Pulse oximetry not reliable for diabetic patients?

Pulse oximeters, which are ubiquitous in nearly all areas of clinical care, provide the means for safe, non-invasive continuous monitoring of blood oxygen saturation. The validity of using pulse oximetry to assess patient blood oxygenation status depends on SpO₂, the parameter measured by pulse oximetry, being a reliable estimate of arterial oxygen saturation (sO₂(a)), the parameter directly measured during blood gas analysis. 

In most clinical contexts SpO₂ is more or less equal to sO₂(a), but that is not always the case and it is important to be aware of the limitations of pulse oximetry and define clinical situations when there is no alternative to blood gas analysis for accurate assessment of blood oxygenation. 

According to the results of a recent clinical study, pulse oximetry may not be suitable for assessment of blood oxygenation among type 2 diabetic patients with poor glycemic control and consequent increased HbA1c. The study population comprised 261 type 2 diabetes patients who were critically ill and required oxygen therapy and/or mechanical ventilation. 

Their care included continuous monitoring of oxygen saturation using a pulse oximeter. Fasting blood was sampled from each study patient for glucose and HbA1c. Arterial blood was sampled for blood gas analysis, including measurement of sO₂(a). As arterial blood was sampled, the patient’s SpO₂ reading from pulse oximeter was recorded. 

For the purposes of this study poor glycemic control was defined as HbA1c > 7.0 %; by this definition 114 of the 261 patients (44 %) had poor glycemic control. There was essentially no difference between SpO₂ and sO₂(a) among those with adequate glycemic control: mean SpO₂ 95.1 % ± 2.8 and mean sO₂(a) 95.3 % ± 2.8. 

However, there was a significant difference (bias) between the two parameters among those with poor glycemic control: mean SpO₂ 98.0 % ± 2.6 and mean sO₂(a) 96.2 % ± 2.9. The magnitude of the bias was demonstrated to correlate with HbA1c, so that the higher the HbA1c the greater is the difference between SpO₂ and sO₂(a). 

The authors were able to conclude that among patients with poorly controlled type 2 diabetes, pulse oximetry overestimates arterial oxygen saturation. It may be more appropriate to use arterial blood gas analysis, rather than pulse oximetry, to monitor treatment of hypoxemia in these patients.