Clinical application and precautions of end-expiratory high pressure monitoring

End tidal carbon dioxide (ETCO2) monitoring technology is simple, real-time, non-invasive, economical, and continuous. With the miniaturization of monitoring equipment, the diversification of sampling methods, and the precision of monitoring results, it is increasingly widely used in clinical practice and is gradually gaining attention from clinicians. The main clinical applications of this technology are:

End-tidal CO2 monitoring continuously monitors a patient's respiratory status. By measuring CO2 concentration, it can assess alveolar ventilation function. If a patient experiences abnormalities such as rapid breathing, shallow breathing, or apnea, monitoring changes in end-tidal CO2 can identify any issues promptly, providing important insights for physicians.

During artificial ventilation, the ventilator delivers oxygen and air to the patient. End-tidal carbon dioxide monitoring can be used to assess the effectiveness of artificial ventilation. By monitoring changes in exhaled carbon dioxide concentration, it can be determined whether the ideal amount of carbon dioxide is expelled. This helps doctors determine the specific values ​​of the ventilator's oxygen concentration and positive expiratory pressure to ensure adequate breathing.

End-tidal carbon dioxide monitoring measures the relationship between alveolar gas and arterial carbon dioxide levels to estimate the patient's alveolar-arterial difference. The smaller the alveolar-arterial difference, the greater the patient's systemic blood flow. Carbon dioxide is an indicator of blood flow. When carbon dioxide is expelled, blood flow is high, while when the end-tidal carbon dioxide concentration is low, systemic blood flow is low.

Under ideal ventilation conditions, alveolar ventilation and blood flow are in a certain ratio, allowing complete diffusion of CO2 from the bloodstream into the alveoli while ensuring oxygen (O2) uptake and no waste of gas or perfusion. This condition is called "ideal" V/Q, and its ratio is considered "1" (Figure A). This "ideal lung" condition is extremely rare. V/Q imbalances are primarily manifested in the following three ways:

●  The ratio increases, where alveolar ventilation exceeds blood perfusion, either due to decreased local blood perfusion or due to normal blood flow but alveolar hyperventilation. Gas that matches the perfusion blood flow is exchanged, and the remaining gas becomes part of the dead space gas (Figure B2). At the other extreme, alveolar ventilation is normal but there is no perfusion blood flow. In this case, the ratio approaches infinity, and the ventilation of this part of the alveoli is no different from the gas in the anatomical dead space. This is commonly seen in pulmonary embolism (PE), and the ventilation of this part of the embolic lung can be regarded as absolute dead space ventilation (Figure B1).

●  The ratio decreases. Here, the alveolar ventilation is less than the blood perfusion, which may be caused by insufficient alveolar ventilation. The blood flow that matches the alveolar ventilation completes the exchange, and the remaining blood becomes useless perfusion.

●  A ratio of zero indicates that the alveoli only have blood flow but no ventilation. The perfused blood is not oxygenated, and CO2 cannot be expelled. This is a right-to-left shunt within the lungs (Figure C). In actual lung ventilation, the above three situations do not exist in isolation. The true pulmonary V/Q imbalance is more complex. Even within a single alveolus, countless different V/Q values ​​may exist. What we can assess is a comprehensive situation, depending on which V/Q situation is dominant. Therefore, the analysis of alveolar dead space is an overall estimate.

In summary, lung disease, whether affecting the airways, lung parenchyma, or pulmonary circulation, disrupts the V/Q balance and increases the VD-ALV ratio. Assessing the dead space size can indirectly provide insights into the severity of lung disease and the effectiveness of treatment.

1. Impact of Inhaled Gases on the Value

For commonly used spectrophotometric ETCO2 monitors, since carbon dioxide has similar absorption spectra to oxygen and nitric oxide, patients who inhale high concentrations of these gases can affect the measurement results, necessitating correction.

2. Impact of Line Filters

If a filter is installed in the breathing circuit between the patient and the monitoring device, it may affect gas monitoring and artificially cause the ETCO2 value to be lower.

3. Impact of Airway Secretions

Airway secretions or excessive humidification can adhere to the inner wall of the monitoring lumen of mainstream devices or clog the sampling tube of sidestream devices, resulting in inaccurate measurements. Patients undergoing long-term continuous monitoring require careful observation of the monitoring device's cleanliness and patency.

4. Infection Factors

Both mainstream and sidestream CO2 monitors can come into contact with patient airway secretions and become contaminated. Reusable devices and accessories should undergo high-level cleaning and disinfection according to the manufacturer's specifications. Monitor surfaces should also be cleaned as needed to prevent cross-contamination.

5. Impact of Respiratory Factors on Measurements

When using sidestream ETCO2 monitoring, if the patient's respiratory rate is too rapid, changes in gas composition can outpace the monitor's response speed, affecting the accuracy of the measurement. High airway resistance and an abnormal I/E ratio can also make sidestream ETCO2 monitors slightly less accurate than mainstream ETCO2 monitors.