Ventilation inhomogeneity in CDH infants – A new attitude within a simulation study

Abstract

Congenital Diaphragmatic Hernia (CDH) is a serious newborn defect requiring mechanical ventilation. Initial ventilation settings should take into account the severity of lungs inhomogeneity (LI), but it is not assessed in everyday clinical practice. We present a new LI index that can be easily determined at the bedside. It is based on a comparison of resistive-elastic properties of lungs and defined as a ratio of time constants T1.T2−1 of gas flows in both lungs (T1 = R1·C1, T2 = R2·C2). We hypothesised that T1.T2−1 index increase causes a rise of lungs impedance (Z) and requires elevation of peak inspiratory pressure (PIP), mean airway pressure (MAP), and work of breathing (WOB). Infant hybrid (numerical-physical) respiratory simulator and a ventilator were used to simulate conventional ventilation of homogeneous and inhomogeneous lungs, and to measure PIP, MAP and WOB. A high correlation was found between Z, WOB, PIP, MAP and the T1.T2−1 index (r = 0.9, P < 0.001). The increase of T1.T2−1 index from 1 to 20 resulted in significant rise of WOB, PIP and MAP, e.g. at RR = 60 bpm, the WOB (1.05 → 1.49 J·l−1), PIP (15.2 → 20.5 cmH2O) and MAP (6.8 → 8.4 cmH2O), P < 0.005. It seems that T1·T2−1 index could be used for prediction of PIP and MAP required to achieve effective ventilation in CDH infants; it also may affect the choice of ventilation strategy (CMV or HFV) as well as ventilator settings on CMV. We show how the relationships between WOB, PIP, MAP and the T1.T2−1 index could be used in clinical practice in the future.