How does positive end‑expiratory pressure (PEEP) change during inspiration and expiration, and what are the underlying respiratory mechanisms?

PEEP Dynamics During the Respiratory Cycle

PEEP (positive end-expiratory pressure) itself remains constant throughout the respiratory cycle when applied externally, but the total end-expiratory pressure and its physiological effects vary significantly between inspiration and expiration, particularly in the presence of intrinsic PEEP (auto-PEEP). 1, 2

Understanding PEEP Mechanics During Expiration

During expiration, PEEP maintains positive alveolar pressure at end-expiration to prevent alveolar collapse and preserve functional residual capacity. 2, 3 The key mechanisms include:

• PEEP prevents complete lung deflation by maintaining a pressure threshold above atmospheric pressure, which keeps alveoli open and increases end-expiratory lung volume (EELV) 2
• In patients with airflow obstruction, intrinsic PEEP (auto-PEEP) develops when insufficient expiratory time prevents complete lung emptying before the next breath begins, creating positive alveolar pressure even without external PEEP 1, 4
• Expiratory muscle activity can artificially elevate measured PEEP without true dynamic hyperinflation—the abdominal pressure rise during active expiration followed by sudden relaxation at end-expiration creates a pressure drop that mimics auto-PEEP 5

A critical pitfall: The measured end-expiratory pressure may not reflect true lung hyperinflation if expiratory muscles are actively contracting, as the pressure drop at end-expiration could represent muscle relaxation rather than inspiratory effort 1, 5

Understanding PEEP Mechanics During Inspiration

During inspiration, PEEP creates an inspiratory threshold load that must be overcome before inspiratory flow can begin. 1 The respiratory mechanisms are:

• Patients with auto-PEEP must generate sufficient negative pleural pressure to counterbalance the intrinsic PEEP before the ventilator can be triggered or spontaneous inspiratory flow can commence 1, 4
• This threshold load is measured as the decrease in pleural pressure preceding the onset of inspiratory flow, representing dynamic PEEPi (PEEPi,dyn), which is significantly less than static PEEPi 1
• External PEEP counterbalances auto-PEEP by reducing the inspiratory pressure gradient patients must overcome, thereby improving patient-ventilator interaction and reducing work of breathing 1, 3

The American Thoracic Society recommends applying external PEEP at 50-85% of measured auto-PEEP to reduce inspiratory effort without exacerbating hyperinflation or causing hemodynamic compromise 3

Respiratory System Compliance Changes Between Phases

In ARDS patients, expiratory elastance (ErsEXP) is substantially higher than inspiratory elastance (ErsINSP), with PEEP diminishing this difference. 6 Specifically:

• Without PEEP, expiratory elastance averaged 45.58 hPa/L compared to inspiratory elastance of 36.76 hPa/L in ARDS patients, suggesting mechanical ventilation with positive pressure influences pulmonary edema and interstitial fluid differently during inspiration versus expiration 6
• Increasing PEEP from 0 to 10 cmH₂O significantly decreased inspiratory resistance (from 16.43 to 13.28 hPa·s/L) by recruiting collapsed alveoli and improving airway patency 6

Breathing Pattern Response to PEEP Changes

PEEP level significantly influences breathing pattern by selectively affecting expiratory time through the Hering-Breuer reflex. 7 The mechanisms include:

• Each 1 cmH₂O change in PEEP results in a 0.4 breaths/min change in respiratory rate in the opposite direction, with effects manifesting within the first breath and fully established by the second breath 7
• Increases in PEEP cause drops in minute ventilation and respiratory rate exclusively through prolongation of expiratory time, not inspiratory time 7
• This response is stronger in patients with impaired respiratory system compliance, suggesting heightened mechanoreceptor sensitivity in diseased lungs 7

Clinical Algorithm for PEEP Management

Step 1: Measure auto-PEEP using end-expiratory airway occlusion (the gold standard), ensuring the patient is passive to avoid artifacts from expiratory muscle activity 1, 3

Step 2: If auto-PEEP is present (>0 cmH₂O):

• Apply external PEEP at 50-85% of measured auto-PEEP value 3
• Never exceed the measured auto-PEEP level with external PEEP 4, 3
• Use flow triggering instead of pressure triggering to reduce work of breathing 4

Step 3: Monitor for adverse effects:

• Decreased cardiac output from reduced venous return 1, 2
• Worsening hyperinflation if external PEEP exceeds intrinsic PEEP 4, 3
• Patient-ventilator asynchrony from inappropriate PEEP levels 1, 4

Step 4: Adjust ventilator settings to minimize auto-PEEP:

• Decrease respiratory rate to allow longer expiratory time 4
• Use shorter inspiratory times with higher flow rates (80-100 L/min) 4
• Target I:E ratios of 1:4 or 1:5 4

Critical caveat: In patients with active expiratory muscle contraction, subtract the expiratory rise in gastric pressure from the measured PEEP to obtain the true value reflecting dynamic hyperinflation 1, 5