Physical therapy is applied to relieve airway obstruction and to decrease the ventilatory effects of increased resistance to flow. The most direct method for reducing resistance through airways and collateral channels for ventilation is to increase lung volume. It is possible to vary the distribution of alveolar volume and airflow by changing the gradient of pleural pressure and the inspiratory flows. Results of physical therapy are frequently difficult to assess, especially if the simplest of end points, such as volume of sputum and arterial blood gases, are used. For example, the volume of sputum may not accurately reflect mucociliary transport either during or after a period of sputum collection, especially if cough is encouraged. Furthermore, arterial blood gases, like measures of airway resistance, may be insensitive indicators of changes in peripheral airways, especially if homeostatic mechanisms for maintaining ventilation-perfusion relationships are effective.