How to Test?

Spirometry measures airflow over time. It is most commonly expressed as two numbers that represent volume expired from the lungs. The forced vital capacity (FVC), is the amount of air that can be blown out of fully inflated lungs. This is the volume test. The forced expiratory volume in one second (FEV1) is the amount of air blown out in the first second of the forced vital capacity. The FEV1 is the flow test. The ratio between the two (FEV1/FVC), should be more than 70%. lf the FEV1/FVC ratio is less than 70%, this is a strong indicator of early airflow obstruction. It is a harbinger of further rapid decline often leading to disabling emphysema.

The determinants of expiratory airflow are illustrated in Figure 1. Expiratory airflow is a function of pressure against resistance. The pressure is generated by elastic recoil and the resistance of the conducting airways. Spirometry is an effort-dependent test. lt takes effort by the patient to fill the lungs completely and a complete uninterrupted effort to empty the lungs. Normal lungs empty in about six seconds.

It is now known that the forced expiratory volume in six seconds (FEV6), is an excellent surrogate for FVC. Thus, doing a six-second expiratory maneuver is more pleasant for the patient and more convenient for the tester. Newer spirometers are now available that use the two parameters: FEV1 and FEV6. Predicted values for FEV6 have been validated and published (see Hankinson and Swanney).

These new office spirometers are small and thus portable. They are inexpensive, easy to use, and accurate. Such a spirometer is illustrated in Figure 2.

FIGURE 1
Factors Associated with Expiratory Airflow
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FIGURE 2
Simple hand-held spirometers are inexpensive, accurate, and easy to use.

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