A capnography waveform is a graphical representation of the partial pressure of carbon dioxide (CO2) in exhaled and inhaled air, plotted over time. The y-axis shows CO2 concentration (measured in mmHg), and the x-axis shows time across each breathing cycle. In a healthy adult, the peak value at the end of each exhaled breath, called end-tidal CO2, falls between 35 and 45 mmHg.
What Each Phase of the Waveform Shows
A single normal capnography waveform has four distinct phases, each corresponding to a specific moment in the breathing cycle. Together they create a roughly rectangular shape that repeats with every breath.
Phase I (inspiratory baseline): The waveform starts flat, near zero. This represents the very beginning of exhalation, when air is flowing out of the large conducting airways (the trachea and bronchi). This air never reached the gas-exchanging parts of the lungs, so it contains little to no CO2. Clinically, this volume is called anatomical dead space, and it accounts for roughly one-third of a normal breath in adults.
Phase II (expiratory upstroke): The line rises sharply as CO2-rich air from deeper in the lungs begins mixing with the dead-space gas. This transition happens quickly in healthy lungs, producing a steep, nearly vertical upstroke.
Phase III (alveolar plateau): The waveform levels off into a gently rising plateau. At this point, the exhaled air is almost entirely coming from the alveoli, the tiny air sacs where oxygen and CO2 are exchanged with the bloodstream. The highest point at the very end of this plateau is the end-tidal CO2 value, representing the peak CO2 concentration of that breath.
Phase IV, or Phase 0 (inspiratory downstroke): The line drops sharply back toward zero as the next inhalation begins and fresh air (which contains virtually no CO2) washes through the airway. In a properly functioning breathing circuit, the baseline returns all the way to zero. If it doesn’t, that signals the person is rebreathing some of their own exhaled gas, often because of equipment issues or very small breath volumes.
More Than Just Breathing
While the waveform directly measures CO2 leaving the lungs, the CO2 itself reflects a chain of three physiological processes: metabolism, circulation, and ventilation. Cells throughout the body produce CO2 as a byproduct of energy use. The bloodstream carries that CO2 to the lungs. The lungs then exhale it. A change at any point in this chain alters the waveform.
A sudden drop in end-tidal CO2 during cardiac arrest, for example, reflects the loss of blood flow carrying CO2 to the lungs. Conversely, during CPR, a sustained rise in end-tidal CO2 can signal that the heart has resumed pumping on its own. Research has shown that when CO2 stays steady or climbs during a pause in chest compressions, it reliably indicates a return of spontaneous circulation, while a drop of around 11% during a pause suggests the heart has not restarted.
This three-link chain (metabolism, perfusion, ventilation) is why capnography provides so much clinical information from a single, noninvasive measurement.
What Abnormal Shapes Mean
Because the waveform’s shape depends on how evenly air empties from the lungs, changes in that shape reveal specific problems. The most recognizable abnormal pattern is the “shark fin” waveform, where the normally steep Phase II upstroke becomes slanted and the Phase III plateau tilts upward rather than staying flat. This pattern appears in conditions like asthma and COPD, where narrowed airways cause different regions of the lung to empty at different rates. The result is an uneven, drawn-out release of CO2 rather than the quick, uniform exhalation seen in healthy lungs.
Research in mechanically ventilated patients with acute bronchospasm has shown that the steepness of the Phase III slope tracks closely with the degree of airway narrowing. As treatment takes effect, the slope flattens before end-tidal CO2 values themselves change, making the waveform shape an early indicator of improvement. The angle between Phase II and Phase III (sometimes called the alpha angle) also widens with obstruction and narrows as airways open up.
The relationship between waveform shape and lung function goes deeper than airway narrowing. Studies in acute lung injury models found a strong correlation (0.85 on a 0-to-1 scale) between the Phase III slope and the degree of mismatch between ventilation and blood flow within the lungs. In practical terms, a steeper slope means some parts of the lung are getting air but not enough blood, or vice versa.
How the Sensor Captures CO2
Capnography devices measure CO2 using infrared light. CO2 molecules absorb infrared radiation at a specific wavelength, so the sensor shines infrared light through a sample of airway gas and measures how much is absorbed. The two main designs differ in where this measurement happens.
Mainstream sensors sit directly in the breathing circuit, close to the patient’s airway. They analyze gas in real time with no transport delay, producing a highly accurate waveform. The tradeoff is that the sensor adds a small amount of weight and dead space to the airway tubing.
Sidestream sensors pull a small, continuous sample of gas through thin tubing to a sensor located in the monitor. This keeps the airway lighter and allows capnography in patients who aren’t on a ventilator (using nasal cannulas, for instance). The tradeoff is a slight delay in readings as gas travels through the tubing, and somewhat more variability in measurements due to mixing of sampled gas with fresh airflow.
Why It Matters in Practice
The most universal application of capnography waveform reading is confirming that a breathing tube is correctly placed in the trachea rather than the esophagus. A normal, repeating waveform with CO2 in the expected range confirms the tube is in the airway. The 2025 American Heart Association guidelines recommend waveform capnography or capnometry for confirming correct airway placement during resuscitation.
Beyond tube placement, the waveform provides a continuous, breath-by-breath window into how well a patient is ventilating. A rising end-tidal CO2 trend suggests the patient isn’t breathing off enough CO2, while a falling trend may indicate hyperventilation or declining cardiac output. Because the waveform reflects metabolism, circulation, and ventilation simultaneously, a single glance can prompt clinicians to investigate the right system rather than guessing.