EtCO2 Normal Range and Capnography Waveforms: A Field Guide
What end-tidal CO2 measures, the normal range of 35 to 45 mmHg, how to read the capnography waveform, and what the common patterns mean: bronchospasm, a lost tube, CPR quality, ROSC, hypoventilation and poor perfusion.
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End-tidal CO2 (EtCO2) is the concentration of carbon dioxide at the end of each exhaled breath. Capnography shows it as a number and a waveform, breath by breath. Because CO2 is made by the body's metabolism, carried to the lungs by the circulation and removed by breathing, one reading tells you something about all three: metabolism, perfusion and ventilation.
The normal range
A normal EtCO2 is 35 to 45 mmHg. In a patient with healthy lungs it runs a few mmHg below the arterial CO2. The gap widens when the lungs or the circulation are not working well, so a normal EtCO2 in a sick patient is reassuring but not proof that the arterial CO2 is normal.
| EtCO2 | What it usually means | Common causes |
|---|---|---|
| Below 35 mmHg | Low CO2 in the exhaled breath | Hyperventilation, or low delivery of CO2 to the lungs: shock, pulmonary embolism, cardiac arrest, poor CPR. Also very small breaths: a low number with a small waveform and slow or shallow breathing is hypoventilation (for example an opioid overdose). Look at the patient and the waveform, not only the number. |
| 35 to 45 mmHg | Normal | |
| Above 45 mmHg | High CO2 in the exhaled breath | Hypoventilation: opioid or sedative effect, fatigue, COPD with CO2 retention, inadequate bagging |
Reading the waveform
A normal capnogram is a series of rectangles with slightly rounded corners. Texts number the phases differently; the shape is what matters.
- Baseline: the flat line at zero during inspiration and the start of exhalation, when dead-space air with no CO2 leaves the airway. It should return to zero.
- Expiratory upstroke: a steep rise as air from the alveoli reaches the sensor.
- Alveolar plateau: a nearly flat top as alveolar air is exhaled. The EtCO2 is the value at the end of the plateau.
- Inspiratory downstroke: a steep fall back to baseline as the next breath brings in fresh air.
Look at the number, the shape and the trend. A single reading means less than the direction it is moving.
The patterns worth knowing
Shark fin: obstruction to exhalation
When the airways are narrowed, air leaves the lungs slowly and unevenly, so the upstroke slopes instead of rising sharply and the plateau slants upward. The result looks like a shark's fin. It is the pattern of bronchospasm in asthma and COPD. As treatment works, the waveform should square back up, which makes capnography a way to watch a bronchodilator take effect.
Sudden loss of the waveform in an intubated patient
A waveform that disappears or falls to near zero in an intubated patient means the tube is out, in the esophagus, disconnected or blocked until you prove otherwise. Check the patient and the circuit immediately. The AHA recommends continuous waveform capnography to confirm and monitor endotracheal tube placement, because it shows a displaced tube the moment it happens rather than when the patient desaturates.
Run the airway checklist, which includes how you will confirm and monitor the tube
A gradual rise: hypoventilation
A rising EtCO2 with a slowing respiratory rate is the pattern of hypoventilation, for example from opioids or sedatives or from a patient tiring out. Capnography often shows it before the oxygen saturation falls, particularly in a patient on supplemental oxygen.
A gradual fall: hyperventilation or falling perfusion
A falling EtCO2 can mean the patient is breathing faster or deeper, including a crew bagging too fast, or that each breath has become so small it barely reaches the alveoli. It can also mean less CO2 is reaching the lungs because cardiac output is falling. If the breathing has not changed, think about perfusion.
Capnography during cardiac arrest
During CPR, the EtCO2 is driven mainly by how much blood the compressions are moving through the lungs, so it is a real-time measure of compression quality. The AHA adult cardiac arrest algorithm says to reassess CPR quality if the EtCO2 is low or falling: check depth, rate, recoil and interruptions, and change compressors.
An abrupt, sustained rise in EtCO2, typically to 40 mmHg or more, is one of the signs of return of spontaneous circulation (ROSC). It often appears before you can feel a pulse. Check for one at the next rhythm check rather than stopping compressions the moment the number moves.
Non-intubated patients
Nasal cannula capnography works on a patient who is breathing on their own. It is useful for watching a patient after an opioid reversal or sedation, for following a patient in respiratory distress, and for tracking a patient whose level of consciousness is falling. The number may read lower than in an intubated patient if the patient is mouth breathing or the cannula is poorly placed, so watch the trend and the shape as well as the value.
Common mistakes
- Reading the number and ignoring the waveform. A number without a good waveform may not be a real reading.
- Assuming a tube is fine because it was fine on placement.
- Bagging too fast during CPR or after ROSC, which drives the EtCO2 down and reduces blood return to the heart.
- Treating a normal EtCO2 in a sick patient as proof the arterial CO2 is normal.
- Stopping compressions to check for a pulse the moment the EtCO2 rises.
The short version
- Normal EtCO2 is 35 to 45 mmHg.
- Low: hyperventilation, poor perfusion, or very shallow breathing. High: hypoventilation.
- Shark fin: bronchospasm. It should square up as treatment works.
- A lost waveform in an intubated patient is a lost tube until proven otherwise.
- During CPR, a low or falling EtCO2 means improve compressions; an abrupt sustained rise suggests ROSC.
- Watch the trend and the shape, not one number.