pulmonic

Lung Volumes and Capacities. The total lung capacity (TLC) is divided into four volumes. The tidal volume (V_T) is the volume inhaled or exhaled in normal quiet breathing. The inspiratory reserve volume (IRV) is the maximum volume that can be inhaled following a normal quiet inhalation. The expiratory reserve volume (ERV) is the maximum volume that can be exhaled following a normal quiet exhalation. The residual volume (RV) is the volume remaining in the lungs following a maximal exhalation. The vital capacity (VC) is the maximum volume that can be exhaled following a maximal inhalation; VC = IRV + V_T + ERV. The inspiratory capacity (IC) is the maximum volume that can be inhaled following a normal quiet exhalation; IC = IRV + V_T. The functional residual capacity (FRC) is the volume remaining in the lungs following a normal quiet exhalation; FRC = ERV + RV.

The vital capacity and its components are measured using a spirometer, which measures the volumes of air inhaled and exhaled. The functional residual capacity is usually measured by the helium dilution method using a closed spirometry system. A known amount of helium is introduced into the system at the end of a normal quiet exhalation. When the helium equilibrates throughout the volume of the system, which is equal to the FRC plus the volume of the spirometer and tubing, the FRC is determined from the helium concentration. This test may underestimate the FRC of patients with emphysema. The FRC can be determined quickly and more accurately by body plethysmography. The residual volume and total lung capacity are determined from the functional reserve capacity.

Forced Vital Capacity (FVC). In the forced vital capacity maneuver, the patient exhales as forcefully and rapidly as possible, beginning at maximal exhalation. Several parameters are determined from the spirogram. The forced vital capacity is the total volume of air exhaled during the maneuver; it is normally equal to the vital capacity. The forced expiratory volume (FEV) is the volume expired during a specified time period from the beginning of the test. The times used are 0.5, 1, 2, and 3 seconds; corresponding parameters are FEV_0.5, FEV_1.0, FEV_2.0, and FEV_3.0. The maximal expiratory flow is the slope of the line connecting the points where 200 ml and 1200 ml have been exhaled; it is also called FEF_200–1200 (forced expiratory flow). The maximal midexpiratory flow is the slope of the line connecting the points where 25 per cent and 75 per cent of the forced vital capacity have been exhaled; it is also called FEF_25–75%.

Maximal Voluntary Ventilation (MVV). This is the maximal volume of air that can be breathed by the patient, expressed in liters per minute; it was formerly called maximal breathing capacity. The patient breathes as rapidly and deeply as possible for 12 to 15 seconds and the volume exhaled is determined by spirometry.

Predicted Values. Because the results of pulmonary function tests vary with size and age, the normal values are calculated using prediction equations or nomograms, which give the normal value for a specific age, height, and sex. The prediction equations are derived using linear regression on the data from a population of normal subjects. The observed values are usually reported as a percentage of the predicted value.

Interpretation. These tests provide evidence of impairment of ventilatory function; they do not point to specific disease processes. Abnormal test results may show either an obstructive or a restrictive pattern; sometimes both are present.