Lung Compliance and Chest Wall Compliance in Adult Respiratory Physiology
Fundamental Definitions and Normal Values
In healthy adults, lung compliance (CL) and chest wall compliance (Ccw) are approximately equal, each contributing about half of the total respiratory system compliance. 1, 2
Lung Compliance (CL)
- Lung compliance measures the change in lung volume per unit change in transpulmonary pressure, reflecting the elastic recoil properties of lung tissue 2, 3
- Normal values in adults: 1.2-2.0 ml/cmH₂O per kilogram body weight 2
- Represents the "stretchability" of the lungs—higher compliance means lungs expand more easily with less pressure 3
Chest Wall Compliance (Ccw)
- Chest wall compliance measures the change in volume per unit change in pleural pressure, reflecting the elastic properties of the rib cage, diaphragm, and abdominal structures 1
- In young adults (20-29 years): approximately 350 ml/cmH₂O with a Ccw/CL ratio around 5 2
- In adolescents and adults, Ccw approximately equals CL (ratio ~1:1), meaning both structures contribute equally to respiratory mechanics 1
Age-Related Changes in Chest Wall Compliance
The relationship between lung and chest wall compliance changes dramatically across the lifespan, which is critical for understanding adult physiology:
- Infants (<1 year): Ccw is 3-6 times greater than CL due to immature bone ossification 1, 4
- School-age children: Ccw is approximately twice CL 1
- Adolescents and young adults: Ccw equals CL 1
- Elderly adults: Ccw becomes approximately half of CL as the chest wall progressively stiffens 1
- Progressive stiffening with age: Ccw decreases from 350 ml/cmH₂O (ages 20-29) to 250 ml/cmH₂O (ages 30-59) to 136 ml/cmH₂O (ages 60-69) 2
Clinical Significance in Disease States
Reduced Lung Compliance
Decreased lung compliance indicates stiffened lungs requiring more pressure for a given volume change, seen in:
- ARDS: Static compliance may be reduced to less than one-fourth of normal values (approximately 20 ml/cmH₂O) 2
- Pulmonary fibrosis: Stiffening of lung tissue directly reduces compliance 2
- Alveolar edema and surfactant dysfunction: Reduces the volume of aeratable lung 2
- Left ventricular dysfunction: Increases lung water content, decreasing compliance 2
Reduced Chest Wall Compliance
Decreased chest wall compliance requires higher inflation pressures to achieve adequate tidal volumes, occurring in:
- Chest wall disorders: Scoliosis, asphyxiating thoracic dystrophy 1
- Increased intra-abdominal pressure: Dramatically reduces Ccw while lung compliance remains relatively normal 5
- Obesity, ascites, abdominal distension: All reduce chest wall compliance by limiting rib cage and diaphragm excursion 1, 5
- Adult neuromuscular disease: Paradoxically decreases Ccw (unlike in children), possibly due to joint contractures from years of low tidal volume breathing 1
Increased Chest Wall Compliance
Abnormally increased chest wall compliance in adults with neuromuscular disease (in infants and young children) predisposes to:
- Inefficient chest wall motion and rib cage deformation 6
- Reduced end-expiratory lung volume 6
- Increased work of breathing 1
Mechanical Ventilation Implications
Pressure Interpretation
Airway plateau pressure reflects the combined elastic properties of both lung and chest wall, not just the lung 5, 7
- With normal lung compliance but reduced chest wall compliance (e.g., increased intra-abdominal pressure), high airway pressures may be necessary and safe 5
- Transpulmonary pressure (airway pressure minus pleural pressure) more accurately reflects lung stress than airway pressure alone 5, 7
- In patients with stiff chest walls, lower transpulmonary pressures occur during positive-pressure ventilation despite high airway pressures 5
Ventilator Management in Chest Wall Disease
Higher inflation pressures are needed in chest wall disorders because of reduced chest wall compliance, while adequate tidal volumes can be achieved with relatively low pressures when only lung compliance is reduced 1
- PEEP strategies: When chest wall compliance is reduced (e.g., increased intra-abdominal pressure), appropriate PEEP is necessary to counterbalance lung compression and prevent atelectasis 1, 5
- Lung recruitment: Should be considered when there is persisting hypoxia and/or evidence of premature small airway closure in dependent lung tissue 1
Critical Pitfall
Setting PEEP greater than intrinsic PEEP (iPEEP) can be harmful in obstructive lung disease, though offsetting iPEEP by increasing ventilator PEEP reduces the effort of triggering 1
Measurement Considerations
Technical Requirements
- Measurements must be made during zero-flow conditions (e.g., inspiratory hold maneuver) to eliminate the resistance component and isolate elastic properties 2, 3
- Esophageal pressure measurements may be needed in spontaneously breathing patients to accurately determine transpulmonary pressure and separate lung from chest wall mechanics 1
- Chest wall compliance measurement requires absolute chest wall muscle relaxation to achieve interpretable results 1
Clinical Application Limitations
Despite its clinical value, static compliance measurement is not routinely performed outside research settings due to technical challenges and the need for specialized equipment 2
Practical Clinical Algorithm
When evaluating respiratory mechanics in mechanically ventilated adults:
- Measure plateau pressure and calculate static compliance (Crs = tidal volume / [plateau pressure - PEEP]) 3
- If compliance is reduced, determine whether the problem is primarily lung or chest wall:
- Adjust ventilator strategy accordingly:
- If reduced lung compliance: Use lung-protective ventilation (tidal volume 6-8 ml/kg, plateau pressure ≤30 cmH₂O) 1
- If reduced chest wall compliance with normal lung compliance: Higher airway pressures may be acceptable if transpulmonary pressure remains safe 5
- Optimize PEEP based on the underlying pathophysiology 1, 5