Spirometer: Lung Function Test

A spirometer allows the measurement of vital capacity and the maximum rate at which air can be expired from the lungs. An individual's vital capacity is largely determined by his or her size rather than fitness level. Although the vital capacity may increase slightly in response to some modes of exercise, it is not likely to act as a limiting factor to exercise performance in normal healthy people. The likely cause of the slight increase in vital capacity is an increase in the strength of the respiratory muscles which allows individuals to expand the rib cage slightly more during inspiration and compress it to a greater degree at the end of expiration.

Of greater importance than the vital capacity is the rate at which one can expire air from the lungs. For asthmatics, for example, bronchoconstriction restricts the rate at which air can be drawn into and expelled from the alveoli and this acts to limit the rate of gas exchange.

With a spirometry test the peak rate of expiration can be measured by determining the slope of the line at its steepest point. However, the forced expiratory volume in one second (FEV) is often expressed as a percentage of the total vital capacity in order to provide an alternative indication of the rate of air flow. A low FEV % indicates what is known as an obstructive airways condition such as asthma.

Generally, the FEV should be 70-80% or more of the vital capacity depending to some extent on the size of the individuals vital capacity (those with larger vital capacities tend to have slightly lower FEV % scores as compared to those with smaller lung capacities).

It is important to realize that a poor spirometer test result may be due to inappropriate technique or lack of motivation on the part of the subject, so some practice (2-4 repetitions) is required prior to the ultimate test.

Whether testing asthmatics or normal healthy individuals, the spirometer test should not be seen as a performance related test, but rather as a screening tool. Vital capacity will increase little, if at all, in response to training and for healthy individuals the FEV % is unlikely to change significantly. It is obviously not appropriate for fitness leaders to diagnose participants as asthmatic; however, a low score may indicate the need for more extensive testing by competent medical personnel.

An alternative tool for indicating possible airways obstruction is a peak flow meter. These allow the measurement of the maximal rate of expiratory airflow which is equivalent to calculating the steepest slope from the spirometry test.

source:Lung Function: Physiology, Measurement and Application in Medicine by John E. Cotes, David J. Chinn, and Martin R. Miller