Capnography is a medical monitoring tool that measures carbon dioxide (CO2) in exhaled breath. It provides real-time data, displayed as a numerical value and a graphical waveform, to assess ventilatory status. This non-invasive method helps healthcare providers understand how effectively a patient is breathing and exchanging gases. It offers a continuous overview of respiratory function in various medical settings.
The Importance of Carbon Dioxide Measurement
Carbon dioxide is a waste product from the body’s metabolic processes, exhaled through the lungs. Measuring exhaled CO2 offers insights into physiological status, reflecting CO2 production, blood circulation (perfusion), and ventilation.
The amount of CO2 at the end of an exhaled breath, known as end-tidal CO2 (ETCO2), indicates the CO2 level in the alveoli, where gas exchange occurs. A normal ETCO2 range is typically 35 to 45 mmHg. Deviations from this range can signal issues with ventilation, such as hypoventilation or hyperventilation, and can also reflect changes in metabolism or circulation. Monitoring these levels helps healthcare providers identify potential breathing complications and respond with appropriate interventions.
Primary Measurement Techniques
Capnography works by utilizing infrared (IR) spectroscopy, as CO2 molecules absorb specific wavelengths of infrared light. A light source emits IR radiation through the exhaled gas sample, and a detector measures the absorbed light. The more CO2 present, the more IR light is absorbed, which is then converted into a CO2 concentration.
There are two primary techniques for measuring CO2: mainstream and sidestream. Mainstream capnography places the CO2 sensor directly in the patient’s airway, providing immediate, real-time readings. Mainstream devices are often used with intubated patients. Sidestream capnography draws a gas sample from the patient’s airway to a sensor located some distance away. While this introduces a slight delay, sidestream capnography is usable with both intubated and non-intubated patients, often via nasal cannulas.
Understanding the Capnogram
The output of a capnography measurement is a capnogram, which is a graphical waveform displaying CO2 concentration over time during a breathing cycle. A normal capnogram typically has a rectangular-like appearance with four distinct phases.
Phase I, the inspiratory baseline, represents the beginning of exhalation, where CO2-free air from the conducting airways (anatomical dead space) is exhaled. During this phase, the CO2 level is near zero. Phase II, the expiratory upstroke, shows a rapid increase in CO2 concentration as CO2-rich air from the alveoli begins to mix with the dead space air. This phase indicates the transition from dead space gas to alveolar gas.
Phase III, known as the alveolar plateau, reflects the continuous exhalation of alveolar gas, which is rich in CO2. The highest point at the end of this plateau is the End-Tidal CO2 (ETCO2) value, representing the maximum CO2 concentration at the end of expiration. Phase IV, or the inspiratory downstroke, occurs as inhalation begins, causing CO2 levels to drop sharply back to the baseline. This rapid decrease signifies the fresh, CO2-free air entering the lungs. The overall shape, height, and frequency of the capnogram provide visual information about the patient’s ventilation and respiratory patterns.
Common Clinical Uses
Capnography is widely used across various medical environments to monitor a patient’s respiratory status. In operating rooms, it is a standard tool for patients under general anesthesia, helping anesthesiologists monitor ventilation effectiveness and detect complications like respiratory depression or issues with the breathing circuit. It also helps confirm the correct placement of an endotracheal tube.
In emergency medical services (EMS) and intensive care units (ICUs), capnography provides real-time information for patients with breathing difficulties. It is used during cardiopulmonary resuscitation (CPR) to assess the quality of chest compressions and to identify the return of spontaneous circulation. Additionally, it helps monitor patients undergoing procedural sedation to detect hypoventilation or airway obstruction. The continuous display of CO2 levels helps healthcare providers make timely decisions to optimize patient care.