PEEP is the application of pressures greater than atmospheric to the airway during the expiratory phase. The term continuous positive airway pressure (CPAP) is usually reserved for constant pressures greater than atmospheric applied to the airway of a spontaneously breathing patient. With CPAP, the patient is responsible for ventilation (no additional pressure during inhalation), whereas inspiratory assistance is provided with PEEP.
PEEP increases P̄aw and mean intrathoracic pressure. This has effects on many physiologic functions (Table 13-1). When applied to appropriate levels for the clinical setting, PEEP improves pulmonary mechanics and gas exchange, and may have varying effects on the cardiovascular system.
Pulmonary mechanics Since pressure and volume in the lungs are related, the application of PEEP increases the functional residual capacity (FRC). In the setting of alveolar collapse, PEEP maintains alveolar recruitment. With recruitment of collapsed lung units, lung compliance improves. The increase in lung volume with PEEP can be the result of alveolar recruitment or an increased volume of already open alveoli. If PEEP overdistends already open alveoli, compliance will decrease. Depending on the overall balance between recruitment and overdistention, the application of PEEP may increase, decrease, or not affect tidal compliance. However, the appropriate application of PEEP in a patient with lung injury generally improves lung compliance. An appropriate level of PEEP also decreases work-of-breathing in spontaneously breathing patients. Excessive PEEP places the lung on the upper flat portion of the pressure-volume curve, thus decreasing compliance and increasing work-of-breathing.
Gas exchange In most clinical applications, PEEP is applied to improve Pao2. This is accomplished through alveolar recruitment and decreasing intrapulmonary shunt. Appropriate PEEP may also improve Paco2 – Petco2 (end-tidal CO2) and Paco2 by decreasing dead space. Excessive PEEP can decrease perfusion to well-ventilated areas of the lungs, causing an increase in dead space and Paco2. For patients with unilateral lung disease, PEEP may result in overdistension of healthy lung units with shunting of blood to the diseased lung units, worsening hypoxemia.
Cardiovascular function The effect of PEEP on the cardiovascular system is dependent on the level of PEEP, the compliance of the respiratory system, and the cardiovascular status. Because PEEP increases P̄aw and mean intrathoracic pressure, venous return and cardiac output may decrease as PEEP is applied. PEEP has the greatest effect on cardiac output in a setting where lung compliance is high, chest wall compliance is low and cardiovascular reserve is low. High levels of PEEP decrease right ventricular preload, increases right ventricular afterload (increased pulmonary vascular resistance), and may shift the interventricular septum to the left. This, along with a reduction in pericardial pressure gradient, limits left ventricular distensibility, reducing left ventricular end-diastolic volume and stroke volume. Thus, both pulmonary and systemic vascular pressures are affected by PEEP. Because PEEP increases pressure outside of the heart, it decreases left ventricular afterload. The net result may be a decreased cardiac output, arterial blood pressure, urine output, and tissue oxygenation. Thus, PEEP may increase arterial oxygenation but decrease tissue oxygenation.
Intracranial pressure As PEEP decreases venous return, intracranial pressure may increase with the application of PEEP. This is usually not an issue unless intracranial pressure is already increased. The effect of PEEP is decreased by elevating the head, which is commonly applied in the care of these patients. PEEP should be used cautiously in any patient where increased intracranial pressure is a concern, but levels less than or equal to 10 cm H2O are usually not a problem.
Barotrauma The amount of overdistention produced with PEEP determines the probability of barotrauma. As lung injury is often heterogeneous, overdistention of an individual lung unit may be achieved at any PEEP level. However, barotrauma occurs due to a high end-inspiratory pressure and, thus, PEEP increases the risk of barotrauma only to the extent that it promotes end-inspiratory overdistention.
Indications for PEEP are shown in Table 13-2.
Table 13-2Indications for PEEP ||Download (.pdf) Table 13-2 Indications for PEEP
• Acute respiratory distress syndrome
• Chest trauma
• Postoperative atelectasis
• Cardiogenic edema
• Acute artificial airway
Acute respiratory distress syndrome Application of 10 to 20 cm H2O PEEP is standard practice in patients with early acute respiratory distress syndrome (ARDS) to maintain alveolar recruitment. In later stages of ARDS, however, fibroproliferation is observed and generally 5 to 10 cm H2O PEEP is used in this setting.
Chest trauma PEEP is used to stabilize the chest wall and prevent paradoxical movement in the setting of flail chest. If ARDS is not present, 5 to 10 cm H2O PEEP is indicated, provided no pulmonary air leak is present and the patient is hemodynamically stable.
Postoperative atelectasis Use of CPAP by facemask may be beneficial to treat postoperative atelectasis. CPAP can be administered continuously or applied for 15 to 30 minutes every 2 to 6 hours at levels of 5 to 10 cm H2O.
Cardiogenic pulmonary edema PEEP decreases preload and afterload. The use of PEEP or CPAP at 5 to 10 cm H2O improves oxygenation, decreases work-of-breathing, increases left-ventricular performance, and improves cardiac output.
Artificial airways Insertion of an artificial airway decreases FRC and may compromise gas exchange. Application of 5 cm H2O PEEP is typically used with intubated patients unless otherwise contraindicated. However, most patients with long-term tracheostomy do not need PEEP or CPAP.
Auto-PEEP The magnitude of auto-PEEP is dependent on the time constant (resistance and compliance), expiratory time, and tidal volume (VT). Auto-PEEP is not observed on the ventilator unless an end-expiratory hold is used. The first indication of auto-PEEP may be inability to trigger the ventilator. Applying PEEP in this setting counterbalances auto-PEEP, decreases the effort needed to trigger, and may not affect end-expiratory alveolar pressure. For the patient who is having difficulty triggering the ventilator, PEEP can be slowly increasing until patient is able to comfortably trigger each breath. At the appropriate level of applied PEEP, patient rate decreases and signs of cardiopulmonary stress subside. PEEP counterbalances auto-PEEP with flow limitation (dynamic airway closure). However, PEEP does not affect auto-PEEP when auto-PEEP is due to high minute ventilation. In volume-controlled ventilation (VCV), if the applied PEEP increases end-expiratory alveolar pressure, peak inspiratory pressure (PIP) and plateau pressure (Pplat) increase. If changes in applied PEEP does not affect PIP with VCV, or tidal volume with PCV (and constant PIP), then auto-PEEP is present.
Ventilator-associated pneumonia Because PEEP raises intratracheal pressure, it decreases the amount of microaspiration around the cuff of the artificial airway. In that way, it decreases contamination of lower respiratory tract and decreases the risk of ventilator-associated pneumonia.
The primary indication for PEEP is ARDS. The goal of PEEP in this setting is prevention of de-recruitment and maintenance of tissue oxygenation. PEEP improves shunt, reverses hypoxemia, and decreases the work-of-breathing. Further, this is accomplished without adversely affecting cardiac output.
The goal is to set PEEP at a level that maximizes alveolar recruitment and avoids overdistention. Higher levels of PEEP may be appropriate for moderate to severe ARDS and modest levels of PEEP may be appropriate for mild ARDS. Because the potential for recruitment is variable among patients with ARDS, it must be titrated for the individual patient. Arterial blood pressure and pulse oximetry are monitored when PEEP is applied. The specific approach used to titrate PEEP is one of the most contentious subjects related to mechanical ventilation. PEEP can be titrated after a recruitment maneuver. Using this approach, PEEP is set higher than necessary to maintain alveolar recruitment and then slowly decreased until the lowest PEEP maintaining the best compliance is identified. Alternatively, PEEP is increased stepwise while monitoring Spo2, Pplat, compliance, and blood pressure. A decrease in Spo2, decrease in compliance, decrease in blood pressure, and Pplat more than 30 cm H2O suggests overdistention. In the setting of a stiff chest wall, PEEP is set to counterbalance the alveolar collapsing effect of the chest wall, and an esophageal balloon may be useful in this setting to estimate pleural pressure. Another approach is to use PEEP/Fio2 combinations as have been used in the ARDS network studies. PEEP for ARDS is generally set between 10 and 20 cm H2O. Hemodynamic monitoring is necessary during PEEP titration due to the potential to adversely affect cardiovascular function.
PEEP should not be abruptly withdrawn. If PEEP is reevaluated on a regular basis, there is usually no need to make large changes in PEEP. Of concern is alveolar de-recruitment and hemodynamic instability with the withdrawal of PEEP. If the Spo2 decreases when PEEP is decreased, the prior level should be re-established rather than increasing the Fio2.