Obstructive diseases can be divided into 3 groups. Fixed obstructions, such as foreign bodies or tumors in large airways, increase airways resistance during both inspiration and expiration (see Figure 2–25A). Variable inspiratory obstructions, which are associated with extrathoracic upper airway problems such as obstructive sleep apnea or a paralyzed vocal cord, affect inspiration more than expiration (see Figure 2–25B). The FEV1/FVC is usually normal. Variable expiratory obstructions (see Figure 2–25C), which are associated with intrathoracic airway problems, result in a low FEV1/FVC and a low peak expiratory flow (PEF). They include asthma, chronic bronchitis, and emphysema. A patient may have 2 or all 3 of these disorders together. If 1 of the 3 diseases predominates, it can be distinguished by the interpretation of the patient’s pulmonary function tests. Asthma is an episodic disease diagnosed by an improvement in the FEV1/FVC after a bronchodilator, and/or a worsening of the FEV1/FVC with bronchoprovocation. Chronic bronchitis, which is characterized by copious sputum production, is associated with a normal carbon monoxide diffusing capacity, high residual volumes and functional residual capacities, low arterial and frequently cyanosis, and high arterial . Emphysema, which is characterized by destruction of alveoli, is associated with a low carbon monoxide diffusing capacity, a leftward shift in the lung compliance curve (see Figure 2–7), very high total lung capacities and functional residual capacities, and very high residual volumes (see Figure 3–3). Arterial are usually only slightly decreased and arterial only slightly increased.