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Very early preterm infants are born at a time when more than 90% of their red blood cells contain fetal haemoglobin (HbF)1 and therefore their blood has a high affinity for oxygen.2 Because of blood sampling, early preterm newborns often receive transfusions for blood volume replacement. These transfusions are carried out with adult red blood cells containing adult haemoglobin (HbA) and decrease the HbO2 affinity.
The change in P50 after transfusion has not previously been published in human preterm infants. A study was therefore planned to be carried out during the first week of life in very early preterm infants to determine the decrease in HbO2 affinity (Po2 required to achieve a saturation of 50% at pH 7.4 and 37°C) after a single transfusion as well as the relation of P50.
Four very early preterm infants of about 26 weeks gestation, free of any congenital anomalies and transfused during the first week of life, were included in the study. The oxygen dissociation curve and the P50 were determined by Hemox-Analyser (TCS Scientific Corp, New Hope, Pennsylvania, USA) as previously described.3 A 50 μl volume of whole blood was added to 4 ml buffer (135 mM NaCl, 30 mM TES, 5 mM KCl, and NaOH adjusted to pH 7.4 ± 0.02; TCS buffer; TCS Scientific Corp), 10 μl antifoam solution, and 20 μl 20% bovine serum albumin. Samples were analysed immediately on collection from the patient. Nitrogen (100%) was bubbled through the sample at a constant rate that resulted in complete deoxygenation within 20 minutes, followed by reoxygenation with air for 15 minutes. The analyser measured the oxygen tension with a standard Clark oxygen electrode (model 5331 Oxygen Probe; Yellow Springs Instrument Co, Yellow Springs, Ohio, USA).
The mean (SD) gestational age was 25.3 (1) weeks and the mean (SD) birth weight was 755 (185) g. The mean (SD) volume of red blood cells transfused (packed cell volume 0.55%) was 21.5 (6) ml (28.47 ml/kg). Haemoglobin levels before and after transfusion were 98 (9) and 134 (10) g/l respectively. The percentage of HbF was 92.9 (1.2) before and 43.3 (5.8), 2.8 (1) days after a transfusion. The value for P50 increased from 18.1 (1.0) to 21.0 (1.1) mm Hg (p = 0.007). The P50, if known after a transfusion, could be useful to predict the range of adequate saturation.
This work was supported by Canadian Institutes of Health Research grant no MOP 49464.
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