Flow-volume curves: 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.