Expansion
Reduced functional residual capacity, blunted drive, and atelectasis
Expansion
Mechanical
- Functional residual capacity falls by about 20 per cent, from loss of respiratory muscle tone and cephalad movement of the diaphragm
- Atelectasis develops in dependent lung within minutes, worsened by high inspired oxygen
- Chest wall compliance falls, and the diaphragm moves differently under paralysis
Control
- Ventilatory response to carbon dioxide is depressed and shifted right
- Ventilatory response to hypoxia is depressed even at subanaesthetic concentrations
- Tidal volume falls and rate rises, giving rapid shallow breathing
Gas exchange
- Hypoxic pulmonary vasoconstriction is inhibited by volatile agents, worsening shunt
- Increased dead space
- Impaired mucociliary clearance and cough
Airway
- Loss of upper airway tone leading to obstruction
- Depressed protective reflexes with a risk of aspiration
These effects are additive with the pre-existing changes of obesity, pregnancy, ageing and lung disease, which is why those groups desaturate rapidly and need preoxygenation, PEEP and careful recovery.