Pulmonary capillary blood flow(Qpc) was measured in 24 term infants (5 hours to 14 days of age) using the infant plethysmograph and nitrous oxide (N2O). The method is a modification of the Lee and DuBois technique, the deep inspiration being replaced by a brief period of rebreathing 60% N2O and 40% O2 from a 100 ml bag. The rate of uptake of N2O is divided by the mean alveolar N2O concentration and the solubility in cord blood to give Qpc. There was a significant increase in Qpc with increasing age from 173 ml/kg/min at 24 hours to 230 ml at 14 days (p < 0.0l). As the method does not measure any shunted blood this change could represent decreasing fetal shunts, but as there was a significant increase in heart rate (125/min at 24 hours to 168/min at 14 days) this seems unlikely. It is suggested that this increase in Qpc follows the known increase in metabolic rate in the newly born infant. In two anesthetized dogs tile mean difference between Qpc measured with this method and pulmonary arterial flow measured directly from a chronically implanted electromagnetic flowmeter was -2.6% S.E.±3.6%.
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Brady et al. (1971) studied this question.