Positive End-Expiratory Pressure

Introduction

• PEEP is a cornerstone of mechanical ventilation and encompasses both extrinsic PEEP applied by the ventilator and intrinsic PEEP resulting from air trapping in obstructive lung disease.
• Understanding how PEEP maintains alveolar stability, improves oxygenation, and influences cardiopulmonary physiology is essential for safe and effective ventilator management.
• Clinicians must balance the benefits of alveolar recruitment with the risks of overdistension and hemodynamic compromise, tailoring both extrinsic and intrinsic PEEP management to the patient’s underlying pathology.

Extrinsic PEEP vs. Intrinsic PEEP

• There are two categories of PEEP: extrinsic PEEP (also called applied PEEP or PEEPE) and intrinsic PEEP (also called auto-PEEP or PEEPI).
• Extrinsic PEEP is a ventilator pressure setting used to maintain alveolar patency during mechanical ventilation. During exhalation, alveolar sacs can collapse, reducing surface area for gas exchange. Applying PEEP can prevent alveolar collapse and increase oxygenation.

• Intrinsic PEEP is the positive alveolar pressure resulting from air trapping in the lungs. Intrinsic or auto-PEEP is often seen in patients with chronic obstructive pulmonary disease (COPD) or other conditions causing high lung compliance.

Physiological Effects of PEEP

Pulmonary Effects of Extrinsic PEEP

• PEEP prevents alveolar collapse by increasing functional residual capacity, splitting alveoli open, and preventing atelectasis.
• It improves oxygenation by increasing alveolar surface area for gas exchange.
• It reduces the work of breathing by offsetting intrinsic PEEP and facilitating triggering in spontaneously breathing patients.
• It reduces atelectrauma by reducing shear injury from repeated opening and closing of alveoli in inflammatory lung conditions such as acute respiratory distress syndrome (ARDS).

Hemodynamic Effects of Extrinsic PEEP

• PEEP increases intrathoracic pressure, reducing venous return and decreasing right-heart filling. It can cause hypotension, especially in patients with hypovolemia, right heart failure, or obstructive shock.
• It increases pulmonary vascular resistance, particularly when excessive PEEP results in alveolar overdistension. This can worsen right heart failure and reduce left heart preload.

Indications for Extrinsic PEEP

Routine Invasive Mechanical Ventilation

• Patients who are sedated and paralyzed while on a ventilator are unable to adequately maintain alveolar patency that arises from the outward recoil of the chest wall and the inward elastic recoil of the lungs (called functional residual capacity). In patients intubated for surgery or to maintain a secure airway, atelectasis is prevented by applying PEEP.

ARDS

• ARDS lungs become rapidly fluid-filled, undergo loss of surfactant, and are heterogeneous with some alveoli collapsed and others overstretched. Applying PEEP helps stabilize and recruit alveoli, improving oxygenation. PEEP in ARDS may also prevent ventilator-induced lung injury by preventing cyclic opening and collapsing of alveoli.¹

Pulmonary Edema

• By increasing intrathoracic pressure, moderate-to-high PEEP can shift hydrostatic forces, pushing fluid out of the alveoli and back into the interstitium. PEEP can also help reduce left ventricular afterload, improving cardiac output in left heart failure and preventing fluid backup in the lungs.²

Contraindications/Precautions for Extrinsic PEEP

Untreated Pneumothorax

• Extrinsic PEEP increases intrathoracic pressure and can rapidly worsen a pneumothorax or precipitate tension physiology. A pneumothorax should be decompressed prior to initiating PEEP.

Hemodynamic Instability from Obstructive Shock

• Extrinsic PEEP increases intrathoracic pressure, decreasing venous return and reducing cardiac output. In patients with massive pulmonary embolism, cardiac tamponade, or right ventricular failure, high PEEP can worsen hypotension.

Profound Hypovolemia

• Likewise, in distributive shock from hypovolemia, PEEP increases intrathoracic pressure, decreases venous return, and may lead to significant hypotension. Volume status should be corrected before increasing PEEP.

Post-Single-Ventricle Palliation

• Patients with Glenn and Fontan physiology rely on passive pulmonary blood flow driven by the pressure gradient between the systemic veins and the pulmonary arteries. Because there is no subpulmonic ventricle, pulmonary blood flow, and therefore ventricular preload, is exquisitely sensitive to increases in intrathoracic pressure.³

Bronchopleural Fistula

• An abnormal connection between the bronchi and pleural space allows a continuous air leak from the airway into the pleural cavity. Similar to a pneumothorax, applying PEEP increases intrathoracic pressure, forcing air into the pleural space and may lead to tension physiology. High PEEP can worsen a bronchopleural fistula by increasing flow through the defect.⁴

Elevated Intracranial Pressure (ICP)

• There is little evidence that PEEP directly impacts cerebral perfusion pressure. However, the effect of PEEP on cardiac output and subsequent changes in mean arterial pressure should be taken into consideration when caring for patients with elevated ICP.⁵

High Intrinsic PEEP

• In patients with significant intrinsic PEEP (e.g., COPD, asthma), care should be taken when applying extrinsic PEEP. Closely monitor for signs of hyperinflation and barotrauma. While a small amount of extrinsic PEEP may be beneficial for treating intubated patients with air trapping, this should be balanced with additional methods (e.g., decreased I:E ratio, reduced respiratory rate, etc.).

Monitoring and Titrating PEEP

• There is uncertainty and variability in optimal PEEP, and no single method consistently improves mortality or ventilator-free days, regardless of the reason a patient is on a ventilator. Current guidelines for simple PEEP titration rely on PEEP/FiO₂ tables, such as the following from ARDSNet:

• Compliance curves (a.k.a. pressure-volume curves) may also assist in titrating PEEP by identifying the pressure range where the lung is most open without becoming overdistended, essentially creating a map of lung recruitability and preventing ventilator-induced lung injury. “Beaking” during the inspiratory phase can indicate overdistension.¹

• Minimizing high driving pressure while applying PEEP may also reduce mortality by reducing ventilator-induced lung injury.¹
• Advanced methods for monitoring PEEP include electrical impedance tomography, esophageal balloon manometry, shunt fraction (Qs/Qt), or end-expiratory lung volume, among others. These may be considered in complex conditions (obesity, chest-wall disease, or heterogeneous lung pathology as seen in ARDS); however, there is limited outcome data on their use to decrease mortality or ventilator-free days.7

Optimal PEEP

• Optimal PEEP is the pressure at which the lung is maximally recruited while avoiding barotrauma and cardiovascular compromise. Too little PEEP results in alveolar collapse and atelectrauma. Too much PEEP results in overdistension, lung injury, and hemodynamic compromise.
• There is little evidence to support a “one-size-fits-all” approach to PEEP; it should be personalized for each patient.

Managing Intrinsic PEEP

• Intrinsic or “auto-PEEP” occurs when air becomes trapped in the lungs during incomplete exhalation. This leads to increased lung volume, increased intrathoracic pressure, and positive alveolar pressure at the end of expiration. Intrinsic PEEP is often seen in COPD, asthma, tachypnea, or mechanical ventilation with high respiratory rates and/or short expiratory times.
• One method to manage intrinsic PEEP in intubated patients is to reduce the respiratory rate and prolong the expiratory phase by decreasing the I:E ratio.
• A small amount of extrinsic PEEP can help promote synchrony by reducing the pressure difference between the expiratory and inspiratory phases of mechanical ventilation.