End-tidal carbon dioxide (EtCO2) measures the maximum concentration of carbon dioxide at the end of an exhaled breath. It indicates how effectively the body produces, transports, and eliminates carbon dioxide. This measurement provides insights into a patient’s ventilation status and offers a real-time assessment of respiratory function.
Why Measure EtCO2
Measuring EtCO2 offers valuable insights into a patient’s respiratory and circulatory status. It is widely used to confirm the correct placement of an endotracheal tube following intubation, providing immediate verification that the airway device is in the lungs and not the esophagus. This immediate feedback is crucial for patient safety. EtCO2 also provides continuous assessment of ventilation effectiveness, allowing for early detection of issues like hypoventilation (too little breathing) or hyperventilation (too much breathing).
Beyond ventilation, EtCO2 monitoring helps evaluate the effectiveness of cardiopulmonary resuscitation (CPR). During CPR, EtCO2 levels can indicate the quality of chest compressions and are a reliable sign of the return of spontaneous circulation (ROSC) when a sudden increase is observed. Furthermore, it is a standard of care during sedation and anesthesia, enabling healthcare professionals to monitor respiratory depression and intervene promptly. This continuous assessment allows for timely adjustments in patient management.
How EtCO2 is Measured
EtCO2 is measured using capnography, which generates both a numerical value (capnometry) and a graphical waveform (capnogram). These devices, known as capnometers or capnographs, typically utilize infrared (IR) spectroscopy. Carbon dioxide molecules absorb specific wavelengths of infrared light, and the device measures the intensity of light absorbed to calculate the CO2 concentration in the exhaled breath.
There are two primary methods for capnography: mainstream and sidestream. Mainstream capnography involves placing a sensor directly in the patient’s airway, typically between the endotracheal tube and the breathing circuit. This method provides real-time CO2 measurements with a rapid response time because the gas is analyzed at the source. While offering immediate data, mainstream sensors can be bulkier and may accumulate moisture.
Sidestream capnography, conversely, samples a small amount of exhaled gas through a thin sampling tube. This tube connects the patient’s airway to a sensor located remotely within the monitoring device. Sidestream systems are often lighter and more adaptable, allowing for use with non-intubated patients via nasal cannulas. However, they may introduce a slight delay in readings due to the time it takes for the gas sample to travel to the sensor, and water traps are often needed to prevent moisture from affecting the analysis.
Interpreting EtCO2 Readings
Interpreting EtCO2 readings involves understanding both the numerical value and the capnogram waveform. A typical normal EtCO2 value for an adult ranges between 35 and 45 millimeters of mercury (mmHg). Values outside this range can indicate changes in a patient’s physiological state. For instance, a high EtCO2 (above 45 mmHg) often suggests hypoventilation, where the body is not eliminating enough CO2, leading to its accumulation. This can occur due to slowed breathing or certain lung conditions.
Conversely, a low EtCO2 (below 35 mmHg) can indicate hyperventilation, where the body is exhaling too much CO2. It might also signal decreased CO2 production or transport to the lungs, such as in cases of reduced blood flow or pulmonary embolism. The capnogram, a graphical representation of CO2 concentration over time, provides further diagnostic clues. A normal capnogram typically shows a rectangular shape with rounded corners, reflecting the distinct phases of the respiratory cycle. Deviations from this normal waveform can signal specific respiratory issues, such as a “shark fin” shape indicating airway obstruction.
The capnogram consists of four phases:
- Phase I: The inspiratory baseline, where CO2 is absent during inhalation.
- Phase II: Represents the rapid rise in CO2 as exhaled gas from the dead space mixes with CO2-rich alveolar gas.
- Phase III: The alveolar plateau, showing the relatively constant CO2 concentration from the alveoli, with the highest point at the end of this phase representing the EtCO2 value.
- Phase IV: The sharp drop in CO2 as inspiration begins and fresh, CO2-free air enters the airway.
Factors Influencing EtCO2 Readings
Several factors, distinct from a patient’s underlying physiological state, can influence EtCO2 readings and their accuracy. Equipment malfunctions are a common cause of altered readings. Issues such as a kinked or blocked sampling tube, leaks in the breathing circuit, or the presence of water or secretions in the tubing can lead to inaccurate EtCO2 values. Proper calibration of the capnometer is also important, as calibration errors can result in consistently high or low readings.
Patient-specific factors also play a role in influencing EtCO2. Changes in metabolic rate, such as during fever or hypothermia, can alter CO2 production, thereby affecting EtCO2 levels. Variations in pulmonary blood flow, like those seen with reduced cardiac output or a pulmonary embolism, can impact the amount of CO2 delivered to the lungs for exhalation. Additionally, a patient’s breathing pattern, including respiratory rate and tidal volume, directly influences how CO2 is exhaled and measured. Environmental conditions, such as altitude, can also subtly affect the partial pressure of gases and thus EtCO2 readings.