Special Feature

PEEP for One and PEEP for All

By Dean R. Hess, PhD, RRT, Assistant Director of Respiratory Care, Massachusetts General Hospital, Associate Professor of Anesthesia, Harvard Medical School, Boston, MA, is Associate Editor for Critical Care Alert.

Dr. Hess serves as a retained consultant for Philips Respironics, ResMed, Breathe, and Pari, and also receives honorarium from Covidien.

Would you like to start a fight? Just ask a colleague how he or she selected the level of positive end-expiratory pressure (PEEP) for a patient. The response is likely to be emotional, but equally likely to lack support from high-level evidence. Although PEEP is beneficial for most, if not all, mechanically ventilated patients, the available evidence is not particularly helpful in guiding selection of PEEP for an individual patient. In this essay, I will discuss the use of PEEP not only in the setting of acute lung injury/acute respiratory distress syndrome (ALI/ARDS), but also to reduce micro-aspiration, to counterbalance auto-PEEP, to aid the failing heart, to splint airways in the presence of tracheomalacia, and to improve speech in tracheostomized patients with cuff-down technique.

PEEP for ALI/Ards

That ventilator-induced lung injury (VILI) can result from inappropriate ventilator settings is well accepted. Tidal volume and inspiratory pressure limitation are standards of care to avoid over-distention. It also is accepted that cyclical alveolar opening and closing throughout the respiratory cycle is injurious; thus PEEP sufficient to maintain alveolar recruitment also is important. There should be no argument that no PEEP (zero) is harmful in patients with ARDS. But the level of PEEP to be used is controversial.

In three clinical trials,1-3 tidal volume was held constant at 6 mL per kg of ideal body weight with patients assigned to receive either a higher or a lower level of PEEP. In the groups of patients receiving higher levels of PEEP, there were benefits such as higher PaO2/FIO2, higher respiratory system compliance, less frequent use of rescue therapies for refractory hypoxemia, and a greater number of ventilator-free days. But there was no significant reduction in mortality for patients receiving higher PEEP.

The value of a meta-analysis is that by pooling data from several studies, statistical power is improved. There have been five meta-analyses published on the use of higher vs lower PEEP in patients with ALI/ARDS.4-8 The results of these meta-analyses are similar: The use of higher levels of PEEP, when compared to use of moderate levels of PEEP, does not lead to lower mortality in groups of unselected patients with ALI/ARDS.9 Why is this?

If alveolar recruitment potential is low, an increase in PEEP may contribute to over-distention of already open alveoli. This results in a decrease in respiratory system compliance, increased dead space, and redistribution of pulmonary blood flow to unventilated alveoli. But when the potential for recruitment is high, the benefit of higher levels of PEEP may outweigh harm due to over-distention. When the PaO2/FIO2 is lower (that is, in ARDS rather than ALI), the potential for recruitment may be greater and this may lead to lower mortality.4 The potential for recruitment also may be greater when chest wall compliance is reduced.10

The available evidence suggests that modest levels of PEEP may be more appropriate for ALI whereas higher levels of PEEP should be used for ARDS.4 Higher levels of PEEP should be reserved for patients in whom alveolar recruitment can be demonstrated. Increasing PEEP while driving up the plateau pressure to harmful levels makes no sense.

A variety of techniques for PEEP selection have been described (see Table).9 Some have suggested a decremental procedure in which PEEP is initially set ≥ 20 cm H2O and then decreased to identify the level that produces the best compliance. The one study that evaluated this approach did not report a benefit.11

Table. Approaches to Setting PEEP in Patients with ALI/ARDS

Gas exchange

Oxygenation: PEEP/FIO2 tables
Dead space

Respiratory mechanics

Compliance (lowest Pplat – PEEP)
Pressure-volume curve
Stress index
Transpulmonary pressure (esophageal balloon)

Imaging

Chest CT
EIT
Ultrasound

Key: acute lung injury (ALI); acute respiratory distress syndrome (ARDS); computed tomography (CT); electrical impedance tomography (EIT); inspired oxygen fraction (FIO2); positive end-expiratory pressure (PEEP); end-inspiratory plateau pressure (Pplat)

PEEP TO REDUCE MICRO-ASPIRATION

It is accepted that the source of contamination of the lower respiratory tract leading to ventilator-associated pneumonia is usually microaspiration of upper airway secretions from around the cuff of the endotracheal tube. This has led to redesigns of the cuff to minimize the creation of longitudinal folds when the cuff is inflated12 and avoidance of a cuff pressure < 20 cm H2O.

There are several in vitro studies that report a reduction in leak past the cuff when PEEP is applied.13-15 Presumably, the tracheal pressure generated by PEEP produces back-pressure, which inhibits leakage from the upper airway. In a clinical study of intubated postoperative patients with normal lungs, it was reported that the rate of ventilator-associated pneumonia with 5 cm H2sub>O PEEP was significantly lower than with a PEEP of zero.16 This suggests that PEEP should be used in a bundle of strategies to reduce the risk of ventilator-associated pneumonia.

PEEP TO COUNTERBALANCE AUTO-PEEP

Auto-PEEP is a threshold pressure that must be overcome by a spontaneously breathing patient before the pressure (or flow) decreases at the proximal airway to trigger the ventilator. Increasing the set PEEP to counterbalance auto-PEEP may improve the patient's ability to trigger the ventilator.17 Typically, the patient with auto-PEEP will be asynchronous with the ventilator, such that there are inspiratory efforts that do not trigger the ventilator. The PEEP setting is increased until the patient can comfortably trigger the ventilator, provided that there is no increase in plateau pressure. PEEP should be used in patients with chronic obstructive pulmonary disease (COPD) only to unload the respiratory muscles from the auto-PEEP due to expiratory flow limitation. If auto-PEEP is not caused by flow limitation, application of PEEP will cause further hyperinflation, worsening respiratory mechanics, muscle activity, and hemodynamics.18

The use of PEEP in the setting of auto-PEEP for acute asthma is more controversial.19 Whereas small airway collapse and flow limitations occur in COPD, with asthma the site of increased resistance is in central, less collapsible airways. The results of one study suggest that the physiology may be variable, so some patients with acute asthma respond to PEEP with an increase in lung volume, some with no change in lung volume, and some with a paradoxical decrease in lung volume.20 This suggests that the use of PEEP to counterbalance auto-PEEP in acute asthma must be individualized, balancing the benefit of improved ability to trigger against the risk of worsening dynamic hyperinflation.

PEEP TO AID THE FAILING HEART

The increase in intrathoracic pressure associated with PEEP can aid the failing heart by decreasing preload and afterload. In addition, PEEP increases lung volume, which decreases the work of breathing and improves arterial oxygenation. This is most commonly appreciated with the use of continuous positive airway pressure (CPAP) by facemask in patients with acute cardiogenic pulmonary edema. In a recent Cochrane review,21 CPAP plus standard medical care compared with standard medical care alone (12 trials, 614 patients) was associated with a significantly lower hospital mortality in CPAP-treated patients (relative risk [RR] 0.58, 95% CI 0.38 to 0.88); this translates into a number-needed-to-treat (NNT) of 9. CPAP plus standard medical care compared with standard medical care alone (12 trials, 616 patients) was associated with a significantly lower rate of endotracheal intubation favoring CPAP-treated patients (RR 0.46, 95% CI 0.32 to 0.65, NNT 6). Available evidence supports the use of mask CPAP as standard therapy for acute cardiogenic pulmonary edema. Similar results occur with noninvasive ventilation.21

PEEP TO SPLINT AIRWAYS

Tracheomalacia is a weakness of the trachea that leaves the airway susceptible to collapse. With tracheomalacia, there is accentuation of the physiologic process in which the intra-thoracic trachea dilates with inspiration and narrows with exhalation due to changes in intrathoracic pressure. Tracheal collapse during exhalation results from this narrowing. CPAP, by creating a "pneumatic stent," prevents the collapse of the airway throughout the respiratory cycle. CPAP and PEEP have been used as an effective treatment for tracheomalacia in both infants and adults.22

PEEP TO FACILITATE SPEECH

In a patient with a tracheostomy tube, gas can escape through the upper airway during the inspiratory phase if the cuff is deflated. Simple manipulations of the ventilator settings allow the patient to speak during both the inspiratory phase and expiratory phase. This avoids the use of a speaking valve, which provides safety if the upper airway becomes obstructed. If the PEEP setting on the ventilator is zero, most of the exhaled gas exits through the ventilator circuit rather than passing around the deflated cuff and through the upper airway. In this situation, there is little ability to speak during the expiratory phase. If PEEP is set on the ventilator, then expiratory flow is more likely to occur through the upper airway when the cuff is deflated, which increases the ability to speak. A longer inspiratory time and higher PEEP are additive in their ability to improve speaking. Tracheal pressure (important for speech) is similar with the use of PEEP and the use of a speaking valve.23

COMPLICATIONS OF PEEP

Some phobia exists related to PEEP with respect to complications such as barotrauma and hemodynamic compromise. However, these complications relate to excessive PEEP, not correctly selected PEEP. If PEEP is appropriately selected, with the risk of over-distention monitored in the individual patient, complications of PEEP are minor or nonexistent.

SUMMARY

PEEP is beneficial in most, if not all, patients receiving invasive or noninvasive mechanical ventilation. But PEEP has potential for both benefit and harm. It is the job of the bedside clinician to balance benefit against harm as the level of PEEP is selected for an individual patient. Determining the appropriate level of PEEP often involves a bedside n-of-1 experiment (that is, varying the PEEP setting and objectively assessing both the positive and negative effects); the level used needs to be reassessed as the patient's condition changes over time.

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