Page 81 - Physiological based CPAP for preterm infants at birth Tessa Martherus

Higher CPAP levels improve functional residual capacity at birth in preterm rabbits
up to 15 cmH2O did not increase the risk of lung bulging between the ribs, the incidence of pneumothoraxes or CPAP belly immediately after birth.
Our findings that CPAP levels markedly improve lung aeration are consistent with previous
studies showing that higher PEEP levels improve lung aeration at FRC in intubated and mechanically ventilated newborns at birth (23-25). However, these modes of respiratory
support are substantially different, particularly as intubation removes the larynx from the respiratory circuit. In contrast, the larynx and upper airways are in circuit when respiratory
support is provided non-invasively. Previously it has been shown that during apnoea, the
larynx closes, which can prevent air from entering or leaving the lungs (4-8), whereas during spontaneous breathing the larynx is mostly open, but may close briefly (expiratory braking manoeuvres) to maintain FRC.36 We have also recently shown that the larynx can regulate
the pressure applied to the sub-glottic airways during CPAP (37). When preterm sheep
received 5-8 cmH2O CPAP, this entire pressure was transmitted to the lungs, while only a proportion (~75%) of the pressure was transmitted into the lungs when 15 cmH2O CPAP was
given non-invasively. While this could be a mechanism that protects the lung from high 3 pressures that could overinflate the lung, it is not clear how and when this protective mechanism is activated during lung aeration (5,37). In this study, kittens that received 15 cmH2O CPAP non-invasively via a face mask had improved lung aeration without giving any indications of overinflation. This was assessed as bulging of the lung between the ribs
(Figure 2d-f) and the incidence of a pneumothorax, which were rare (2/77), only occurring in the 0 and 8 cmH2O groups. This finding suggests that higher pressures may have reached the lower airways initially to facilitate lung aeration, but following lung aeration, volume receptors in the lung may have triggered laryngeal protective mechanisms that prevented over-expansion of the lung.
In addition to the benefits we observed, it is possible that our study underestimated the true benefit of high CPAP levels on lung aeration and respiratory function immediately after birth in very preterm rabbits. All kittens were required to breath spontaneously while on CPAP, but if the kittens became apnoeic, rescue interventions (e.g. IPPV and tactile stimulation) were required. However, apnoea occurred much more frequently in kittens receiving 0, 5 and 8 cmH2O CPAP, compared with 12 and 15 cmH2O. Indeed, as the apnoeic kittens required rescue interventions (iPPV and/or tactile stimulation) to increase breathing and FRC, it is possible that these rescue interventions lessened the differences between groups. While we were unable to acquire reliable oxygen data, due to the well-known relationship between hypoxia and apnoea in fetuses and newborns, it is highly likely that breathing, or lack thereof, in these kittens is a sensitive indicator of oxygenation status (38,39).
Although IPPV was successful in re-establishing breathing in 42% of kittens that were assessed as apnoeic (breath intervals >4-5 s), the presence of spontaneous breaths, albeit at a low rate (i.e. <12-15 breaths/min), were essential to re-establishing regular breathing. As IPPV alone was not able to re-establish breathing in 58% of apnoeas, it is the combined effect of
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