Since very low birth weight preterm newborns are prone to oxygen toxicity and have red blood cells that have a high oxygen affinity, the knowledge of the P90 (the PaO2 required for 90% saturation of hemoglobin) could result in a more optimal oxygenation. This information could diminish the incidence of bronchopulmonary dysplasia and retrolental fibroplasia. To determine P90 and P50 (the PaO2 required for 50% saturation of hemoglobin), cord blood was obtained from 10 preterm newborns who were of less than 30 weeks' gestation (26.4 +/- 1.4 [SD] weeks) weighing less than 1000 g (816 +/- 119 g). The P50 and P90 were determined by gas mixing tonometry and expressed in millimeters of mercury at a temperature of 37 degrees C, a pH of 7.40, and PCO2 of 40 mm Hg. The tonometer permitted an equilibration and sampling of successive aliquots of blood at different oxygen tensions. Measurement of oxygen saturation, pH, and PaO2 provided the information for plotting the oxygen dissociation curve, the P50 and P90. There were at least six experimental points for each oxygen dissociation curve. The results showed that the mean P50 was 18.3 +/- 1.9 mm Hg (2.5 +/- 0.3 kPa) and the P90 was 40.8 +/- 3.6 mm Hg (5.4 +/- 0.5 kPa). This study demonstrates the fetal nature of the oxygen dissociation curve of very low birth weight newborns; but most importantly it also shows that when treating very low birth weight newborns with oxygen, a PaO2 of 41 mm Hg (5.5 kPa) is enough to saturate 90% of the hemoglobin at a physiological pH. This information could be of clinical importance to minimize oxygen toxicity.
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