Figure 5-17 The ratio of delivered to alveolar concentrations (FD/FA) required to sustain the alveolar concentration constant at, for example, the MAC, is a function of several factors. First, it is determined by uptake (see Fig. 5-16 ). Second, it is determined by rebreathing. The ratio therefore decreases with increasing inflow rates (A to D). Although these graphs have the same shape, notice the progressive decrease in the scale on the ordinate as inflow rate increases. The lowest values for any anesthetic result from a nonrebreathing system (i.e., when the inflow rate equals minute ventilation).