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