Severe asthma becomes dangerous when airflow obstruction turns ventilation into a mechanical trap. The immediate concern is often oxygen saturation, but the physiology that kills many intubated asthma patients is dynamic hyperinflation.
In asthma, airway smooth muscle constriction, mucosal edema, and mucus plugging increase airway resistance. Exhalation becomes prolonged because air leaves the lungs slowly. If the next breath begins before the previous breath has fully exhaled, gas becomes trapped. End-expiratory lung volume rises.
Intrinsic PEEP develops. This is auto-PEEP. Dynamic hyperinflation increases intrathoracic pressure, which can reduce venous return and cause hypotension. In severe cases, the physiology can resemble obstructive shock.