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Spirometry results

Spirometry provides several important measures of lung volumes, which are presented in a diagram with time, measured in seconds, on the x-axis, and volume, measured in mL, on the y-axis. From this diagram (Figure 1), the lung volumes can be used to calculate lung capacities like the vital capacity (VC).

The VC, also called FVC, is the greatest amount of air that a person can strongly breathe out after breathing in as deeply as possible.

This is a graph that measures volume in milliliters on the y-axis and time in seconds on the x-axis. At a time of 0 seconds, small waves are registered going from 0 milliliters to 0,5 milliliters, which are labeled as tidal volume. Later, a bigger wave is registered, which reaches the top of the graph and then it goes down a little bit below the inferior limit of the tidal volume. Immediately after, that wave goes back up until the top limit of the total volume so then it can stabilize. The volume from the top of the graph until the top limit of the tidal volume is labeled as inspiratory reserve volume. The expiratory reserve volume refers to the part of the graph between the low limit of the tidal volume and the lowest point of the wave in the graph. The space between the lowest point of the wave and the rest of the graph is the residual volume. The sum of the inspiratory reserve volume and the tidal volume is the inspiratory capacity. The sum of the expiratory reserve volume and the residual volume is the functional residual capacity. The sum of all the volumes in the graph is the total lung capacity and the vital capacity is the total lung capacity minus the residual volume.

Figure 1. Diagram of lung volumes and capacities

When taking a spirometry test, you need to breathe into the spirometer mouthpiece and perform three types of breathing. First, you must breathe normally, also called quiet breathing, which will be registered as a slight increase in volume when inhaling and a small decrease in volume when exhaling. Quiet breathing corresponds to the Tidal Volume (TV). After that, you do a deep inhale, which will be represented in the diagram as a big increase in volume. A big inhale like this, in addition to the amount of air that was already inhaled during quiet breathing, is referred to as the Inspiratory Reserve Volume (IRV). Finally, you must perform a forced exhale, which is the maximum amount of air possible to exhale from the lungs at once. This deep expiration will be recorded as a large decrease in volume which corresponds to the Expiratory Reserve Volume (ERV).

  • The FVC is calculated by summing up the TV, the IRV, and the ERV. Among people with obstructive lung diseases like asthma, the FVC is lower than people with no obstructive disease.

Other volumes and capacities can be extracted from this test.

Referred from:

Article
Spirometry
Article
Lung capacities