A fourth-degree equation has been employed to describe the capnogram. By means of its derivative functions two flexion points can be determined, VeI and VeII, the former corresponding to the maximal increase in |FeCO2|, the latter to the minimal increase, i.e., to the beginning of the alveolar phase. The dead space exhaled can be expressed as a continuous function by an equation using the integral function of the empirical curve. The experimental curve (capnogram) has been divided into four phases: lag, where |FeCO2| = |FiCO2; transition, where |FeCO2| increases steadily; log, where |FeCO2| increases exponentially; and alveolar, where FaCO2 increases steadily again. The slope of the record is related to flow rate and total anatomical dead space in the transition phase, to FaCO2 in the log phase, to the physiological dead space and the distribution of inspired gas in the alveolar phase. In the lag phase the dead space arising from the upper airways is exhaled. In the transition phase before VeI the increase in |FeCO2| is almost constant; after VeI it becomes smaller, with formation of a dead-space tail or of a rather flame-shaped front. At the point VeII, the whole anatomical dead space is washed out, and air arising from the alveoli is exhaled, presumably uncontaminated with dead-space gas. Submitted on October 12, 1960

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