Spirometry, which includes measurement of forced expiratory volume in one second (FEV1) and forced vital capacity (FVC), is the most readily available and most useful pulmonary function test. The flow-volume loop (also called a spirogram) is a plot of inspiratory and expiratory flow (on the Y-axis) against volume (on the X-axis) during the performance of maximally forced inspiratory and expiratory maneuvers.
The normal expiratory portion of the flow-volume curve is characterized by a rapid rise to the peak flow rate, followed by a nearly linear fall in flow as the patient exhales toward residual volume (figure 1). The inspiratory curve, in contrast, is a relatively symmetrical, saddle-shaped curve. The flow rate at the midpoint of exhalation (between total lung capacity and residual volume) is normally approximately equivalent to the flow rate at the midpoint of inspiration.
Changes in the contour of the loop can aid in the diagnosis and localization of airway obstruction . Characteristic flow-volume loop patterns are also often found in certain forms of restrictive disease, although flow-volume studies are not considered primary diagnostic aids in the evaluation of these disorders.
An overview of flow-volume loops will be presented here. A general review of pulmonary function testing, including the indications for spirometry, is provided separately. (See "Overview of pulmonary function testing in adults".)
OBSTRUCTIVE PULMONARY DISEASE
A frequently recognized abnormality in the flow-volume loop is the concave upward or "scooped-out," or "coved" pattern encountered in asthma or chronic obstructive pulmonary disease (COPD) (figure 2). Maximal expiratory flow rates during the latter two-thirds of an expiratory maneuver are largely effort independent (ie, flow cannot be accelerated by increased expiratory effort), and vary directly with the elastic recoil of the lung and inversely with the airway resistance upstream of the equal pressure point.