Kapnograms

Kapnograms, also known as capnograms, are graphical representations of carbon dioxide (CO2) concentration in the airway over time during respiration. They are produced by capnography, the continuous measurement and display of exhaled CO2, and are used to assess ventilation and the efficiency of gas exchange in patients.

Capnography devices sample exhaled gas either by mainstream sensors placed directly in the airway or by sidestream sensors that withdraw a small portion of gas to a proximal sensor. The resulting capnogram is a waveform with end-tidal CO2 (EtCO2) indicating the CO2 level at the end of expiration. Measurements are typically reported in millimeters of mercury (mmHg) or as a percentage.

Normal EtCO2 values are commonly about 35–45 mmHg in healthy adults, and EtCO2 generally tracks arterial CO2 (PaCO2) under normal conditions, though there can be a discrepancy in certain situations. The capnogram waveform is typically described in phases: Phase I represents gas from the anatomical dead space with low CO2; Phase II shows a rapid upstroke as CO2-rich alveolar gas mixes with dead space air; Phase III is the alveolar plateau where CO2 concentration levels off near the end-tidal value.

Clinical uses include monitoring ventilation during anesthesia and critical care, confirming endotracheal tube placement by the sudden appearance of CO2 after intubation, detecting hypoventilation or hyperventilation, and identifying airway obstruction, disconnection, rebreathing, pulmonary embolism, or other ventilation–perfusion disturbances. Limitations include potential inaccuracy in low-perfusion states, significant ventilation-perfusion mismatch, high FiO2, or technical factors such as leaks, condensation, or motion artifacts that can distort the waveform and EtCO2 reading.