Accuracy of pulse oximetry affected by arterial pCO₂

Arterial oxygen saturation sO₂(a) is a parameter calculated during blood gas analysis that is useful for monitoring respiratory function. Pulse oximetry provides a safer and more convenient, non-invasive means of measuring oxygen saturation that is used in many clinical settings. 

The value of pulse oximetry depends on many studies that have confirmed acceptable agreement between SpO₂ (oxygen saturation determined by pulse oximetry) and sO₂ (oxygen saturation determined during arterial blood gases). 

A recent study has demonstrated that agreement between SpO₂ and SaO₂ is diminished in patients with raised pCO₂. Simultaneous determination of sO₂(a) and SpO₂ was made on 846 adult patients with chronic lung disease under assessment for long-term oxygen therapy. 

On the basis of measured pCO₂ these patients were divided into three equally sized groups (tertiles). The first tertile comprised 279 patients whose pCO₂ was < 42 mmHg (5.6 kPa); second tertile comprised 274 patients whose pCO₂ was within the range 42–48 mmHg (5.6–6.4 kPa) and the third tertile comprised 293 patients whose pCO₂ was greater than 48 mmHg (6.4 kPa). 

Difference between SpO₂ and sO₂(a) was determined for each patient. Statistical analysis revealed significant positive correlation between pCO₂ and magnitude of the difference between SpO₂ and sO₂(a). 

The mean difference (sO₂(a) minus SpO₂) increased from 0.08 for the first tertile to -0.28 for the second tertile and to -1.24 for the third tertile. The authors conclude that “pCO₂(a) status can contribute to impairing agreement between sO₂(a) and SpO₂ particularly in patients with hypercapnia.”