The components of the normal capnogram

Figure. No Caption Available.

Phase 1

During the early part of the exhaled breath anatomical dead space and sampling device dead space gas are sampled. There is negligible CO2 in phase 1.

Phase 2

As alveolar gas begins to be sampled there is a rapid rise in CO2 concentration.

Phase 3

Phase 3 is known as the alveolar plateau and represents the CO2 concentration in mixed expired alveolar gas. There is normally a slight increase in PCO2 during phase 3 as alveolar gas exchange continues during expiration. Airway obstruction or a high rate of CO2 production will increase the slope. End-tidal PCO2 will be less than the PCO2 of ideal alveolar gas since the sampled exhaled gas is mixed with alveolar dead space gas.

Phase 4

As inspiration begins there is a rapid fall in sample PCO2.

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