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As neonatal resuscitation critically depends upon lung aeration at birth, knowledge of the progression of this process is required to guide ongoing care. We investigated whether expired CO2 (ECO2) levels indicate the degree of lung aeration immediately after birth in two animal models and in preterm infants. Lambs were delivered by caesarean section and ventilated from birth. In lambs, ECO2 levels were significantly (p<0.0001) related to tidal volumes and CO2 clearance/breath increased exponentially when tidal volumes were greater than 6 mL/kg. Preterm (28 days of gestation; term = 32 days) rabbits were also delivered by caesarean section and lung aeration was measured using phase contrast X-ray imaging. In rabbit kittens, ECO2 levels were closely related (p<0.001) to lung volumes at end-inflation and were first detected when ∼7% of the distal lung regions were aerated. ECO2 levels in preterm infants at birth also correlated with tidal volumes. In each infant, ECO2 levels increased to >10 mmHg 28 (median) (21-36) seconds before the heart rate increased above 100 beats per minute. These data demonstrate that ECO2 levels can indicate the relative degree of lung aeration after birth and can be used to clinically assess ventilation in the immediate newborn period.

AIM: To retrospectively investigate the changes of SpO2 and respiratory drive in preterm infants at birth after administration of 100% oxygen. METHODS: Respiratory parameters, FiO2 and oximetry of infants <32 weeks gestation before and after receiving FiO2 1.0 were reviewed during continuous positive airway pressure (CPAP) or positive pressure ventilation (PPV). RESULTS: Results are given as median (IQR) or percentages where appropriate. Suitable recordings were made in 50 infants (GA 27 (26-29) weeks), 17 received CPAP and 33 PPV. SpO2 increased rapidly in the first minute after FiO2 1.0 and remained stable. The duration of FiO2 1.0 tended to be shorter in the CPAP group than in the PPV group (CPAP vs. PPV: 65 (33-105) vs. 100 (40-280) s; p = 0.05), SpO2 >95% occurred more often in PPV group (53% vs. 69%) and lasted longer (70(40-95) vs. 120(50-202) s). In CPAP group, minute volume increased from 134 (76-265) mL/kg/min 1 minute before to 240 (157-370) mL/kg/min (p<0.01) 1 minute after start FiO2 1.0 and remained stable at 2 minutes (252 (135-376) mL/kg/min; ns). The rate of rise to maximum tidal volume increased (from 13.8 (8.0-22.4) mL/kg/s to 18.2 (11.0-27.5) mL/kg/s; p<0.0001) to 18.8 (11.8-27.8) mL/kg/s; ns). In the PPV group respiratory rate increased from 0(0-4) to 9(0-20) at 1 minute (p<0.001) to 23 (0-34) breaths per minute at 2 minutes (p<0.01). CONCLUSION: In preterm infants at birth, a rapid increase in oxygenation, resulting from a transient increase to 100% oxygen might improve respiratory drive, but increases the risk for hyperoxia.