End-tidal carbon dioxide (ETCO2) as a non-invasive monitoring technology has been increasingly used in clinical practice, and it has become a standard equipment in ambulances due to its small size and easy connection. The uses of ETCO2 include confirming the gold standard of advanced airway placement, assessing the survival rate of patients with cardiac arrest, evaluating the severity of sepsis patients, identifying pulmonary embolism earlier than SpO2, and detecting respiratory depression after the application of midazolam, among others.
Carbon dioxide is produced by tissue cellular metabolism, transported to the lungs through capillaries and veins, and released into the atmosphere during exhalation. Under normal conditions, the venous blood partial pressure of CO2 is 46mmHg, and the alveolar CO2 partial pressure is 40 (35-45) mmHg. When venous blood (deoxygenated blood) enters pulmonary capillaries, it can diffuse and exchange with the gas in the alveoli and reach equilibrium. Therefore, the arterial blood CO2 partial pressure (PaCO2) value is equal to the alveolar CO2 partial pressure value of 40mmHg. The body adjusts the level of pulmonary CO2 partial pressure by changing the frequency and depth of breathing, with increased breathing frequency expelling more CO2. The respiratory center in the brain also adjusts breathing frequency and rhythm based on PaCO2 levels.
The name ETCO2 comes from the beginning of expiration (the rising branch of end-tidal CO2), which represents the gas exhalation process in the trachea and bronchi. This part of the gas is part of the dead space and cannot represent the CO2 partial pressure level in the alveoli. Only at the end of expiration is the gas exhalation process from the alveoli, hence the name end-tidal CO2, and the value measured by the sensor is ETCO2 (in this example, 40mmHg). The frequency of the CO2 waveform represents the respiratory frequency.
When cardiac output (blood flow) is normal, ETCO2 reflects ventilation.
When cardiac output decreases, the use of ETCO2 is to reflect cardiac output. Generally speaking, when a person's cardiac output is normal, ETCO2 can reflect the efficiency of ventilation. When cardiac output decreases, the CO2 that returns to the lungs from the tissues decreases. When ventilation remains constant, compared with patients with normal cardiac output, their ETCO2 will decrease, so under constant ventilation, ETCO2 has a certain indication of cardiac output. In patients with cardiac arrest, very little blood can flow back to the lungs, and ETCO2 will drop sharply. It should also be noted that when patients have lung disease, due to the decrease in diffusion function, there may be a large difference between ETCO2 and PaCO2, but we can observe changes in trends and confirm the placement of advanced airways, among others. Understanding these principles is more meaningful than memorizing specific waveforms when interpreting ETCO2.