Pulse oximetry

When a case is presented, you probably like to assume some things are a given – that capillary refill time is a universal constant no matter who performs it, or that the way one person measures the respiratory rate is the same as the next. I’m always a little intrigued as to how things are the way they are, and so this time, I’m going to take a closer look at pulse oximetry.

How does pulse oximetry work?

Whilst large fixed machines that could measure SaO2 have been around since the end of the Second World War, the more portable machine sprang into existence in the mid-1970s. Aoyagi gives a fascinating account of its development in which he describes using two tungsten bulbs as the light sources (at wavelengths of 630nm and 900nm) shone at the plucked ears of anaesthetised dogs before sampling arterial pH and PaO2. Now, fancy electronic jiggery-pokery measures the absorption of light at two frequencies (660nm and 940nm). As the light passes through, it is absorbed differently by pulsatile and oxygenated tissue. Differences are averaged over time, and a number that is hopefully nearer 100  than 80 is produced.

The oximeter uses the time difference between peaks to calculate the pulse rate in beats per minute.

What might affect its accuracy?

In both the ED and NICU/PICU, we are constantly bombarded by alarms. Many tragic cases have occurred due to alarm fatigue in healthcare workers, and given that most pulse ox alarms are false, we need to think about what might make the SpO2 less accurate.

Ambient light

If the probe is not placed just so, emitted light can bypass the digit and go straight to the photoreceptor leading to a falsely elevated reading.

Anaemia

Anaemia shouldn’t have much effect on SpO2 as both oxygenated and deoxygenated blood are reduced in equal amounts.

Colours

You might think that jaundiced skin would impact light absorption, and you would be right. It peaks at 460nm, 560nm and 500nm, though, so it is not affected by the standard 660/940nm light used in pulse oximeters.

Dyshaemoglobins

Both carboxy and methaemoglobinaemia absorb red and infrared light differently, leading to inaccurate readings. HbCO can lead to false reassurance with normal-appearing sats. Given that machines are calibrated using normal, healthy volunteers with a normal level of HbCO and MetHb, it makes sense that if you have non-functioning haemoglobin, the standard probe will be less accurate. Fortunately, the fetal haemoglobin that many of our patients rely on has similar absorption to 660 to 1000nm of light.

Low peripheral perfusion

The readings are more accurate with good tissue perfusion and so cannot be relied upon to be accurate in poor perfusion states, whether pathological (sepsis) or iatrogenic (inflating a BP cuff).

Motion

Motion artefact seems to be one of the common causes of false alarms, with it taking just a wriggle to set the machine bleeping. Newer signal extraction technology has made this less of an issue, but when the alarm goes for low sats, it is worth looking for potential motion artifacts.

Nail Varnish

Hopefully, our younger patients are not wearing nail varnish, but I have seen many acrylic nails in adolescents. There is conflicting evidence surrounding the impact of nail varnish. It depends on the colour. Some data suggests that varnish with similar absorbance to deoxygenated blood (around the 660nm range) might lead to an artificial lowering of the reading. The studies have only been carried out in small patient sets, so there is a lot of room for error. Suffice it to say, if the patient in front of you has nail varnish on, take it off.

What about detecting hypoventilation?

It’s interesting, isn’t it, that we place an oxygen saturation probe on our soon-to-be sedated patients and then pop on supplemental oxygen. Whilst this is much more common in the adult environment, it still does occur in paediatrics. A lovely two-phase study by Fu et al. showed the expected rise in ETCO2 and PaCO2 with post-operative hypoventilation. In the second phase of the experiment, the ‘volunteers’ were randomised to breathe room air or supplemental oxygen. They found that when a patient spontaneously breathed room air (FiO2 = 0.21), they could not hypoventilate to a PaCO2 > 70 mmHg without their SpO2 dropping below 90%. If they even had a trickle of oxygen (FiO2 = 0.25) their PaCO2 could rise to around 100 mmHg without a drop in oxygen saturation. Pulse oximetry should not be used as the sole means of detecting hypoventilation, especially in the presence of supplemental oxygen therapy.

Children can have tiny fingers, so where can you put the probes?

Incorrectly placed probes have caused blisters. Recently, the NHS Improvement body sent a Patient Safety Alert, citing the risks of harm from incorrect placement. Adult probes on children can produce falsely reassuring readings. Site-specific probes – fingers, toes, ears, and forehead should be used.

What’s the future?

Do you have a portable sats probe in your pocket? There is a good chance you do but didn’t know it. With most of the medical workforce owning a smartphone, you might be surprised that you can use it for something more than catching up on the latest tweets from @DFTBteam. There are camera-based apps that use the strobing flash and camera lens to determine heart rate and oxygen saturations. Some probes can plug directly into the phone, though these are much more costly than the app-based option.

Selected references

Alexander CM, Teller LE, Gross JB. Principles of pulse oximetry: theoretical and practical considerations. Anesthesia & Analgesia. 1989 Mar 1;68(3):368-76.

Aoyagi T. Pulse oximetry: its invention, theory, and future. Journal of anesthesia. 2003 Nov 1;17(4):259-66.

Barker SJ, Shah NK. The effects of motion on the performance of pulse oximeters in volunteers (revised publication). Anesthesiology: The Journal of the American Society of Anesthesiologists. 1997 Jan 1;86(1):101-8.

Brand TM, Brand ME, Jay GD. Enamel nail polish does not interfere with pulse oximetry among normoxic volunteers. Journal of clinical monitoring and computing. 2002 Feb 1;17(2):93-6.

Chan ED, Chan MM, Chan MM. Pulse oximetry: understanding its basic principles facilitates appreciation of its limitations. Respiratory medicine. 2013 Jun 1;107(6):789-99.

Coté CJ, Goldstein EA, Fuchsman WH, Hoaglin DC. The effect of nail polish on pulse oximetry. Anesthesia and analgesia. 1988 Jul;67(7):683-6.

Fu ES, Downs JB, Schweiger JW, Miguel RV, Smith RA. Supplemental oxygen impairs detection of hypoventilation by pulse oximetry. Chest. 2004 Nov 1;126(5):1552-8.

Hay WW, Brockway JM, Eyzaguirre M. Neonatal pulse oximetry: accuracy and reliability. Pediatrics. 1989 May 1;83(5):717-22.

Paterson E, Sanderson PM, Paterson NA, Loeb RG. Effectiveness of enhanced pulse oximetry sonifications for conveying oxygen saturation ranges: a laboratory comparison of five auditory displays. BJA: British Journal of Anaesthesia. 2017 Oct 13;119(6):1224-30.

Ralston AC, Webb RK, Runciman WB. Potential errors in pulse oximetry III: effects of interference, dyes, dyshaemoglobins and other pigments. Anaesthesia. 1991 Apr;46(4):291-5.

Rubin AS. Nail polish color can affect pulse oximeter saturation. Anesthesiology: The Journal of the American Society of Anesthesiologists. 1988 May 1;68(5):825-.

Salyer JW. Neonatal and pediatric pulse oximetry. Respiratory care. 2003 Apr 1;48(4):386-98.

Severinghaus JW, Honda Y. History of blood gas analysis. VII. Pulse oximetry. Journal of clinical monitoring. 1987 Apr 1;3(2):135-8.

Sinex JE. Pulse oximetry: principles and limitations. The American journal of emergency medicine. 1999 Jan 1;17(1):59-66.

Tomlinson S, Behrmann S, Cranford J, Louie M, Hashikawa A. Accuracy of Smartphone-Based Pulse Oximetry Compared with Hospital-Grade Pulse Oximetry in Healthy Children. Telemedicine and e-Health. 2018 Jul 1;24(7):527-35.

Van Gastel M, Stuijk S, De Haan G. Camera-based pulse-oximetry-validated risks and opportunities from theoretical analysis. Biomedical optics express. 2018 Jan 1;9(1):102-19.

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