capnometrie

Capnométrie is the measurement of the concentration or partial pressure of carbon dioxide (CO2) in respiratory gases. In clinical use, capnometry provides a numerical value for CO2, most commonly end-tidal CO2 (ETCO2), which is the CO2 concentration at the end of expiration. Capnography, by contrast, refers to the graphical display of CO2 versus time (the capnogram). Together, capnometers and capnographs are key tools for monitoring ventilation and circulation.

Principle and methods

Most modern capnometers rely on non-dispersive infrared (NDIR) spectroscopy to detect CO2. Sensors can be mainstream, placed directly in the airway, or sidestream, sampling a portion of gas through a tube to a detector. The technology may also use alternative sensors, but NDIR remains the standard. The ETCO2 value approximates alveolar CO2 and, with a known gradient, provides insight into ventilation-perfusion status.

Clinical use

Capnometry is widely used in anesthesia, critical care, emergency medicine, and during resuscitation to monitor ventilation, verify endotracheal tube placement, guide ventilator settings, and assess circulatory status. A typical ETCO2 in healthy adults is about 35–45 mmHg. The capnogram waveform offers immediate cues about ventilation efficiency, airway patency, rebreathing, and sudden changes in perfusion.

Interpretation and limitations

Elevated ETCO2 can indicate increased CO2 production or hypoventilation; lowered ETCO2 may reflect hyperventilation, reduced cardiac output, or increased dead space. Absence or loss of waveform can signal disconnection or esophageal intubation. Limitations include sensor drift, condensation, motion artifacts, and delayed correspondence with arterial CO2; certain pathologies can alter the CO2–arterial relationship.

History

Capnometry and capnography developed with infrared CO2 sensing in the mid- to late 20th century and have become standard in perioperative and critical-care monitoring.