PEEP Explained
A thorough clinical reference on positive end-expiratory pressure — its physiology, benefits, titration strategies, complications, and role in lung-protective ventilation.
Written for respiratory therapists, students, and clinicians.
What Is PEEP?
Positive end-expiratory pressure (PEEP) is pressure that is maintained in the airways above atmospheric pressure at the end of expiration during mechanical ventilation. Rather than allowing the circuit pressure to return to zero (atmospheric) at end-expiration, PEEP keeps the airway and alveolar pressure at a positive level throughout the entire respiratory cycle, including the expiratory phase.
PEEP was first described clinically in 1967 by Ashbaugh and Petty as a treatment for acute respiratory distress — the condition we now call ARDS. It has since become a cornerstone of lung-protective ventilation and is applied in virtually every mechanically ventilated patient to some degree.
A physiologic PEEP of 3–5 cmH₂O is standard practice in most ventilated patients and mimics the effect of the glottis, which maintains a small amount of natural PEEP during normal breathing. Therapeutic PEEP is applied in higher amounts to treat specific conditions.
Physiologic Effects of PEEP
The primary physiologic effect of PEEP is alveolar recruitment and stabilization. By maintaining positive pressure at end-expiration, PEEP:
Increases FRC
Raises the functional residual capacity by preventing alveolar collapse at end-expiration
Recruits Collapsed Alveoli
Opens atelectatic lung units, increasing the effective gas exchange surface area
Improves V/Q Matching
Reduces intrapulmonary shunting by keeping alveoli open for gas exchange
Improves Compliance
Recruited alveoli operate on a more favorable portion of the compliance curve
Reduces Work of Breathing
In spontaneously breathing patients, PEEP reduces the threshold to trigger a breath
Reduces Cardiac Preload
High PEEP increases intrathoracic pressure, which can reduce venous return and cardiac output
Intrinsic PEEP (Auto-PEEP)
Auto-PEEP (also called intrinsic PEEP or iPEEP) is inadvertent PEEP that develops when the expiratory time is insufficient to allow full exhalation to the set PEEP level before the next breath begins. Trapped gas accumulates in the lungs, raising end-expiratory alveolar pressure above the set PEEP — without the clinician setting it.
Causes of Auto-PEEP
- • Short expiratory time (high RR, high I:E ratio, long Ti)
- • High minute ventilation demands
- • Obstructive lung disease (COPD, asthma) — slow exhalation due to dynamic airway collapse
- • Patient-ventilator dyssynchrony with breath stacking
Clinical Consequences
- • Increased work of breathing (patient must overcome iPEEP to trigger)
- • Dynamic hyperinflation — risk of pneumothorax
- • Hemodynamic compromise from reduced venous return
- • Inaccurate PEEP and compliance measurements
Detection and Management
Auto-PEEP can be measured with an expiratory hold maneuver on the ventilator. Management includes: reducing RR, increasing expiratory time (lower I:E ratio), increasing bronchodilator therapy in obstructive disease, and applying extrinsic PEEP at 50–80% of the measured auto-PEEP value to reduce the triggering threshold.
PEEP Titration Strategies
Determining the optimal PEEP level is one of the most challenging aspects of ventilator management, particularly in ARDS. Multiple strategies exist:
ARDSNet Low PEEP / High FiO₂ Table
The original ARMA trial protocol paired PEEP levels with FiO₂ targets. It provides a simple, validated starting framework. PEEP of 5 cmH₂O with FiO₂ 0.30 at the low end, up to PEEP 24 with FiO₂ 1.0 at the high end.
Best Compliance (Static) Method
PEEP is titrated from high to low (decremental titration) while measuring static compliance (Vt/[Plateau Pressure − PEEP]). The PEEP level that achieves the highest static compliance before it begins to fall is selected as optimal PEEP. Requires paralysis or minimal patient effort.
Stress Index Method
Analyzes the shape of the pressure-time curve during constant-flow volume control breaths. A linear increase = optimal; upward concavity = overdistension (reduce PEEP or Vt); downward concavity = ongoing recruitment (increase PEEP).
Esophageal Manometry
Uses an esophageal balloon catheter to estimate pleural pressure. Transpulmonary PEEP is calculated as airway PEEP minus pleural pressure, titrated to keep end-expiratory transpulmonary pressure positive (0–3 cmH₂O) to maintain recruitment.
PEEP Complications
While PEEP is beneficial when appropriately applied, excessive PEEP causes harm:
- • Barotrauma / Volutrauma: Overdistension of already-open alveoli at high PEEP levels can cause pneumothorax, pneumomediastinum, or subcutaneous emphysema
- • Hemodynamic compromise: High intrathoracic pressure reduces venous return → decreased cardiac output, hypotension — especially in hypovolemic patients
- • Increased intracranial pressure: Reduced venous drainage in patients with TBI or elevated ICP
- • Right heart strain: High PEEP increases pulmonary vascular resistance, potentially causing right ventricular dysfunction
- • Reduced renal perfusion: Decreased cardiac output and venous congestion may reduce urine output
Frequently Asked Questions
What is the minimum PEEP used in mechanically ventilated patients?
A physiologic or basal PEEP of 3–5 cmH₂O is standard in most ventilated patients. This prevents atelectasis in dependent lung regions without meaningful hemodynamic impact. Reducing PEEP below 3–5 cmH₂O is generally not recommended even in patients being weaned.
Does higher PEEP always improve oxygenation in ARDS?
Not necessarily. PEEP improves oxygenation by recruiting collapsed alveoli. In patients with minimal recruitable lung ("non-recruiters"), high PEEP mainly overdistends open alveoli without recruiting closed ones — worsening dead space and potentially worsening oxygenation. PEEP response is highly individualized.
How do I detect hemodynamic compromise from PEEP?
Monitor blood pressure, heart rate, cardiac output (if pulmonary artery catheter or thermodilution available), and urine output. Passive leg raise or fluid challenge can differentiate PEEP-induced preload reduction from other causes. A decrease in MAP or cardiac output with PEEP increase suggests clinically significant hemodynamic impact.
What is extrinsic PEEP and how does it help with auto-PEEP?
Applying set (extrinsic) PEEP at approximately 50–80% of the measured auto-PEEP level counterbalances the auto-PEEP threshold that the patient must overcome to trigger the ventilator. This reduces triggering effort without significantly worsening hyperinflation, provided total PEEP does not exceed safe limits.
Summary
- PEEP maintains positive airway pressure at end-expiration to prevent alveolar collapse and improve oxygenation
- Auto-PEEP is inadvertent trapped gas — detect with expiratory hold, manage by reducing respiratory rate or increasing expiratory time
- Optimal PEEP titration balances alveolar recruitment against overdistension and hemodynamic impact
- Excessive PEEP causes barotrauma, hemodynamic compromise, and right heart strain
- PEEP response varies significantly between patients — individualized assessment is essential
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