Respiratory difficulty may gradually increase with signs of upper airway obstruction, in other words, suprasternal and intercostal recession.

伴随着上呼吸道阻塞，呼吸困难可能会逐渐加重，胸部三凹征明显。

Since the bronchioles are smaller, even a slight increase in mucus can lead to airway obstruction, so this contributes to the majority of the air trapping.

因为细支气管管径更小，即使少量黏液增加也会使其阻塞，大部分气体滞留的原因即此。