Page 83 - Physiological based CPAP for preterm infants at birth Tessa Martherus

Higher CPAP levels improve functional residual capacity at birth in preterm rabbits
respiratory activity in kittens receiving high CPAP via activation of neural receptors on the face (trigeminal nerve) or activation of lung volume receptors (45-48). In a previous study we have shown that, after establishing lung aeration, increasing CPAP ≥7 cmH2O greatly reduces breathing activity in preterm rabbit kittens (4). It appears that immediately after birth, CPAP levels do not affect breathing rates, but when the pressure is suddenly increased after lung aeration it may trigger a Hering-Breuer or trigeminal reflex that protects the airways from high pressures (45-48).
Current recommendations suggest using CPAP levels of 5-8 cmH2O, as higher CPAP levels may
increase the risk of lung over-expansion injury and pneumothoraces. We found no indications
that, when applied immediately after birth, CPAP levels of 12-15 cmH2O caused lung over-expansion or increased the risk of pneumothorax or CPAP belly in preterm rabbits. We
have also recently found that 15 cmH2O CPAP does not impede the increase in pulmonary
blood flow at birth in spontaneously breathing preterm sheep (37). In contrast, a PEEP of ≥8 cmH2O in intubated and mechanically ventilated preterm lambs significantly reduces pulmonary blood flow and increases the risk of a pneumothorax (49-52). These discrepancies 3 highlight the fundamental differences between the two respiratory modes. When infants are supported with CPAP, the CPAP level is effectively the highest external pressure applied to
the airways and pressures in the lower airways only transiently exceed the CPAP level to effect expiration. With each breath, the intrathoracic pressures phasically decrease below the CPAP level, thereby reducing the mean airway pressure below the CPAP level. During IPPV, the PEEP level is the lowest externally applied pressure. As the pressures phasically increase above this with each inflation, the mean airway pressure is higher than the PEEP level and is substantially higher than occurs with a similar CPAP level during spontaneous breathing.
It is also important to consider that the timing after birth when the support is applied is also relevant, with the lung being more resistant to the adverse effects of higher pressures when liquid-filled. This is consistent with the finding that the risk of a pneumothorax was not increased when using PEEP levels ≤15 cmH2O in very preterm infants (53-55). Nevertheless, it would appear that there is a limit to how much pressure is appropriate in the first few minutes after birth. Indeed, a retrospective comparison study between two hospitals has shown that CPAP levels of 12-35 cmH2O increased the risk of pneumothoraxes compared to 5-8 cmH2O, although other procedural differences between hospitals could not be excluded.56 As such, we suggest that CPAP ≤15 cmH2O applied within the first minutes after birth improves lung aeration, but does not cause lung over-expansion and does not impede the circulatory transition in preterm lambs and rabbits immediately after birth. However, as it is likely that the risk of adverse events caused by high pressures increases as the lung aerates after birth, it is important to understand when and by how much this pressure can be weaned.
In two of the three 15 cmH2O CPAP groups, CPAP levels were reduced from 15 cmH2O to either 5 or 8 cmH2O when the lungs were visibly aerated. We found that kittens were able to maintain FRC and regular breathing when CPAP levels were reduced to 8 cmH2O, but not
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