Positive end-expiratory pressure (PEEP) is not a ventilator mode itself, but rather an adjunctive treatment that can be combined with all forms of mechanical ventilation, both controlled and assisted,1–7 or applied to spontaneous breathing throughout the entire respiratory cycle, so-called continuous positive airway pressure (CPAP).8–10 Following the pioneering work of Poulton and Oxon11 and Barach and associates12 who demonstrated in the mid-1930s that application of positive pressure to the airway can effectively treat patients with pulmonary edema, several pathological conditions were proved to benefit from PEEP, which is today considered by intensive care unit physicians as one of the most powerful treatments available for acute respiratory failure (ARF).13
Since publication of the second edition of this book in 2006, several relevant studies have been published on PEEP and CPAP, primarily concerning lung injury and cardiogenic pulmonary edema. To update the chapter with this new information, we have condensed material that was covered in greater depth in the second edition.
The reversal of hypoxemia caused by intrapulmonary shunt and venous admixture requires interventions that recruit more aerated lung units for ventilation. In patients with an acute reduction of lung volume secondary to lung edema and/or atelectasis, PEEP can improve arterial oxygenation1,8 by increasing functional residual capacity (FRC),14–19 reducing venous admixture,20–24 shifting tidal volume (VT) to a more compliant portion of the pressure–volume curve,25 preventing the loss of compliance during mechanical ventilation,5,26 reducing intratidal alveolar opening and closing27 and the work of breathing.28 Figure 10-1A summarizes the rationale for PEEP in patients with ARF secondary to acute lung volume reduction.
Respiratory effects of PEEP/CPAP in acute respiratory failure secondary to (A) lung volume reduction caused by edema and/or atelectasis and (B) airway obstruction and expiratory flow limitation. FRC, functional residual capacity; PaO2, partial pressure of arterial oxygen; PEEP, positive end-expiratory pressure.
Although there is general consensus about the potential of PEEP in treating patients with hypoxemic ARF secondary to lung edema and/or atelectasis, several aspects are controversial. For many years, it has been recognized that the actions of PEEP on gas exchange and pulmonary mechanics are variable,29,30 depending on the type and severity of lung injury, which may be sustained by different pathways31,32 and pathophysiologic mechanisms.31,33 As with any treatment, PEEP is not free of side effects.34 In patients with acute respiratory distress syndrome (ARDS), PEEP may recruit nonaerated regions, but also distend normally aerated regions,35,36 contributing to barotrauma through increase in end-inspiratory plateau pressure.22,37–40 High levels of PEEP ...