, then lose their righting reflex and become immobile. To minimise this period of excitement, we should aim to fill the chamber rapidly, so the animal loses consciousness quickly.
Calculate the volume of the chamber you intend using (for a mouse this should be less than 2 litres). The aim is to reach the safe maximum induction concentration of the anaesthetic as rapidly as possible (5% isoflurane or 8% sevoflurane).
Using a flow of 4 litres per minute will achieve this within 1 minute in a chamber of fewer than 2 litres volume.
Using a face mask
Following induction of anaesthesia the animal can be removed from the chamber and brief (<30secs) procedures carried out.
It is usually more convenient to maintain anaesthesia by placing the animal on a face mask. Suitable masks can either be purchased commercially or constructed from plastic syringes.
When the animal is transferred from the chamber, the anaesthetic concentration should be reduced (to 1.5-2.5% for isoflurane, 3.5-4.5% for sevoflurane) and the gas flow rate turned down.
Although it is possible to induce anaesthesia with inhalational anaesthetics in larger species, the animal may resent being restrained and having a face-mask placed over its nose. For this reason, anaesthesia is often induced with injectable agents and then maintained using a volatile agent, or a heavy sedative or tranquilliser is given as pre-anaesthetic medication (“pre-med”). Once it is heavily sedated, anaesthesia can be induced using a face mask.
Oxygen flow rates
When anaesthesia is maintained using an inhalational agent using a face mask, sufficient gas flow must be provided so that when the animal breathes in, it only inhales anaesthetic vapour and not the surrounding room air. Sufficient gas also needs to be provided to remove exhaled carbon dioxide from within the mask.
To achieve this the flow rate (in litres per minute) of anaesthetic gases should be approximately 3 times the volume that the animal breathes in one minute (the animal’s minute volume). This can be calculated by multiplying the respiratory rate by the volume of one breath (the tidal volume).
Most animals have a tidal volume of 7-10ml/kg of body weight. So for example, a 200g rat would have a volume of approximately 2ml, and when anaesthetised is likely to breathe at a rate of around 60 breaths per minute. Its minute volume would be 120ml (2 x 60) and so a flow rate of 360ml/min should be sufficient to maintain anaesthesia, and prevent the rat rebreathing gas that is has exhaled. A 25g mouse would need only 40-50ml/min.
A practical difficulty of using these low flow rates (<100ml) is that some vaporisers become inaccurate, although this can be overcome by adjusting the setting depending upon the animal’s response. A second problem is that most of the commercially available systems used to remove waste anaesthetic gas apply powerful suction around the face mask. This can result in excessive gas being removed, and anaesthesia no longer being maintained, unless flow rates of 1–2 litres per minute are used.
To avoid this, an alternative is to use specially designed low flow masks. It is also possible to purchase specialist vaporisers that can deliver anaesthetics using very low flows.