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Positive End-Expiratory Pressure (PEEP) refers to the pressure applied to the lungs at the end of expiration during mechanical ventilation to maintain alveolar recruitment and prevent alveolar collapse (atelectasis).

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In other words, PEEP maintains a positive baseline airway pressure throughout the respiratory cycle, improving oxygenation and functional residual capacity (FRC).

Physiologic Basis

During normal spontaneous breathing, alveoli tend to collapse partially during expiration due to surface tension and elastic recoil.

PEEP counteracts this by maintaining end-expiratory pressure above atmospheric level, keeping alveoli open, thereby improving gas exchange and reducing shunt fraction.

Types of PEEP

Type Description Clinical Use
Extrinsic (Applied PEEP) Set by the ventilator ARDS, pulmonary edema, atelectasis
Intrinsic (Auto-PEEP) Unintended air trapping due to incomplete expiration (dynamic hyperinflation) COPD, asthma exacerbation

Mechanisms of Action

  1. Prevention of alveolar collapse: Maintains end-expiratory lung volume, especially in dependent zones.
  2. Recruitment of collapsed alveoli: Opens atelectatic units → increases gas exchange surface area.
  3. Improved oxygenation: Increases mean airway pressure → better V/Q matching → higher PaO₂.
  4. Decreased intrapulmonary shunting: Reduces perfusion of non-ventilated alveoli.
  5. Redistribution of lung water: In pulmonary edema, PEEP decreases interstitial edema by increasing perivascular pressure.

Physiologic Effects

| Respiratory Effects | • ↑ Functional residual capacity (FRC). • ↑ Alveolar recruitment. • ↓ Shunt fraction. • ↑ Compliance (up to an optimal PEEP). • ↓ Work of breathing (when optimally set). | | --- | --- | | Hemodynamic Effects | Excessive PEEP may compromise circulation: • ↑ Intrathoracic pressure → ↓ venous return → ↓ preload → ↓ cardiac output. • ↑ Right ventricular afterload (especially in pulmonary hypertension). • ↓ Coronary perfusion pressure. | | Other Effects | • ↓ Renal perfusion → ↓ urine output (due to ↓ venous return and ADH release). • ↑ Intracranial pressure (due to impaired venous drainage). |

Indications