The measurement of End-Tidal Carbon Dioxide (EtCO2) offers a non-invasive way to assess the concentration of CO2 at the conclusion of each exhaled breath. This measurement provides real-time insights into a person’s physiological state. Understanding how EtCO2 is measured and what its waveform indicates can offer important information about respiratory function.
Understanding End-Tidal Carbon Dioxide
End-Tidal Carbon Dioxide (EtCO2) refers to the maximum concentration of carbon dioxide in the exhaled breath. This value reflects how effectively CO2 is transported by the blood to the lungs and then expelled. EtCO2 measurements provide insights into three key physiological aspects: the body’s metabolic activity, the efficiency of blood circulation, and the adequacy of ventilation. Normal EtCO2 values in healthy individuals typically fall within a range of 35 to 45 millimeters of mercury (mmHg) or approximately 4.5 to 6.0 kilopascals (kPa). This range serves as a benchmark for interpreting measured values and identifying potential deviations from a healthy respiratory status.
The Underlying Principle of Measurement
The fundamental principle behind EtCO2 measurement in capnography devices is infrared (IR) absorption. Carbon dioxide molecules uniquely absorb specific wavelengths of infrared light. This characteristic forms the basis for how capnographs detect and quantify CO2 levels. A capnograph operates by directing an infrared light beam through a sample of exhaled gas. A detector positioned on the opposite side measures the amount of light that successfully passes through the gas sample. The more carbon dioxide present in the exhaled gas, the greater the absorption of the infrared light. Consequently, less light reaches the detector, and this reduction in detected light is directly proportional to the concentration of CO2. This process allows for continuous, non-invasive, real-time monitoring of CO2 levels.
Mainstream Capnography Devices
Mainstream capnography integrates the CO2 sensor directly into the patient’s airway. This sensor is typically placed between the endotracheal tube and the breathing circuit. This direct placement allows for immediate and continuous CO2 readings as the exhaled breath passes through the sensor. An advantage of mainstream devices is their rapid response time due to the sensor’s direct position in the airway. These devices also do not require a water trap because the sensor is often heated to prevent condensation. However, mainstream sensors can be bulky or heavy, potentially adding drag on the airway, and may require calibration.
Sidestream Capnography Devices
Sidestream capnography continuously draws a small sample of gas from the patient’s airway. This sample travels through a thin tube to a remote CO2 sensor. This method is versatile and can be used for both intubated patients and those breathing spontaneously, often via a nasal cannula. A benefit of sidestream devices is that the sensor itself is not directly at the patient’s airway, making the connection at the patient lighter and less cumbersome. Potential drawbacks include a slight delay in readings as the gas sample travels through the tubing, the necessity for a water trap, and the possibility of sample dilution if the sampling rate is too high or if there are leaks.
Decoding Capnography Waveforms
Capnography waveforms visually represent CO2 concentration changes throughout the respiratory cycle, providing more information than a numerical value. A typical waveform has distinct phases reflecting the progression of exhalation and inspiration.
The waveform begins with Phase I, the inspiratory baseline, which is flat and at or near zero, indicating that inspired air contains minimal CO2. Phase II, the expiratory upstroke, shows a rapid increase in CO2 as dead space gas mixes with alveolar gas. Phase III, the alveolar plateau, represents the exhalation of CO2-rich alveolar gas, and the highest point of this plateau indicates the End-Tidal CO2 value. Finally, the inspiratory downstroke marks a sharp drop in CO2 as inspiration begins and fresh air enters the lungs.
Deviations from this normal, typically rectangular waveform can signal various physiological changes. For example, a sudden drop in EtCO2 or a flatline can indicate a disconnected airway, cardiac arrest, or apnea. A prolonged or “shark fin” shape in Phase III might suggest airway obstruction, such as in asthma or chronic obstructive pulmonary disease (COPD), where air trapping occurs. An elevated baseline can indicate rebreathing of CO2, possibly due to issues with the CO2 absorber in a breathing circuit. These waveform alterations provide diagnostic clues.