What is dynamic compliance in a mechanically ventilated adult patient with chronic obstructive pulmonary disease (COPD) or acute respiratory distress syndrome (ARDS)?

Dynamic Compliance in Mechanically Ventilated Patients

Dynamic compliance (Cdyn) is the ratio of tidal volume to the difference between peak inspiratory pressure and PEEP, measured during active airflow, and reflects both the elastic properties of the respiratory system AND the resistance of the airways—making it fundamentally different from static compliance which isolates only elastic recoil.

Definition and Calculation

Dynamic compliance = Tidal Volume (mL) ÷ [Peak Inspiratory Pressure - PEEP (cmH₂O)]

• This measurement is obtained during normal ventilator cycling with continuous airflow, unlike static compliance which requires an inspiratory hold maneuver to eliminate flow 1
• Normal values in healthy adults range from approximately 50-100 mL/cmH₂O, though this varies with body size and measurement conditions 1
• The key distinction: dynamic compliance includes both elastic recoil forces AND airway resistance, while static compliance (measured at zero flow) reflects only elastic properties 1

Clinical Significance in COPD and ARDS

In COPD Patients

• Dynamic compliance is markedly reduced due to increased airway resistance from airflow obstruction 2
• The expiratory time must be prolonged to reduce dynamic hyperinflation (gas-trapping), which directly impacts compliance measurements 2
• Intrinsic PEEP (iPEEP) creates an inspiratory threshold load that must be overcome before triggering a breath, artificially worsening the measured dynamic compliance 2
• Setting external PEEP to offset iPEEP (but not exceed it) can improve triggering effort and apparent compliance, though setting PEEP greater than iPEEP is harmful 2

In ARDS Patients

• Dynamic compliance falls dramatically—often to less than 25% of normal values (approximately 20 mL/cmH₂O or less) due to decreased aerated lung volume, alveolar edema, and surfactant dysfunction 1
• The compliance profile changes throughout the tidal breath, with intratidal recruitment causing compliance to increase during inspiration at low PEEP, or overdistension causing compliance to decrease at high lung volumes 3
• With inadequate PEEP (5 cmH₂O), 92% of patients show intratidal recruitment/derecruitment, which improves to only 46% at PEEP 9 cmH₂O 4

Factors That Decrease Dynamic Compliance

Lung factors:

• Alveolar edema and surfactant dysfunction reduce aeratable lung volume 1
• Pulmonary fibrosis stiffens lung tissue 1
• Atelectasis and lung collapse decrease functional lung volume 5

Airway factors:

• Increased airway resistance from bronchospasm or secretions 2
• Dynamic hyperinflation in obstructive disease 2

Chest wall factors:

• Elevated intra-abdominal pressure decreases chest wall compliance while lung compliance may remain relatively normal 5
• In chest wall deformity, higher pressures (>15 cmH₂O) are needed to achieve adequate tidal volumes due to reduced chest wall compliance 2

Monitoring and Interpretation

The intratidal compliance profile reveals critical pathophysiology:

• Increasing compliance during the breath (up to 20% above early inspiration) indicates intratidal recruitment—the lung is opening up during the breath, suggesting inadequate PEEP 3
• Decreasing compliance during the breath (up to 60% below maximum) signals overdistension—the lung is being overstretched, particularly with tidal volumes >5 mL/kg at high PEEP 3
• Stable, high compliance throughout the breath suggests optimal PEEP and tidal volume combination (typically achieved at PEEP 10 cmH₂O with appropriate tidal volume) 3

Practical Application at the Bedside

To optimize ventilation using dynamic compliance:

1. Calculate baseline Cdyn = VT ÷ (PIP - PEEP) at current settings 1

2. Assess for dynamic hyperinflation in COPD: Prolong expiratory time by reducing respiratory rate (10-15 breaths/min) and using I:E ratios of 1:2 to 1:4 2

3. Titrate PEEP to improve compliance: In obstructive disease, PEEP may increase tidal volume and improve compliance by offsetting iPEEP (but never exceed iPEEP) 2. In restrictive disease/ARDS, higher PEEP (>12 cmH₂O) recruits collapsed alveoli and improves compliance 6

4. Monitor the trend: Serial measurements track disease progression and treatment response better than single values 1

5. Integrate with driving pressure: Since driving pressure (ΔP = plateau pressure - PEEP) reflects VT/compliance, maintaining ΔP ≤15 cmH₂O ensures you're not overdistending the functional lung 7

Critical Pitfalls

• Do not confuse dynamic and static compliance—dynamic compliance will always be lower due to the resistance component 1
• Dynamic compliance measured during patient-ventilator asynchrony is unreliable—ensure adequate sedation or address trigger issues before interpreting values 2
• In patients with elevated intra-abdominal pressure, low dynamic compliance may reflect chest wall restriction rather than lung pathology—esophageal pressure monitoring can partition these components 8, 5
• Attempting to "normalize" compliance with excessive pressures risks barotrauma—accept permissive hypercapnia (pH >7.2) rather than using peak pressures >30 cmH₂O 2