Capnography is a non-invasive technique that measures carbon dioxide (CO2) concentration in a patient’s exhaled breath. It provides valuable information about ventilatory status, helping understand breathing patterns and detect potential changes in respiratory function.
Understanding Capnography
Capnography measures the partial pressure or concentration of CO2 in exhaled air. CO2 is a byproduct of cellular metabolism, transported by the circulatory system to the lungs for exhalation. The amount of CO2 in exhaled breath acts as an indirect, real-time indicator of a patient’s metabolism, blood circulation, and ventilation effectiveness.
Importance of Capnography Monitoring
Monitoring exhaled CO2 offers real-time feedback on a patient’s physiological status, allowing immediate detection of breathing changes. This makes it a valuable tool in various settings, from emergency care to operating rooms.
Capnography is used to confirm correct placement of breathing tubes, such as endotracheal tubes, ensuring they are in the airway. It also helps assess ventilation adequacy, indicating hyperventilation or hypoventilation. Capnography can offer clues about blood circulation effectiveness, as CO2 transport relies on adequate blood flow. During cardiopulmonary resuscitation (CPR), it helps assess chest compression quality and indicates when spontaneous circulation has returned.
Techniques for Measuring Capnography
Capnography devices, or capnographs, use infrared technology to measure CO2 levels. CO2 molecules absorb specific infrared light wavelengths, which the device detects to quantify concentration. Data is displayed as a numerical value and a graphical waveform. Two primary methods exist: mainstream and sidestream.
Mainstream capnography involves placing the CO2 sensor directly at the patient’s airway, typically between the breathing circuit and an endotracheal tube. This direct placement allows immediate, real-time CO2 measurement with minimal delay. The sensor unit is often heated to prevent condensation interference. Mainstream devices are generally used for intubated patients.
Sidestream capnography positions the CO2 sensor within the monitoring device, away from the patient’s airway. A continuous sample of exhaled gas is drawn through a thin sampling tube for analysis. This less intrusive method suits both intubated and non-intubated patients, often with nasal cannulas or masks. However, gas travel through the sampling tube causes a slight measurement delay compared to mainstream devices.
Deciphering Capnography Readings
Capnography readings include a numerical End-Tidal CO2 (EtCO2) value and a capnogram waveform. EtCO2 represents the partial pressure of CO2 at the end of an exhaled breath. A normal EtCO2 range is 35 to 45 mmHg. Values outside this range can suggest underlying issues.
The capnogram waveform graphically displays CO2 levels throughout the breathing cycle. A normal capnogram typically has a rectangular shape with four distinct phases. Phase I represents the inspiratory baseline, where CO2 levels are minimal. Phase II shows a rapid increase in CO2 as dead space air mixes with CO2-rich alveolar air during exhalation. Phase III is the alveolar plateau, indicating the exhalation of CO2-rich gas from the lungs, where the EtCO2 value is measured at its peak. Phase IV is the inspiratory downstroke, where CO2 levels sharply drop as inhalation begins.
Deviations from this normal waveform shape can indicate specific conditions; for example, a “shark fin” shape may suggest airway obstruction, while a consistently elevated baseline could indicate rebreathing of CO2. Low or high EtCO2 values, along with changes in waveform shape, can provide immediate information about conditions like hypoventilation, hyperventilation, or issues with circulation.