Page 148 - Physiological based CPAP for preterm infants at birth Tessa Martherus

General discussion
venous pressure were left unaffected. 15 cmH2O clearly provided more cardiovascular stability and all lambs in this group reached the 30 minute end point as opposed to 50% in the 5 cmH2O group. Post-mortem examinations showed no increase in the risk of pneumothoraxes. From the animal data we conclude that 15 cmH2O CPAP does not lead to lung over-expansion and provides more cardiovascular stability as compared to lower CPAP levels.
Expansion of the lungs could not be measured in our clinical study (Chapter 5), but we did not observe cardiovascular signs of over-expanded lungs. On the contrary, infants receiving PB-CPAP showed an earlier and higher increase in heart rate when compared to 5-8 cmH2O CPAP. Although this was a small study, none of the infants (n=8) in the PB-CPAP group developed pneumothoraxes during NICU admission and short-term respiratory outcomes were similar between groups.
Once lung aeration has been established, the role of CPAP in supporting the respiratory effort of the infant likely changes. Previous studies (39-42) demonstrated that 8-12 cmH2O of PEEP during positive pressure ventilation reduces PBF and increases the risk of pneumothoraxes in intubated preterm lambs with already aerated lungs. These findings are not conflicting but emphasize the difference between CPAP during spontaneous breathing and PEEP during positive pressure ventilation as well as the importance of timing the reduction in CPAP. When 15 cmH2O CPAP is given immediately at birth to facilitate lung aeration, the activation of a neural reflex in response to lung aeration presumably overwhelms the increase in alveolar pressure associated with high CPAP to prevent the reduction in PBF (43, 44). This has also been shown in a previous lamb study whereby 30 second sustained inflations using pressures of 35 cmH2O did not adversely affect the increase PBF (52). In Chapter 4, we found that after lung aeration, a sudden increase in CPAP from 5 to 15 cmH2O, led to a ~10% reduction in PBF. This is in line with a previous study with intubated preterm lambs where increasing PEEP from 4 to 12 cmH2O resulted in a ~40% decrease in PBF (40). This again demonstrates that high CPAP levels can be beneficial, but decreases in CPAP levels are needed and the timing of this is essential.
As high CPAP levels would be expected to increase intrathoracic pressure, this could also increase jugular venous pressure and thereby reduce or increase fluctuations in cerebral blood flow (53), which would then increase the risk of IVH. However, in our preterm lamb study (Chapter 4) high CPAP did not affect jugular venous pressure or cerebral blood flow. We speculate that this was because the intrathoracic pressure did not exceed the jugular venous pressure due to selective closure of the larynx and the mechanics of spontaneous breathing. During spontaneous breathing, the pressure transmitted to the lungs is effectively the highest intrathoracic pressure but with every inspiration this pressure decreases allowing air to flow into the lungs and thereby phasically decreases below the JVP. Our clinical trial (Chapter 5) was too small to make appropriate conclusions on the risk of IVH.
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