Page 84 - Physiological based CPAP for preterm infants at birth Tessa Martherus

Chapter 3
when they were reduced to 5 cmH2O. Although FRC and breathing rates were statistically similar between all three 15 cmH2O CPAP groups at the end of the experiment, the 15 to 5 cmH2O group showed a wider interquartile range. Indeed, the low 25th percentile indicates that some of these rabbit kittens were not able to maintain FRC and breathing rates. Thus, CPAP levels of 5 cmH2O may be too low to sustain lung aeration for up to 10 min after birth in some kittens and that 8 cmH2O is more appropriate. This also suggests that CPAP level should be lowered gradually and that the optimal level may need to be tailored to suit the needs of the individual. This is consistent with a previous study suggesting that small and gradual pressure changes may avoid a loss of FRC (25), but the parameters used to guide these reductions are unknown. In preterm lambs, decreasing CPAP from 15 to 8 cmH2O at a median of 13.4 (8.4-20.3) min after birth was found to increase oxygen requirement, which is indicative of FRC loss and suggests that the reduction in CPAP should have been delayed (37). In that study, CPAP was decreased after lambs reached pulmonary blood flows >300 mL/min, oxygen saturations >90% and supplemental oxygen ≤0.7. However, as it is not feasible to assess aeration or other parameters, such as pulmonary blood flow, in the delivery room, these results may not be directly applicable in humans. Nevertheless, they do indicate that CPAP levels should not be based solely on oxygenation status or breathing effort, as these appear to be dependent on, and are not a determinant of, lung aeration. Indeed, we have previously shown that 100% oxygen can increase breathing rates without necessarily improving FRC in preterm rabbits (12). Similarly, the results of this study show that high CPAP levels increase FRC but do not affect breathing rates, and that a small proportion of kittens became apneic even with FRCs of up to 15.5 mL/kg. This indicates that while breathing rates and oxygenation are closely inter-related, they are not a good measure for lung aeration in the delivery room.
In summary, we found that 15 cmH2O CPAP improves lung aeration in spontaneously breathing preterm rabbits. While high CPAP levels (≥12 cmH2O) facilitate spontaneous breathing and reduce the risk of apnoea, CPAP levels do not influence breathing rates. We found no indication that CPAP levels of up to 15 cmH2O causes lung over-expansion, as indicated by bulging of the lung between the rib or increases in the risk of pneumothoraxes or CPAP belly. Once the lungs are aerated, CPAP can be decreased, but future studies are required to determine when to decrease the pressure and by how much in a clinical setting.
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