CLINICAL SPIROMETRY
Vital Capacity
The most common measurement of lung function is the vital capacity
(VC). John Hutchinson is given the credit
for inventing the spirometer and for coining the term vital
capacity in the mid-19th century.[1]
The VC is the largest volume measured after an individual inspires deeply and maximally
to total lung capacity (TLC) and then exhales completely to residual volume (RV)
into a spirometer. The maneuver is performed without concern for rapidity of effort.
Normal values for VC are lower in supine subjects than in sitting ones and vary
directly with height and inversely with age. In general, a given VC is suspected
of being abnormal if it falls below 80% of the predicted value. Patients with abnormally
low values for VC are said to have restrictive disease. The decreased VC associated
with restrictive disease may result from lung pathology such as pneumonia, atelectasis,
or pulmonary fibrosis. It may also occur with a loss of distensible lung tissue
(e.g., after surgical excision). Decreased VC is also seen in the absence of lung
disease. In this case, muscle weakness, abdominal swelling, or pain may prevent
the patient from obtaining a full inspiration or a maximum expiratory effort.
