Cough is caused by a failure of Lung qi to descend, either because the Lung qi is too weak to move in its natural direction or because the downward movement of otherwise healthy Lung qi is blocked by a pathogen or obstruction to qi movement. Normally, robust Lung qi descends, assisting the downward movement of qi in the body in general, particularly in the Stomach and Large Intestine, as well as helping fluids move to the Kidneys for reprocessing. When the Lung qi is too weak to descend properly, it simply wafts upward. Yin deficiency can cause cough by drying out the Lung fluids and generating heat from deficiency, which rises and takes the Lung qi with it. Cough caused by deficiency is chronic and persistent.
Obstruction of Lung qi descent by external or internally-generated pathogens, constrained qi, or by a failure of qi descent in the yang ming organ systems is a disease of excess. The resulting excess-type cough can be acute or chronic. In practice, however, many coughs are a mix of excess and deficiency. For example, the chronic cough of Spleen and Kidney yang deficiency occurs because of a failure of fluid metabolism — pathological fluids accumulate in the Lungs and obstruct the Lung qi. And the persistent cough of a lingering qi-level heat pathogen is always accompanied by qi and fluid deficiency.