Evaluation of respiratory function in health and disease

From a conceptual standpoint, the tests of pulmonary function can be divided into those that assess the ventilatory function of the lungs and those concerned with gas exchange. Tests of ventilatory function reflect alterations of the elastic resistance and flow resistance of the respiratory apparatus. The elastic properties of the lungs are assessed by determining the position and shape of the curve representing the relationship between the pressure across the lungs and absolute lung volume. When there is reduced distensibility of either the lungs or the chest wall, the volume-pressure curve is shifted down and to the right. The slope of the curve is reduced in the patient with pulmonary fibrosis, while it is normal in the patient with obesity. In asthma (or chronic bronchitis) and emphysema, the volume-pressure curve is shifted up and to the left. In emphysema, the slope of the curve is increased, while it is normal in patients with asthma or bronchitis. In practice, lung volume is used as an index of alterations of the volume-pressure characteristics of the lungs and/or chest wall. The vital capacity is often used as a surrogate for the TLC but it is lower than expected in both restrictive and obstructive disorders. The FEV1.0 reflects the degree of expiratory flow limitation. In a restrictive disorder, lung volume and the FEV1.0 are reduced, but the FEV1.0/FVC ratio is normal. In airflow limitation, lung volume, the FEV1.0, and the FEV1.0/FVC ratio are lower than expected. In airflow limitation, the reversibility with inhaled bronchodilator should be determined. Tests of airway responsiveness are indicated when evaluating patients with unexplained chronic cough, chest tightness, or wheezing, particularly if other lung function tests are normal. The adequacy of gas exchange is assessed by determining the arterial blood gas tensions--PaO2 and PaCO2--and the alveoloarterial pO2 gradient--P(A-a)O2. A lower-than-expected PaO2 can result from several different physiologic disturbances. When alveolar hypoventilation is the sole disturbance, the oxygen in the alveoli and in the blood perfusing them virtually comes into equilibrium, so that the P(A-a)O2 is normal. An elevated P(A-a)O2 is caused by either mismatching of ventilation and perfusion, true venous admixture, a diffusion abnormality, or a combination of these disturbances. Because dyspnea on exertion is a cardinal symptom of respiratory disease, exercise tolerance should be assessed. A reduced exercise tolerance may result from ventilatory limitation, impaired gas exchange, cardiac impairment, impaired delivery of the oxygen to the working muscles, or an inability to use the energy.