Expansion
Bronchoconstriction, gas trapping and hyperinflation with V/Q mismatch
Expansion
The sequence:
- Bronchoconstriction, mucosal oedema and mucus plugging raise airway resistance
- Expiratory flow limitation with dynamic compression traps gas
- Hyperinflation raises functional residual capacity; this splints airways open but forces breathing onto the flat, stiff part of the compliance curve
- Work of breathing rises sharply and the diaphragm is flattened and mechanically disadvantaged
- V/Q mismatch causes hypoxaemia
Carbon dioxide follows a characteristic course: low initially from hyperventilation, then normalising, then rising as the patient tires. A “normal” carbon dioxide in a distressed asthmatic is therefore a warning sign, not reassurance.
Other markers of severity: silent chest (too little airflow to generate wheeze), pulsus paradoxus from exaggerated inspiratory swings, inability to complete sentences, and exhaustion.
Auto-PEEP from incomplete exhalation impedes venous return and complicates mechanical ventilation, which requires long expiratory times and permissive hypercapnia.