Differential diagnosis of cyanosis in the neonate is difficult and cardiac catheterisation may be required for a correct diagnosis. It has been suggested that the response of PaO2 to continuous positive airway pressure (CPAP) with 100% oxygen may be useful. The purpose of this study was to test further this hypothesis by studying all neonates investigated for cyanosis with a PaO2 less than or equal to 50 torr in 0-8 to 1-0 F1O2. Arterial blood samples were obtained in an F1O2 of 0-21-0-4 and 0-8-1-0, and in an F1O2 of 0-8-1-0 with 8-10 cm CPAP, and were analysed for PaO2, PaCO2, and pH, bicarbonate being calculated. The final diagnoses were congenital heart disease (CHD) 21 cases, pulmonary parenchymal disease (PD) 10 cases, and persistent fetal circulation (PFC) 3 cases. No significant difference in pH, bicarbonate, or PaCO2 was observed among the three groups or with CPAP. In the CHD and PFC infants CPAP produced no significant change in PaO2. In the PD babies PaO2 increased by an average of 33 torr (P less than 0-05). Despite thus attaining statistical significance 2 PD infants had no increase in PaO2 with CPAP. An increase of PaO2 greater than 10 torr with CPAP suggests PD, and a nonsignificant increase in PaO2 does not rule out PD. Irrespective of initial PaO2, final PaO2 in 0-8-1-0 F1O2 with CPAP greater than 50 torr suggests PD, and less than 50 torr suggests CHD. The results indicate that CPAP may be used as an adjunct in differentiating cardiac from pulmonary disease.
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