Systolic pressure differences of 6 to 45 mm Hg were found between the main and branch pulmonary arteries in 37 of 242 infants undergoing diagnostic cardiac catheterization. Eleven of these 37 infants were restudied, and all had lost the pressure difference regardless of the underlying cardiac defect. A similar pressure difference was noted in lambs studied at thoracotomy with direct measurement of main and branch pulmonary arterial pressures. Increasing pulmonary blood flow caused a rise in pressure differences and also evoked a difference in some older lambs when none was originally present. Cineangiography in infants with a pressure difference showed no discrete narrowing or poststenotic dilatation of the pulmonary arteries. The main pulmonary artery was a large domed structure from which relatively small right and left branches arose at sharp angles from the inferior and posterior surfaces. The size, disparity, and angulation probably caused turbulence with, at times, a loss of pressure. Systolic flow murmurs may be common. In assessment of pulmonary vascular resistance, it is therefore important to recognize that a portion of the total pressure drop across the lungs after birth reflects maturational changes of the larger pulmonary arteries as well as of the muscular arterioles.

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