Capnographs

Capnographs are medical devices that continuously monitor and display the concentration or partial pressure of carbon dioxide in exhaled air. They provide a capnogram waveform and a numeric end-tidal CO2 (EtCO2) value, which represents the CO2 level at the end of expiration. Capnography is the broader process; capnometry refers to the numeric measurement, while capnography includes the waveform.

Most capnographs use infrared spectroscopy to detect CO2 or gas-analysis methods, and they can be integrated into anesthesia machines, ventilators, or operate as standalone monitors. Sampling can be sidestream, drawing a small sample of gas through a tube to a sensor, or mainstream, with the sensor placed directly in the airway.

The capnogram waveform typically consists of phases corresponding to the respiratory cycle and a plateau phase reflecting alveolar gas. The end-tidal CO2 value is usually around 35–45 mmHg in healthy adults and serves as an indicator of effective ventilation. EtCO2 values help confirm endotracheal tube placement, monitor ventilation during anesthesia or critical care, and assess changes in metabolic or circulatory status.

Clinical uses include confirming airway security, monitoring ventilatory status during surgery or in the ICU, and guiding resuscitation efforts in cardiac arrest. Interpretation requires awareness of potential artifacts: leaks or disconnections, sample-line issues, humidification, low perfusion, high inspired CO2, or abnormal blood chemistry such as severe carboxyhemoglobinemia or methemoglobinemia can affect accuracy and waveform shape.