Shut in the darkness unloved and underused...... Part 1

............The tragic life of an anaesthetic monitor

I am guessing there are hundreds of anaesthetic monitors across the country suffering the tragedy of underutilisation. They were created with such high hopes and enthusiasm that they would have the potential to improve patient safety and care and yet without support and training they are pushed to the side, silently beeping, shut away in the darkness!

So why are we underutilising them? Why do we find it hard to get them out of the cupboard?

When we travel around to practices to provide in house CPD, we are often told that:

  1. "I want to use them but I don't understand what they are telling me"

  2. "The procedures we see are too short, it takes longer to set up the monitor than perform the procedure"

  3. "The rest of the team don't see the importance of using them"

  4. "We do use them but only in our critical patients"

We know all patients should have a dedicated person whose role is solely to monitor anaesthesia. Brodbelt reported that adverse events can be prevented with adequate patient monitoring and documentation to help highlight trends and initiate appropriate treatment.

Whilst we are the best piece of monitoring equipment available if we use a combination of hands on physical monitoring and anaesthetic monitoring equipment it can help us identify common complications and initiate early intervention, improving patient safety and outcome.

In the first of this 4 part series we are going to focus on pulse oximetry

Pulse oximeters

Pulse oximetry is considered a significant technologic advance that has improved patient safety. Most of us have a pulse oximeter we can use and because they are quick and easy to attach to the patient these machines are often utilised regularly. They detect hypoxaemia earlier than human eyesight and are non-invasive and portable.

In human hospitals anaesthetic complications have been avoided with the use of pulse oximetry, this is highlighted through the work done by the charity, Lifebox who work with resource poor countries to ensure that all patients undergoing general anaesthesia anywhere in the world, receive at least pulse oximetry.

Pulse oximeters give us information about the % of haemoglobin saturated with oxygen using a probe that shines a light through the tissue over pulsated blood flow (which means that the SpO2 number you see will be based on arterial blood levels).

In humans they are beginning to use pulse oximeters which utilise reflectance technology where the probe is placed on a flat surface, such as the forehead. This can be extremely useful in sedated patients or critical patients where the tongue is not easily accessible or if the patient has pigmented hair and skin that does not allow the transmission probe to work.

Pulse oximeters also give us a pulse rate and some also give us a waveform called plethysmography (Pleth).

The pleth can be used to assess accuracy of equipment and detect abnormalities with pulse rhythm and quality

The pleth can also help early detection of some cardiac arrhythmias by comparing the waveform with the ECG.

Pleth changes in response to ECG is this due to a reduction in cardiac output/peripheral perfusion? Check the peripheral pulse, does it match the heart?

The Confidential Enquiries into Perioperative Small Animal Fatalities (CEPSAF) study found that the use of pulse oximeters resulted in a reduction in odds of anaesthetic related deaths.

Some of the largest organisations in anaesthesia (Association of Anaesthetists of Great Britain and Ireland, the American Society of Anesthesiologists and The Association of Veterinary Anaesthetists) state that pulse oximetry is essential for the safe conduct of anaesthesia because a failure to recognise hypoxaemia is a major cause of preventable death.

As always there are limitations....

A weak signal, indicated by a low amplitude of the waveform, can be caused for several reasons

These machines are not perfect and it is important that we learn how to troubleshoot what we are seeing. They can struggle to pick up a signal in certain situations including:

- Vasoconstriction (medetomidine, hypothermia, shock)

- Hypoperfusion (low blood pressure)

- Movement

- Smoke inhalation

- Some lighting conditions

- Pigmented skin

- Anaemia

Top tips: Pulse ox probes don't work as well when the tissue dries out , so try wrapping the tongue with a warm wet swab to improve signal.

It may be useful (depending on the procedure and level of pigmented skin) to position the probe at alternative location such as the lip, pinna, toe, flank, vulva and prepuce.

Whats with maintaining oxygen % above 95%???

We are all taught that over SPO2 reading should be over 95%, but why is this?

Looking at mucous membrane colour alone to assess oxygen saturation is not an accurate indicator; an abnormal bluish discolouration of skin and mucous membranes from high levels of deoxygenated haemoglobin, is a late indicator of decreased oxygenation and can be very subjective.

Pulse oximetry, provides an objective measurement of the patient’s oxygenation and can alert us to a problem early.

The oxygen dissociation curve (ODC)

Normal partial pressure of oxygen (PaO2) in arterial blood for a patient breathing room air is 86.5-97.7mmHg in dogs and 101-112.5mmHg in cats. If we look at the ODC we can see that a pulse oximetry value of 95% or higher (left hand axis) is normally clinically acceptable, whereas a value of 90% or lower is a red flag. On the ODC, a Sao2 value of 90% correlates to a Pao2 level of 60 mm Hg. Pao2 pushes or loads the oxygen onto hemoglobin. So if this level isn’t adequate, suspect the patient’s overall oxygenation is abnormally low. By administering oxygen we have the potential to increase our patients PaO2.

Hyoxaemia verses Hypoxia

As we have already said, the pulse oximeter measures the percentage of haemoglobin saturated with oxygen. When the % falls below 95% it indicates that our patient is hypoxaemic (low oxygen in the blood), it doesn’t tell us if we should be concerned about our patient developing hypoxia (Deficiency in the amount of oxygen reaching the tissues.

For this reason, a patient may not be hypoxaemic (pulse Ox reading >95%) but may be hypoxic – e.g. poor circulation. The blood is well oxygenated but the circulation is insufficient to distribute the oxygenated blood. A human intensive care study showed that pulse oximetry was more accurate when the systolic blood pressure (SAP) was greater than 80 mmHg and the lower the SAP, the more inaccurate the readings were, supporting the importance for monitoring and maintaining blood pressure in our anaesthetised patients.

Pulse oximetry during CPR

  • Pulse oximetry during CPR: During true cardiac arrest, when there is no pulse the pulse oximeter will be unable to pick up a signal. Pulse oximetry is useful during CPR to assess the adequacy of chest compressions in producing a pulse

Pulse oximetry in the recovery patient

The Cepsaf equiry highlights most anaesthetic related deaths occur in the recovery phase. Hypoxaemia is one of the most dangerous complications that may be encountered during general anaesthesia and the recovery period. Crude assessment of mucous membrane colour to accurately assess the patient’s oxygenation state can be difficult for even the most trained eye and in the recovery phase when we move our patients from breathing 100% oxygen to 21% room air they are at a greater risk of desaturation.

Probe maintenance

Pulse oximeter probes pose a significant risk for bacterial contamination. In a human study looking at bacteria cultured from pulse ox probes 44 sensors (66%), including 20 that had been cleaned with alcohol or an antibacterial/antiviral agent had positive cultures. Among the isolated organisms were Staphylococcus aureus, Staphylococcus haemolyticus, Enterococcus faecalis, and Klebsiella oxytoca. For this reason it is important that you contact the manufacturers of your pulse oximeter and confirm their recommended cleaning protocols.


Earlier this year we donated a pulse oximeter to the World Veterinary Society for their Goan training hospital.

The fund were raised through our monthly webinar series.

If you still want to know more about pulse oximetry, check out Volume 5 Issue 7 of Feline Focus.

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