Depth of Anaesthesia: Part 1

An animal patient under anesthesia
An animal patient under anesthesia
Author:
Nichola Frampton
Published:
December 2, 2020

The use of anaesthetics has been around for a long time and is continuing to advance in both its complexity and its technology. If we go back to 1875, the well-known Charles Darwin used ether to “anaesthetise” Venus Flytraps. He found that by varying the amount of ether applied to the Flytrap, the length/duration of “anaesthesia’ would differ. A few years earlier, in 1846, Dr Morton had created the Morton’s inhaler. With this instrument, ether would be applied to a song within the glass bottle/ball and the patient would then breathe it in. We have come a long way since then in terms of technology but the basis of all our modern anaesthetics and anaesthetic systems still comes from these very early experiments.

Stages of anaesthesia

Initially when ether was used as the sole anaesthetic agent, the depth of anaesthesia could be classified into 4 basic stages.

Stage 1:

This stage is the initial administration of the induction agent until the point of loss of consciousness.

Stage 2:

This is often known as the excitement/delirium or involuntary movement stage. It starts from the initial loss of consciousness until the onset of a regular breathing pattern.

Stage 3:

This is the stage of surgical anaesthesia. Again, the patient is unconscious and progressively develops depression of their reflexes. It can be divided itself into 3 parts: light, medium, and deep.

Stage 4:

The last of the anaesthetic stages is characterised by further depression of the cardiovascular and respiratory systems and is often linked to impending respiratory and circulatory failure.

As anaesthetists, our animals go through stages 1 – 3 (we try to avoid stage 4) but the amount of time they spend in each stage will depend on your premedication, induction technique, maintenance plan, and the procedure itself.

Each of those stages can be related to anaesthetic depth and the methods by which we can assess anaesthetic depth are extensive. Heart rate, respiratory rate, and blood pressure are all useful indicators of anaesthetic depth. If the animal is too light then often we see an increase in HR, RR, and BP. However, it is important to recognise that hormonal and other physiological changes can also alter these parameters. Therefore, if we identify changes in these parameters our next step is to look at other factors for depth assessment. These can include eye position, muscle tone and reflexes.

Eye position particularly in dogs and cats is a very useful way of assessing anaesthetic depth. As the animal changes plane of anaesthesia, their eye position will also change. For example, when they are in stage 3, their eye is often rolled ventromedially. As they enter a lighter plane of anaesthesia again, then their eye will roll back to a more central position. It is important to note though that as an animal becomes more deeply anaesthetised in stage 3, their eye will also move more centrally. So, it is important to properly identify whether the animal is too deep or too light. To do this we have to assess the animal overall including assessment of some of the other reflexes, which can include palpebral reflex, pupillary light reflex, nystagmus, withdrawal reflex, ear flick, jaw tone, pharyngeal reflex, laryngeal reflex and visceral reflex. Each of these reflexes will become stronger as the animal moves towards a lighter plane of anaesthesia.

If you assess all the animal’s reflexes and combine these with your assessment of the animal’s physiological parameters, then you can get an idea as to whether the animal is too light or too deep. Once we know how to assess the depth of our patient, we can use this to alter our settings on either the vaporiser, the flowmeter, or any infusions that we are administering.

In part 2 we will go back and revise flow rates and breathing circuits.

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