The value and limitations of pulse oximetry

Whilst arterial blood gas analysis is the gold standard method for determination of arterial oxygen saturation (sO₂(a)), pulse oximetry provides an alternative, more convenient method that does not depend on sampling arterial blood. 

Because of its ease of operation and ability to monitor oxygen saturation in real time non-invasively, the pulse oximeter has become ubiquitous in nearly all clinical settings and is, by far, the most commonly used method of assessing oxygen saturation and thereby, patient oxygen status. 

The validity of pulse oximetry depends on the notion that the parameter measured – oxygen saturation of (capillary) peripheral blood (SpO₂) - is an acceptable estimate of the oxygen saturation of arterial blood (sO₂(a)) measured by the blood gas analyzer. 

Although this is indeed the case in most clinical contexts, there are important exceptions and it is important that clinical staff is aware of the limitations of pulse oximetry and the situations when there is no alternative to arterial blood gas analysis. A recently published review article addresses all aspects of pulse oximetry and serves as a valuable clinical guideline for the safe and effective use of pulse oximeters.

By way of introduction the article sets out the application of pulse oximetry in a range of clinical settings, including: prehospital ambulatory care, emergency and critical care, perinatal and neonatal care and outpatient investigation of sleep disorders (a particular interest for one of the authors). 

There follows explanation of how a pulse oximeter works, and the factors (e.g. dyshemoglobinemia, reduced perfusion, reduced oxygenation – sO₂(a) <70 %) that reduce its accuracy.

In a section headed “interpretation issues” the authors highlight the inherent limitation of pulse oximetry by discussing what is not measured during pulse oximetry and consequently how a normal SpO₂ value can, under some circumstances, provide a falsely optimistic picture of tissue oxygen delivery. 

For example pulse oximetry, unlike blood gas analysis, provides no information about acid-base balance or alveolar ventilation; abnormality of either can reduce oxygen delivery, even if oxygen saturation (SpO₂) is within normal limits. Additionally, oxygen delivery is compromised by anemia, a condition that does not affect SpO₂.

There are particular recommendations from the authors in the final section of the article, addressing the technical use of the pulse oximeter. For example, the authors recommend that pulse oximeters should be assessed at least every 2 years for their accuracy, using ”dedicated validated technology”. 

(They cite a recent study that suggests that 30 % of oximeters used in clinical practice fall short of manufacturers’ accuracy specification). Other issues discussed in this section include: motion-induced SpO₂ inaccuracy, positioning of the pulse oximeter probe, signal averaging time and alarm settings.  

The authors cite a meta-analysis of 14 questionnaire-based studies that revealed poor understanding of pulse oximetry by both medical and nursing clinicians. Along with the 63 references it contains, this comprehensive review article/clinical guideline is a valuable resource that can help plug the knowledge gap for those who rely on pulse oximetry for making clinical decisions.