Basic principles and measurement methods of end-expiratory pressure monitoring

End tidal carbon dioxide (ETCO2), like blood oxygen saturation monitoring, is a relatively new and non-invasive clinical monitoring technology with high sensitivity. It can not only monitor ventilation conditions, but also reflect the body's circulation and pulmonary blood flow conditions.

End-tidal carbon dioxide (ETCO2) monitoring refers to the use of a CO2 monitoring device to monitor the CO2 concentration or pressure in a patient's exhaled gas. In addition to displaying the CO2 concentration or pressure in real time, the device can also plot the CO2 concentration or pressure onto a graph. Combining concentration indicators with waveform characteristics, these instruments can calculate and derive a series of ETCO2-related parameters, thereby providing information on the patient's ventilation/ventilation function, cardiac output, ventilation/perfusion (V/Q), and metabolism.

End-tidal carbon dioxide can be measured using methods such as infrared absorption, mass spectrometry, Raman spectroscopy, gas chromatography, and carbon dioxide chemical electrode analysis.

The most commonly used method in clinical practice is infrared absorption, which uses the infrared absorption properties of CO2 molecules to determine their concentration. When gas passes through an infrared sensor, a beam of infrared light passes through the gas sample, and an infrared detector measures the beam intensity. Because CO2 absorbs infrared light of a specific wavelength (4.3μm), the degree of beam attenuation is proportional to the CO2 concentration.

Instruments can be divided into mainstream and sidestream types based on the gas sampling method. The mainstream type is commonly used.

-Features: Airflow passes directly through the measurement chamber, and the detection pipeline is part of the artificial airway.
-Advantages: Test results are less affected by airway moisture and secretions.
-Disadvantages: Continuous monitoring can only be used in closed airways, and some manufacturers' products significantly increase airway tubing load and respiratory dead space.

Features: Airflow enters the measurement chamber passively. Exhaled air is drawn into the measurement chamber by a suction pump for measurement, with a suction flow rate of 20-300 mL/min.
Advantages: Applicable to non-sealed airways and diverse sampling locations.
Disadvantages: The sampling port is susceptible to airway moisture and secretions. For low-flow ventilation or children, airflow loss caused by suction sampling may affect tidal volume measurement and ventilator triggering.

Depending on the instrument's waveform display parameters, it can be categorized as either a time-based partial pressure of carbon dioxide waveform or a volume-based partial pressure of carbon dioxide waveform. Because volume-based partial pressure of carbon dioxide waveform meters need to monitor both partial pressure of carbon dioxide and airway flow velocity, they both use a mainstream sampling method.

The vertical axis represents CO2 partial pressure; the horizontal axis represents time.The waveform is continuous and can be divided into four phases:

-Phase I, where the waveform is at baseline, represents inspiration and dead space ventilation.

-Phase II, the ascending limb, represents the time when dead space ventilation and alveolar gas mix and are exhaled.

-Phase III, where the waveform appears as a high horizontal line, represents the time when alveolar gas is exhaled.

-Phase IV, where the waveform reaches baseline at the end of Phase III, represents the start of the next inspiration.

2. Volume-Partial Pressure of Carbon Dioxide Waveform

The vertical axis represents partial pressure of carbon dioxide; the horizontal axis represents expired volume. The waveform is discontinuous and can be divided into three phases:

-Phase I is the baseline, representing the dead space ventilation phase.

-Phase II is the ascending limb, representing the transition from dead space ventilation to alveolar ventilation.

-Phase III is the high horizontal line, representing the alveolar exhalation phase. Since inspiration is not monitored, there is no phase IV.

Normal end-tidal carbon dioxide values ​​are 35-45 mmHg (4.67-6.0 kPa). Because ETCO2 is influenced by basal metabolism, circulation, and respiration, comprehensive analysis of ETCO2 values ​​within a certain range, combined with other relevant indicators, can accurately guide clinical treatment.