Capnography is a non-invasive monitoring tool that measures the concentration of carbon dioxide (CO2) in a patient’s exhaled breath. It provides continuous, real-time information about a person’s respiratory status. It assesses ventilation and overall respiratory function. Understanding capnography helps healthcare providers evaluate how effectively carbon dioxide is eliminated from the body.
What Capnography Measures
Capnography provides a numerical value and a graphical waveform. The numerical value is known as End-tidal Carbon Dioxide (EtCO2), which represents the maximum concentration of CO2 at the very end of an exhaled breath. A typical EtCO2 value for a healthy individual ranges between 35 and 45 millimeters of mercury (mmHg). Monitoring CO2 levels provides insights into a patient’s ventilation, metabolic activity, and circulatory efficiency. Capnography devices use infrared light, which CO2 molecules absorb at specific wavelengths, allowing measurement of gas concentration.
The Normal Capnography Waveform
A healthy capnography waveform displays a characteristic rectangular shape with rounded corners. This waveform has four phases reflecting carbon dioxide during a breath. Phase I, the inspiratory baseline, represents the beginning of exhalation, where only CO2-free gas from the anatomical dead space is exhaled, resulting in a flat line near zero.
Phase II, known as the expiratory upstroke, shows a rapid increase in CO2 concentration. This occurs as CO2-rich air from the alveoli mixes with dead space gas moving out of the lungs. The sharp rise in this phase indicates the efficient expulsion of CO2 from the airways.
Phase III, the alveolar plateau, is where the CO2 concentration reaches its peak and remains relatively constant. This plateau reflects exhalation of gas predominantly from the alveoli, where gas exchange occurs. The very end of this plateau is the point where the EtCO2 value is measured.
Finally, Phase IV, the inspiratory downstroke (sometimes referred to as Phase 0), marks the rapid decrease in CO2 concentration back to zero. This sharp downward slope signifies the beginning of the next inspiration, as fresh, CO2-free air is drawn into the lungs.
Interpreting Abnormal Waveforms
Deviations from the typical capnography waveform provide visual cues about underlying physiological problems. A prolonged expiratory upstroke and an upward-sloping alveolar plateau, often described as a “shark fin” appearance, can indicate airway obstruction. This pattern is seen in conditions like bronchospasm, asthma, or chronic obstructive pulmonary disease (COPD), where air is trapped in the lungs.
An elevated baseline in Phase I, where the waveform does not return to zero, suggests rebreathing of exhaled carbon dioxide. This can happen if there is insufficient fresh gas flow in a breathing circuit or if the CO2 absorbent material in a ventilation system is exhausted. An absent waveform, appearing as a flat line, indicates no CO2 exhalation. This sign can result from apnea, accidental disconnection of the breathing circuit, esophageal intubation, or cardiac arrest.
A notched waveform in Phase III, sometimes called a “curare cleft,” can be observed when a patient attempts to initiate spontaneous breaths while receiving mechanical ventilation. This notch appears as the effects of muscle relaxants begin to wear off, signifying a return of some spontaneous respiratory effort. A sudden drop in the EtCO2 value to near zero, while the waveform shape remains otherwise normal, points to a sudden loss of pulmonary blood flow or cardiac output. This pattern can occur in massive pulmonary embolism or during cardiac arrest.
Beyond the Waveform: Clinical Context
Both the numerical EtCO2 value and the capnography waveform provide a comprehensive assessment in clinical practice. This combined information offers continuous insights into a patient’s ventilatory status. Capnography is useful for verifying endotracheal tube placement immediately after intubation and continuously monitoring its position.
The tool also monitors patients receiving sedation, helping to detect early respiratory depression. During cardiopulmonary resuscitation (CPR), capnography helps assess the effectiveness of chest compressions by reflecting cardiac output. Capnography’s continuous nature allows healthcare providers to make timely decisions based on dynamic changes in CO2 levels and breathing patterns.