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In vivo oxygen dissociation curves in tranfused and untransfused newborns with cardiopulmonary disease

Abstract:

We used a new, rapidly responding in vivo catheter oximeter to continuously measure arterial oxygen saturation (Sa(O 2)) in 34 newborns with cardiopulmonary disease. The Sa(O 2) was compared with the oxygen tension (Pa(O 2)) of blood sampled through the catheter in 2 groups. We evaluated the oxygen dissociation curves of 24 untransfused infants whose hemoglobin F (HbF) concentration was ≥ 80%. We also examined the Sa(O 2) curves of 20 extensively transfused infants who had ≥ 80% hemoglobin A (HbA). We found a wide variation in Sa(O 2) at levels of arterial P(O 2) between 30 and 90 mmHg. Although the oxygen dissociation curve of the transfused group was significantly to the right of the untransfused group, there was considerable overlapping between the 2 curves. This dispersion reflected the wide range of clinical states encountered in newborns with cardiopulmonary disease, and the changes in blood oxygen affinity. In some newborns, when Pa(O 2) was less than 50 mmHg, Sa(O 2) reflected arterial oxygen contents more precisely because of the shape of the individual oxygen dissociation curves in both groups. Opinions vary as to the safe upper limit of Pa(O 2) at which retrolental fibroplasia does not occur. The Sa(O 2) at 2 upper limits of Pa(O 2) (100 and 80 mmHg) was estimated by probit analysis of simultaneous measurements of Pa(O 2) and in vivo Sa(O 2). In both the untransfused and extensively transfused groups of newborns, Pa(O 2) was < 100 mm Hg when Sa(O 2) was < 96%. The Pa(O 2) was < 80 mmHg when the SaO 2 was < 92%, if the HbF was ≥ 80%, and when Sa(O 2) was < 91%, if HbA was ≥ 80%, (95% confidence limits). The findings suggested a caveat against the practice of calculating Sa(O 2) from measurements of pH, PaO 2, and Pa(CO 2). This can be quite inaccurate when taking samples of arterial blood from newborns.