A capnography machine or device is a medical machine that monitors the carbon dioxide in a patient's respiratory gas or exhaled air (CO2). It is typically used to monitor patients who are under anesthesia, and is an essential equipment in facilities during surgeries and other procedures that require sedation. Using a capnography machine can help detect potential respiratory events such as hypoventilation.
The value of a capnography machine in the operating room has been well recognized, and anesthesiologists are well-trained to rely on this valuable source of information in their decision making. However, when patients are admitted to the intensive care unit (ICU), they will not be able to enjoy the benefits of this valuable monitor. This is likely due to the lack of capnography machines in the ICU, lack of practice experience by critical care physicians and ICU nurses, insufficient maintenance of the capnography machine system resulting in malfunction and subsequent non-use, and the entrenched protocol-driven historical excessive reliance on arterial blood gas testing in the ICU.
Direct measurement of CO2 in exhaled air directly indicates the changes in the clearance of CO2 from the lungs. Indirectly, it indicates the changes in the tissue-level production of CO2 and the circulation system delivering CO2 to the lungs. A capnography machine is a non-invasive monitoring technique that can quickly and reliably provide information on ventilation, circulation, and metabolism. The capnography machine provides waveform and digital readings of end-tidal CO2 (ETCO2 - the maximum concentration of CO2 at the end of the breath). The digital readings of ETCO2 are usually displayed as mmHg (the partial pressure of CO2 in exhaled air) or as a percentage of CO2 in exhaled air.
Most commonly used devices use infrared absorption of CO2 as a working principle. The known concentration of infrared light passes through exhaling gas. CO2 is a multi-atomic gas that absorbs infrared radiation. The remaining beam is detected by the detector and calculates the exhaled CO2 value. Depending on the location of the sensor, a capnography machine is divided into two types: mainstream sensor and sidestream sensor.
ICU patients need to change positions frequently. The capnography machine will help to monitor the integrity of the patient's airway during position changes. Partial endotracheal tube obstruction caused by endotracheal tube kinking or secretions can be detected by continuous monitoring of CO2 waveform. Prolongation of phase II and tilt of phase III indicate endotracheal tube obstruction.