EndtidalCO2

EndtidalCO2, also written end-tidal CO2 or EtCO2, is the concentration of carbon dioxide at the end of an exhaled breath. It is typically measured by capnography, which provides a numeric EtCO2 value and a capnogram waveform.

Measurement methods include mainstream capnography, where a sensor is placed directly in the airway, and sidestream capnography, which samples gas through a line to a remote sensor. EtCO2 values are usually reported in mmHg or percent; normal EtCO2 is roughly 35–45 mmHg in healthy adults, but may vary.

Clinically, EtCO2 monitoring is standard in anesthesia and widely used in critical care, emergency medicine, and prehospital care. It helps confirm endotracheal tube placement, guides ventilator settings, and provides a rapid indication of changes in ventilation, perfusion, or metabolism. A capnogram can help detect airway obstruction, disconnection, rebreathing, or pulmonary embolism.

Interpretation: Stable EtCO2 within a target range and a normal capnogram suggest adequate ventilation. Rising EtCO2 can reflect increased CO2 production or reduced ventilation, while falling EtCO2 may indicate hyperventilation, reduced cardiac output, or disconnection. Sudden loss of waveform indicates a leak or tube removal.

Limitations include discordance with arterial CO2 in certain conditions (e.g., severe COPD, acute respiratory distress syndrome with dead-space ventilation, or pulmonary embolism) and potential errors from leaks, sampling line kinks, or sensor malfunction.

History: Capnography and end-tidal CO2 monitoring became common in anesthesiology in the late 20th century and have since been adopted in critical care and emergency settings as a noninvasive, continuous measure of ventilatory status.

See also Capnography; Capnogram.