Capnography is a medical monitoring technique used to assess a patient’s breathing and respiratory status. It offers continuous, real-time insights into how effectively air moves in and out of the lungs. This non-invasive method provides immediate feedback on respiratory function, helping healthcare providers make informed decisions about patient care.
What Capnography Measures
Capnography measures the concentration or partial pressure of carbon dioxide (CO2) in a patient’s exhaled breath. CO2 is a byproduct of metabolism, produced by the body’s cells and transported by the blood to the lungs for exhalation. Monitoring CO2 levels provides information about a patient’s metabolic activity, blood circulation, and breathing effectiveness.
The most common measurement is end-tidal carbon dioxide (ETCO2). ETCO2 represents the peak CO2 concentration present at the very end of a full exhalation. This value reflects the amount of CO2 eliminated by the lungs and indirectly indicates arterial blood CO2 levels. Normal ETCO2 values typically range between 35 and 45 millimeters of mercury (mmHg).
How Capnography Works
Capnography operates on the principle of infrared absorption spectroscopy. Carbon dioxide molecules absorb infrared light at specific wavelengths. A capnograph directs an infrared light beam through a sample of exhaled gas. As CO2 molecules absorb some of this light, the remaining intensity is detected by a sensor.
The device converts these changes into electrical signals. These signals are processed and displayed on a monitor as a numerical ETCO2 value and a graphical waveform called a capnogram. Two methods sample exhaled gas: mainstream, where the sensor is placed directly in the airway, and sidestream, where gas is continuously drawn from the airway to a sensor within the monitor. Both provide continuous breath-by-breath CO2 measurements throughout the respiratory cycle.
Where Capnography is Used
Capnography is used across various medical settings for its immediate, objective ventilation data. In anesthesia, it confirms correct breathing tube placement and continuously monitors respiratory status during surgery. It ensures adequate ventilation and detects issues like tube dislodgement or changes in breathing patterns.
Emergency medical services (EMS) and critical care units rely on capnography to assess patients with respiratory distress or altered mental status. It confirms effective chest compressions during cardiopulmonary resuscitation (CPR) and provides an early warning system for changes in a patient’s condition. During procedural sedation, capnography offers more comprehensive information on ventilation frequency and regularity than pulse oximetry alone. Paramedics use it in pre-hospital care to monitor both intubated and non-intubated patients, providing real-time insights into their ventilatory status.
Understanding Capnography Readings
Understanding capnography involves interpreting both the numerical ETCO2 value and the waveform, known as the capnogram. The capnogram is a graphical representation of CO2 concentration over time during a breath cycle. A typical capnogram waveform has four distinct phases.
Phase I, the inspiratory baseline, shows minimal CO2 as fresh air is inhaled. Phase II, the expiratory upstroke, indicates the rapid rise in CO2 as dead space air mixes with CO2-rich alveolar air during exhalation. Phase III, the alveolar plateau, represents the exhalation of CO2-rich gas from the alveoli, forming a relatively flat top. Phase IV, the inspiratory downstroke, shows a sharp drop in CO2 as inhalation begins again.
Deviations from this normal waveform or ETCO2 value can indicate various physiological changes. For instance, a sudden loss of the waveform or a drop in ETCO2 might suggest ventilator disconnection or breathing tube displacement. A prolonged or elevated alveolar plateau could indicate airway obstruction, such as bronchospasm.
Elevated ETCO2 values typically signal hypoventilation, meaning slow or shallow breathing, which leads to CO2 retention. Conversely, low ETCO2 values often suggest hyperventilation, meaning excessive breathing, or reduced blood flow to the lungs. Monitoring these changes allows healthcare providers to identify and address respiratory issues.