ABSTRACT
Flow-Volume Loop A useful measure derived from spirometry is the flow-volume loop, formed by plotting both inspiratory and expiratory flow vs. volume (Fig. 2). The flow-volume loop gives some indication of the degree of effort used in generating it. A good effort is usually manifested as a sharp rise in expiratory flow, showing a distinct peak, followed by a more gradual decay in flow as lung volume is expired. The loop also allows visual pattern recognition of various disease states. In restriction, the loop is foreshortened, reflecting the loss of vital capacity, and narrow and tall, reflecting the enhanced early airflow from increased elastic recoil forces contributing to expiratory flow (Fig. 6). In intrathoracic
obstruction, as occurs in asthma or chronic bronchitis, expiratory airflow is limited, re sulting in a loop that has a concave expiratory limb (Figs. 7 and 8A). In extrathoracic obstruction, commonly caused by laryngeal tumors, foreign bodies, or vocal cord dysfunc tion, the loop has a truncated inspiratory limb, usually with preserved expiratory flow (Fig. 8B). Fixed obstruction, occurring anywhere in the large airways and not varying with the respiratory cycle, from such processes as airway tumors, foreign bodies, or steno sis, results in a squared-off flow-volume loop (Fig. 8C). Clinically, it is very important to recognize that extrathoracic obstruction, especially vocal cord dysfunction, commonly masquerades as asthma (see Case 6, below). Vocal cord dysfunction (VCD) is a syndrome manifest by variable extrathoracic obstruction at the level of the vocal cords due to inap propriate closure of the vocal cords during inspiration. Patients are often misdiagnosed as having asthma and present with severe dyspnea, inspiratory wheezing, and stridor. However, up to 50% of VCD patients also have coexistent asthma. A definitive diagnosis of VCD requires laryngoscopy to directly visualize the cords during an attack.
