How can pursed lip breathing reduce the symptoms of an asthma attack?

Obstruction...: In an asthma attack, there is airway obstruction so expiration becomes difficult and, eventually, there is an increase in carbon dioxide since it can't be expired. Pursed lip breathing prolongs the expiratory phase of respiration so that, potentially, more air and carbon dioxide can be exhaled. That way, there is more room for oxygen to be inhaled.