UNDERESTIMATION OF PaO₂ BY tcPO₂ WITH INCREASING POSTNATAL AGE

Abstract

This study was performed to find out whether the relation between transcutaneous oxygen tension (tcPO₂) and arterial oxygen tension (PaO₂) changed with increasing gestational and postnatal age.

160 simultaneous measurements of tcPO₂ and PaO₂ were made at weekly intervals on 42 babies born at 24-41 weeks of gestation and aged 0-32 weeks from birth. tcPO₂ was measured using a Drãger Transoxode tcPO₂ electrode set at 44°C and connected to a chart recorder. Arterial blood samples were obtained with minimal disturbance of the baby as judged by the stability of the tcPO₂ record. PaO₂ was measured using an IL 1303 blood gas analyser.

At 0-1 week of postnatal age, mean tcPO₂/PaO₂ in 15 babies born at <30 weeks of gestation was 1.11 ± SD 0.11, n=19, and in 11 babies of ⋝30 weeks of gestation it was 1.02 ± 0.09, n=13 (n.s.). The tendency towards higher values for tcPO₂/PaO₂ in less mature babies persisted during the succeeding weeks but was not statistically significant. Irrespective of gestation, a progressive underestimate of PaO₂ by tcPO₂ with increasing postnatal age was found. For example, mean tcPO₂/PaO₂ from observations on 16 babies ⋝8 weeks old was 0.83 ± 0.15, n=72, significantly lower than in 36 babies <8 weeks old, 1.00 ± 0.13, n=88 (p<0.001).

We conclude that (1) tcPO₂ underestimated PaO₂ with increasing postnatal age and (2) this underestimate could lead to older babies being nursed in an inappropriately high ambient oxygen concentration.

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