Capnography is one of the terms commonly used throughout the medical world. Capnography machines are commonly used to monitor patients under anesthesia and are an important piece of equipment in facilities when patients undergo surgery and other procedures that require sedation. The use of a capnography machine can help detect potentially dangerous respiratory events, such as hypoventilation.
There are several ways to monitor carbon dioxide. It can be displayed using visual or numerical data, and can be read using a sensor attached directly to the patient's airway, called mainstream monitoring, or a sensor located inside the capnography, called sidestream monitoring.
A capnogram specifically refers to the visual waveform of exhaled carbon dioxide displayed on a capnography machine. This allows healthcare professionals to have continuous visibility into the patient's respiratory status. Capnography, on the other hand, measures end-tidal carbon dioxide, or ETCO2. This is a number that summarizes the patient's exhaled carbon dioxide. If it is below the normal ETCO2 of 30-45 mm Hg, the patient cannot exhale effectively.
Mainstream Capnography: Mainstream CO2 monitoring should only be used in intubated patients. It uses a sensor that connects directly to the airway and sends its data to a capnography machine for display. Because mainstream sensors read CO2 levels directly from ET tubes, providing the most accurate data. Its waveform readings are in-phase, meaning they occur exactly as the patient exhales.
Sidestream capnography: In sidestream capnography, the sensor is located inside the capnography machine. A portion of the tubing diverts from the main breathing circuit to the monitor, allowing the capnography machine to analyze the patient's breathing inside the monitor. This eliminates the need for a second machine, an external mainstream sensor.
Lateral flow sensors can be used with nasal cannulae and thus can be used on non-intubated patients. They can also be used on intubated patients if required. However, with lateral flow monitoring, the waveform is out of phase because it takes longer for the patient's exhaled air to reach the sensor. This is an accurate reading, but with a slight delay.