As a non-invasive monitoring technology, end-expiratory carbon dioxide (EtCO2) has been used more and more in clinic, and has become a common monitor in ambulances due to its small area and easy connection. EtCO2 can determine survival in patients with cardiac arrest, assess the severity of sepsis, identify pulmonary embolism earlier than SpO2, and respiratory depression after midazolam, among other things.
The main flow (non-fractional flow) monitor uses infrared light emitted by a photodetector placed in a heating box (to prevent liquid condensation) to distinguish respiratory gas components and analyze local respiratory gases at the junction of endotracheal intubation and respiratory system. In contrast to side-flow instruments, main-flow monitors provide fast results (less than 100 ms) with fewer problems due to secretions or water accumulation.
However, main-flow monitors also have some disadvantages. Due to the weight and location of the monitor, components are vulnerable to accidental disconnection, leakage, and damage, and may cause kinks in the endotracheal tube.
By placing a small sensor joint at the junction of endotracheal intubation and respiratory system, the side-flow monitor pumps respiratory gas into the measuring chamber through a tube for analysis. Can be operated remotely (e.g. MRI). In addition, both intubated and non-intubated patients can be used.
Disadvantages of side-flow monitors include response delays of 2-3 seconds, the need for regular calibration, frequent replacement of disposable consumables, and the potential for blockage of the sample tube due to breathing moisture, blood, or secretions.
Note that either the main flow technique or the side-flow technique is used to obtain 50-150mL/min (or more) of exhaled gas. This is particularly important when using low-flow anesthesia techniques.
