Changes in lung volumes caused by various disease processes

Normal persons have an FRC/TLC ratio of about 50 percent and an RV/TLC ratio of 20 to 30 percent. Patients with restrictive pulmonary disorders (first column) can be expected to have reductions in FRC, TLC, and RV, whereas patients with neuromuscular weakness (fourth column) have an elevated RV, reduced TLC, and normal FRC. Asthma (third column) is characterized by elevations in RV, FRC, and in more severe cases, TLC.