Single-breath analysis of carbon dioxide concentration records

¹ Istituto di Patologia medica, University of Siena, Siena, Italy,

A fourth-degree equation has been employed to describe the capnogram. By means of its derivative functions two flexion points can be determined, Ve_I and Ve_II, the former corresponding to the maximal increase in Fe_COCO2 , 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 Fe_COCO2 = Fi_COCO2; transition, where Fe_COCO2 increases steadily; log, where Fe_COCO2 increases exponentially; and alveolar, where Fa_COCO2 increases steadily again. The slope of the record is related to flow rate and total anatomical dead space in the transition phase, to Fa_COCO2 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 Ve_I the increase in Fe_COCO2 is almost constant; after Ve_I it becomes smaller, with formation of a dead-space tail or of a rather flame-shaped front. At the point Ve_II, the whole anatomical dead space is washed out, and air arising from the alveoli is exhaled, presumably uncontaminated with dead-space gas.

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