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# Normal Capnogram A capnogram is a graph of the concentration of carbon dioxide (CO2) in exhaled gas over time during respiration. The graph shows four phases: **Phase I:** Pure gas from the anatomical dead space. This space contains no CO2. **Phase II:** A mixture of anatomical and alveolar g...

# Normal Capnogram A capnogram is a graph of the concentration of carbon dioxide (CO2) in exhaled gas over time during respiration. The graph shows four phases: **Phase I:** Pure gas from the anatomical dead space. This space contains no CO2. **Phase II:** A mixture of anatomical and alveolar gas. **Phase III:** Pure alveolar gas (alveolar plateau). **Phase IV:** The end of the respiration cycle. Expiration begins before CO2 appears. This is because the anatomical dead space gas, which contains no CO2, is exhaled before the alveolar gas. **BOX 7-1 V/Q Conditions** This box describes the relationship between ventilation (V) and perfusion (Q) and their effect on the partial pressure of CO2 (PCO2) in exhaled gas (PETCO2), arterial blood (PaCO2), and venous blood (PvCO2). * **If V/Q = ∞ (no perfusion):** Then PETCO2 ≈ 0 * **If V/Q is normal:** Then PETCO2 ≈ PaCO2 * **If V/Q ≈ 0 (no ventilation):** Then PETCO2 ≈ PvCO2 V/Q represents the ratio of ventilation to perfusion. The values of V/Q and their effects on PCO2 are shown in the table below. | V/Q | PETCO2 | |---------------|-------------| | ∞ (no perfusion) | ≈ 0 | | Normal | ≈ PaCO2 | | ≈ 0 (no ventilation) | ≈ PVCO2 | **Note:** PETCO2 = Partial pressure of CO2 in exhaled gas; PaCO2 = Partial pressure of CO2 in arterial blood; PVCO2 = Partial pressure of CO2 in venous blood; V/Q = Ventilation/perfusion ratio.