Page 108 - Physiological based CPAP for preterm infants at birth Tessa Martherus

Chapter 4
Figure 7. Pressure generated during CPAP and iPPV. Transpulmonary pressures (ΔP) are the primary factor that drives lung aeration after birth. During spontaneous breathing, thoracic expansion generates sub-atmospheric intrathoracic pressures, which when combined with CPAP increase the effective transpulmonary pressure applied to the lung. While intrathoracic pressures must transiently increase above the CPAP level to effect expiration, the CPAP level is effectively the highest external pressure applied to the airways and the mean intrathoracic airway pressure is considerably lower than the CPAP level. In contrast, during iPPV, PEEP is the lower limit of the applied pressure to the airways and the transpulmonary pressure that drives airway liquid clearance results from pressure increases above PEEP resulting in a mean airway pressure considerably higher than the PEEP.
As adduction of the larynx can seal the airways and prevent air from enter or leaving the lungs (6-8), it can regulate the pressure in the lower airways and protect the lungs from high pressures and hyperinflation. It also allows pressurization of the airways and thorax during active expiration, as occurs during a Valsalva maneuver, for example. This is consistent with the concept that preterm newborns commonly use expiratory breaking maneuvers, either generated by post-inspiratory diaphragmatic contractions and/or laryngeal closure to help defend end-expiratory gas volumes during expiration (22, 51). We found that, when giving 15 cmH2O of CPAP or iPPV, only a proportion (~75%) of the pressure applied at the nasal prongs was transmitted through the larynx into the upper trachea. In contrast, at lower CPAP levels (at 5 and 8 cmH2O), all of the pressure applied at the nasal prongs was transmitted through the larynx into the upper trachea. Thus, it is possible that the larynx was closing to seal the airways near the end of expiration during high-CPAP, but remained open throughout
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