capnography

Capnography is the continuous measurement and graphical display of the concentration or partial pressure of carbon dioxide (CO2) in exhaled air, represented as a capnogram. It provides a real-time assessment of ventilation and the effectiveness of gas exchange. The end-tidal CO2 (EtCO2) value, read from the end of a normal exhalation, is a key parameter and typically serves as a surrogate for arterial CO2 (PaCO2) in many clinical settings, though differences can occur in certain conditions.

A capnogram shows characteristic waveform phases. Phase I is the baseline with low CO2 representing inspired air. Phase II is the expiratory upstroke as dead space air mixes with alveolar gas. Phase III is the alveolar plateau, reflecting CO2-rich gas from the alveoli. In some monitors, a brief Phase 0 may precede the baseline during inspiration. Normal EtCO2 in healthy adults is about 35–45 mmHg, usually slightly lower than PaCO2.

Technologies and modalities include mainstream and sidestream capnography. Mainstream sensors measure CO2 directly in the breathing path, providing rapid, real-time values. Sidestream systems aspirate gas through a tube to a sensor located remotely, with a slight sampling delay but greater flexibility for integration with existing equipment.

Clinical uses span anesthesia, emergency and critical care, and intensive care. Capnography helps confirm endotracheal tube placement, monitor ventilation and circulation during surgery and resuscitation, guide ventilation settings, and assess the adequacy of chest compressions during CPR.

Limitations include artifacts from leaks, disconnections, poor airway seal, rapid ventilation changes, high FiO2, or respiratory motion. Interpreting EtCO2 requires consideration of clinical context and device-specific characteristics.