Errors in Capnography

Because water vapor is often the major source of error, several "water trap" devices or heated suction tubes are used to deliver moisture-free gas to the reading cell. The effect of gas temperature can be calculated. Assume body temperature is 37°C and PaCO₂ = PETCO₂ = 40 mm Hg. If the sampling tube and detector are maintained at a temperature above 37°C, there will be no condensation or "rain out" of water, and the measured PCO₂ will be 40 mm Hg. However, if the expired gas is allowed to cool to room temperature (e.g., 20°C), water will condense in the tubing, and the new water vapor pressure (PH₂ O) will decrease to the saturated PH₂ O at room temperature (typically 20 mm Hg). If a drying agent is used in the sample line, PH₂ O will drop to zero. In either case, the measured PCO₂ will increase slightly.