What is the difference between Positive End-Expiratory Pressure (PEEP) and Peak Inspiratory Pressure (PIP) in patients requiring mechanical ventilation, particularly in those with underlying lung diseases such as Chronic Obstructive Pulmonary Disease (COPD) or Acute Respiratory Distress Syndrome (ARDS)?

Understanding PEEP and PIP in Mechanical Ventilation

Core Definitions and Timing

PEEP (Positive End-Expiratory Pressure) is the pressure maintained in the airways at the end of expiration, while PIP (Peak Inspiratory Pressure) is the maximum pressure reached during inspiration. These represent fundamentally different phases of the respiratory cycle with distinct physiological effects and clinical implications.

PEEP: End-Expiratory Pressure

• PEEP maintains alveolar recruitment at end-expiration, preventing cyclic collapse and reopening of alveolar units that causes atelectrauma 1
• Applied continuously throughout the expiratory phase to keep airways and alveoli open 1
• In ARDS patients with moderate-to-severe disease (PaO2/FiO2 < 200), higher PEEP levels (mean 15.1 ± 3.6 cm H2O) reduce mortality compared to lower levels (mean 9.1 ± 2.7 cm H2O) 1
• For moderate or severe ARDS, use higher PEEP strategies (typically 12-15 cm H2O or above) rather than lower levels 1

PIP: Peak Inspiratory Pressure

• PIP represents the highest pressure during the inspiratory phase, reflecting the sum of driving pressure, PEEP, and resistive forces 1
• Occurs at the moment of maximum lung inflation during tidal breath delivery 1
• Should be limited to ≤30 cm H2O (plateau pressure) in ARDS to prevent overdistention injury 1

Disease-Specific Considerations

ARDS Management

• Higher PEEP (15-18 cm H2O) improves oxygenation by 61 mm Hg and reduces mortality in moderate-to-severe ARDS 1
• PEEP prevents atelectrauma by maintaining end-expiratory lung volume and reducing stress at the interface between collapsed and aerated lung 1
• Recruitment maneuvers (transient PIP elevations to 30-40 cm H2O) combined with adequate PEEP reduce mortality (RR 0.81) 1
• Monitor plateau pressure (end-inspiratory hold) rather than PIP alone, keeping it ≤30 cm H2O 1

COPD Exacerbations

• In COPD with dynamic hyperinflation, apply external PEEP at 4-8 cm H2O to counterbalance intrinsic PEEP (PEEPi) without increasing lung volume 1, 2, 3
• PEEPi creates an inspiratory threshold load that increases work of breathing and can cause ventilator dyssynchrony 1, 2, 3
• External PEEP up to a critical value (Pcrit) reduces inspiratory effort without further hyperinflation in patients with expiratory flow limitation 2, 3
• For noninvasive ventilation in COPD, combine CPAP (4-8 cm H2O) with pressure support (10-15 cm H2O) 1
• Intubate if pH <7.25 despite optimal NIPPV, or if PaCO2 >60 mmHg with respiratory rate >35 breaths/min 1, 4, 5

Critical Pitfalls to Avoid

PEEP-Related Errors

• Do not use low PEEP (<8 cm H2O) in moderate-to-severe ARDS, as this increases mortality and allows repetitive alveolar collapse 1
• Avoid excessive PEEP in COPD that exceeds Pcrit, as this worsens hyperinflation and hemodynamic compromise 2, 3
• In COPD, failure to recognize and counterbalance PEEPi leads to increased work of breathing and ventilator dyssynchrony 1, 2

PIP-Related Errors

• Limiting PIP alone is insufficient—monitor plateau pressure (end-inspiratory hold pressure), which better reflects alveolar distention risk 1
• High PIP from airway resistance (secretions, bronchospasm) differs from high plateau pressure (overdistention) and requires different interventions 1
• In ARDS, allowing plateau pressure >30 cm H2O increases ventilator-induced lung injury regardless of PIP 1

Practical Monitoring Algorithm

For ARDS Patients

1. Set PEEP based on disease severity: mild ARDS use lower PEEP (8-10 cm H2O); moderate-to-severe ARDS use higher PEEP (12-18 cm H2O) 1
2. Measure plateau pressure with end-inspiratory hold, maintaining ≤30 cm H2O 1
3. Calculate driving pressure (plateau pressure minus PEEP)—lower driving pressure improves outcomes 1
4. Monitor oxygenation response: higher PEEP should improve PaO2/FiO2 by approximately 60 mm Hg 1

For COPD Patients

1. Measure PEEPi using end-expiratory occlusion technique 1
2. Apply external PEEP at 80% of measured PEEPi (typically 4-8 cm H2O) 1, 2, 3
3. Monitor for increased hyperinflation by inspecting flow-volume curves—expiratory flow should not decrease 2
4. Assess work of breathing reduction and patient-ventilator synchrony 1, 2
5. Target oxygen saturation 88-92% to avoid worsening hypercapnia 4

Hemodynamic Considerations

• Both high PEEP and high PIP can reduce cardiac output by increasing intrathoracic pressure and decreasing venous return 1, 3
• In COPD with PEEPi, appropriate external PEEP may actually improve cardiac function by reducing negative intrathoracic pressure swings during inspiration 3
• High PEEP increases pulmonary vascular resistance, potentially causing cor pulmonale in susceptible patients 1