Response of the pulmonary vasculature to hypoxia and H+ ion concentration changes.

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Abstract
The pulmonary vascular responses to variations in blood O2 tension and H+ ion concentration levels have been studied in the newborn calf. The difficulties in measuring pulmonary blood flow in the newborn animal are discussed. In this study, flow was measured directly with an electromagnetic flowmeter. When arterial O2 tension (P02) is 100 mm Hg or above, reduction of pH below 7. 30 results in a small increase in pulmonary vascular resistance. A progressive and more dramatic increase in vascular resistance occurs as pH drops to lower levels. At normal levels of blood pH (above 7. 35) reduction of arterial Po2 produced by inhalation of low O2 gas mixtures resulted in minimal increases in pulmonary vascular resistance even when arterial Po2 fell to 18 to 20 mm Hg. When pH was lowered, reduction of P02 produced an increase of pulmonary vascular resistance which was of significant degree when Po2 fell below about 50 mm Hg. Further reductions of Po2 resulted in very marked increases in pulmonary vascular resistance. A curvilinear relationship between pulmonary vascular resistance and Po2 was established, with extreme sensitivity of pulmonary vascular resistnace to Po2 changes in the range 18 to 20 mm Hg. Also, the lower the pH, the greater was the pulmonary vascular resistance response to Po2 reduction. The importance of this relationship on shunting mechanisms through the ductus arteriosus and foramen ovale in the neonatal period is discussed, with particular reference to the respiratory distress syndrome and to postoperative complications of thoracotomy in infancy.