Functional diagnosis usually begins by identifying ventilatory dysfunction through forced spirometry. Forced spirometry is the simplest and most consolidated test of pulmonary function, and so it is very useful for an initial approximation to evaluation of the ventilatory status of the lung and respiratory system. Forced spirometry has three main parameters: Forced Vital Capacity (FVC), Forced Expiratory Volume in one second (FEV1) and the percent ratio between both (FEV1%FVC). FVC is the maximal volume that can be exhaled forcibly after a maximal deep inspiration; FEV1 is defined as the amount of gas that can be exhaled in one second after a maximal inspiration and during a maximal forced expiratory maneuver.
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Pulmonary function tests explore different aspects of pulmonary (respiratory) function. Usually several categories of parameters can be identified: those exploring ventilation, gas exchange, pulmonary circulation, oxygen transport or response to exercise. Obviously all these parameters interact with one another in such a way that, for instance, ventilatory dysfunction nearly always influences gas exchange or response to exercise, and so on.
Diagnosing the functional status of the lung and respiratory system generally requires several tests that measure or evaluate different aspects of respiratory system function. The ensemble of parameters provided by respiratory function tests composes a matrix of data. These data have their own meaning (usually in relation to predicted values) but also a meaning derived from internal relationships in the context of our pathophysiological knowledge, in opposition to numeric (or calculation) dependence. Therefore, functional changes in disease hardly ever affect a single parameter but, on the contrary, the whole functional situation is depicted by a more or less characteristic distortion of the functional matrix. The more complete the functional study, the more precise the functional diagnosis. Each parameter is, therefore, a piece in a complex puzzle with no predicted pattern, but one that we compose in order to make the pieces fit into a functional picture.