A capnometer is a medical device that monitors a patient’s breathing by measuring the concentration of carbon dioxide (CO2) in their exhaled breath. While “capnometer” often refers to the numerical display of CO2 levels, “capnography” is the more comprehensive term, encompassing both the numerical value and a continuous graphical display of CO2 over time.
The Science Behind the Measurement
A capnometer measures end-tidal carbon dioxide (EtCO2), the CO2 concentration at the very end of an exhaled breath. This measurement reflects the amount of CO2 that has traveled from the body’s cells, through the bloodstream, to the lungs, and then out into the exhaled air. The device operates based on the principle that carbon dioxide molecules absorb specific wavelengths of infrared light.
An infrared light source emits a beam through the patient’s breath. A detector then measures the amount of light that passes through. More CO2 in the breath sample absorbs more infrared light, allowing the capnometer to precisely calculate the CO2 concentration. Measuring EtCO2 offers indirect information about a patient’s metabolism (CO2 production), circulation (CO2 transport to the lungs), and ventilation (CO2 elimination from the lungs).
The Capnogram
The output of capnography is called a capnogram, a continuous waveform displaying CO2 concentration over the breathing cycle. This graphical representation offers a breath-by-breath visual of a patient’s respiratory pattern. A typical capnogram waveform has distinct phases, each providing specific information about the patient’s ventilation.
Phase I, the inspiratory baseline, represents the beginning of exhalation where CO2 levels are minimal. Phase II, the expiratory upstroke, shows a rapid increase in CO2 as dead space gas mixes with CO2-rich alveolar gas. Phase III, the alveolar plateau, reflects the exhalation of gas primarily from the alveoli, where CO2 concentration is highest and relatively stable. Phase IV, the inspiratory downstroke, indicates the start of inhalation as CO2 levels rapidly drop back to baseline. Analyzing the shape and numerical values of this waveform, typically with normal EtCO2 values ranging from 35 to 45 mmHg, helps medical professionals assess respiratory function and detect abnormalities.
Why Capnometry is Essential
Capnometry provides immediate, real-time insights into a patient’s respiratory status, circulation, and metabolic activity, making it a valuable tool in various medical settings. It is considered the gold standard for confirming proper placement of an endotracheal tube in the trachea, as the presence of a CO2 waveform confirms the tube is in the airway and not the esophagus. This immediate confirmation is crucial for patient safety during intubation procedures.
During cardiopulmonary resuscitation (CPR), capnometry helps assess the effectiveness of chest compressions by monitoring EtCO2 levels; higher values often correlate with better blood flow and a greater chance of successful resuscitation. An EtCO2 level less than 10 mmHg after 20 minutes of high-quality chest compressions indicates a poor prognosis. The device can also provide an early indication of the return of spontaneous circulation (ROSC), often showing a sudden increase in EtCO2 before a pulse is detectable. Capnometry is also used to monitor patients under sedation, detect respiratory depression before other signs appear, and guide ventilation settings for critically ill patients.