Weaning from Mechanical Ventilation in ARDS: Aspects to Think about for Better Understanding, Evaluation, and Management

Abstract

Acute respiratory distress syndrome (ARDS) is characterized by severe inflammatory response and hypoxemia. The use of mechanical ventilation (MV) for correction of gas exchange can cause worsening of this inflammatory response, called "ventilator-induced lung injury" (VILI). The process of withdrawing mechanical ventilation, referred to as weaning from MV, may cause worsening of lung injury by spontaneous ventilation. Currently, there are few specific studies in patients with ARDS. Herein, we reviewed the main aspects of spontaneous ventilation and also discussed potential methods to predict the failure of weaning in this patient category. We also reviewed new treatments (modes of mechanical ventilation, neuromuscular blocker use, and extracorporeal membrane oxygenation) that could be considered in weaning ARDS patients from MV.

Figure 1

Double‐triggering occurs when a spontaneous effort triggers a (second) ventilator breath before the initial breath has completely exhaled (arrow). The pressure-time trace (upper panel) and flow-time trace (mid panel) demonstrate the occurrence of the additional breath, but do not give a sense that both inspirations are summed; this is apparent from the volume-time (lower panel) trace indicating that the double-triggering results in a substantially larger (potentially injurious) VT (red) compared with regular triggering (blue). Legend: VT: tidal volume. With authors permission [15].

Figure 2

Dynamic CT scan in end-expiration (left panel) demonstrates that the aerated lung (blue) is nondependent, while the dependent lung is densely atelectatic (red). At end-inspiration during a spontaneous breath (mid panel), there is little change in the nondependent aerated lung (blue); the dependent lung, previously densely atelectatic (red) is now partially aerated (green/red). The inspiratory pleural pressure traces (right panel), measured at the arrow tips, show the negative deflections (“swings”) in regional Ppl and global Pes during inspiration. However, the “swing” in regional Ppl is greater (x2) than the “swing” in Pes, indicating that diaphragm contraction results in greater distending pressure applied to the regional lung near the diaphragm compared with the pressure transmitted to the remainder of the lung (i.e., Pes). Ppl: pleural pressure; Pes: esophageal pressure; HU: Hounsfield Units, with authors permission [15].

Figure 3

Weaning failure in ARDS. ARDS: “Acute Respiratory Distress Syndrome”; VILI: “Ventilator-Induced Lung Injury”; MV: “Mechanical Ventilation”; TV: “Tidal Volume”; DP: “Driving Pressure”; P-SILI: “patient self-inflicted lung injury”.

Figure 4

Electrical impedance tomography (EIT) waveforms in experimental lung injury, spontaneous versus ventilator breaths. In an anesthetized pig model of acute lung injury assist pressure-controlled ventilation (IP, 15 cm H2O; f, 25 min21; PEEP=13 cm H2O; triggering threshold, 22 cm H2O) was used. The EIT image was divided into four zones, each covering 25% of the ventrodorsal diameter (zones 1–4). During controlled ventilation (under muscle paralysis), simultaneous inflation of each of the different lung regions was observed, although at different inflation rates. In contrast, when spontaneous efforts were present, two observations were noted. First, in the initial stages of the breath, spontaneous efforts caused inflation of dependent lung regions (red in zones 3 and 4), which was greater with controlled breaths. Second, the early inflation in the dependent region was accompanied by concomitant (transient) deflation of nondependent region (red in zone 1), indicating movement of gas from nondependent to dependent lung regions, because this was not associated with alterations in tidal volume it indicates a pendelluft phenomenon. This finding was always present during spontaneous breathing efforts in all animals with experimental lung injury: f = respiratory frequency; IP: inspiratory pressure; PEEP: positive end-expiratory pressure, with authors permission [16].

Figure 5

Electrical Impedance Tomography. Example of the visualization of the variation in pulmonary ventilation seen through EIT of MV weaning in two ARDS patients in the first 2 hours. Image on the left indicates the gain in ventilation in green with an increase in TV from 6 to 12 ml/kg. The patient on the right showed weaning failure and prolonged weaning from MV. Image displays an increase in TV from 6 to 8 ml/kg and loss of ventilation variation in red; the patient showed simple weaning from MV.