The waveform tells you more than the number. A student watches the EtCO2 value. A therapist reads the shape. This is how you read the shape.
What capnography measures
Capnography is the continuous measurement of exhaled carbon dioxide. End-tidal CO2 (EtCO2) is the peak value at the end of exhalation — the number your monitor displays. Normal EtCO2 runs 35–45 mmHg, which tracks a few mmHg below PaCO2 in a patient with healthy lungs. The waveform is the plot of that CO2 over time. The number is a snapshot. The waveform is the story.
The normal waveform: four phases
A normal capnogram is a rounded rectangle. Learn its parts and every abnormal shape becomes a deviation you can name.
- Phase I — baseline. Start of exhalation, CO2-free gas from anatomical dead space. Sits at zero. If it doesn't sit at zero, suspect rebreathing.
- Phase II — expiratory upstroke. A fast, steep rise as alveolar gas reaches the sensor. Should be sharp and near-vertical.
- Phase III — alveolar plateau. A slight upward slope. The end of this plateau is the end-tidal point — the EtCO2 value.
- The inspiratory downstroke. A sharp fall back to baseline as fresh gas washes in and the next breath begins.
Sharp corners, flat-ish plateau, return to zero. That's a clean capnogram. Commit it to memory so the abnormals jump out.
Shark fin: obstruction
Bronchospasm slurs the waveform. The sharp Phase II–III angle rounds off and the upstroke slopes. The result looks like a shark fin. It signals obstructive airflow — asthma, COPD exacerbation, a kinked or partially obstructed tube. Treat the obstruction and the corner sharpens back up. This is one of the few real-time, objective readouts of bronchodilator response you'll get at the bedside.
The curare cleft: the patient is trying to breathe
A notch dipping down into the Phase III plateau is a curare cleft. In a mechanically ventilated, paralyzed patient, it means the neuromuscular blockade is wearing off and the diaphragm is twitching against the vent. It's a heads-up to reassess sedation and paralysis — not a monitor glitch.
Loss of waveform: an emergency until proven otherwise
A waveform that drops flat to zero is never a nuisance alarm. Work the causes fast: tube dislodged or esophageal, circuit disconnection, complete obstruction, apnea — or no cardiac output. This is why sustained capnography is the standard for confirming endotracheal tube placement: an esophageal tube gives you no persistent waveform. Confirm a good waveform over several breaths after every intubation. A number without a waveform is not confirmation.
Trending: gradual changes carry information too
- Rising EtCO2 — hypoventilation, rising CO2 production (fever, sepsis), or rebreathing. Check the baseline: if Phase I lifts off zero, suspect rebreathing, an exhausted absorber, or a faulty expiratory valve.
- Falling EtCO2 — hyperventilation, dropping cardiac output, hypothermia, hypovolemia, or pulmonary embolism. A sudden drop with stable ventilation points at circulation or a leak, not the lungs.
Capnography in a code
During CPR, EtCO2 reflects the cardiac output your compressions are generating. Good compressions push CO2 to the lungs and the number rises; poor compressions let it fall. A persistently low EtCO2 (under 10 mmHg) after 20 minutes of resuscitation is associated with poor outcomes and is one data point in the decision to stop. An abrupt jump in EtCO2 during a code — from single digits up toward normal — often signals return of spontaneous circulation before you feel a pulse. Watch the number; it will tell you the compressions are working before anything else does.
How to drill this
Pull up waveforms and name the phase before you read the value. Then name the abnormal: shark fin, cleft, flatline, rising baseline. When you can label the shape in under a second, the number becomes a confirmation instead of a surprise. That's the difference between watching a monitor and reading it.
More reference sheets like this in Field Notes. If you want the mindset on your chest, the O2 BOOST tee and the rest of the RESPTAC shirt line are built for people who read the waveform, not just the numbers.
Study reference, not medical advice. Follow your institution's protocols and your medical director's orders. Specific thresholds and code protocols vary by facility and guideline edition — confirm locally.