Abstract
A multibreath N2 washout method is presented for evaluating the gas distribution (or ventilation inhomogeneity) and absolute lung volume (FRC) of mechanically ventilated subjects. Because a mechanical ventilator cannot deliver a step change of the input gas composition to subjects, an analysis has been developed to deal with a variable inspired N2 fraction. The N2 washout data are transformed so that they are the same for either variable or step inputs of inspired N2 fraction. The transformed washout curve (end-tidal N2 fraction against dilution number) has a characteristic shape which is indicative of the inhomogeneity of gas distribution and mixing. As a quantitative index of ventilation inhomogeneity a mean dilution number (MDN), which is the ratio of the first to the zeroth moments of the washout curve, is computed. Computer simulations and experiments with subjects having normal or obstructed lungs showed that the method is valid. The MDN and FRC provide a means of assessing the effects of therapeutic interventions on mechanically ventilated subjects.
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