Fig. 1. A , Time capnogram showing segments, phases and angles. Inspiratory segment is phase 0, and expiratory segment is divided into three phases: I, II, and III. Maximum value of carbon dioxide at the end of the breath is designated as PETCO2. It is lower than PaCO2by about 5 mmHg because of alveolar dead space. The angle between phase II and phase III is α angle, and between phase III and inspiratory downstroke is β angle. B , Volume capnogram (PCO2vs. expired volume): Volume capnogram showing subdivisions of tidal volume. Area under the carbon dioxide curve is effective alveolar ventilation. Area above the carbon dioxide curve and below the arterial PaCO2line indicates physiological dead space. A vertical line is drawn across phase II such that the two triangles p and q are equal in area. This divides physiologic dead space into anatomic and alveolar dead space. C , A time capnogram recorded during cesarean delivery general anesthesia showing phase IV (For details of phase IV, refer to reference 4). PETCO2= end-tidal PCO2.