Durgical
EN
Open the app

Capnography (end-tidal CO2): tube confirmation, the waveform and what a change means

  • Assessment

Capnography shows exhaled CO2 breath by breath: it confirms the tube, warns of disconnection and reflects ventilation and blood flow to the lungs.

Updated

What it is

Capnometry measures the carbon dioxide at the end of each exhaled breath (EtCO2) as a number; capnography adds the waveform, CO2 over time. Mainstream sensors sit in line at the endotracheal tube and are used in intubated patients; sidestream systems draw a gas sample and also work in non-intubated patients through a nasal or oral-nasal cannula. Colorimetric detectors only change colour and give a range, not a value. Capnography confirms tube placement, monitors circuit continuity and the adequacy of ventilation, and during CPR reflects blood flow produced by chest compressions.

Reference values

  • Normal waveform: a square shape with a flat top (plateau).
  • Phase 0: inspiration; the CO2 falls to zero.
  • Phase I: start of expiration, dead-space gas with no CO2.
  • Phase II: a rapid rise as mixed gas is exhaled.
  • Phase III: the plateau of alveolar gas; its end value is the EtCO2.
  • There is no single target: the team compares EtCO2 with the patient's own PaCO2.

What you need

  • Mainstream sensor in line at the endotracheal tube, or a sidestream sampling line.
  • For non-intubated patients, a nasal or oral-nasal sampling cannula.
  • A monitor that shows both the number and the waveform.
  • A colorimetric detector where capnography is not available.

How to measure it (and why)

  1. Choose mainstream for an intubated patient, or sidestream (also by cannula for a non-intubated one), as the protocol says.
  2. Fit a mainstream sensor in line at the tube hub, as the protocol says — it is designed for intubated patients.
  3. After intubation, look for a waveform that persists over about six breaths — a few early breaths can come from the oesophagus.
  4. Check the shape every time: a square wave with a plateau — without a plateau the number is unreliable.
  5. Compare EtCO2 with the PaCO2 on the blood gas and note the gap — EtCO2 underestimates PaCO2 when lung blood flow falls.
  6. Watch the trend with ventilator changes, sedation and transport — rising EtCO2 can mean hypoventilation; falling can mean low output.
  7. If the waveform disappears, check the patient, the tube and the circuit first — disconnection or displacement must be ruled out.
  8. During CPR, use EtCO2 to judge compressions and watch for an abrupt rise — it reflects cardiac output and return of circulation.
  9. Record the value, the waveform and the context with each check — the trend matters more than one number.

How to read it

  • Low EtCO2 with low blood pressure: reduced blood flow to the lungs, as in shock.
  • Low EtCO2 with a wide gap to PaCO2: more dead space, as in pulmonary embolism.
  • A prolonged, sloping expiratory phase: bronchospasm, as in COPD or asthma.
  • No CO2 after intubation: the tube is in the oesophagus until proven otherwise.
  • Colorimetric detectors can give a false positive after bag-mask ventilation or recent carbonated drinks or antacids; the reading is usually reliable after about six breaths. Sodium bicarbonate raises EtCO2 for 5–10 minutes.

Next step: when to call for help

  • No waveform after intubation, or a waveform that disappears: tell the provider or call for help at once.
  • A sudden fall in EtCO2 with low blood pressure: report it now.
  • EtCO2 rising with drowsiness or sedation: report it; the patient may be hypoventilating.
  • A new change in the waveform shape: report it and check the airway.

Test yourself

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

Why is a CO2 waveform watched over several breaths after intubation?

Because a little CO2 can appear for a few breaths even when the tube is in the oesophagus; a waveform that persists confirms the trachea.

In normal lungs, how does EtCO2 compare with PaCO2?

EtCO2 is about 3–5 mmHg lower. The gap widens when lung blood flow falls or dead space increases.

What does an abrupt rise in EtCO2 during CPR suggest?

Return of spontaneous circulation; it is often the first sign.

What is the first thing to do if the capnography waveform suddenly disappears?

Check the patient, the tube and the circuit for disconnection, displacement or obstruction before blaming the monitor.

Add to my cards (4)

Elsewhere in DurgiPedia

Connections

← All of nursing

Educational content for students; it does not replace your institution's protocols, supervised clinical training or a prescriber's order. Dose-calculation examples use made-up numbers: they are educational, not a clinical order.

Sources: Clinical care for severe acute respiratory infection: toolkit, update 2022 (COVID-19 adaptation), tools 4.4 and 9.4 (World Health Organization, 2022); Applications of end-tidal carbon dioxide (ETCO2) monitoring in emergency department; a narrative review (Emergency (Tehran) (open access, PMC), 2018); Use of capnography during resuscitation of patients in intensive care units (SAGE Open Nursing (open access, PMC), 2025).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.