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Figure E3. (A) Paw, flow, and volume tracings representing the procedure used to determine respiratory mechanics during controlled ventilation and to establish the presence of flow limitation. The ventilator is slowed down to allow return of lung volume toward passive FRC. A ventilator cycle is then triggered and the breath was occluded at the end of the inflation phase. VT delivered after the ventilator slowing was varied. E and R were computed using the usual calculations employed with the interrupter technique (E1). From the computed E for this breath, the time course of Pel during the following expiration is determined at different points during the expiratory phase (x symbols, Paw tracing). From this Pel and the R measured from the preceding occlusion, predicted expiratory flow is estimated at several points during expiration (x symbols, flow tracing). In this patient, actual flow is considerably less than predicted expiratory flow and there is an early expiratory flow spike. Both these features are taken to indicate flow limitation. (B) Tracings from the same patient during PAV showing the determination of expiratory flow immediately preceding inspiratory effort (event marker). The beginning of inspiratory effort is defined as the point at which the trajectory of expiratory flow deviates upward from its trajectory established through most of expiration. To determine the magnitude of DH, the point at which expiratory flow during CMV (A) reaches the same expiratory flow observed immediately before inspiratory onset during PAV is identified (event marker, flow tracing in Panel A). The decrease in volume from this point to the end of expiration is then measured (DH).
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