Page 21 - Physiological based CPAP for preterm infants at birth Tessa Martherus

had already transitioned and lungs were already aerated as they were ventilated prior to the start of the experiment.
Considering the current physiological knowledge of the neonatal transition at birth and the scientific evidence of different CPAP/PEEP levels in preclinical studies, likely that a CPAP strategy that is different from that currently used will be required. Directly after birth airway-liquid needs to be cleared for the lungs to allow the entry of air, and the primary purpose of all respiratory support modes during this stage of transition is to enhance this process. During lung aeration, the lungs are liquid-filled (6, 7) and this liquid moves distally through the airways and across the distal airway wall as air enters the lung. The high viscosity of this liquid means that its movement through the distal airways and across the epithelium, create a high resistance in the airways to the entry of air (20, 21). The only way of overcoming this high resistance is to increase the pressure gradient or to increase the time over which the pressure gradient is applied. As increasing the CPAP level can increase the transpulmonary pressure gradient, a high-CPAP strategy could more easily overcome the high resistance in the airways, clear liquid and promote lung aeration (18, 19, 79, 80). Once the lungs become more aerated and the liquid accumulates in surrounding tissue, the resistance in the airway markedly decreases and the lungs become more compliant and so the required pressure gradient is less. However, with the clearance of liquid into the tissue, the pressure in the interstitial tissue increases, and combined with the formation of surface tension at the air/liquid interfaces alveolar recoil and the pressure gradient for liquid to re-enter the airways increases (16, 23-25). In this phase of the transition, the infant needs to maintain its established lung aeration by preventing re-entry of lung liquid during expiration and alveolar collapse at end-expiration (18, 19, 26). Thus, at this time, purpose of CPAP shifts from promoting lung aeration to maintaining lung aeration by prevention of liquid re-entry, alveolar collapse and supporting spontaneous breathing (81).
Based on the scenario described above, it appears that the applied CPAP level should be dynamic and needs to be changed following the course of lung aeration and its changing role during the transition. Indeed, maintaining high-CPAP levels following lung aeration could cause pulmonary overexpansion as the lungs become more compliant. When considering the low viscosity of air and the decreased airway resistance, lower CPAP levels should be sufficient to maintain aeration and support spontaneous breathing (75-80). A physiological based (PB)-CPAP strategy will match the dynamic nature of the lung’s changing physiological state as the infant transitions to newborn life. Initial high-CPAP could enhance lung aeration, but after the lungs become aerated, CPAP can be reduced to maintain aeration while avoiding pulmonary overexpansion (79, 80).
The general hypothesis of this thesis is that preterm infants at birth will benefit from the use of a dynamic CPAP approach with an initial high CPAP level. A CPAP titration strategy with an initial higher CPAP level at birth will enhance lung aeration and better stimulate and support spontaneous breathing when compared to the currently used CPAP range of 4-8 cmH2O.
GI
General introduction
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