What is the cause of dyspnea in asthma and emphysema?

A group of 50 asthmatics and another of 27 subjects with emphysema were interviewed with regard to the nature of dyspnea. They were in particular asked whether their difficulty was during inspiration or in expiration. Asthmatics chiefly described dyspnea as inspiratory while the majority of the emphysema group were unable to relate their distress to either phase of respiration. It is not clear why these two types who share common physiologic abnormalities and form the two well defined polar groups in the spectrum of airways obstruction experience clearly different sensations of dyspnea.