pulmonary disorders
Airflow, Lung Volumes, and Flow Volume Loop
Patterns of Abnormalities
Most common respiratory disorders can be categorized as obstructive or restrictive on the basis of airflow and lung volumes (see table Characteristic Physiologic Changes Associated With Pulmonary Disorders ). Obstructive disorders are characterized by a reduction in airflow, particularly the FEV1 and the FEV1 expressed as a percentage of the FVC (FEV1/FVC). The degree of reduction in FEV1 compared with predicted values determines the degree of the obstructive defect (see table Severity of Obstructive and Restrictive Lung Disorders*, † ). Obstructive defects are caused by Increased resistance to airflow due to abnormalities within the airway lumen (eg, tumors, secretions, mucosal thickening) Changes in the wall of the airway (eg, contraction of smooth muscle, edema) Decreased elastic recoil (eg, the parenchymal destruction that occurs in emphysema) With decreased airflow, expiratory times are longer than usual, and air may become trapped in the lungs due to incomplete emptying, thereby increasing lung volumes (eg, TLC, RV). Improvement of FEV1 and/or FVC ≥ 12% and 200 mL with the administration of a bronchodilator confirms the diagnosis of Asthma or airway hyperresponsiveness. However, some patients with asthma can have normal pulmonary function and normal spirometric parameters between exacerbations. When suspicion of asthma remains high despite normal spirometry results, Exercise Testing may be used to detect exercise-induced bronchoconstriction but is less sensitive than Eucapnic voluntary hyperventilation (EVH) may also be used to diagnose exercised-induced bronchoconstriction. EVH involves hyperventilation of a gas mixture of 5% carbon dioxide and 21% oxygen at 85% of maximum voluntary ventilation for 6 minutes. FEV1 is then measured at specified intervals after the test. As with other bronchial provocation tests, the drop in FEV1 that is diagnostic of exercise-induced bronchospasm varies by laboratory. Restrictive disorders are characterized by a reduction in lung volume, specifically a TLC<80% of the predicted value. However, in early restrictive disease, the TLC can be normal (as a result of strong inspiratory effort) and the only abnormality might be a reduction in RV. The decrease in TLC determines the severity of restriction (see table). The decrease in lung volumes causes a decrease in airflow (reduced FEV1—see figure). However, airflow relative to lung volume is increased, so the FEV1/FVC ratio is normal or increased. Restrictive defects are caused by the following: Loss in lung volume (eg, lobectomy) Abnormalities of structures surrounding the lung (eg, pleural disorder, kyphosis, obesity) Weakness of the inspiratory muscles of respiration (eg, neuromuscular disorders) Abnormalities of the lung parenchyma (eg, Idiopathic Pulmonary Fibrosis ) The feature common to all is a decrease in the compliance of the lungs, the chest wall, or both. 