I am a paramedic. Asthma is a two-fold problem that includes both smooth muscle constriction in the lower airway and inflammation of the epithelial lining in the airway lumen. A severe attack can be life-threatening.

I am not aware of any intervention involving slapping the back.

An emergency responder will usually administer nebulized albuterol and ipratropium bromide along with oxygen according to their protocol. More severe attacks may also require epinephrine.

If someone is experiencing a severe asthma attack and does not have their rescue inhaler available it is imperative to call emergency responders immediately.