Abstract
IntroductionThe inspiratory flow pattern influences CO2 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 CO2 elimination. The objective was to test the hypothesis that effects of inspiratory gas delivery pattern on CO2 exchange can be comprehensively described from the effects of MDT and EIF in patients with acute respiratory distress syndrome (ARDS).MethodsIn 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 (TP), as well as the shape of the inspiratory flow wave. From the single-breath test for CO2, the volume of CO2 eliminated by each tidal breath was derived.ResultsA long MDT, caused primarily by a long TP, led to importantly enhanced CO2 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 CO2 elimination, and in individuals, up to 35%.ConclusionsIn ARDS, CO2 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.
Highlights
The inspiratory flow pattern influences CO2 elimination by affecting the time the tidal volume remains resident in alveoli
In acute respiratory distress syndrome (ARDS), CO2 elimination is importantly enhanced by an inspiratory flow pattern with long mean distribution time (MDT) and high end-inspiratory flow (EIF)
The results indicated that positive effects were related to a high value of MDT and to an abrupt cessation of end-inspiratory flow (EIF) that follows from shortening time for gas insufflation (TI) or using an increasing flow pattern [12]
Summary
The inspiratory flow pattern influences CO2 elimination by affecting the time the tidal volume remains resident in alveoli. Ventilator-induced lung injury is an important problem in the acute respiratory distress syndrome (ARDS) It may be caused by barotrauma related to high airway, alveolar, and transpulmonary pressures or by shear forces at lung collapse and opening during tidal breaths. Bruhn et al [6] showed by dynamic CT that cyclic collapse and opening is reduced by lower VT, dead-space reduction plays a role in a rational lung-protection strategy This suggests an optimal pattern of inspiratory gas delivery [7,8,9,10]. The results indicated that positive effects were related to a high value of MDT and to an abrupt cessation of end-inspiratory flow (EIF) that follows from shortening TI or using an increasing flow pattern [12].
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