Page 109 - Physiological based CPAP for preterm infants at birth Tessa Martherus

High-CPAP does not impede cardiovascular changes at birth in preterm sheep
the respiratory cycle at lower CPAP pressures. This may explain why studies in intubated preterm lambs observed higher pneumothorax rates when using PEEP ≤15 cmH2O (38, 41), while a recent study (37) found that the use of similar pressures applied non-invasively during iPPV in preterm infants did not increase the risk of adverse events.
In both the fetus and newborn, apnea or unstable breathing (varying in depth and rate), causes the larynx to close and only open during a breath (5-8). In contrast, during regular stable breathing, the larynx remains mostly open, but can briefly close to effect expiratory braking. However, lambs supported with 15 cmH2O were breathing continuously in a regular stable pattern and, as such we would expect the larynx to be mostly open. Nevertheless, glottic closure during expiration could be part of complicated breathing maneuvers induced by high-CPAP levels. Indeed, we noted that active expirations were common in these lambs, which likely reflects a physiological response to the increase in pressure that may also have included expiratory braking maneuvers during the expiratory phase of the breathing cycle (51).
It is also pertinent to note that in this study, the high-CPAP was applied from birth, when the
lungs are initially liquid filled. In most previous studies, the lungs were aerated before the
effect of high-PEEP levels on PBF was examined. This is consistent with our finding that increasing CPAP levels from 5 to 15 cmH2O at 30 min after birth caused a small (~10%) 4 decrease in PBF, although this was substantially less than the decrease (~40%) observed when
PEEP was increased from 4 to 12 cmH2O during iPPV at 20 min after birth (39). Nevertheless, the timing for when high-CPAP levels are applied to the airways may influence whether or not high pressures adversely affect PBF. Indeed, a sustained inflation (to 35 cmH2O) for up to 1 min does not adversely affect PBF when applied during lung aeration (52). Why this should occur is unclear, but it is possible that the initial stimulus for the increase in PBF overwhelms the adverse effect of increased alveolar pressure (52). It has recently been shown that a neural reflex, activated in response to liquid leaving the airways and entering lung tissue, triggers a global increase in PBF, which overrides all other influences such as oxygenation (19, 53). Thus, both the type and timing of application to the airways appear to determine how high airway pressures influence the cardiovascular system at birth.
At birth, pulmonary gas exchange is dependent on the available surface area (i.e., how much of the lung is aerated) and the partial pressure gradient for the respiratory gases, which for oxygen is largely determined by the FiO2. When the surface area is small, a large partial pressure gradient for oxygen is required for adequate oxygen exchange, whereas with increasing lung aeration, the surface area increases and so the required partial pressure gradient decreases. This finding is consistent with previous studies that have shown high-PEEP levels increase lung aeration and oxygenation and lowers the oxygen requirement (38, 39, 42, 43, 54). While we found no statistically significant differences between groups in FiO2, SaO2, or AaDO2, there was a clear trend toward a higher AaDO2 and FiO2 requirement and lower SaO2 in the low-CPAP group. The absence of a significant difference likely results
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