Abstract Background Very preterm infants often require respiratory support after birth with current recommendations suggesting the use of continuous positive airway pressure (CPAP) of 4–8 cmH 2 O and an initial fraction of inspired oxygen (FiO 2 ) of 0.21–0.3. We have examined the interaction of high and low CPAP and FiO 2 levels on breathing rates and lung aeration in preterm rabbits. Methods Prematurely delivered rabbits (29/32 days gestation) received CPAP of either 5cmH 2 O (5CPAP; n = 12) or 15 cmH 2 O (15CPAP; n = 14), and a FiO 2 of either 0.3 (5CPAP/0.3, n = 6 or 15CPAP/0.3, n = 7) or 0.6 (5CPAP/0.6, n = 6 or 15CPAP/0.6, n = 7). Breathing rates, lung aeration (functional residual capacity; FRC), lung bulging and air accumulation in the stomach were measured using phase-contrast X-ray imaging. Results Kittens receiving 0.6 FiO 2 had higher breathing rates (5CPAP/0.6: 32.6±6.4 breaths/min; p = 0.0064 and 15CPAP/0.6: 36.9±3.5breaths/min; p = 0.0010) than 5CPAP/0.3 kittens (11.8±4.1breaths/min). Kittens receiving 15CPAP/0.6 tended to have higher FRC volumes (34.9±4 mL/kg) than kittens receiving 5 cmH 2 O CPAP (5CPAP/0.3: 13.1±6mL/kg; p = 0.0675 and 5CPAP/0.6: 13.5±6 mL/kg; p = 0.1720) and 15CPAP/0.3 (22.5 ± 6.6 mL/kg; p = 0.4245). Lung bulging and air accumulation in the stomach were not different between groups. Conclusion Preterm rabbits supported with both 15 cmH 2 O CPAP and 0.6 FiO 2 increased spontaneous breathing rates and lung aeration without increasing the risk of air in the stomach or lung bulging. Impact While current guidelines recommend the use of low CPAP (4–8 cmH 2 O) and low FiO 2 levels (0.21–0.3 FiO 2 ) to support preterm infants at birth, the optimum levels are unknown. This study has shown that 15 cmH 2 O of CPAP and FiO 2 of 0.6 improved lung aeration and breathing in preterm rabbits, compared with a CPAP of 4 cmH 2 O and FiO 2 of 0.3. These results add to the evidence indicating that initial high CPAP and high FiO 2 levels, followed by titration of both, enhance respiratory support for preterm newborns.

Optimising CPAP and oxygen levels to support spontaneous breathing in preterm rabbits