Mechanics of ventilation

Compliance

The change in lung volume per unit change in pressure

Both the lungs and the chest wall contribute to compliance.

To calculate the combined compliance, add the reciprocals:

\frac{1}{C_{total}}=\frac{1}{C_{thorax}}+\frac{1}{C_{parenchyma}}=\frac{1}{C_{200}}+\frac{1}{C_{200}}=\frac{1}{100}\therefore C_{total}=100ml/cmH_{2}O

Static compliance

Measured when gas flow is absent

Typically calculated by performing an end-inspiratory hold

C_{static}=\frac{V_{T}}{P_{Plat}-PEEP}

Static compliance is decreased by lung parenchymal disease, chest wall disease or raised intra-abdominal pressure

Dynamic compliance

Measured during rhythmic breathing

Determined by peak pressure rather than plateau pressure

C_{dyn}=\frac{V_{T}}{P_{Peak}-PEEP}

Peak pressure is higher than plateau pressure as it represents the compliance of the lung and chest wall, plus the pressure required to overcome airway resistance.

Dynamic compliance is therefore lower than static compliance

This discrepancy is larger in the context of obstructive airway disease

Lung compliance curve.
Lung compliance curve. The small central loop represents a tidal volume breath.
The larger loop represents a vital capacity breath and demonstrates the low compliance at low and high lung volumes.