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ObjectiveTo characterise the transitional pulmonary physiology of infants with congenital diaphragmatic hernia (CDH) using measures of expiratory tidal volume (TV) and end-tidal carbon dioxide (ETCO2).DesignProspective single-centre observational study.SettingQuaternary neonatal intensive care unit.PatientsInfants with an antenatal diagnosis of CDH born at the Children’s Hospital of Philadelphia.InterventionsTV and ETCO2were simultaneously recorded using a respiratory function monitor (RFM) during invasive positive pressure ventilation immediately after birth.Main outcome measuresTV per birth weight and ETCO2values were summarised for each minute after birth. Subgroups of interest were defined by liver position (thoracic vs abdominal) and extracorporeal membrane oxygenation (ECMO) treatment.ResultsRFM data were available for 50 infants from intubation until a median (IQR) of 9 (7–14) min after birth. TV and ETCO2values increased for the first 10 min after birth, but intersubject values were heterogeneous. TVs were overall lower and ETCO2values higher in infants with an intrathoracic liver and infants who were ultimately treated with ECMO. On hospital discharge, survival was 88% (n=43) and 34% (n=17) of infants were treated with ECMO.ConclusionRespiratory function immediately after birth is heterogeneous for infants with CDH. Lung aeration, as evidenced by expired TV and ETCO2, appears to be ongoing throughout the first 10 min after birth during invasive positive pressure ventilation. Close attention to expired TV and ETCO2levels by 10 min after birth may provide an opportunity to optimise and individualise ventilatory support for this high-risk population.