Physiological differences and similarities in asthma and COPD--based on respiratory function testing.

Physiological differences and similarities in asthma and COPD are documented based on respiratory function testing. (1) The airflow reversibility is usually important for the diagnosis of asthma. However, patients with long disease histories may have poor reversibility. The reversibility test in COPD is useful for predicting the treatment response. (2) In some of the stable asthmatic patients without attack, the concave downslope of flow-volume curve is present. In severe COPD, the flow in the second half of the curve is smaller than that of rest-breathing. (3) Inspiratory capacity (IC) is a good estimator of air trapping and of predicting the exercise capacity in COPD or persistent asthma. (4) Peak expiratory flow (PEF) can be an important aid in both diagnosis and monitoring of asthma. PEF is not used in COPD because the main disorder is in the peripheral airway. (5) Measurements of airway responsiveness may help to a diagnosis of asthma. However, many COPD cases also have it. (6) Impulse oscillation system (IOS) revealed that the predominant airway disorders in asthma and COPD are central and peripheral respiratory resistance, respectively. However, some asthma patients have larger values of peripheral component. (7) D(LCO) reflects the extent of pathological emphysema and it is useful for the follow-up of COPD, whereas D(LCO) is not decreased in asthma. (8) The patient with widened A-aDO(2) and alveolar hypoventilation may lead to the life threatening hypoxia in severe asthma attack or severe COPD. When PaCO(2) overcomes PaO(2), the patient should immediately be treated by mechanical ventilation.