Changes in Positive End-Expiratory Pressure Alter the Distribution of Ventilation within the Lung Immediately after Birth in Newborn Rabbits

Marcus J Kitchen,Arjan B Te Pas,Melissa L Siew,Kentaro Uesugi,Megan J Wallace,Andreas Fouras,Naoto Yagi,Stuart B Hooper,Robert A Lewis,Edgardo Szyld

doi:10.1371/journal.pone.0093391

Abstract

Current recommendations suggest the use of positive end-expiratory pressures (PEEP) to assist very preterm infants to develop a functional residual capacity (FRC) and establish gas exchange at birth. However, maintaining a consistent PEEP is difficult and so the lungs are exposed to changing distending pressures after birth, which can affect respiratory function. Our aim was to determine how changing PEEP levels alters the distribution of ventilation within the lung. Preterm rabbit pups (28 days gestation) were delivered and mechanically ventilated with one of three strategies, whereby PEEP was changed in sequence; 0-5-10-5-0 cmH2O, 5-10-0-5-0 cmH2O or 10-5-0-10-0 cmH2O. Phase contrast X-ray imaging was used to analyse the distribution of ventilation in the upper left (UL), upper right (UR), lower left (LL) and lower right (LR) quadrants of the lung. Initiating ventilation with 10PEEP resulted in a uniform increase in FRC throughout the lung whereas initiating ventilation with 5PEEP or 0PEEP preferentially aerated the UR than both lower quadrants (p<0.05). Consequently, the relative distribution of incoming VT was preferentially directed into the lower lobes at low PEEP, primarily due to the loss of FRC in those lobes. Following ventilation at 10PEEP, the distribution of air at end-inflation was uniform across all quadrants and remained so regardless of the PEEP level. Uniform distribution of ventilation can be achieved by initiating ventilation with a high PEEP. After the lungs have aerated, small and stepped reductions in PEEP result in more uniform changes in ventilation.

Highlights

The lungs of premature newborns are structurally and functionally immature and so have a greater tendency to collapse than the adult lung [1,2,3]
Our findings indicate that ventilating preterm rabbits with different and changing Positive end-expiratory pressure (PEEP) levels markedly effects the distribution of ventilation within the lung and on lung compliance during the immediate newborn period
We have used the quantitative capability [18] and high spatial resolution of phase contrast X-ray imaging to measure the distribution of air in the lung at both functional residual capacity (FRC) and end-inspiration (VPIP) as well as the relative distribution of the incoming VT with each inflation

Summary

Introduction

The lungs of premature newborns are structurally and functionally immature and so have a greater tendency to collapse than the adult lung [1,2,3]. The lung tissue is inelastic and the distal airways lack surfactant [4], which is required to reduce surface tension and promote uniform lung aeration [5,6]. As their chest walls are highly compliant [7], the premature newborn’s ability to oppose lung recoil and maintain functional residual capacity (FRC) is reduced, which may contribute to a lower FRC [8]. End-expiratory pressures improve FRC development, gas exchange, lung compliance and reduce lung injury and inflammation [9,10,11]. Little is known about how these pressure changes alter the distribution of ventilation within the immature lung

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