Lung volumes are measured by a gas dilution method using either nitrogen or helium. The most important measurements are residual volume (RV) and total lung capacity (TLC). A high total lung capacity demonstrates hyperinflation, which is is consistent with emphysema. A low total lung capacity suggests restrictive disease and is consistant with fibrosis among other problems. Increased residual volume indicates air trapping. These tests are very important in understanding the individual patient's problem and point the way to optimal therapy.