Compliance

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Lung Compliance

Definition

Lung compliance is the measure of the lung's ability to stretch and expand. It is defined as the change in volume per unit change in pressure:

C = ΔV / ΔP

A high compliance means the lung distends easily; a low compliance means it is stiff and resists expansion. Compliance is the inverse of elastance (recoil):

Elastance = 1 / Compliance

Costanzo Physiology 7th Edition, p. 205

Normal Values

Component	Compliance
Lung alone	~150-220 mL/cmH₂O
Chest wall alone	~200 mL/cmH₂O
Total respiratory system (lung + chest wall in series)	~85-130 mL/cmH₂O

Because the lung and chest wall are arranged in series, total compliance is calculated as:

1/C_RS = 1/C_L + 1/C_W

Murray & Nadel's Textbook of Respiratory Medicine, p. 207

Static vs. Dynamic Compliance

Static compliance - measured when there is no airflow; reflects the pure elastic properties of the lung and chest wall. It is measured at points of zero flow (end-inspiration and end-expiration).
Dynamic compliance - measured during breathing, when air is moving. It is influenced by both elastic properties AND airway resistance. Dynamic compliance will be lower than static compliance when airway resistance is increased (e.g., asthma, COPD).

Pressure-Volume Curve and Hysteresis

The compliance curve of an air-filled lung shows hysteresis - the inspiration and expiration limbs follow different paths:

Lung compliance curves showing hysteresis - air-filled vs saline-filled lung

Fig. 5.7 from Costanzo Physiology: Compliance of the lung. The air-filled lung shows different inspiration and expiration curves (hysteresis), while the saline-filled lung does not.

Key observations:

For a given pressure, lung volume is greater during expiration than inspiration (expiration limb is steeper = higher compliance)
The saline-filled lung has a single, steeper curve (no hysteresis) because saline eliminates the liquid-air surface tension responsible for hysteresis
The comparison proves that surface tension, not elastic tissue, is the primary cause of hysteresis
Costanzo Physiology 7th Edition, p. 205-206

Surface Tension and the Law of Laplace

Alveoli are lined with a thin fluid film. The liquid molecules attract each other (surface tension), generating a collapsing pressure described by the Law of Laplace:

P = 2T / r

Where:

P = collapsing pressure on the alveolus
T = surface tension
r = radius of the alveolus

This has an important implication: smaller alveoli have higher collapsing pressures. Without a mechanism to counter this, small alveoli would empty into larger ones (alveolar instability). This is where surfactant comes in.

Role of Surfactant

Surfactant is a phospholipid mixture produced by type II alveolar cells. Its key component is dipalmitoylphosphatidylcholine (DPPC).

Surfactant works by:

Reducing surface tension - DPPC molecules align at the alveolar surface, with hydrophobic ends attracted to each other and hydrophilic ends repelled, breaking up water-water intermolecular forces
Preventing alveolar collapse - by reducing T in the Laplace equation, small alveoli no longer have dangerously high collapsing pressures
Increasing lung compliance - lower surface tension = less force needed to expand the lung = easier breathing = reduced work of breathing

Surfactant synthesis begins around gestational week 24 and is reliably present by week 35. Premature infants born before week 24 will lack surfactant entirely.

Costanzo Physiology 7th Edition, p. 210

Diseases Affecting Lung Compliance

Changes in compliance in emphysema vs. fibrosis with effects on FRC

Fig. 5.11 from Costanzo Physiology: Changes in lung compliance and FRC in emphysema (increased compliance) vs. fibrosis (decreased compliance).

Increased Compliance (Floppy Lung)

Condition	Mechanism	Effect on FRC
Emphysema	Loss of elastic fibers	Increased FRC ("barrel chest")
Aging	Loss of elastic tissue	Mildly increased

In emphysema, the lungs' elastic recoil is reduced. At any given volume, the collapsing force of the lung is less than the expanding force of the chest wall. The system reaches a new equilibrium at a higher FRC - the patient breathes at higher lung volumes.

Decreased Compliance (Stiff Lung)

Condition	Mechanism	Effect on FRC
Pulmonary fibrosis	Stiffening of lung tissue (fibrotic scarring)	Decreased FRC
Neonatal RDS	Surfactant deficiency; atelectasis	Decreased
Pulmonary edema	Fluid replaces air; alveolar flooding	Decreased
Acute Lung Injury / ARDS	Surfactant dysfunction; inflammation	Decreased
Obesity	Reduced FRC; abdominal loading	Decreased
Pneumothorax	Loss of transmural pressure	Lungs collapse

In fibrosis, lung stiffness means the collapsing force at the original FRC is now greater than the chest wall's expanding force. The system seeks a new lower FRC - the patient breathes at lower lung volumes.
Costanzo Physiology 7th Edition, p. 208

Compliance vs. Age

Lung compliance changes across the lifespan (Miller's Anesthesia, 10th ed.):

At birth: Low compliance - alveolar precursors have thick walls, decreased elastin, and potentially deficient surfactant
Childhood to adulthood: Compliance increases as alveolar structure matures
Old age: Compliance increases again (emphysema-like changes from elastic fiber loss), but dynamic compliance falls due to small airway collapse

Clinical Measurement

In mechanically ventilated patients, static compliance is directly measurable:

Record end-expiratory pressure (PEEP) and end-inspiratory plateau pressure (P_plat)
C_static = Tidal Volume / (P_plat - PEEP)
Normal: ~50-80 mL/cmH₂O on mechanical ventilation
Falling compliance is an early sign of worsening lung pathology (ARDS, pneumothorax, mucus plugging)

Compliance monitoring guides PEEP titration - a recent meta-analysis (PMID 39825438) confirmed that individualized PEEP titration guided by lung compliance during one-lung ventilation improves oxygenation compared to fixed PEEP strategies.

Summary

Parameter	Low Compliance	High Compliance
Lung stiffness	High (stiff)	Low (floppy)
Work of breathing	Increased	Decreased (but gas trapping ↑)
FRC	Decreased	Increased
Examples	Fibrosis, RDS, ARDS, edema	Emphysema, aging
Pressure-volume slope	Flat	Steep

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