pO₂ versus SpO₂ in premature neonates

Premature neonates frequently have inadequate respiratory function and require oxygen therapy during the weeks after birth. Since they are also exquisitely vulnerable to oxygen toxicity, careful monitoring of oxygen status is an essential component of neonatal intensive care. 

Measuring % oxygen saturation by pulse oximetry (SpO₂) provides the most convenient means for continuous monitoring of oxygen status, but there remains a lack of consensus on the safest possible SpO₂ target range for these vulnerable babies. 

A series of coordinated international studies are now underway to compare outcome for babies whose oxygen therapy is adjusted to maintain SpO₂ within a low target range of 85-90 % with those whose SpO₂ is maintained within a higher target range of 91-95 %. Blood gas analysis includes measurement of pO₂ (oxygen tension of arterial blood) and thus provides an alternative means (once the only means) of monitoring oxygen status in the neonatal intensive care unit. 

Current guidelines suggest that in this clinical setting pO₂ should be maintained within the range of 6.7-10.7 kPa (50-80 mmHg). Surprisingly few studies have compared SpO₂ with contemporaneous pO₂ values among neonatal intensive care patients. That situation is corrected with the publication of a recent work from the neonatal intensive care unit at the Royal Infirmary, Edinburgh. 

The focus of this study was 98 very premature infants admitted to intensive care. Mean (SD) gestational age at birth was 26.7 (1.56) weeks. All had pulse oximetry probes fitted and arterial lines inserted on admission. On average, during the first week of life, blood was sampled for blood gas analysis from each of these infants on 21 occasions. 

At the time of each blood sampling SpO₂ was recorded, so that a total of 2076 time-matched pairs of pO₂ and SpO₂ results were recorded. Analysis of these matched pairs revealed inconsistency between the guidelines relating to pO₂ compared with those related to SpO₂. For example, many neonatal intensive care units adopt the lower recommended SpO₂ target range of 85-90 %. 

The study revealed that when SpO₂ is actually maintained within this range, 95 % of pO₂ values fall within the range of 3.8-7.1 kPa (28-53 mmHg), well below the recommended pO₂ range of 6.7-10.7 kPa. The higher SpO₂ target range of 91-95 % was found to be associated with pO₂ values that ranged from 4.6 to 8.9 kPa (34-67 mmHg) in 95 % of cases.

The sigmoidal oxygen dissociation curve, familiar to students of respiratory function, relates pO₂ to % oxygen saturation and suggests that it is possible to roughly predict SpO₂ from pO₂. This study has revealed a counterintuitive disconnect between the two values for this population of very premature infants. For example, SpO₂ of 90 % was found to be compatible with a pO₂ value of anything from 3 to 10 kPa. (22-75 mmHg).