Flow oscillations on the flow-volume loop: clinical and physiological implications.

In 1981, SANDERS et al [1) described the "saw-tooth" sign, a sequence of rapid oscillations of the flow-volume loop tracing, as an aid for the detection of sleep apnea in awake patients. Such flow oscillations had been recognised before in fact ever since the flowvolume loop came into clinical practice at the beginning of the sixties but generally were discarded as a noisy disturbance, created in the measuring instruments or in the subject's airway and blurring the signal contained in the flow-volume loop [2-5). To illustrate this, in MILLER and HvArr's important paper [4) on the usefulness of flow-volume loops in the evaluation of obstructing lesions of the larynx and trachea it can be literally read that the flow plateaux characteristic of upper airway obstruction "... often contained noise or rapid oscillations because of marked flow turbulence ... " and that the figures shown in the article"... show smooth contours for simplicity of display." Smoothing of flow-volume loops has been the preferred approach to display flow-volume loops, not only to meet the need of both manufacturers and users of pulmonary function equipment to get rid of the noisy disturbances, but also to minimize the possible contributions of oscillations to the variability of derived forced expiratory flow rates [3, 6). Some oscillations seen on flow-volume loops may indeed be nothing else but noise. However, recent evidence indicates that flow oscillations in some instances, may signal the presence of a functional or structural disorder, usually located in the upper airways. In this paper we will try to collect the evidence that should change our view on flow oscillations, an example of how noise may become signal.