pulmonary compliance

DEFINITION: Compliance (C) = ΔV/ΔP, the slope of the pressure-volume (P-V) curve. Normal total respiratory system compliance is ~100 mL/cmH2O (lung alone ~200 mL/cmH2O; chest wall ~200 mL/cmH2O). Lung compliance is the inverse of elastance (elastic recoil). Total system: 1/C_total = 1/C_lung + 1/C_chest wall.

DETERMINANTS: (1) Elastic tissue (elastin and collagen) of the lung parenchyma. (2) Surface tension at the air-liquid interface of alveoli; surfactant (DPPC/dipalmitoylphosphatidylcholine, made by type II pneumocytes) reduces surface tension, increasing compliance and preventing alveolar collapse. Per Laplace's law (P = 2T/r), small alveoli with high surface tension tend to collapse without surfactant.

P-V CURVE: Inflation and deflation limbs differ (hysteresis) because of surface tension effects and the recruitment of collapsed alveoli. Compliance is greatest in the mid-range of the curve and falls at high volumes (flattening of the curve at total lung capacity). The deflation limb has higher volume at a given pressure, and surfactant increases this hysteresis effect.

DECREASED COMPLIANCE (stiff lung; steeper work of breathing, small low-volume breaths, restrictive pattern with decreased FVC and TLC, normal or increased FEV1/FVC): pulmonary fibrosis (e.g., IPF, asbestosis, silicosis), pulmonary edema, ARDS, neonatal respiratory distress syndrome (surfactant deficiency), atelectasis, pneumonia, and kyphoscoliosis/obesity (chest wall component).

INCREASED COMPLIANCE (loss of elastic recoil; obstructive pattern with air trapping and increased TLC/RV): emphysema (e.g., alpha-1 antitrypsin deficiency, smoking; elastase destroys elastin) and normal aging (senile emphysema). The lung is easily inflated but difficult to deflate, because of decreased elastic recoil and airway collapse during expiration. This produces a higher FRC and barrel chest.

FRC: Functional residual capacity is the equilibrium point where the inward elastic recoil of the lung equals the outward recoil of the chest wall. Pleural pressure is negative at FRC. In emphysema, decreased lung recoil shifts the equilibrium so FRC rises. In fibrosis, increased lung recoil lowers FRC.

CLINICAL: In mechanically ventilated patients, static compliance = tidal volume / (plateau pressure − PEEP); dynamic compliance = tidal volume / (peak inspiratory pressure − PEEP). A fall in static compliance suggests parenchymal or chest wall problems (ARDS, pneumothorax, mainstem intubation, pulmonary edema, abdominal compartment). A widening gap between peak and plateau pressure suggests increased airway resistance (bronchospasm, mucus plugging, kinked tube) rather than reduced compliance. In ARDS, low tidal volumes (6 mL/kg predicted body weight) with plateau pressure <30 cmH2O are used to limit ventilator-induced lung injury.

HIGH-YIELD: Premature infants lack surfactant (the lecithin:sphingomyelin ratio is <1.5-2 before ~35 weeks), which gives low compliance, atelectasis, and a ground-glass appearance on radiograph. Treatment includes exogenous surfactant, CPAP, and antenatal corticosteroids to accelerate type II pneumocyte maturation.