Physiology in Medicine: Understanding dynamic alveolar physiology to minimize ventilator-induced lung injury

Abstract

Acute respiratory distress syndrome (ARDS) remains a serious clinical problem with the main treatment being supportive in the form of mechanical ventilation. However, mechanical ventilation can be a double-edged sword: if set improperly, it can exacerbate the tissue damage caused by ARDS; this is known as ventilator-induced lung injury (VILI). To minimize VILI, we must understand the pathophysiologic mechanisms of tissue damage at the alveolar level. In this Physiology in Medicine paper, the dynamic physiology of alveolar inflation and deflation during mechanical ventilation will be reviewed. In addition, the pathophysiologic mechanisms of VILI will be reviewed, and this knowledge will be used to suggest an optimal mechanical breath profile (MB: all airway pressures, volumes, flows, rates, and the duration that they are applied at both inspiration and expiration) necessary to minimize VILI. Our review suggests that the current protective ventilation strategy, known as the "open lung strategy," would be the optimal lung-protective approach. However, the viscoelastic behavior of dynamic alveolar inflation and deflation has not yet been incorporated into protective mechanical ventilation strategies. Using our knowledge of dynamic alveolar mechanics (i.e., the dynamic change in alveolar and alveolar duct size and shape during tidal ventilation) to modify the MB so as to minimize VILI will reduce the morbidity and mortality associated with ARDS.

Prevention and treatment of acute lung injury with time-controlled adaptive ventilation: physiologically informed modification of airway pressure release ventilation

“…Indeed, the concept that the pulmonary parenchyma falls into three crudely differentiated categories according to the gravitational axis is being challenged. The current understanding is that open and collapsed tissues are not delineated into compartments, but are rather intermingled throughout the entire lung [17][18][19][20][21]. The unchanged mortality associated with the LVt method may also reflect the fact that maintaining lung tissue collapse ("resting") may not be protective [1,[3][4][5][6][7][8][9][10][11].…”

“…Cereda et al hypothesized that VILI is not caused by overdistension of normal lungs, but rather develops in multiple areas of excessive regional strain located throughout the lung and caused by the primary insult [17]. They showed that tissue adjacent to the primary lesion was most susceptible to secondary VILI, an outcome supported by dynamic modeling of interdependent parenchyma during ALI [21]. This suggests that to effectively reduce VILI at the bedside, the clinician needs to know how to adjust ventilator settings (e.g., Vt, Pplat, PEEP, inspiratory and expiratory duration) to reduce R/D and S-M [50][51][52].…”

“…Alveoli are often misunderstood as elastic and modeled as rubber balloons with immediate size changes (volumetric distortion or strain) with application or removal of pressure (physical stress) during inspiration and expiration [15]. In reality, alveoli behave in a viscoelastic rather than an elastic manner [21,[61][62][63]. Viscoelastic systems exhibit a time-dependent strain and can be conceptualized by the spring-and-dashpot model ( Fig.…”

“…Viscoelastic systems exhibit a time-dependent strain and can be conceptualized by the spring-and-dashpot model ( Fig. 2) [21]. Figure 2 illustrates the strain/time curve of elastic (spring), viscous (dashpot), and viscoelastic (spring-and-dashpot) behaviors.…”

“…7 Gross lung photos with corresponding driving pressure (ΔP), tidal volume (Vt), and respiratory system compliance (C RS ) values over time [21]. Two protective mechanical ventilation strategies, the TCAV method (a) and the ARDSNet (LVt) method (b), were tested in a clinically applicable 48-h porcine ARDS model of peritoneal sepsis (PS) and gut ischemia/reperfusion (I/R) injury [21]. The evolution of C RS , ΔP, and Vt with time in each group occurred over the 48-h study period (c, d).…”