Final answer:
Clinical signs of lower airway obstruction occur primarily during exhalation. Obstructions in the lower parts of the respiratory system, such as the respiratory bronchiole and alveolar sacs, result in trapped air and difficulty breathing out. This is measurable through diagnostic ratios like FEV1/FVC, which are reduced in obstructive lung diseases.
Step-by-step explanation:
The major clinical signs of lower airway obstruction typically occur during the exhalation phase of the respiratory cycle. Lower airway obstruction is characterized by difficulty in expelling air out of the lungs. When there is an obstruction in parts of the lung such as the respiratory bronchiole, terminal portion of the bronchiole tree, or the alveolar sacs, and alveoli, air becomes trapped in the lungs making it hard to breathe out, which is especially evident after a maximal expiration.
Obstructive lung diseases like asthma or emphysema increase the resistance in the airways, causing symptoms like shortness of breath, wheezing, stridor, and cough. Diagnosis can be aided by the use of the FEV1/FVC ratio which is markedly decreased in obstructive lung disease due to a greater reduction in FEV1 (the amount of air that can be forcibly exhaled in one second after taking a deep breath) compared to the reduction in FVC (the total amount of air that can be forcibly exhaled).