Fifty Years of Research in ARDS. Setting Positive End-Expiratory Pressure in Acute Respiratory Distress Syndrome

Abstract

Positive end-expiratory pressure (PEEP) has been used during mechanical ventilation since the first description of acute respiratory distress syndrome (ARDS). In the subsequent decades, many different strategies for optimally titrating PEEP have been proposed. Higher PEEP can improve arterial oxygenation, reduce tidal lung stress and strain, and promote more homogenous ventilation by preventing alveolar collapse at end expiration. However, PEEP may also cause circulatory depression and contribute to ventilator-induced lung injury through alveolar overdistention. The overall effect of PEEP is primarily related to the balance between the number of alveoli that are recruited to participate in ventilation and the amount of lung that is overdistended when PEEP is applied. Techniques to assess lung recruitment from PEEP may help to direct safer and more effective PEEP titration. Some PEEP titration strategies attempt to weigh beneficial effects on arterial oxygenation and on prevention of cyclic alveolar collapse with the harmful potential of overdistention. One method for PEEP titration is a PEEP/FiO2 table that prioritizes support for arterial oxygenation. Other methods set PEEP based on mechanical parameters, such as the plateau pressure, respiratory system compliance, or transpulmonary pressure. No single method of PEEP titration has been shown to improve clinical outcomes compared with other approaches of setting PEEP. Future trials should focus on identifying individuals who respond to higher PEEP with recruitment and on clinically important outcomes (e.g., mortality).

Keywords: acute respiratory distress syndrome (ARDS); mechanical ventilation; positive end-expiratory pressure (PEEP).

Figures

Figure 1.

Mechanisms of ventilator-induced lung injury. Left panel shows lung regions at end-expiration. Right panel shows the same lung regions at end-inspiration. (A) Patent alveoli are overdistended or stretched to injurious volumes. (B) Some tissue may be injured by excessive stress at the margins between atelectatic and aerated alveoli. (C) Small bronchioles and alveoli may be injured by mechanical forces involved in repeated opening and closing. Reprinted with permission from Reference .

Figure 2.

Example of pressure–volume (P–V) curves from a representative patient at two different positive end-expiratory pressure (PEEP) levels (darkblue line, PEEP of 5 cm H2O and pink line, PEEP 14 cm H2O). P–V curves are plotted on the same volume axis. The vertical solid line indicates end-expiratory lung volume (EELV) measured using a nitrogen washout/washin technique. The dashed lines indicate volume expired from PEEP to zero end-expiratory pressure (PEEP-vol 1 and PEEP-vol 2). Recmes is the measured recruitment induced by the increase in PEEP from 5 to 14 cm H2O. Recestim is the predicted recruitment derived from the change in EELV minus the minimum predicted increase in lung volume based on compliance and the increment in PEEP. Gray inset is a representation of Recestim. Reprinted with permission from Reference .

Figure 3.

Sample inspiratory static pressure–volume curve of the respiratory system showing two inflection points (PFLEX). Positive end-expiratory pressure (PEEP) is set above the lower inflection point to avoid alveolar collapse. Smaller black triangles represent static inflation pressure points measured from a representative patient. Small gray shaded triangle represents the calculated compliance with a small tidal volume. Larger gray shaded triangle represents the calculated compliance with a high tidal volume. Reprinted with permission from Reference .

Figure 4.

Airway (Paw), esophageal (Pes), and transpulmonary (PL) pressure waveforms in three different clinical circumstances. The resulting PL is similar for all three patients; however, the Paw and Pes are different. The Pes, as an estimate of pleural pressure, varies based on the contribution of the chest wall. Reprinted with permission from Reference . ARDS = acute respiratory distress syndrome.