Optimizing Positive End-Expiratory Pressure

Introduction^1,2

• PEEP is the pressure remaining in the lung at the end of the expiratory phase that is greater than the atmospheric pressure.
• General anesthesia and supine positioning are well recognized for reducing FRC, thereby predisposing dependent lung regions to collapse and increasing the risk of atelectasis
• Intrinsic PEEP (auto-PEEP) is generated spontaneously by the patient due to inadequate expiratory time or high airway resistance, leading to dynamic hyperinflation.
  • Incomplete return to baseline on the volume-time curve during mechanical ventilation, elevated plateau pressure, use of accessory muscles during exhalation, decreased blood pressure, prolonged expiratory phase, and signs of respiratory distress may suggest the presence of intrinsic PEEP.
• Extrinsic PEEP is the external pressure set by the mechanical ventilator that can be utilized to maintain open alveoli, reduce the tendency for collapse, and facilitate better gas exchange.
  • Extrinsic PEEP is recognized as a significant part of ventilation for treating conditions with reduced lung compliance or significant shunt physiology, such as acute respiratory distress syndrome (ARDS).²
• The primary goals of PEEP application are^1,2
  • To prevent the repetitive opening and closing of dependent alveoli (atelectrauma) and preserve functional residual capacity.
  • To improve arterial oxygenation by recruiting collapsed alveoli and decreasing intrapulmonary shunting.
  • To reduce the work of breathing if auto-PEEP is present.

Mechanism of Action^1,2,9

• PEEP works by shifting the end-expiratory volume of the lung upwards on the pressure-volume (P/V) curve, moving it out of the low-compliance region where atelectasis occurs.
  • At these levels, the greatest gain in lung compliance is observed with minimal risk of overdistension or barotrauma.
  • By operating above the lower inflection point, PEEP counteracts the natural tendency of the lungs and chest wall to recoil towards a lower volume as small airway closure is prevented, minimizing shear stress and reducing ventilator-induced lung injury (VILI).^2,9
  • PEEP also promotes a more uniform distribution of ventilation throughout the lung, particularly in dependent regions at higher risk of atelectasis. This redistribution reduces ventilation-perfusion mismatch, which is a common complication induced by both surgery and anesthesia.

Indications for PEEP^1-5,7

• In the surgical and anesthetic contexts, PEEP has been shown to reduce intraoperative atelectasis and is frequently used during general anesthesia to improve postoperative pulmonary outcomes.
  • Multiple randomized trials have demonstrated improved arterial oxygenation postoperatively with intraoperative PEEP; these improvements are presumed to result from reduced atelectasis, although a consistent effect on mortality or major pulmonary complications has not been observed.³
• ARDS usually calls for extrinsic PEEP intervention.^1,2
  • While ARDSNet protocols remain the standard PEEP reference, more studies recommend a more individualized approach, with higher PEEP strategies in patients with moderate to severe ARDS, as these patients may derive greater benefit from recruiting non-aerated lung units.^1,7
  • Higher PEEP may also benefit ARDS patients with obesity or other chronic conditions that may impair pulmonary function, thereby counteracting low FRC and reducing the risk of atelectasis.
  • Select cases, such as cardiogenic pulmonary edema, may benefit from PEEP, which reduces preload and afterload, thereby improving cardiac function.^1,5
• The indication can be extended to chronic obstructive pulmonary disease (COPD), acute respiratory failure, and conditions associated with pulmonary edema or shunt physiology.^1,2,4
  • In COPD, PEEP can be used to counterbalance intrinsic PEEP (auto-PEEP), facilitate effective triggering of breaths, and improve patient-ventilator interaction in mechanically ventilated patients with hyperinflation.

Methods for Determining Optimal PEEP^2,6,7

• The selection of an appropriate PEEP level depends on assessing the patient’s lung recruitability, defined as the potential for collapsed lung tissue to reopen under pressure.
• A highly recruitable lung (common in early, severe ARDS) will benefit from higher PEEP. In comparison, a poorly recruitable lung (common in mild ARDS or after several days) may be sufficient with lower PEEP.⁷
• Methods to assess recruitability and titrate PEEP include driving pressure, imaging, bedside mechanical tests, and monitoring hemodynamics.^2,6
  • Driving pressure (TV/CRS), a ratio of tidal volume (TV) to the static compliance of the respiratory system (CRS), can serve as a strong indicator for PEEP titration as well as predicting mortality, as it represents dynamic pulmonary stress and strain during mechanical ventilation (the higher the driving pressure, the higher the mortality).

• • Compliance-based titration involves incremental or decremental PEEP trials aimed at achieving the highest static or dynamic compliance. This is widely adopted due to its bedside feasibility, but it may be confounded by patient activity and procedural variability.
  • PEEP-FiO₂ tables are algorithms correlating required oxygenation with recommended PEEP for ARDS patients, as used in ARDSNet protocols. These are simple to implement and well validated, but they do not account for individual variation in lung mechanics, which may limit their utility in heterogeneous lung pathology.

• • Imaging-guided approaches such as computed tomography (CT) and electrical impedance tomography can directly measure recruitment and overdistension. These methods offer insight into regional lung mechanics; CT may be considered the gold standard for assessing recruitment, but its use may be limited by equipment and resource constraints.
  • Esophageal pressure monitoring uses balloon catheters to estimate pleural pressure and partition lung/chest wall mechanics, allowing for transpulmonary pressure targeting. This may be important in complex cases in which chest wall effects predominate, but it requires specialized equipment and training.
  • Oxygenation and gas exchange can be optimized by adjusting PEEP incrementally or decrementally to improve PaO₂ or SpO₂. This method is accessible but may not account for the risk of overdistension or mechanical impact.

Clinical Application and Titration^1-5,7,8

Initiation and Setting

• PEEP is commonly initiated at ~5 cmH₂O in otherwise healthy individuals undergoing general anesthesia.⁹
• In critically ill patients or those with moderate to severe ARDS, higher PEEP levels may be beneficial for maintaining adequate oxygenation; however, in those with mild ARDS, higher PEEP levels are not necessarily beneficial and may cause harm.³
• In cardiac dysfunction, moderate PEEP levels (5–10 cm H_₂O) may enhance left ventricular function by reducing preload; however, high PEEP is contraindicated in right ventricular failure or tamponade, and continuous cardiac output monitoring is recommended.
• To prevent perioperative complications, a PEEP of ≥5 cm H_₂O is usually adequate, with adjustments as needed based on patient comorbidities, such as obesity.
• In COPD, PEEP is typically set between 3–8 cm H_₂O to match external PEEP to the patient’s intrinsic PEEP, thereby reducing the work of breathing while avoiding excessive PEEP (greater than 10 cm H_₂O) that could worsen hyperinflation.

Monitoring and Troubleshooting

• Careful monitoring should include:
  • Oxygenation indices (PaO₂/FiO₂ ratio, SpO₂)
  • Static and dynamic compliance
  • Hemodynamic response (blood pressure, heart rate, central venous pressure)
  • Evidence of auto-PEEP on ventilator waveforms
• Troubleshooting involves:
  • Reducing PEEP if there is hypotension, increased dead space, or lung overdistension
  • Considering recruitment maneuvers in refractory hypoxemia (with careful medical oversight)
  • Adjusting FiO₂ and tidal volume in tandem with PEEP to maintain lung-protective ventilation

Complications^1,9,10

• Complications with PEEP include VILI such as barotrauma and volutrauma, which will manifest as pneumothorax, subcutaneous emphysema, or air leak syndromes.^1,2,9
  • Barotrauma is a lung injury caused by excessive pressure; increases in mean airway pressure can overdistend more compliant lung regions.
  • Similarly, volutrauma results from excessive tidal volumes, leading to alveolar overdistension and shear stress, independent of airway pressure.
• Hemodynamic compromise is also possible, presenting as hypotension, increased requirement for vasopressors, and, in rare cases, decreased end-organ perfusion.^1,10
  • PEEP increases intrathoracic pressure, which can, in turn, reduce venous return, followed by decreased cardiac output and precipitation of hypotension, especially in patients with right ventricular dysfunction.
• In obstructive lung disease, PEEP may lead to further air trapping and hemodynamic deterioration unless carefully diagnosed and managed.¹