Saturation versus transcutaneous monitoring: a randomised crossover study

Oxygen therapy is usually adjusted with the aim of keeping the infant's oxygenation within a target range in preterm infants, since hyperoxia and variability of oxygenation have been linked with increased risk of morbidity. Oxygen saturation (SpO₂) and transcutaneous oxygen tension (TcPO₂) monitors are used to monitor oxygen levels in preterm infants. SpO₂ monitors are non-invasive, easy to use and do not require calibration or cause heating of the skin. However, they have a relatively high rate of false alarms; moreover, because of small changes in oxygen saturation above 95% can mask large increases in oxygen tension, saturation monitoring may not be reliable in preventing hyperoxia. On the other hand, TcPO₂ monitors require calibration and need to be resited regularly to avoid skin damage from heating. Although both SpO₂ monitors and TcPO₂ monitors are used in clinical practice, neither monitoring method is clearly superior in terms of minimising adverse outcome; however, no comparative studies have been conducted.
On this basis, an English group performed a randomized crossover study to determine whether infants are exposed to more cumulative hyperoxia or hypoxia or to more variable oxygen tension when oxygen therapy is controlled on the basis of TcPO₂ monitoring or SpO₂ monitoring.
SpO₂ and TcPO₂ were measured simultaneously during two 3-h study periods allocated in random order. During one period supplemental oxygen was adjusted according to TcPO₂ (target range 6.0-9.0 kPa) and during the other according to SpO₂ (target range 86-94%).
During each period, readings from the second monitor were not displayed. For each period level of SpO₂ and TcPO₂ and the percentage of time spent above and below target range were calculated and compared.
In total, 19 infants were studied at mean corrected gestational age of 27.2 weeks and mean postnatal age of 6.8 days. Care based on SpO₂ monitoring was associated with more time spent with high oxygen tension (median increase 2.62%, p=0.01), more time with low oxygen tension (median increase 17.41%, p=0.01), more variability in oxygen tension (median increase 0.28 kPa, p=0.02) and more variability in oxygen saturation (median increase 0.82%, p=0.01) than care based on TcPO2 monitoring.
These results have important implications for practice, since there has been a trend recently to reject transcutaneous monitoring in favour of saturation monitoring. It is widely accepted that high oxygen tensions or saturations and increased variability of oxygenation may be harmful: the differences in oxygen tension variability observed in this study between the two monitoring methods were large enough to be relevant to the risk of developing retinopathy of prematurity.
In conclusion, the results of this study may suggest that, within the target ranges studied, the use of TcPO₂ monitoring is associated with less variability in oxygen tension and saturation and less time spent with low and high oxygen tension than the use of SpO₂ monitoring. This is potentially relevant for the clinical outcome.

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