Updated July 2026 · 9 min read
Part of the ICU Devices Hub — browse every related guide in one place.
Pulse oximetry tells you about oxygenation, and it lags — a saturation doesn't fall until seconds to minutes after a problem starts. Capnography tells you about ventilation, breath by breath, in real time. The end-tidal CO2 number matters, but the waveform is where the diagnosis lives. Learning to read its shape turns a squiggle on the monitor into an early-warning system for airway loss, bronchospasm, falling cardiac output, and a displaced tube.
Oxygenation (what the SpO2 reflects) and ventilation (moving air in and out to clear CO2) are different problems. A patient on high-flow oxygen can stop breathing effectively and hold a normal saturation for a while because their lungs are full of oxygen — the SpO2 buys you a false sense of security. EtCO2 falls the instant ventilation drops, so it catches apnea, sedation-induced hypoventilation, and airway obstruction well before the oximeter does. That is why capnography is standard for intubated patients, procedural sedation, and increasingly for any patient on a PCA opioid or heavy sedation.
A normal capnogram is a repeating box. Reading its four phases lets you name what is happening:
| Phase | What it is | Normal look |
|---|---|---|
| I — baseline | Start of exhalation, dead-space gas (no CO2) | Flat at zero |
| II — upstroke | Alveolar gas begins mixing out | Steep, near-vertical rise |
| III — plateau | Alveolar gas, CO2-rich | Flat or gently rising to the peak |
| 0 — inspiration | Fresh gas in, CO2 drops | Sharp fall back to zero |
The peak at the end of the plateau is the end-tidal CO2 (EtCO2), normally 35–45 mmHg and usually 2–5 mmHg below the arterial PaCO2. A crisp box with a flat baseline and a clean plateau is a healthy ventilation picture. When the box distorts, the shape names the problem.
Three abnormal patterns cover most of what you will see:
| Waveform change | What it usually means | Nursing action |
|---|---|---|
| Sloped, "shark-fin" upstroke (loss of the sharp phase II/III angle) | Airway obstruction — bronchospasm (asthma/COPD), kinked or partially blocked ETT | Assess breath sounds, suction, bronchodilators, check the tube |
| Sudden drop to zero | Complete loss of ventilation — extubation/tube out of trachea, circuit disconnect, apnea, or cardiac arrest | Look, listen, and confirm the tube; call for help; start resuscitation if no pulse |
| Gradual fall in EtCO2 over minutes | Falling cardiac output/perfusion (less CO2 delivered to lungs), hyperventilation, or hypothermia | Check hemodynamics and vent settings; think about shock or over-ventilation |
| Rising baseline (phase I no longer at zero) | Rebreathing — exhausted CO2 absorber, faulty valve, inadequate expiratory time | Check the circuit/absorber and ventilator settings |
| "Curare cleft" notch in the plateau | Spontaneous breath breaking through — neuromuscular blockade wearing off | Reassess paralytic dosing and sedation |
During CPR, EtCO2 is one of the most useful numbers on the monitor because CO2 only reaches the lungs if blood is circulating. That makes it a live quality gauge:
A persistently low EtCO2 (under ~10 mmHg) during CPR signals inadequate compressions — push harder, push faster, rotate the compressor. A sudden jump in EtCO2 (for example from 12 to 35+) is often the earliest sign of return of spontaneous circulation (ROSC), frequently before you feel a pulse. And after prolonged arrest, a stubbornly low EtCO2 despite good compressions is a poor-prognosis marker teams weigh in resuscitation decisions. This is why capnography belongs on every intubated arrest.
EtCO2 usually runs a few mmHg below arterial PaCO2, but that gradient widens when dead space increases — pulmonary embolism, low cardiac output, severe lung disease. So a normal EtCO2 does not guarantee a normal arterial CO2 in a sick patient; correlate with the ABG when it matters. A sudden fall in EtCO2 with stable ventilation is a classic clue to a pulmonary embolism (blood suddenly not reaching ventilated lung). And remember the number depends on both metabolism and perfusion: fever and sepsis raise CO2 production, while shock lowers delivery.
Read the shape first. A clean box with a plateau of 35–45 is normal ventilation; a shark fin is obstruction; a flatline is a lost airway or lost circulation until you prove otherwise; a slow drift down is falling perfusion or over-ventilation. Use capnography to confirm the tube and then to watch it continuously, and in a code let the EtCO2 coach your compressions and announce ROSC. The waveform gives you minutes of warning that the oximeter cannot — learning to read it is one of the highest-yield monitoring skills at the ICU bedside.
Related: ABG interpretation in 5 steps · Ventilator weaning & the SBT · Train-of-four monitoring
Educational content for licensed clinicians. Always follow your facility's protocol and provider orders. Not medical advice.
Get The ICU Notebook Newsletter
Clinical tools and career insights for ICU nurses. One email per week, no fluff.
Yes, send it freeNo spam. Unsubscribe any time.