Waveform capnography is a patient monitoring technique that provides a real-time measurement of carbon dioxide (CO2) concentration in a patient’s exhaled breath. This method is widely used in medical settings, from emergency rooms to operating theaters, to assess a person’s ventilatory status. Unlike a simple numerical reading, the capnography waveform provides a continuous, visual graph of CO2 levels throughout the entire breathing cycle. This graphic representation, known as the capnogram, offers immediate and detailed insight into a patient’s breathing, circulation, and metabolism. Visualizing these changes allows healthcare providers to detect respiratory problems sooner than other monitoring tools, such as pulse oximetry.
How Capnography Measures Carbon Dioxide
Capnography relies on the specific properties of carbon dioxide gas. Carbon dioxide molecules absorb infrared (IR) light at a precise wavelength, most commonly around 4.26 micrometers. The capnograph directs an infrared beam through the gas sample and measures how much of that light is absorbed by the CO2 molecules present. A higher concentration of CO2 in the exhaled air results in greater absorption of the IR light, which the device’s photodetector registers as a lower signal.
The monitor then translates this light absorption measurement into a partial pressure of carbon dioxide, expressed in millimeters of mercury (mmHg). This process generates two outputs: capnometry, which is the numerical value of CO2 concentration, and capnography, which is the continuous waveform. Devices use two main approaches for sampling: mainstream and sidestream. Mainstream sensors directly measure the gas flow at the patient’s airway, while sidestream devices continuously draw a small sample of gas to a remote sensor for analysis.
Interpreting the Normal Waveform Cycle
The normal capnogram appears as a distinctive, nearly rectangular waveform, representing a single breath cycle. It is plotted with CO2 concentration on the vertical axis and time on the horizontal axis. This cycle is divided into four distinct phases, each corresponding to a specific physiological event during breathing. Phase I (A-B) is the inspiratory baseline, which is flat and rests at zero CO2, reflecting the air just inhaled.
Phase II (B-C) marks the beginning of exhalation and is characterized by a sharp, rapid upstroke. This rising slope occurs as the CO2-free air from the anatomical dead space—the conducting airways that do not participate in gas exchange—begins to mix with CO2-rich air from the alveoli. The slope rises quickly as the alveolar gas dominates the expired volume.
Phase III (C-D) is the alveolar plateau, where the waveform flattens into a constant pressure. This plateau reflects the exhalation of gas primarily from the alveoli and represents the maximum concentration of CO2. The peak of this plateau, point D, is the End-Tidal CO2 (ETCO2) value, the highest concentration of CO2 at the very end of exhalation.
In healthy individuals, the ETCO2 value normally falls within the range of 35 to 45 mmHg. The final segment, Phase IV (or Phase 0), is the inspiratory downstroke (D-E), where the waveform abruptly drops back to the zero baseline. This rapid descent is caused by the patient inhaling fresh, CO2-free gas, preparing for the next respiratory cycle.
Essential Uses in Patient Monitoring
The waveform provides immediate feedback beyond simple respiratory rate, offering information in various clinical settings. A primary application is the verification of artificial airway placement, such as an endotracheal tube. If the tube is correctly placed in the trachea, a clear waveform appears because the tube samples air containing CO2 from the lungs. If the tube is mistakenly placed in the esophagus, the capnograph displays a flat line, immediately alerting clinicians to the misplacement.
Capnography is also a fundamental tool for monitoring the effectiveness of cardiopulmonary resuscitation (CPR). The ETCO2 value directly reflects pulmonary blood flow and correlates with cardiac output during chest compressions. An ETCO2 reading consistently below 10 mmHg suggests that compressions are not effectively circulating blood, prompting the team to improve quality. A sudden, sustained increase in the ETCO2 value, typically rising into the normal range of 35 to 45 mmHg, is often the earliest sign of the Return of Spontaneous Circulation (ROSC).
The shape of the waveform is also used to monitor a patient’s overall ventilation status. Obstructive lung conditions, such as severe asthma or chronic obstructive pulmonary disease (COPD), produce a characteristic “shark fin” appearance. This sloped, prolonged Phase III occurs because air is released inconsistently from the lungs due to bronchospasm or airway obstruction. Capnography provides an earlier indication of hypoventilation (an increase in CO2) than pulse oximetry, which measures oxygen saturation.