[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Nursing](https://durgical.com/en/nursing/) › [Critical care](https://durgical.com/en/nursing/areas/critical-care/) › Capnography (end-tidal CO2): tube confirmation, the waveform and what a change means # 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 Nov 13, 2026 ## Key points - A persistent CO2 waveform after intubation means the tube is in the trachea; CO2 can appear for a few breaths when the tube is in the oesophagus. - In normal lungs EtCO2 is about 3–5 mmHg lower than PaCO2; the gap widens with low blood pressure, pulmonary embolism, high PEEP, severe ARDS or emphysema. - If the waveform suddenly disappears, check the patient, tube and circuit before the monitor. - During CPR, an abrupt rise in EtCO2 is the first sign of return of spontaneous circulation. - A long, sloping rise in the expiratory part of the curve suggests bronchospasm or airway obstruction. ## 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) - Choose mainstream for an intubated patient, or sidestream (also by cannula for a non-intubated one), as the protocol says. - Fit a mainstream sensor in line at the tube hub, as the protocol says — it is designed for intubated patients. - After intubation, look for a waveform that persists over about six breaths — a few early breaths can come from the oesophagus. - Check the shape every time: a square wave with a plateau — without a plateau the number is unreliable. - Compare EtCO2 with the PaCO2 on the blood gas and note the gap — EtCO2 underestimates PaCO2 when lung blood flow falls. - Watch the trend with ventilator changes, sedation and transport — rising EtCO2 can mean hypoventilation; falling can mean low output. - If the waveform disappears, check the patient, the tube and the circuit first — disconnection or displacement must be ruled out. - During CPR, use EtCO2 to judge compressions and watch for an abrupt rise — it reflects cardiac output and return of circulation. - 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)](https://durgical.app/estudiar/?add=/en/nursing/capnography-tube-confirmation-the-waveform-and-what-a-change-means/) ## Elsewhere in DurgiPedia - [Dead space and alveolar ventilation](https://durgical.com/en/physiology/dead-space-and-alveolar-ventilation/) ## Connections ## Related 3 - [Assisting endotracheal intubation: preparing, rapid sequence from the nursing side and confirming tube position](https://durgical.com/en/nursing/assisting-endotracheal-intubation-preparing-rapid-sequence-from-the-nursing-side/) - [Pulse oximetry at the bedside: placement and pitfalls](https://durgical.com/en/nursing/pulse-oximetry-at-the-bedside-placement-and-pitfalls/) - [Basic life support from the nursing side](https://durgical.com/en/nursing/basic-life-support-from-the-nursing-side/) [← All of nursing](https://durgical.com/en/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](https://www.who.int/publications/i/item/clinical-care-of-severe-acute-respiratory-infections-tool-kit) (World Health Organization, 2022); [Applications of end-tidal carbon dioxide (ETCO2) monitoring in emergency department; a narrative review](https://pmc.ncbi.nlm.nih.gov/articles/PMC5827051/) (Emergency (Tehran) (open access, PMC), 2018); [Use of capnography during resuscitation of patients in intensive care units](https://pmc.ncbi.nlm.nih.gov/articles/PMC12126665/) (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. --- Source: https://durgical.com/en/nursing/capnography-tube-confirmation-the-waveform-and-what-a-change-means/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/nursing/capnography-tube-confirmation-the-waveform-and-what-a-change-means/