A Method of Assuring Maximal Flow in Forced Expiratory Maneuvers

¹ Cardiopulmonary Laboratory, Grasslands Hospital, Valhalla, New York; The Department of Medicine, New York Medical College

The most common forms of pulmonary function testing require a forced expiratory effort, recorded either as a forced vital capacity or a maximum expiratory flow volume curve. Proper performance is always in doubt, with regard to the volume used as well as the flow rate. This work offers a practical approach to assuring maximal flow, at least in the later portion of the expiratory curve. During the later portion of the curve, the airways behave as a Starling resistor. One feature of such a resistance is that the addition of a downstream resistance does not alter flow. If a resistance is rapidly added and removed at the mouth and flow does not change, then the airways are behaving as Starling resistors and maximum flow has been achieved. This approach appeared validated by the following: 1. During the later portion of the expiratory effort, flow was altered only during efforts which were consciously limited by the subject. 2. These consciously limited efforts also resulted in lower flow rates than when the Starling effect was operative. 3. The early, or effort-dependent portion of the curve, is altered by the resistor. 4. Beginning the forced expiratory effort at something less than total lung capacity truncates or eliminates the portion of the curve altered by the external resistance.