Capnography is a medical monitoring technique that provides real-time insights into a person’s breathing and respiratory health. It continuously measures the concentration of carbon dioxide (CO2) in exhaled air. This allows healthcare providers to quickly assess how effectively an individual is ventilating and exchanging gases within their lungs. By tracking CO2 levels, capnography offers immediate, objective information about a patient’s respiratory status, making it a widely used tool in diverse medical situations.
Understanding Quantitative Waveform Capnography
Quantitative waveform capnography measures carbon dioxide (CO2) in a patient’s exhaled breath. The “quantitative” aspect means the system provides a precise numerical value for CO2 concentration, most often as end-tidal CO2 (EtCO2). EtCO2 reflects the CO2 level at the conclusion of an exhaled breath, typically expressed in millimeters of mercury (mmHg) or kilopascals (kPa), with a normal range between 33 mmHg and 43 mmHg.
The “waveform” component means CO2 concentration is visually presented as a continuous graph over time, called a capnogram. This dynamic display plots CO2 levels throughout the entire respiratory cycle. Unlike a single numerical reading, the waveform provides a detailed, breath-by-breath picture of CO2 elimination. This allows healthcare providers to observe changes in CO2 concentration as air moves in and out of the lungs, offering a more complete understanding of a patient’s ventilatory performance.
How Capnography Works
Capnography operates on the principle that carbon dioxide molecules absorb infrared light. A capnograph directs infrared light through exhaled breath, and a sensor detects the amount of light that passes through. The presence of CO2 reduces the light reaching the sensor, directly proportional to CO2 concentration. This measured light intensity is converted into an electrical signal, which the device processes to display the numerical EtCO2 value and the capnogram waveform.
There are two primary methods for measuring CO2 in capnography: mainstream and sidestream. In mainstream capnography, the sensor is placed directly in the patient’s airway, typically between the breathing tube and the ventilator circuit. This method provides immediate, real-time measurements because the CO2 is analyzed as it exits the airway. Mainstream sensors are heated to prevent condensation, which can interfere with readings.
Sidestream capnography draws a small continuous sample of gas from the patient’s airway through a thin sampling line to a sensor located within the main monitor unit. While there might be a slight delay in readings due to the gas traveling through the tubing, sidestream systems are versatile and can be used with non-intubated patients, such as those breathing through a nasal cannula. Both methods provide accurate CO2 measurements, with the choice depending on the clinical situation and patient needs.
Decoding the Capnogram Waveform
The capnogram waveform visually represents CO2 changes during each breath, offering crucial diagnostic information. A typical normal capnogram has four distinct phases, resembling a rectangle with rounded corners.
Phase I, the inspiratory baseline, is a flat line near zero CO2. This phase represents the inhalation of fresh, CO2-free air and the exhalation of anatomical dead space, which is the air in the airways that does not participate in gas exchange.
As exhalation begins, Phase II, the expiratory upstroke, shows a rapid increase in CO2 concentration. This rise reflects the transition from dead space air to a mixture of dead space and CO2-rich alveolar gas from the lungs.
Following this sharp increase, Phase III, the alveolar plateau, appears as a relatively flat segment. During this phase, CO2-rich gas from the alveoli, where gas exchange occurs, is being exhaled. The end of this plateau marks the end-tidal CO2 (EtCO2) value, which is the maximum CO2 concentration at the end of exhalation.
Finally, Phase IV, the inspiratory downstroke, is a sharp drop back to the baseline as the patient begins to inhale fresh air, rapidly clearing CO2 from the sensor.
Deviations from this normal waveform can indicate various physiological conditions or issues. For instance, in conditions like asthma or chronic obstructive pulmonary disease (COPD), the waveform might display a “shark fin” appearance, where Phase II and Phase III show a prolonged, upward-sloping curve instead of a sharp upstroke and flat plateau. This shape suggests airway obstruction, as the exhalation of CO2 is slowed due to narrowed airways.
Changes in a patient’s breathing rate also alter the capnogram. Hyperventilation, or breathing too rapidly, results in a lower EtCO2 value because CO2 is exhaled more quickly than the body produces it. The waveform itself may appear normal in shape but with smaller “rectangles” due to the shorter duration of each breath. Conversely, hypoventilation, or slow and shallow breathing, leads to a buildup of CO2 in the body, resulting in a higher EtCO2. In this case, the waveform “rectangles” appear stretched horizontally due to longer breath times. The absence of a waveform can indicate serious issues like esophageal intubation or cardiac arrest.
Key Applications in Patient Care
Quantitative waveform capnography serves several functions in patient care, aiding healthcare providers in making timely decisions.
One primary application is verifying and continuously monitoring the correct placement of an endotracheal tube (ETT). After insertion, CO2 in exhaled breath confirms the tube is in the trachea (windpipe) rather than the esophagus, which would show little to no CO2. This immediate feedback enhances patient safety.
Capnography is also used during cardiopulmonary resuscitation (CPR). The EtCO2 value estimates cardiac output and pulmonary blood flow during chest compressions. An EtCO2 consistently below 10 mmHg during CPR may suggest ineffective compressions, while a sudden increase can signal the return of spontaneous circulation (ROSC). This feedback allows rescuers to adjust technique and assess resuscitation effectiveness.
When patients receive sedation, capnography helps monitor respiratory status. Sedative medications can depress breathing, and capnography can detect hypoventilation earlier than pulse oximetry, often by 30 to 60 seconds. This early warning allows for interventions before oxygen levels drop, enhancing patient safety during procedures. Capnography also assists in managing patients with respiratory distress, as waveform changes can quickly identify conditions like airway obstruction or apnea. It guides treatment and assesses a patient’s response to therapies.