Forced Vital Capacity (FVC) is a fundamental measurement used during a pulmonary function test (PFT) to assess the mechanical health of the lungs and airways. It is used for individuals experiencing symptoms like chronic cough, wheezing, or shortness of breath. The FVC test determines the total volume of air a person can move, providing a valuable snapshot of their overall lung capacity. This simple, non-invasive maneuver helps differentiate between conditions where the airways are narrowed and those where the lung tissue itself is restricted.
Defining Forced Vital Capacity
Forced Vital Capacity is defined as the total volume of air a person can forcefully and completely exhale after taking the deepest possible breath. This measurement is expressed in liters and represents the maximum capacity of the lungs to move air. The term “Forced” is significant because the exhalation must be performed as quickly and vigorously as possible, providing information about airway resistance.
FVC differs from a simple Vital Capacity (VC), where exhalation is performed slowly and steadily. FVC is measured alongside the Forced Expiratory Volume in 1 second (FEV1), which is the amount of air expelled during the first second of the maneuver. Comparing FVC and FEV1 creates the FEV1/FVC ratio, a calculated value essential for classifying lung diseases.
How FVC is Measured
FVC is measured using a device called a spirometer, in a procedure known as spirometry. This test requires the patient to participate actively and follow specific instructions precisely to ensure accurate results. The patient is typically seated with a nose clip placed on their nostrils to ensure all air passes through the mouth.
The technician instructs the patient to take a maximum, deep breath, filling the lungs completely. Following inhalation, the patient seals their lips tightly around a mouthpiece and exhales as hard and fast as possible until the lungs feel completely empty. The exhalation phase usually lasts for at least six seconds to ensure all possible air is expelled.
To ensure reliability, the test is usually repeated at least three times, with the two best measurements needing to be within a close range. The quality of the final measurement depends heavily on the patient’s maximal effort and cooperation. This process provides the absolute volume, measured in liters.
Interpreting FVC Results
The FVC value is compared to a predicted normal value, calculated based on the person’s age, sex, height, and ethnicity. Results are reported as a percentage of this predicted value; a result above 80% is generally considered within the normal range. A low FVC suggests the lungs are unable to hold a normal amount of air, pointing toward a restrictive pattern of lung disease.
A low FVC combined with a normal or high FEV1/FVC ratio is the classic indicator of a restrictive defect. This pattern suggests the lung tissue is stiff or scarred, as seen in conditions like pulmonary fibrosis, or that chest wall movement is restricted due to issues like obesity or neuromuscular disease. In these cases, both the FEV1 and FVC are reduced proportionally, keeping the ratio intact.
Conversely, an obstructive lung disease, such as asthma or Chronic Obstructive Pulmonary Disease (COPD), is defined by a low FEV1/FVC ratio, indicating difficulty with the speed of exhalation. While FVC measures lung volume, its clinical meaning is only fully understood when evaluated in conjunction with the FEV1 and the resulting ratio.