Expansion
Elastic work plus resistive work, minimised at an optimal respiratory rate
Expansion
Two components:
- Elastic work - expanding lung and chest wall against recoil. Rises with larger tidal volumes
- Resistive work - overcoming airway and tissue resistance. Rises with faster flow rates
The body selects the rate that minimises total work, normally about 12 to 15 breaths per minute.
Disease shifts the optimum:
- Restrictive disease (fibrosis) raises elastic work, so patients breathe rapidly and shallowly
- Obstructive disease raises resistive work, so patients breathe slowly and deeply, often with pursed lips to splint airways open
At rest the oxygen cost of breathing is only about 2 to 3 per cent of total consumption, but in severe disease it can exceed 30 per cent, creating a vicious cycle in which the respiratory muscles consume much of the oxygen they deliver. That is a key reason for instituting ventilatory support.