part0012

Positive end-expiratory pressure (PEEP) is used during mechanical ventilation to maintain alveolar patency and to improve oxygenation. When the ventilator delivers a breath, the airway pressure is raised to the peak airway pressure (or peak inspiratory pressure). Expiration is passive, but instead of allowing the lungs to exhale until the pressure is atmospheric (i.e., zero), the ventilator stops expiratory flow when the pressure reaches the preset end-expiratory pressure. This is analogous to breathing out while a large fan blows air into your lungs—it provides a pressurized splint of air, which in turn maintains airway and alveolar patency when they would otherwise collapse. During conventional mechanical ventilation, this is known as PEEP. During spontaneous and noninvasive ventilation, it is called continuous positive airway pressure (CPAP). Since both have the same physiologic effect, they are essentially synonymous.

PEEP is most often applied and adjusted to improve a patient's oxygenation. The primary effect of PEEP in hypoxemic respiratory failure is to reduce the fraction of intrapulmonary shunting; that is, areas of the lungs that are perfused but not ventilated. The more alveolar flooding there is (from pulmonary edema, bleeding, pneumonia, exudate, etc.), the more pressure is needed to open up collapsed lung units. The patient's chest X-ray can be used as a rough guide for the initial PEEP.

The majority of patients with hypoxemic respiratory failure can be managed rather easily with a PEEP in the range of 5-10 cm H 2 O. In those with moderate to severe acute respiratory distress syndrome (ARDS), a more intensive regimen may be required. This is known as trying to set the "best PEEP" or "optimal PEEP"—that is, the PEEP that attains the best oxygenation and compliance while minimizing the risk of ventilator-induced lung injury. Numerous clinical approaches to the problem of finding the "optimal PEEP" for a patient have been described in the medical literature, and each method has both adherents and detractors. As you might expect, each method has its strong points and drawbacks, and no one approach is superior to the rest (otherwise everyone would use it and ignore the others). These will be discussed in turn.

The tables used in the ARDSNet studies have the advantage of simplicity and titratability to oxygenation, which can be measured easily with an arterial blood gas or a pulse oximeter. Two tables have been published—one that uses a high-PEEP approach, and one using lower levels of PEEP. The two methods were compared head-to-head in the ALVEOLI study, 12 which did not demonstrate improved outcomes from either approach as long as a lung-protective tidal volume of 4-6 mL/kg PBW was used. This is actually advantageous to the physician—it suggests that either table can be used, depending on the patient's condition. A patient who is morbidly obese, or who has abdominal compartment syndrome, has reduced chest wall compliance and might benefit from a higher-PEEP strategy. The extrinsic compression of the lungs, combined with the poor lung compliance due to ARDS, means that a higher expiratory pressure should be used to prevent alveolar collapse and derecruitment.

On the other hand, using a lower level of PEEP might be indicated in some cases. A patient with a bronchopleural fistula, or one with tenuous hemodynamics, or someone with one lung significantly more injured than the other, may get worse with a high-PEEP strategy. Since one table doesn't have any proven advantage over the other, the physician can pick whichever one seems to fit the patient better.

Chest X-Ray	Initial PEEP
Clear	5 cm H2 O
Scattered Infiltrates	10 cm H2 O
Diffuse Infiltrates Dense	15 cm H2 O
Bilateral White Out	20 cm H2 O