The different pulmonary capacities include the functional residual capacity, inspiratory capacity, total lung capacity and vital capacity, determined by spirometry. Forced vital capacity is an additional function that can be determined by the flow-volume loop, according to The Merck Manual Professional Edition.
Spirometry values are important in the calculation of lung volumes that cannot be determined directly and are useful in pulmonary function tests, according to The Merck Manual Professional Edition. Functional residual capacity is the sum of residual volume and expiratory reserve volume. Inspiratory capacity is calculated by adding tidal volume to inspiratory reserve volume.
Total lung capacity can be calculated in many ways, including the sum of residual volume, expiratory reserve volume and inspiratory reserve volume. Vital capacity is the sum of tidal volume, inspiratory reserve volume and expiratory reserve volume, explains The Merck Manual Professional Edition. The flow-volume capacity is helpful in diagnosing obstructive, restrictive and mixed lung diseases. In obstructive disorders, the ratio between forced expiratory volume in one second and forced vital capacity is decreased. In restrictive disease, this ratio can be decreased but is more often normal or increased. In mixed disorders, it is decreased.
To calculate this ratio, a patient typically takes a deep breath and exhales forcefully into a tube that is attached to a machine that makes the calculation. The forced expiratory volume in one second is the amount of air the patient is able to exhale into the tube in the first second; the forced vital capacity is the total amount of air exhaled in this process, notes The Merck Manual Professional Edition.