Pulse oximetry is a very useful diagnostic and monitoring tool that has become commonplace in veterinary clinics. It measures the percentage of haemoglobin saturated with oxygen, and is an indirect measure of arterial oxygen levels.

pulse ox
Dog with pulse oximetry.

However, here are several important points to help you understand the limitations of pulse oximetry.

Causes for false readings

Falsely low readings:

  • motion artefact
  • peripheral vasoconstriction/low tissue perfusion from hypothermia or shock
  • pigmentation of mucous membranes
  • thick hair coat

Falsely high readings:

  • haemoglobin abnormalities (carboxyhaemoglobin and methaemoglobin, for example)

False sense of security

Pulse oximetry can give us a false sense of security. We hold on to the adage “95% and above means everything is going along swimmingly”, but that couldn’t be further from the truth:

Dog with pulse oximetry
Pulse oximetry can give us a false sense of security.
  • It does not detect hypoventilation or apnoea: it can take several minutes for apnoea to result in hypoxaemia that is detected on pulse oximetry; therefore, it cannot be used as a sole measure of respiratory adequacy. This is best measured by capnography.
  • A common misconception is the oxygen saturation will drop with patients with anaemia. This is incorrect. The haemoglobin present in the decreased number of red blood cells will still be saturated to normal levels. However, this cannot be interpreted as the patient having adequate oxygen delivery to its tissues.
  • One last point: due to the oxyhaemoglobin dissociation curve, any drop below 94-95% is significant and warrants investigation. At 95% SpO2 the partial pressure of oxygen in the arterial blood is 80mmHg (normal), but at 90% SpO2 the partial pressure is 60mmHg (severe hypoxaemia) – for only a small percentage decrease, there is an exponential reduction in arterial oxygen content. This is even more important when patients are receiving oxygen therapy as the patient’s SpO2 should be 99-100% normally. So when a patient has an SpO2 of 95%, but is on high rates of oxygen, then significant respiratory compromise/disease must be present for an SpO2 of 95% or lower to occur.
View your activity >

Leave a Reply

Be the First to Comment!

Notify of
avatar

wpDiscuz

related content

Celia Marr describes methods of treating the virus and spotting the clinical signs.

29 mins

Lotfi El Bahri discusses how this low-cost medication can be dangerous for small animals and how to spot signs of intoxication.

13 mins

The global nature of the equine industry makes it vital to consider the constant risk of notifiable exotic diseases. Josh Slater lists those relevant to the UK and emphasises the importance of awareness and risk-based biosecurity.

25 mins

Sarah Heath looks at the emotional motivation for dog bites and explains the profession’s role in improving understanding of canine mental health.

23 mins

RVN Samantha Feighery discusses veterinary nursing protocols to help stabilise equine patients after this gastrointestinal procedure.

20 mins

Patrick Pollock discusses a development in equine veterinary medicine that has led to a fundamental change in understanding the airway – both in terms of health and disease.

22 mins