Waveform Capnography: The Gold Standard

Why this matters

The sat tells you about oxygen at the fingertip. It does not prove the tube is in the trachea. It does not prove the bag is working. It does not show how carbon dioxide is moving from the tissues to the lungs. Waveform capnography fills that gap. It shows carbon dioxide continuously, with a shape you can read on scene and in the ambulance.

Any time you place an advanced airway, plan a continuous waveform to confirm it and to keep watching it, when you have the equipment. That includes an endotracheal tube, a supraglottic airway, or another device your protocol treats as an advanced airway. A color-change device is better than nothing for a first check in some systems. It is not continuous monitoring. No sustained waveform means the tube is not in the trachea.

Clinical sequence

1. Turn it on and stage the adapter before the tube or the supraglottic airway goes in. Do not hunt for the adapter after the tube is in.
2. On the first breath you give, look for a waveform, not just a number.
3. After every move, if the trace goes flat, treat it as a displaced tube until you prove otherwise. Use DOPE-S: displacement, obstruction, pneumothorax, equipment, and seal. Check tube depth and the circuit.
4. A shark-fin wave means bronchospasm. Slow the bag and treat the lungs. A sudden zero means check the tube and the circuit first. Do not assume the monitor failed.

Set It Up Early

Capnography often needs time to warm up and a zero or calibration cycle. If you wait until after the tube, you lose the minutes when confirmation matters most.

• Turn on the end-tidal channel while you prepare the airway kit.
• Zero it the way the manufacturer and your service require.
• Put the adapter in place so the first breaths after placement go through the sensor.
• Know whether your system is mainstream or sidestream. Mainstream means the sensor sits at the airway. Sidestream means a sample line pulls gas away. Secretions can block either signal.

Treat the airway kit and the waveform as one package.

What the Words Mean

• Capnography is the continuous measurement of carbon dioxide in exhaled gas, shown as a waveform.
• Capnometry is a number only, without a full waveform.
• EtCO₂ is the peak carbon dioxide at the end of exhalation, often in millimeters of mercury. A common teaching range for an adult with a pulse is about 35 to 45. Use your system’s reference and the whole picture: rate, perfusion, and waveform shape.
• The waveform is the graph of one breath. Read the number and the shape together. That is the standard way to confirm and monitor the airway.

Why you need the waveform

• It is continuous. A one-time check misses a tube that moves during packaging or while the ambulance is moving.
• It updates with every breath. It reflects ventilation and, with limits, blood flow to the lungs and metabolism.
• You can see a flat line, a sudden loss, or a bad shape, and act before the sat falls. The sat lags.
• An esophageal tube usually shows no reliable alveolar waveform. Act immediately.

What a Normal Waveform Looks Like

1. Phase I. Exhalation starts at a near-zero baseline. That first gas is dead space from the airway, not the alveoli.
2. Phase II. The line rises fast as gas from the alveoli mixes in.
3. Phase III. The plateau is alveolar gas. The peak at the end of that plateau is the end-tidal number.
4. Inspiration. The line drops sharply back toward the baseline.

On scene and in the ambulance, you want a clear rise, an alveolar plateau, and a return toward baseline with inspiration. A flat line or a sudden loss is an emergency until you prove otherwise. A normal waveform is not a random scribble, and it is not one continuous flat line at zero.

Waveforms to recognize

• A sudden flat line after a good waveform. The tube moved, the patient stopped breathing, the circuit disconnected, or cardiac arrest has no output. Fix it now. Use DOPE.
• A shark fin, with a sloping upstroke. That is bronchospasm or obstructed exhalation, as in asthma or chronic obstructive pulmonary disease. It is not proof of an esophageal tube, and it is not perfect breathing.
• A low number with a good waveform. Possible causes include bagging too fast, low metabolism, or poor blood flow, as in shock or arrest.
• A rising number during cardiopulmonary resuscitation (CPR). Blood flow to the lungs may be improving, or circulation may have returned. Still confirm a pulse per protocol. Do not stop CPR for a number alone, without a plan to check the pulse.
• The baseline does not return to zero. The patient may be rebreathing carbon dioxide, the absorbent in some circuits is used up, or the calibration is off. Know your device.

What You Use It For

• Confirm an advanced airway, and keep monitoring it.
• Catch a tube that moves during a transfer or packaging.
• Guide rate and depth. Bagging too fast often drives the number down.
• In arrest and after circulation returns, follow the trend as one clue to CPR quality and to return of a pulse.
• During sedation or opioid pain control, use a continuous waveform to watch for slow breathing when your protocol supports that.
• It does not replace the sat. Use both when you have them. The waveform is about ventilation and blood flow to the lungs. The sat is an estimate of oxygen in the blood.

Common errors

• Placing the tube before the channel is ready, then spending a minute searching for the adapter.
• Trusting one number without looking at the shape.
• Assuming a number of 35 means the tube is correct when the waveform is poor or comes and goes.
• Ignoring a sudden loss of waveform after you move the patient onto the stretcher.

Practice

Related: Failed oxygenation (DOPE-S), Post-intubation care.

Sources & Further Study

Named guidelines for further study. They are not clinical orders and do not replace your protocol or medical direction.

• NASEMSO National Model EMS Clinical Guidelines (2022) — U.S. EMS model language; follow your protocol
• AHA 2025 Part 9: Adult Advanced Life Support — continuous waveform capnography to confirm and monitor advanced airways (primary ALS cite)
• Your airway protocol and monitor-specific training.