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# Capnography: the waveform and what it tells you

- Concept

A capnograph draws the carbon dioxide of each breath as a wave, showing at once whether a breathing tube is in place and whether breathing and circulation work.

Updated Nov 13, 2026

## Key points

- A sustained capnography waveform over several breaths is the gold standard for confirming tracheal tube placement; no waveform means oesophageal or displaced tube until proved otherwise.
- Normal EtCO2 is about 35 to 45 mmHg, a few mmHg below arterial PaCO2; the gradient widens with dead space (low cardiac output, pulmonary embolism, COPD).
- In CPR, a persistently very low EtCO2 suggests poor compressions or a poor prognosis; a sudden sustained rise suggests return of spontaneous circulation.
- A sloping 'shark-fin' plateau means expiratory obstruction (bronchospasm, COPD); a baseline that does not return to zero means rebreathing.
- Capnography detects apnoea and hypoventilation during sedation earlier than pulse oximetry, especially when supplemental oxygen is given.

On this page

- The basics Key points
- What it is
- Key points
- How it works
- Step by step
- Patterns and likely causes
- In practice
- Next step
- Elsewhere in DurgiPedia

- Study Test yourself

- Connections Connections

## What it is

Capnometry measures the concentration of carbon dioxide in exhaled gas, and capnography displays it continuously as a waveform against time. Most monitors use infrared absorption, either in a sensor placed in the airway (mainstream) or by drawing a small sample of gas to the monitor (sidestream, which also works with nasal cannulas in non-intubated patients). Because carbon dioxide is produced by metabolism, carried by the circulation and eliminated by ventilation, the waveform reflects all three. The end-tidal value (EtCO2) normally lies a few mmHg below the arterial value and between about 35 and 45 mmHg; the gap widens when dead space increases. A sustained waveform is the most reliable confirmation that a tracheal tube is in the trachea, and it is required during anaesthesia and for intubated patients. During cardiopulmonary resuscitation it shows the quality of chest compressions and gives an early sign of return of spontaneous circulation. During sedation it detects apnoea and hypoventilation before pulse oximetry falls. Reading the shape of the waveform and its trend, not only the number, is the key skill.

## Key points

- Phase I: inspiratory baseline, dead-space gas without carbon dioxide (near zero).
- Phase II: rapid expiratory upstroke as dead-space gas mixes with alveolar gas.
- Phase III: alveolar plateau with a slight upslope; its end point is the EtCO2.
- Phase 0 (or IV): inspiratory downstroke as fresh gas enters.
- Mainstream sensors respond faster; sidestream sampling adds a short delay and can be blocked by water or secretions.
- Colourimetric detectors change colour with carbon dioxide but give no waveform and can mislead after carbonated drinks or in cardiac arrest.

## How it works

- EtCO2 depends on carbon dioxide production (metabolism), delivery to the lungs (cardiac output) and elimination (alveolar ventilation).
- With a stable metabolism and ventilation, a fall in EtCO2 reflects falling pulmonary blood flow, which is why it tracks cardiac output in arrest and shock.
- Uneven emptying of obstructed lung units makes the plateau slope upward, producing the shark-fin shape.
- Dead space ventilates without perfusion, so it adds carbon dioxide-free gas and lowers EtCO2 relative to PaCO2.

## Step by step

- Attach capnography before induction or sedation and check that a waveform appears — a monitor set up late misses the critical moment.
- After intubation, confirm a sustained waveform for several breaths — this is the most reliable sign of tracheal placement.
- Read the shape as well as the number — obstruction, rebreathing and asynchrony show in the shape first.
- Compare EtCO2 with an arterial blood gas — the gap reveals dead space and shows how far the number can be trusted.
- Follow the trend over time — gradual changes reveal hypoventilation, rising metabolism or falling cardiac output.
- In CPR, use EtCO2 to guide compression quality and to spot return of circulation — it responds before a pulse is felt.
- If the waveform is lost, assess the patient first, then the tube, circuit and sampling line — the patient may be in danger.

## Patterns and likely causes

| Pattern | Likely causes |
| --- | --- |
| No waveform after intubation | Oesophageal intubation, displaced tube, total obstruction, disconnection, cardiac arrest without CPR |
| Shark-fin plateau | Bronchospasm, COPD, kinked or partly blocked tube |
| Baseline above zero | Rebreathing: exhausted absorber, faulty valve, too little fresh gas |
| Gradual rise | Hypoventilation, fever, malignant hyperthermia, carbon dioxide absorption in laparoscopy |
| Gradual fall | Hyperventilation, hypothermia, falling cardiac output |
| Sudden fall, not to zero | Pulmonary embolism, sudden hypotension, leak, partial disconnection |
| Cleft in the plateau | Patient effort against the ventilator; returning muscle function |

## In practice

- Sudden loss of the waveform: disconnection, extubation, tube obstruction, apnoea or cardiac arrest; check the patient first, then the equipment.
- Gradual rise: hypoventilation, fever, sepsis, malignant hyperthermia, absorption of carbon dioxide during laparoscopy, or bicarbonate given.
- Gradual fall: hyperventilation, hypothermia, falling cardiac output or increasing dead space.
- Sudden fall to a low but non-zero value: massive pulmonary embolism, sudden hypotension or cardiac arrest, a large leak or partial disconnection.
- A notch in the plateau (curare cleft) shows the patient breathing against the ventilator, often as neuromuscular blockade wears off.

## Next step

- If there is no waveform after intubation, assume the tube is in the oesophagus: look again, and if in doubt take it out and ventilate by mask; call for help.
- Report a sudden fall in EtCO2 with hypotension to the senior at once: think of pulmonary embolism, haemorrhage or arrest.
- Check an arterial blood gas whenever ventilation decisions rely on EtCO2, and recheck after major changes.
- Keep capnography on during transport, sedation outside the operating room and for every intubated patient.
- Document EtCO2 and the waveform at key moments: after intubation, during CPR and at handover.

## Test yourself

Think of the answer, then tap the question to see it.

## What is the most reliable way to confirm tracheal tube placement?

A sustained capnography waveform over several breaths; its absence means oesophageal or displaced intubation until proved otherwise.

## What does a shark-fin capnography waveform suggest?

Obstruction to expiration with uneven emptying of lung units, as in bronchospasm, COPD or a partly blocked tube.

## Why does EtCO2 fall in low cardiac output states?

Less blood reaches the lungs to deliver carbon dioxide, and dead space rises, so less carbon dioxide appears in exhaled gas.

## What does a sudden sustained rise in EtCO2 during CPR suggest?

Return of spontaneous circulation, because pulmonary blood flow and carbon dioxide delivery to the lungs increase abruptly.

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## Elsewhere in DurgiPedia

- [CO₂ transport and the Haldane effect](https://durgical.com/en/physiology/co-transport-and-the-haldane-effect/)
- [Dead space and alveolar ventilation](https://durgical.com/en/physiology/dead-space-and-alveolar-ventilation/)
- [Reading an arterial blood gas step by step](https://durgical.com/en/physiology/reading-an-arterial-blood-gas-step-by-step/)

## Connections

## Related 8

- [Standard intraoperative monitoring](https://durgical.com/en/anaesthesia/standard-intraoperative-monitoring/)
- [Tracheal intubation](https://durgical.com/en/anaesthesia/tracheal-intubation/)
- [Advanced life support in hospital](https://durgical.com/en/anaesthesia/advanced-life-support-in-hospital/)
- [Care after cardiac arrest](https://durgical.com/en/anaesthesia/care-after-cardiac-arrest/)
- [Malignant hyperthermia](https://durgical.com/en/anaesthesia/malignant-hyperthermia/)
- [Bronchospasm and laryngospasm](https://durgical.com/en/anaesthesia/bronchospasm-and-laryngospasm/)
- [Ventilator alarms, auto-PEEP and the DOPE approach](https://durgical.com/en/anaesthesia/ventilator-alarms-auto-peep-and-the-dope-approach/)
- [Levels of sedation and procedural sedation safety](https://durgical.com/en/anaesthesia/levels-of-sedation-and-procedural-sedation-safety/)

## Related conditions 4

- [Pulmonary embolism](https://durgical.com/en/conditions/pulmonary-embolism/)
- [Cardiac arrest (VF, pulseless VT, asystole, PEA)](https://durgical.com/en/conditions/cardiac-arrest/)
- [Asthma](https://durgical.com/en/conditions/asthma/)
- [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/)

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Educational content; it does not replace supervised training, your institution's protocols or the anaesthetist's or intensivist's decision. It contains no doses and no ventilator or infusion settings.

Sources: [Capnography](https://www.ncbi.nlm.nih.gov/books/NBK482169/) (StatPearls (NCBI Bookshelf), 2026).

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anaesthesia/capnography-the-waveform-and-what-it-tells-you/