Correctly performed FVC maneuver, no pathology
An example of a correctly performed FVC maneuver. Upon producing
the peak expiratory flow, flow diminishes in a nearly linear
fashion. Repeated maneuvers lead to reproducible flows, albeit that the FVC maneuver appears to be terminated prematurely at least once.
The second figure shows 5 reproducible
maximum expiratory flow-volume curves. In some cases the middle
portion of the curve is convex towards the volume axis, in
other cases it displays ‘a shoulder’ instead.
These variations often relate to variations in body position
during the maneuver. If trunk, neck and head are kept straight
while looking forward the shape of the curve may be different
from the one obtained when the subject bends over during the
maneuver. This is because the former position may straighten
the trachea, thereby stretching it and increasing its stiffness
(consult reference below). It is therefore recommended to standardize
body posture; it is best if the subject sits or stands straight
up, with the chin tilted slightly upwards throughout the
maneuver. This also prevents saliva from dripping into the
equipment or tubing.
Posture and trachea
Melissinos CG, Mead J. Maximum
expiratory flow changes induced by longitudinal tension on
trachea in normal subjects. J Appl Physiol 1977; 43: 537-544.