capnograph

A capnograph is an instrument that displays a graph of CO2 concentration in exhaled air as a function of time, providing a continuous measure of ventilation. Capnography refers to the ongoing measurement and graphical display, known as the capnogram, of end-tidal CO2 (EtCO2). A capnograph typically presents both a numerical capnometry value (EtCO2) and the waveform.

Measurement methods rely on infrared absorption of CO2. Modern devices are either mainstream, with a sensor placed directly in the airway, or sidestream, drawing a small gas sample via a tube to an external sensor. EtCO2 on the display approximates arterial CO2 (PaCO2) but can differ, especially in conditions with ventilation–perfusion mismatch.

Interpretation centers on the capnogram waveform and the EtCO2 value. Normal EtCO2 in awake adults is about 35–45 mmHg. The waveform comprises a baseline, expiratory upstroke, a plateau, and an inspiratory downstroke. The shape and numerical value provide information about ventilation status, airway patency, and circulatory efficiency. Changes in the waveform can indicate disconnections, airway obstruction, rebreathing, apnea, or changes in cardiac output.

Applications and limitations: capnography is widely used in anesthesia, emergency medicine, and critical care to confirm endotracheal tube placement, monitor ventilation, guide resuscitation, and detect airway problems. Limitations include differences between EtCO2 and PaCO2, especially with dead space ventilation or poor perfusion, and potential artifacts from motion or equipment issues. Overall, capnography is considered a valuable, real-time monitor of respiratory and circulatory status.