Ventilatory support and mechanical properties of the fibrotic lung acting as a "squishy ball"

Abstract

Protective ventilation is the cornerstone of treatment of patients with the acute respiratory distress syndrome (ARDS); however, no studies have yet established the best ventilatory strategy to adopt when patients with acute exacerbation of interstitial lung disease (AE-ILD) are admitted to the intensive care unit. Due to the severe impairment of the respiratory mechanics, the fibrotic lung is at high risk of developing ventilator-induced lung injury, regardless of the lung fibrosis etiology. The purpose of this review is to analyze the effects of mechanical ventilation in AE-ILD and to increase the knowledge on the characteristics of fibrotic lung during artificial ventilation, introducing the concept of "squishy ball lung". The role of positive end-expiratory pressure is discussed, proposing a "lung resting strategy" as opposed to the "open lung approach". The review also discusses the practical management of AE-ILD patients discussing illustrative clinical cases.

Keywords: Acute respiratory distress syndrome; Interstitial lung diseases; Mechanical ventilation; Respiratory failure; Ventilator-induced lung injury.

Fig. 1

Relationship between stress and strain in healthy, ARDS and fibrotic lungs. The specific elastance (K) is the slope of the curve in its linear portion. Although ARDS lungs are characterized by low compliance, its elastic properties follow those of healthy lungs provided that the deformation induced by tidal ventilation is normalized to the end-expiratory lung volume. In ARDS, the “baby lung” (gray area) inflates until a certain level where hyperinflation occurs and the linearity of the stress–strain relation is lost, approaching the breakdown limit of the extracellular matrix constituents (lightning). In fibrotic lungs, the specific elastance is higher thus the stress–strain curve is steeper. During inflation, the healthy regions protrude through the fibrotic walls, as illustrated by the hand progressively squeezing the “squishy ball”. Compared to ARDS, the breakdown is reached at lower stress and lower strain. ARDS acute respiratory distress syndrome, V_T tidal volume, EELV end-expiratory lung volume, P_L transpulmonary pressure

Fig. 2

a Histological evidence of spatial heterogeneity with relatively spared alveolar spaces surrounded by patchy areas of fibrosis with multiple fibroblastic foci in a patient with IPF. b CT appearance of UIP pattern in a patient with IPF. c Graphical appearance of a “squishy ball” depicting the elastic features of fibrotic lung in resting position. d Squishy ball subjected to the application of an internal pressure: the increase of the pressure inside the object causes throttling of the elastic part of the body through the inelastic net that wraps the ball determining a mechanical disadvantage during the expansion

Fig. 3

CT scan images and transpulmonary pressure monitoring of a representative patient with UIP pattern and superimposed ground-glass during an AE-ILD, with PEEP set according to a “lung resting strategy” (left, PEEP 4 cmH₂O) or with an “open lung approach” titrated to achieve positive end-expiratory transpulmonary pressure (right, PEEP 12 cmH₂O). End-inspiratory transpulmonary pressure values significantly rise when higher values of PEEP are applied. Purple areas represent lung collapse, opacities and fibrous regions. Red circles highlight areas of over-inflation. AE acute exacerbation, ILD interstitial lung disease, UIP usual interstitial pneumonia, PEEP positive end-expiratory pressure