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)
- 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!