Reading capnography means understanding two things: the shape of the waveform on the monitor and the number it produces, called end-tidal CO2 (EtCO2). A normal EtCO2 in adults falls between 35 and 45 mmHg, and the waveform should look like a clean, roughly rectangular wave that rises, plateaus, and drops back to zero with each breath. Deviations from that shape or that number range tell you something specific about what’s happening in the lungs, airway, or circulation.
The Four Phases of a Normal Waveform
Every capnography waveform is built from the same repeating cycle, broken into four phases. Understanding these phases is the foundation for spotting abnormalities.
Phase I (baseline): The flat line at the very bottom of the waveform. This represents the beginning of exhalation, when the air leaving the mouth is still “dead space” gas from the upper airway, which contains almost no CO2. The baseline should sit at zero.
Phase II (rapid rise): A sharp upstroke as CO2-rich air from the lungs begins mixing with dead space gas. In a healthy person, this rise is steep and smooth.
Phase III (alveolar plateau): The relatively flat portion at the top of the waveform. This represents gas coming almost entirely from the alveoli, the tiny air sacs deep in the lungs. The highest point at the end of this plateau is your EtCO2 value.
Phase 0 (inspiratory downstroke): A sharp drop back to zero as the patient inhales fresh air with no CO2. This descent should be quick and return cleanly to the baseline. If it doesn’t reach zero, something is wrong.
The overall shape resembles a rounded rectangle or a box with slightly softened corners. Once you’ve seen a normal waveform enough times, abnormal ones jump out immediately.
What EtCO2 Numbers Tell You
The number matters as much as the shape. EtCO2 reflects how much carbon dioxide the body is producing, how well the lungs are clearing it, and how effectively blood is circulating to deliver CO2 to the lungs in the first place.
A reading above 45 mmHg suggests hypoventilation, meaning the patient isn’t breathing fast or deeply enough to clear CO2. The waveform will look taller than usual. Common causes include sedation, opioid use, and respiratory fatigue. A reading below 35 mmHg suggests hyperventilation, where the patient is blowing off CO2 faster than the body produces it. The waveforms become shorter and more frequent. Anxiety, pain, and compensation for metabolic acidosis can all drive this pattern.
In children, normal EtCO2 runs slightly lower, averaging around 34 mmHg in studies of healthy kids, though the same general range of 35 to 45 is used clinically. The interpretation principles are identical to adults.
The Shark Fin: Obstructive Lung Disease
One of the most recognizable abnormal waveforms looks like a shark fin. Instead of a steep Phase II rising into a flat Phase III plateau, the entire waveform becomes a long, sloping triangle. This pattern appears in asthma exacerbations and COPD flare-ups.
The reason is uneven lung emptying. During bronchoconstriction, some areas of the lung are more obstructed than others. The less obstructed regions empty first, releasing lower concentrations of CO2. The more obstructed regions hold onto their air longer, eventually releasing higher concentrations of CO2 late in the breath. This staggered emptying, sometimes called desynchronization, eliminates the flat plateau and creates that characteristic upward slope. The angle between Phase II and Phase III widens noticeably.
Tracking the shark fin over time is clinically useful. As bronchodilator treatment takes effect, the waveform gradually returns toward a normal rectangular shape. If the shark fin is getting worse, the obstruction is worsening.
A Baseline That Won’t Return to Zero
If the waveform doesn’t drop all the way back down to zero between breaths, the patient is rebreathing CO2. In other words, they’re inhaling some of the carbon dioxide they just exhaled.
The most common mechanical cause is exhausted soda lime in a rebreathing anesthesia circuit. As the absorbent loses its ability to scrub CO2, the baseline gradually creeps upward and the overall height of each waveform increases. A faulty expiratory valve can produce a similar picture, sometimes with a distinctive slant on the downstroke. In ventilated patients, inadequate expiratory time (not enough time between breaths for all the gas to leave) can also elevate the baseline.
The key visual cue is simple: if the tracing never touches zero, CO2 is present in the inspired gas, and the cause needs to be identified.
Sudden Loss of the Waveform
A capnography tracing that abruptly drops to a flat line at zero is one of the most urgent patterns you can see. It means no CO2 is being detected, and the possible causes range from equipment problems to life-threatening emergencies.
- Breathing tube dislodgement: The tube has come out of the airway entirely.
- Esophageal intubation: The tube is in the stomach instead of the trachea. You may see a few small, rapidly shrinking waveforms before the trace goes flat, because the stomach can contain small amounts of CO2 initially.
- Cardiac or respiratory arrest: No circulation means no CO2 delivery to the lungs.
- Complete airway obstruction: The patient may have bitten down on the tube or a mucus plug may be blocking airflow.
- Equipment issues: A disconnected sensor, water droplet contamination of the module, or a hole in the breathing tube that lets gas escape before reaching the sensor.
The clinical context usually narrows the cause quickly, but the first step with any sudden flat line is confirming the airway is intact and the patient is still breathing.
The Curare Cleft
A small notch or dip in the middle of the Phase III plateau is called a curare cleft. It indicates that a patient on mechanical ventilation is making their own breathing effort, typically because a paralytic medication is wearing off. The patient’s diaphragm contracts briefly during the machine’s exhalation phase, momentarily interrupting the smooth flow of CO2 past the sensor.
This pattern got its name from curare, a paralytic agent. When it appears, it often signals that the patient needs additional sedation or another dose of a paralytic if continued muscle relaxation is required. One caveat: a partially disconnected mainstream sensor can mimic a curare cleft by briefly disrupting airflow past the detector, so it’s worth checking equipment before assuming the patient is waking up.
Capnography During CPR
During cardiac arrest, capnography becomes a real-time indicator of how well chest compressions are circulating blood. Effective compressions push blood through the lungs, delivering CO2 to be exhaled and detected. Poor compressions produce very low EtCO2 readings.
In pediatric resuscitation, EtCO2 values of 20 mmHg or greater during CPR have been associated with better odds of return of spontaneous circulation (ROSC) and survival to hospital discharge in a multicenter study. When ROSC occurs, you’ll typically see a sudden, dramatic spike in EtCO2, sometimes doubling or tripling within a few breaths, because restored circulation floods the lungs with CO2 that had been building up in the tissues. This spike can be the earliest sign that the heart has restarted, often appearing before a pulse is palpable.
The American Heart Association notes that a specific EtCO2 cutoff should not be used alone to decide when to stop resuscitation, because survival has been documented even in patients with average EtCO2 values below 20 mmHg.
Mainstream vs. Sidestream Monitors
Capnography monitors come in two main designs, and understanding which one you’re using affects how you interpret certain details.
Mainstream monitors attach a sensor directly to the breathing circuit at the airway. They measure CO2 in real time as it passes the sensor, which means faster response and more accurate waveform shape. Sidestream monitors pull a small sample of gas through thin tubing to a sensor located inside the machine. This adds a slight delay and can smooth out some waveform details.
For basic monitoring, both types perform nearly identically. Studies comparing the two in spontaneously breathing adults show excellent agreement for EtCO2 values. However, sidestream monitors are less reliable for advanced measurements like dead space fraction and the fine details of uneven lung emptying. If you need to assess ventilation-perfusion matching precisely, mainstream capnography is the better tool. For routine monitoring of EtCO2 trends and basic waveform shape, either type works well.
Sidestream setups are more common in non-intubated patients because they can use a simple nasal cannula to collect exhaled gas, making them practical for sedation monitoring and emergency departments where patients are breathing on their own.