Page 82 - Physiological based CPAP for preterm infants at birth Tessa Martherus
P. 82

Chapter 3
spontaneous breathing and IPPV, particularly when a breath and inflation coincided, that led to the increase in lung aeration and the re-establishment of breathing (11,40). When this occurs, the larynx is open4 and so the pressure in the face mask is up to 35 cmH2O (in this study) during inspiration (i.e. at PIP during IPPV), which is substantially greater than a CPAP level of 15 cmH2O. While the resulting large pressure gradients, generated by inspiration plus the inflation pressure, partially explains the increase in lung aeration we observed following rescue interventions, it also risks lung over-expansion and injury. Indeed, as the pressure in the face mask cycles between 5 and 35 cmH2O which may or may not coincide with spontaneous breathing, the inherent physiological feedback controls that normally regulate tidal volumes will be ineffective. We suggest that this is a normal scenario that occurs in the delivery room when very preterm infants receive IPPV and inflations come into and out of synchrony with inspiratory efforts. This may contribute to the large variation in tidal volumes that have been reported in very preterm infants (41-43).
The incidence of apnoea was not statistically different between groups, yet the incidence of apnoea appeared to be markedly higher in the rabbits receiving 0-8 cmH2O CPAP (36-46%) as compared to those rabbits receiving 12-15 cmH2O CPAP (16-20%). This difference was even more pronounced in preterm lambs, with 83% of lambs supported with 5 cmH2O CPAP becoming apnoeic compared to only 17% of lambs supported with 15 cmH2O CPAP (37). We suggest that higher CPAP levels better support spontaneous breathing by improving lung aeration, which increases the lung’s surface area for gas exchange and thereby improves oxygenation, which sustains breathing activity. Indeed, hypoxia is a potent inhibitor of breathing in the foetus and newborn and is likely to be a major cause of apnoea (38,39). Alternatively, CPAP may also reduce alveolar collapse and thereby reduce breathing efforts, making it easier for an infant to breathe (44). We speculate that this may explain our anecdotal observation that the amplitude of the intrathoracic pressure reductions associated with breathing increased with decreasing CPAP level; these data have not been analysed as the pressure amplitude is highly sensitive to positional changes.
The effect of CPAP levels on breathing rates is unclear and appear contradictory, likely because they are influenced by multiple competing factors. We have shown that breathing rates are higher when preterm rabbits are supported by CPAP, but there seemed to be little difference between different CPAP levels. Intubated preterm lambs showed no difference in breathing rates when supported by 5 or 8 cmH2O (26), but lambs achieved higher breathing rates with 15 compared to 5 cmH2O CPAP (37). While these results may appear contradictory, there were major differences in the level and timing of interventions that stimulate breathing. For example, while tactile stimulation is hard to standardize even when there is direct access to the newborn, access to the animal was greatly limited in these synchrotron studies. Indeed, tactile stimulation could only be applied by ceasing imaging and gaining access to the imaging hutch. Thus, differences in the degree of tactile stimulation may have obscured the effect of increased lung aeration and oxygenation on breathing rates. Alternatively, increasing pressure applied to the face and/or airways may have countered the stimulation of
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