Practical application of the flow-volume loop

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Diminished airflow is the abnormality most frequently noted on spirometry. This finding is not always caused by intrinsic airways obstruction, but can be caused by a variety of other pathogenetic mechanisms. Apart from diffuse bronchial obstruction, these include small airways disease, upper airway obstruction, suboptimal effort of the patient, muscular weakness, and restrictive lung diseases. Such problems may not be readily distinguishable by spirometry, but a special physiologic test, the flow-volume loop can be helpful in their differentiation.

Standard spirogram versus flow-volume loop

The flow-volume loop is merely one kind of graphic display of breathing mechanics during the forced vital capacity maneuver. The vital capacity (VC) is the maximum volume of air that an individual can expel after maximum inspiration (Fig. 1). Residual volume (RV) is that volume of air that remains after maximal exhalation. Total lung capacity (TLC) is the sum of those two volumes. When the vital capacity maneuver is performed at a maximal rate, it is called a forced vital capacity (FVC). The standard spirogram (V-t) is the most commonly used graphic display of the forced vital capacity maneuver. The measured parameters are volume (V) on the vertical axis and time (t) on the horizontal axis. From this curve, one can derive the forced expiratory volume in one second (FEV1), the peak expiratory flow (PEF), the forced expiratory flow at 200 to 1200 ml (FEF200–1200 ml), and the maximum mid-expiratory flow rate (FEF25%–75%). Together, these flows can provide information about the patient’s effort, neuromuscular strength, large airways.



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