Manual Techniques for Anaesthesia Monitoring

Introduction

Although monitors are becoming more available in veterinary practices, the most readily available source of monitoring during an anaesthetic is the veterinary surgeon, or veterinary nurse. Using our senses, it is possible to get a good indication of cardiovascular and respiratory system status, an idea about the temperature of the patient and most importantly we are the best monitors for depth of anaesthesia.

Depth of Anaesthesia

Before any procedure is performed on a patient it is vital to confirm with the anaesthetist that the animal is adequately anaesthetised, as well as during the procedure checking that the patient is neither to light or deep. There are a variety of parameters that can be used as discussed below.

Eye Position

In small animals, the eye position is frequently used to check the depth of anaesthesia. When a patient is adequately anaesthetised, the eye is rotated ventrally, exposing the white of the eye (i.e. the eye has effectively roled such that it is as if the animal is looking down). When an animal is too light the eye is in a central position, so that none or very little of the white of the eye is showing. However, this is also true if the patient is too deeply anaesthetised. It is therefore important to look at other parameters to fully assess the depth.

In horses, this cannot be used as an indicator for the depth of anaesthesia, as the eye rolls throughout an anaesthetic and so can be found in any position regardless of the depth. Therefore, other parameters should be used to assess depth of anaesthesia in these patients.

Palpebral Reflex

Another easy indicator of depth of anaesthesia to assess is the presence or abscence of the palpebral reflex. This is tested by gently touching the lateral or medial canthus of the eye and observing whether there is a blink reaction. If the reflex is present, i.e. there is a blink reaction, this indicates that the patient is too light. If the reflex is negative this indicates the animal is either adequately anaesthetised or too deep. Again, this is a parameter that should be used in conjunction with others to fully assess anaesthetic depth.

Heart Rate

As discussed later on, heart rate is easily monitored and frequently performed. If the patient is too light then there is usually an increased heart rate. If the patient is too deep then there is usually a decrease in heart rate.

Respiratory Rate

Respiratory rate can be easily monitored and is important in assessing anaesthesia as if an inhalation agent is being used then it is important to check that the patient is breathing sufficiently to be receiving and exhaling the anaesthetic agent. If the patient is too light then usually there is an increase in respiratory rate, where as if the patient is too deep then there is usually a decrease in rate.

Other parameters include jaw tone, movement and pain response.

Cardiovascular Status

There are a number of different parameters that can be used to assess the cardiovascular status of the patient during an anaesthetic.

Mucous Membranes

• This is a good indicator of perfusion of the patient.
• Ideally, mucous membranes should in pink in colour and moist to touch.
• If the membranes become darkened (Red-Brick Red) it indicates local congestion or haemoconcentration.
• If the membranes become paler it indicates hypoperfusion or anaemia.
• If the membranes become blue it can indicate cyanosis.
• Commonly observed membranes include the oral gums, and anus.

Capillary Refill Time

• This is usually assessed at the same time as mucous membrane colour.
• Again, it is a good indicator of patient perfusion.
• Ideally, capillary refill time (CRT) should be 1-2 seconds.

Pulse

• When assessing pulses, not only rate should be noted but the quality of the pulse.
• It indicates heart rate and rhythm, and some indication of blood pressure.
• Whenever possible, it should be assessed while auscultating the heart to check that they are synchronised.
• Pulse quality includes amplitude, length of pulse wave and time between waves.
• Common pulses detected are femoral, sublingual, metacarpal and metatarsal.

Cardiac Ausculatation

• Used to assess heart rate, rhythm and presence of any mumurs.
• Can be done via a stethoscope or an oesophageal stethoscope.
• Can be used to assess the heart intergity, responses to noxious stimuli and depth of anaesthesia.

Respiratory Status

In most anaesthetic procedures, inhalation agents are used for maintainance and so it is important that the patient respiratory rate and rhythm is sufficient to allow for effective exchange of agent within the alveoli. However, it is also important during any anaesthetic to make sure the patient does not become cyanotic.

Respiratory rate and rhythm can be assessed by watching either the patient's thorax for movements, watching the reservoir bag on the breathing system (if attached to an anaesthetic machine) and auscultation via a stethoscope or oesophageal stethoscope. Changes in rate are also related to depth of anaesthesia and response to pain.

It is also important to look at the size of the breaths being taken to assess tidal volume as it will play a role in the intake of anaesthetic agent in patients.

During auscultation, or just when listening to a patient, abnormal sounds may also be detected.

Temperature

One of the biggest concerns with anaesthesia is the risk of hypothermia to patients. It is therefore important to continually monitor the temperature of the patient throughout the procedure as well as post operatively. This can be done either manually using a thermometer rectally, or using a thermometer probe that can be found on some monitors placed either rectally or down the oesophagus if the procedure permits.