FIGURE 3.

Effect of ventilation heterogeneity on wash-in progress. Wash-ins are all from a model with a first lung compartment volume (V1) of 0.7 and fractional ventilation 0.3 (giving a model lung clearance index of 10). Three different wash-in scenarios are presented: open circuit, closed circuit with tidal breaths and closed circuit with six deep breaths at the start of wash-in. a) Wash-in progression, as shown by expired end-tidal SF₆. Target end tidal SF₆ (representing complete gas equilibrium throughout the model) is shown as a horizontal dotted line. Closed circuit wash-in to 0.1% SF₆ and open circuit wash-in to 0.2% SF₆ are shown to same scale on the different axes. b) Apparent wash-in incompleteness (measured as difference between inspired and expired SF₆ concentrations at the mouth). c) True wash-in incompleteness, expressed as the percentage difference between SF₆ concentrations in the two compartments.