What does vital capacity mean?

The “vital capacity” (VC) is the maximum breathing volume that can be inhaled and exhaled. In general, this value is determined by the performance (or health restriction) of the lung tissue and the motor ability to inhale and exhale. In ALS, the lung tissue itself is not usually affected – the focus is on the weakness of the respiratory muscles (hypoventilation).

Various test methods are available to determine vital capacity (VC). The measurement is carried out by patients breathing in and out as quickly and forcefully as possible (forced vital capacity, FVC) or slowly and forcefully (slow vital capacity, SVC). The results are compared with standard values (reference values) that have been established for normal healthy people (depending on gender, weight, height and age).

The result of the SVC measurement shows an absolute value (respiratory volume in liters) and a comparative value (as a percentage of the reference values of the “normal population”). An optimal value is 100 % and above. SVC values above 80 % are considered unproblematic. For measured values below 80%, the possibility of a breathing aid should be considered, taking individual factors into account.

When evaluating SVC values, not only the absolute value must be considered, but also the dynamics of the SVC progression (decrease in SVC percentage points per month) must be taken into account. Certain methodological limitations of the SVC measurement must also be included in the evaluation, as patients with bulbar syndrome in particular have difficulties in performing the measurement in a comparable manner. This can result in measured values that are lower than the “real” vital capacity. The measurement of SVC (in addition to the measurement of the cough) is one of the important examination methods used in specialized ALS centers to assess respiratory function and offer treatment options.