Anaes · Applied cardiovascular & respiratory physiology

Respiratory mechanics: compliance, resistance and the work of breathing

Also known as Lung compliance · Chest wall compliance · Airway resistance · Poiseuille law · Reynolds number · Time constant · Surfactant · Laplace law · Work of breathing · Pressure-volume curve · Hysteresis

Ventilation is a mechanical act: the respiratory muscles do work to move gas against two loads, the elastic recoil of the lung and chest wall (compliance) and the frictional resistance of the airways. Compliance is the change in volume per unit change in pressure (delta V over delta P) and is the reciprocal of elastance; the lung, the chest wall and the total respiratory system each have a compliance, and because lung and chest wall are arranged in series the total compliance (about 100 mL per cmH2O) is less than either component alone (each about 200 mL per cmH2O). The pressure-volume curve is sigmoidal, with low compliance at low volumes (atelectasis and airway closure) and at high volumes (tissue overdistension) and the highest compliance around functional residual capacity, the operating point; the inflation and deflation limbs differ (hysteresis) because of surface tension and the recruitment of surfactant. By Laplace law the pressure collapsing an alveolus is two times surface tension over radius, so small alveoli would empty into large ones; surfactant (type II pneumocytes) prevents this, raises compliance and prevents atelectasis. Airway resistance is the pressure drop per unit flow (about 1 to 2 cmH2O per L per s during nose breathing, less than 1 during mouth breathing); for laminar flow Poiseuille law makes resistance proportional to one over radius to the fourth power, so small changes in radius cause large changes in resistance, while turbulent flow (a Reynolds number above about 2000) is density dependent and needs pressure proportional to flow squared. Most resistance sits in the medium-sized bronchi (generations 2 to 8), NOT the small airways, because total cross-sectional area rises enormously toward the periphery. Each lung unit fills and empties with a time constant (tau equals resistance times compliance, about 0.3 s normally, with three time constants giving 95 per cent of a volume change); long time constants in obstructive disease cause gas trapping when respiratory rate is high and expiration short. The work of breathing has an elastic component (about two thirds) and a resistive component (about one third), and there is an energetically optimal frequency the respiratory controller normally finds. Anaesthesia reduces compliance (atelectasis, loss of tone, cephalad diaphragm) and raises resistance (the endotracheal tube and breathing circuit); PEEP recruits lung and improves compliance; bronchodilators and a larger airway device lower resistance.