Lung volumes and transpulmonary pressure are decreased with expiratory effort and restored with passive breathing in ARDS: a reapplication of the traditional Campbell diagram

Dear Editor,

Neuromuscular blockade (NMB) in patients with severe acute respiratory distress syndrome (ARDS) may improve oxygenation, respiratory mechanics, and 90-day survival [1]. The role of active breathing in patients with ARDS may depend upon the type of activity as diaphragmatic contraction during expiration may be beneficial to prevent lung collapse [3], while excessive expiratory efforts [2] and strong inspiratory efforts may be harmful [4]. Guervilly et al. proposed that the benefit of NMB may be secondary to removal of spontaneous efforts as patients without NMB in their study had lower mean transpulmonary pressures (encouraging collapse), secondary to expiratory efforts [2]. They proposed that expiratory activity might counteract positive end-expiratory pressure (PEEP) by promoting collapse at end-expiration.

The Campbell diagram in respiratory physiology illustrates the pressure–volume (PV) characteristics of the chest wall [5]. PV loops are generated by plotting esophageal pressure (Pes) changes (pleural pressure) versus volume. The relaxation volume (Vrel) is the intersection of the chest wall curve with the lung-elastic-recoil curve determined by points of zero flow and will shift up or down as PEEP changes. We have reapplied these PV loops in patients with ARDS from an ongoing study to show the deflating effect of expiratory efforts. Six patients out of 35 analyzed have shown expiratory activity. Figure 1 illustrates examples of patients with expiratory effort, which increase Pes and result in negative transpulmonary pressures during expiration. Figure 1a, b illustrate two patients with subtle end-expiratory effort pushing volumes below the Vrel. Figure 1c, d show two patients transitioning from active to fully passive breathing. After becoming passive, Pes decreases during expiration leading to increased transpulmonary pressures and an increase in end-expiratory lung volume back to the Vrel. Notably these patients also have active inspiratory efforts which may lead to excessive lung stress from increased regional transpulmonary pressures [4].

Fig. 1

Campbell diagrams (pressure–volume loop of the chest wall) of four patients with acute respiratory distress syndrome (ARDS) with active breathing. Active inspiratory efforts are characterized by a negative deflection in esophageal pressure (Pes) relative to the passive chest wall and expiratory efforts are characterized by a positive deflection in Pes relative to the passive chest wall. Passive ventilation is characterized by a positive Pes deflection during inspiration and negative deflection during expiration. The relaxation volume (Vrel) on a Campbell diagram can be determined as the intersection of the passive chest wall curve with the lung compliance curve (estimated by connecting the points at roughly zero flow) and is dependent on the applied positive end-expiratory pressure (PEEP). a, b Patients with active inspiration followed initially by passive exhalation along the passive chest wall curve, with an end-expiratory effort. The expiratory effort pushes lung volumes below the Vrel, thus negating the effect of PEEP in maintaining lung volume. c, d Patients with strong active inspiration and expiration, followed by fully passive ventilation after several breaths. Expiratory efforts push lung volumes below the Vrel. Patient C has five active breaths that generate swings in Pes up to 40 cmH₂O, with decreasing inspiratory and expiratory efforts from breath 1 to breath 5 before becoming fully passive with increased end-expiratory volume. The subsequent passive breaths without any inspiratory or expiratory effort characterize the patients passive pressure–volume curve of the chest wall. Patient D illustrates a patient with weaker expiratory efforts which push lung volumes below the relaxed volume to a lesser degree

The Campbell diagram of the chest wall in patients with ARDS may be used to illustrate breathing patterns and patient–ventilator interaction. Subtle inspiratory and expiratory efforts are easily missed with monitoring of airway pressures alone and even with Pes changes via the usual time tracings. More importantly, these results show that expiratory muscle activity can cause patients to push their lungs below the relaxation volume expected if they were passively breathing. This expiratory effort appears to counteract the expected benefit of PEEP in maintaining lung volume and recruitment. This reduction in end-expiratory volume in theory could be harmful by causing derecruitment with worsened hypoxemia, degrading lung mechanics and promoting atelectrauma. Removal of these expiratory efforts by deep sedation and paralysis with NMB, thereby assuring a truly passive chest wall, may partially explain the benefit found in the Papazian study [1], but this needs further investigation. We do not currently know the prevalence of expiratory muscle activity in patients with ARDS, nor the actual clinical effects of these efforts. Our group is currently exploring these findings in more detail using the Campbell diagram to improve our detection and analysis of these events.