August 2013

In this VETgirl online veterinary continuing education blog, we review the importance of the pulse oximeter in veterinary medicine.

The Role of the Pulse Oximetry in Veterinary Medicine

Pulse oximetry is a non-invasive method of measuring the oxygen saturation of hemoglobin (SpO2) in arterial blood. It it perfect?  Is it the gold standard?  No: an arterial blood gas (ABG) is considered the gold standard.  Due to the complexity of arterial blood gas sampling, it is often reserved for critically ill canine patients, leaving pulse oximetry the common bedside test for evaluating oxygenation. That said, in VetGirl’s opinion: pulse oximeter + venous blood gas (VBG) = ABG.

So, how exactly does the pulse oximeter work?
The pulse oximeter calculates the percent of oxygen saturation of hemoglobin in arterial blood using spectrophotometry. The probe passes light through the tissues at two different wavelengths: a red and infrared light absorption.  Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through. Deoxygenated (or reduced) hemoglobin absorbs more red light and allows more infrared light to pass through. The difference in light absorption is calculated and the final figure is displayed as a percentage (SpO2%).

So, why should you care? Because with the non-invasive pulse oximeter, you can correlate your  SpO2% to the partial pressure of oxygen in the blood (PaO2) based off the oxyhemoglobin dissociation curve. Bear with us and think back to third-year veterinary school…

The oxyhemoglobin dissociation curve (OHDC) relates oxygen saturation (SpO2) and partial pressure of oxygen in the blood (PaO2).  This is determined by “hemoglobin’s affinity for oxygen”… in other words, how readily hemoglobin acquires and releases oxygen molecules from its surrounding tissue.

Although many of us would be thrilled with a 90% as a grade on an exam in veterinary school, it’s actually a really, really bad pulse oximeter reading. Looking at the relationship between SpO2 and PaO2 on the OHDC/graph, note that an SpO2 of 90% (e.g., your pulse oximeter reading seen on the Y axis) is really quite bad, correlating with only a PaO2 of 60 mmHg (on the X axis).  As your PaO2 in your blood should be about 80-100 mmHg (at sea level), that means that your patient is severely hypoxemic.

In a future blog, we’ll talk about how just because your patient’s gums are pink, doesn’t mean it isn’t hypoxemic. As the pulse oximetry of 90% shows, you only have about 2/3 of the amount of oxygen that you should! Cyanosis doesn’t show up until a PaO2 of about 40-50 mmHg.

So, how do I use a pulse oximeter on my patient?

A pulse oximeter has a probe which is placed on the patient.  The common locations include hairless, minimally-pigmented areas of the body including the lip, pinnae, prepuce, vulva, and interdigital space.  Essentially, the probe can be placed on any area with a pulsating arteriolar bed.


The machine you are using may solely display a reading on the screen or it may be more advanced.  Some machines have a waveform reading, while others have a simple pulse rate monitor.  If a waveform is present, the waveform (or barcode) represents the quality of the pulse being measured.  In these more advanced machines, the amplitude (or size) of the wave (or bar) correspond directly to the pulse quality (or strength).


If there is not a waveform to assess pulse quality,  it’s imperative that you make sure that the machine is reading the heart rate correctly.  If the machine shows a heart rate of 40 bpm and the actual patient has a heart rate of 150 bpm, the machine is not likely reading the patient’s SpO2 correctly either! Unless the waveform or heart rate are strong/matching, the pulse oximetry reading is likely to be inaccurate. Always take the time to verify its accuracy!

Understanding the numbers is only part of the battle.  Pulse oximeters may display false and inaccurate readings in the presence of the following:

  • Patient movement (e.g., shivering, tremors, seizure)
  • Poor perfusion (e.g., hypothermia, shock, vasoconstriction)
  • Anemia
  • Skin pigmentation
  • Very thin tissue (e.g., cat’s ear pinnae)
  • Interference (e.g., extraneous overhead light, electromagnetic energy)
  • Abnormal hemoglobin content (Pulse oximeters can’t distinguish between carboxyhemoglobin and oxyhemoglobin) and will therefore display a falsely elevated reading.

So what is normal and how do I use this thing?

Ultimately, we need to focus on the reading.  In health, the reading should be > 95-96% at minimum (with 98-100% preferred).  More importantly, make sure  you assess for the variables above, as well as the patient itself (in other words, does it match the patient’s clinical signs? Is the patient breathing normally but the pulse oximeter is reading 62%? Machine fail!).

While there are drawbacks of the pulse oximeter,  this device is a great bedside, non-invasive measurement of SpO2 and pulse rate and is a very important tool to have in your practice!

  1. Are there any pulse oximeters that are decent for $200? My friend has a small animal practice in Africa, and she asked me to get one for her. I have no idea what to look for in a brand, and I’ve seen prices range from $80 to $900.

  2. With vet pulse oximeters, be very careful since even $80 is a lot to spend if the unit doesn’t work. We have a lot of experience with pulse oximeters going back over 20 years. With LCD display oximeters, the colour displays tend to fade over time – around 2-3 years or less in hot/humid conditions. This is because the internal display connector is glued on and this tends to delaminate over time. With monchrome LCD displays, these can go off if left in direct sunlight. We recommend only using LED display pulse oximeters or high temperature LCD displays.Also, remember that places like Ebay and Alibaba will not provide any warranty or support. Have a look at or for more info.

  3. I’m currently studying Cert lV vet nursing & found VetGirl website to be extremely informative & written in a language I can understand. Been a very useful resource.
    Thank you 🙂

  4. What are the normal readings for Blood Pressure in dogs and cats? Systolic, Diastolic, and MAP. I keep getting different answers in different websites with Systolic ranging from 110-160 in dogs. Please help!

    • Typically, a systolic of 120 mm Hg. In a stressed pet in the hospital, up to 160 mm Hg for a systolic is acceptable.

  5. Hello Veterinarians.
    I found this page looking for a gift for my cousin who is a veterinarian. His wife is a veterinarian too, and they have a veterinary clinic in Colombia, my country.

    I found an oximeter on the internet and I thought it would be a good gift for them.
    Do you have any other suggestions for about $100.00 gift?

    Germán Perilla

  6. I follow you on Instagram and love your posts! Thank you for explaining things thoroughly but also accurately in a way that is easily understood! I am a third year vet student and find your page to be extremely helpful! THANK YOU!

  7. Thanks. Great post. If oximeters are accurate in all mammals, this implies that their blood is the same colour (more specifically their oxygenated blood and deoxygenated blood are all the same colour). Is that so?
    Is it also accurate with birds and reptiles?

  8. Does it really matter which side of the tongue the red light is placed on when monitoring SPO2 on your canine patient? Can the red light be on the top of the tongue vs. the bottom of the tongue ?

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