End-Tidal carbon dioxide (EtCO2) is a non-invasive measurement that determines the maximum concentration of carbon dioxide at the end of an exhaled breath. The procedure, known as capnography, uses infrared light to detect and measure the amount of CO2 in the respiratory gas. EtCO2 provides real-time, breath-by-breath information about a patient’s ventilation status and how effectively CO2 is cleared from the body. This monitoring tool gives immediate feedback, making it an effective early warning system for changes in a patient’s physiological condition. This article explains the methods used to capture and interpret this data.
Why Measuring EtCO2 is Essential
EtCO2 measurement reflects the interaction of three physiological systems: metabolism, circulation, and ventilation. Since CO2 is produced by metabolism and transported by circulation before being expelled by ventilation, a change in the EtCO2 value can signal a problem in any of these three areas.
The measurement is a quick, objective indicator of gas exchange adequacy and is sometimes called the “ventilation vital sign.” A sudden drop in EtCO2 can indicate a circulation problem, such as a severe reduction in cardiac output or the onset of cardiac arrest. During cardiopulmonary resuscitation (CPR), EtCO2 monitoring assesses the quality of chest compressions, as effective compressions deliver CO2 to the lungs. A persistently low reading (10 mmHg or less) during CPR is associated with poor outcomes.
EtCO2 is also used to confirm the correct placement of a breathing tube during advanced airway management. A measurable CO2 waveform confirms the tube is in the lungs, while a sudden drop to zero suggests tube displacement or disconnection. Furthermore, monitoring EtCO2 assists in the early detection of respiratory depression in patients receiving sedation or pain medication.
Devices and Sampling Techniques
EtCO2 measurement uses an infrared light absorption technique to detect carbon dioxide molecules in exhaled air. A capnometer provides a numerical EtCO2 value, while a capnograph provides both the numerical value and a continuous graphical display, known as a capnogram. Quantitative capnography is the standard method for continuous monitoring.
There are two main sampling technologies: mainstream and sidestream capnography. Mainstream capnography places the sensor directly into the breathing circuit at the patient’s airway, offering a non-diverting, real-time measurement. This method is highly accurate with a fast response time, but the sensor assembly at the airway can be bulky and heavy.
Sidestream capnography, or diverting capnometry, uses a small sampling tube to continuously aspirate gas from the airway to a remote sensor inside the monitor. This technique reduces bulk at the patient’s face, making it suitable for both intubated and non-intubated patients using nasal cannulas or face masks. However, the travel time for the gas sample causes a slight delay in the reading, and results can be affected by the aspiration of ambient air.
A simpler, low-technology alternative is colorimetric EtCO2 detection, which uses a chemical indicator that changes color when exposed to carbon dioxide. This method offers a quick, qualitative confirmation of CO2 presence, usually to verify tube placement. However, it does not provide the continuous, breath-by-breath numerical and waveform data that quantitative capnography delivers. For ongoing assessment, continuous waveform capnography is the accepted standard.
Interpreting the Capnography Waveform
The capnogram waveform is the visual display of EtCO2, charting the concentration of CO2 over the respiratory cycle. A normal waveform has a characteristic shape divided into four distinct phases. Phase I represents the beginning of expiration, where CO2-free gas from the upper airway dead space is exhaled, keeping the waveform at the baseline.
Phase II is the sharp expiratory upstroke, representing the mixing of dead space gas with CO2-rich gas from the alveoli. Phase III, the alveolar plateau, is a relatively flat segment reflecting gas primarily from the alveoli where gas exchange occurs. The peak of this plateau is the actual End-Tidal CO2 value, which normally falls between 35 and 45 mmHg.
Phase 0 is the inspiratory downstroke, where the CO2 level rapidly drops back to the baseline as the patient inhales fresh, CO2-free gas. Deviations from this normal shape indicate specific physiological problems. For example, a rounded, sloped Phase II upstroke resembling a “shark fin” suggests an airway obstruction, such as bronchospasm or chronic obstructive pulmonary disease.
Conversely, a sudden loss of the waveform, where the line drops to zero, is a serious sign. This can indicate that the breathing tube is dislodged or that the patient has experienced cardiac arrest.