1. Mainstream method
In the mainstream method, a measuring cuvette between the endotracheal tube and the Y-piece measures the absorption of infrared light.
There is no time delay in measurement, and the measurement encompasses the entire volume of air.
The additional measuring cuvette between the patient valve and the tube results in an increased dead space volume when applying the mainstream method. This is particularly relevant when ventilating infants or young children.
Moreover, the measuring cuvette must be constantly heated to 39 °C in order to prevent the measurement from being affected by condensation, and so avoid incorrect measurements.
The mainstream method also poses an increased risk of disconnection due to the additional measuring cuvette.
2. Sidestream method
In the sidestream method, a small amount of air is permanently extracted and directed to the detector, where the measurement is taken.
The sidestream method can also be applied to non-intubated patients via a nasal cannula with a CO₂ suction line. In addition, the sidestream method offers the major advantage that no additional measuring cuvette is required. Consequently, the dead space volume of the breathing circuit is significantly reduced, and the ventilation hose is also lighter in weight at the patient end.
In the sidestream method, etCO₂ measurement is slightly delayed, meaning it is somewhat slower than the mainstream measurement method.