Lung Opening and Closing during Ventilation of Acute Respiratory Distress Syndrome

TLDR

High positive end‑expiratory pressure (PEEP) effects in ARDS depend on lung recruitability, as increasing PEEP can worsen injury by raising alveolar strain or by reducing intratidal opening and closing, which have opposing impacts. The study aimed to determine how lung recruitability modulates alveolar strain and intratidal opening and closing after high PEEP application. Using whole‑lung computed tomography from 68 ARDS patients at 5, 15, and 45 cm H₂O, the authors calculated alveolar strain and opening/closing tissue at 5 and 15 cm H₂O PEEP from pressure–volume curves. In patients with high recruitable lung, raising PEEP markedly decreased intratidal opening and closing tissue while leaving alveolar strain unchanged, whereas in patients with low recruitability no such reduction occurred; moreover, opening/closing tissue was predominantly in dependent and hilar regions and independently predicted mortality (OR 1.10 per 10‑g increase).

Abstract

Rationale: The effects of high positive end-expiratory pressure (PEEP) strictly depend on lung recruitability, which varies widely during acute respiratory distress syndrome (ARDS). Unfortunately, increasing PEEP may lead to opposing effects on two main factors potentially worsening the lung injury, that is, alveolar strain and intratidal opening and closing, being detrimental (increasing the former) or beneficial (decreasing the latter).Objectives: To investigate how lung recruitability influences alveolar strain and intratidal opening and closing after the application of high PEEP.Methods: We analyzed data from a database of 68 patients with acute lung injury or ARDS who underwent whole-lung computed tomography at 5, 15, and 45 cm H2O airway pressure.Measurements and Main Results: End-inspiratory nonaerated lung tissue was estimated from computed tomography pressure–volume curves. Alveolar strain and opening and closing lung tissue were computed at 5 and 15 cm H2O PEEP. In patients with a higher percentage of potentially recruitable lung, the increase in PEEP markedly reduced opening and closing lung tissue (P < 0.001), whereas no differences were observed in patients with a lower percentage of potentially recruitable lung. In contrast, alveolar strain similarly increased in the two groups (P = 0.89). Opening and closing lung tissue was distributed mainly in the dependent and hilar lung regions, and it appeared to be an independent risk factor for death (odds ratio, 1.10 for each 10-g increase).Conclusions: In ARDS, especially in patients with higher lung recruitability, the beneficial impact of reducing intratidal alveolar opening and closing by increasing PEEP prevails over the effects of increasing alveolar strain.