Effects of Inspired Oxygen on Echocardiographic Assessment of Pulmonary Vascular Resistance and Myocardial Contractility in Bronchopulmonary Dysplasia

We studied the effects of minor changes of inspired oxygen in ten preterm infants (birth weight: 700 to 1,780 gm; postnatal age: 4 to 18 weeks) with severe bronchopulmonary dysplasia. We compared the echocardiogram, capillary blood gases, and calculated venous admixture when the oxygen level was raised or lowered by 10% of the initial value. Mean ambient inspired oxygen was 0.31, capillary Po₂ was 62 torr, and calculated venous admixture was 40%. The ratio of right ventricular preejection period to right ventricular ejection time (RPEP/RVET) was 0.32, suggesting mild increase in pulmonary vascular resistance. In all instances, there was echocar-diographic evidence of further increase in pulmonary vascular resistance with a minor decrease in inspired oxygen. Lowering the inspired oxygen to 0.28 resulted in a reduction in Pao₂ to 54 torr, an increase in calculated venous admixture to 47%, and an increase in RPEP/RVET to 0.36 (P <.001). Raising the inspired oxygen to 0.34 increased Pao₂ to 72 torr but had no effect on RPEP/RVET. Calculated venous admixture decreased to 33% (P <.001). Myocardial contractility did not change with decrease in inspired oxygen but improved slightly with increase in inspired oxygen. These preliminary findings suggest that in infants with bronchopulmonary dysplasia the Pao₂ should be kept > 55 torr and the RPEP/RVET < 0.35 to avoid the development of right heart failure.