Dead space and CO(2 )elimination related to pattern of inspiratory gas delivery in ARDS patients

INTRODUCTION: The inspiratory flow pattern influences CO(2 )elimination by affecting the time the tidal volume remains resident in alveoli. This time is expressed in terms of mean distribution time (MDT), which is the time available for distribution and diffusion of inspired tidal gas within resident alveolar gas. In healthy and sick pigs, abrupt cessation of inspiratory flow (that is, high end-inspiratory flow (EIF)), enhances CO(2 )elimination. The objective was to test the hypothesis that effects of inspiratory gas delivery pattern on CO(2 )exchange can be comprehensively described from the effects of MDT and EIF in patients with acute respiratory distress syndrome (ARDS). METHODS: In a medical intensive care unit of a university hospital, ARDS patients were studied during sequences of breaths with varying inspiratory flow patterns. Patients were ventilated with a computer-controlled ventilator allowing single breaths to be modified with respect to durations of inspiratory flow and postinspiratory pause (T(P)), as well as the shape of the inspiratory flow wave. From the single-breath test for CO(2), the volume of CO(2 )eliminated by each tidal breath was derived. RESULTS: A long MDT, caused primarily by a long T(P), led to importantly enhanced CO(2 )elimination. So did a high EIF. Effects of MDT and EIF were comprehensively described with a simple equation. Typically, an efficient and a less-efficient pattern of inspiration could result in ± 10% variation of CO(2 )elimination, and in individuals, up to 35%. CONCLUSIONS: In ARDS, CO(2 )elimination is importantly enhanced by an inspiratory flow pattern with long MDT and high EIF. An optimal inspiratory pattern allows a reduction of tidal volume and may be part of lung-protective ventilation.