A capnograph is a medical device that continuously measures the amount of carbon dioxide (CO2) in a person’s breath. It works by shining infrared light through exhaled air and calculating how much CO2 is present based on how much light gets absorbed. The result appears as both a number and a real-time waveform on a monitor, giving clinicians a moment-by-moment picture of how well someone is breathing.
How a Capnograph Works
CO2 molecules absorb specific wavelengths of infrared light. A capnograph exploits this property by passing infrared light through a sample of the patient’s exhaled air. A sensor on the other side, called a photodetector, measures how much light makes it through. The device compares the amount of light it sent out to the amount that arrived at the detector, and the difference tells it exactly how much CO2 is in the sample. More CO2 means more light absorbed.
The key measurement is called end-tidal CO2 (EtCO2), which is the peak concentration of carbon dioxide at the very end of an exhaled breath. In a healthy adult, this value typically falls between 35 and 45 mmHg. That number alone is useful, but the real power of capnography is the waveform it draws in real time.
Reading the Waveform
The capnograph plots CO2 levels against time, producing a distinctive wave shape with each breath cycle. This waveform has four phases, and each one tells a different part of the story.
- Phase I (inspiratory baseline): CO2 sits at zero because the air coming out first is from the upper airway, where no gas exchange happens.
- Phase II (rapid rise): CO2 climbs steeply as air from deeper in the lungs, where oxygen and carbon dioxide are swapped, begins to flow out.
- Phase III (alveolar plateau): The line levels off with a gentle upward slope. This represents air from the deepest parts of the lungs. The highest point at the end of this plateau is the EtCO2 value.
- Phase IV (rapid descent): The person inhales, fresh air flushes through, and CO2 drops back to zero.
A normal waveform looks like a rounded rectangle. When it doesn’t, that’s often the first sign that something is wrong.
What Abnormal Waveforms Reveal
Changes in the shape of the waveform can flag respiratory problems before other monitors pick them up. In asthma or bronchospasm, the normally steep rise in Phase II becomes a gradual, sloping climb, sometimes called a “shark fin” pattern. This happens because narrowed airways slow the flow of CO2-rich air from the lungs. The worse the obstruction, the more gradual the slope.
A sawtooth pattern layered on top of the waveform can indicate that the patient is trying to breathe against a ventilator, a sign that sedation or muscle relaxation is wearing off. Clinicians sometimes call this a “curare cleft” because it historically signaled that a paralytic drug was fading.
In emphysema, the alveolar plateau can actually reverse its usual upward slope because the damaged lungs exchange gas so rapidly. A bifid, or double-peaked, waveform can suggest that a breathing tube has slipped too far into one side of the airway, causing the two lungs to empty at different rates. Even heartbeats can show up on the waveform. In patients with enlarged hearts, the pulsing cardiac muscle nudges the lungs just enough to create small oscillations in the CO2 trace.
A slowly rising baseline, where CO2 never quite returns to zero between breaths, points to CO2 rebreathing. In an operating room, this often means the chemical absorber that scrubs CO2 from the breathing circuit is used up and needs replacing.
Mainstream vs. Sidestream Devices
Capnographs come in two main designs based on where the CO2 sensor sits.
In a mainstream capnograph, a small sensor clips directly onto an adapter placed in the breathing circuit, right at the patient’s airway. Infrared light passes through tiny windows in the adapter, and the reading happens instantly with no delay. This design is fast and accurate but requires the patient to have a breathing tube in place.
In a sidestream capnograph, a thin tube continuously draws a small sample of air from near the patient’s nose or mouth and routes it to a sensor inside the main monitor, typically through about six feet of tubing. The trade-off is a slight delay in readings, but sidestream devices have a major practical advantage: they can monitor people who are not intubated. A simple nasal cannula with a built-in sampling port can capture exhaled CO2 from someone breathing on their own. This makes sidestream capnography the go-to choice for sedation monitoring, emergency departments, and recovery rooms. Mainstream devices, on the other hand, provide more precise measurements of certain parameters like ventilation dead space, which matters in critical care and research settings.
Why It Matters More Than Pulse Oximetry Alone
Most people are familiar with the finger clip that measures oxygen levels, the pulse oximeter. Capnography and pulse oximetry monitor two fundamentally different things. Pulse oximetry tells you how much oxygen is in the blood. Capnography tells you how well the lungs are actually moving air in and out, which is ventilation.
This distinction matters because a person can stop breathing adequately, retaining dangerous levels of CO2, while their oxygen levels remain normal for several minutes. Sedation drugs like opioids and benzodiazepines commonly cause this kind of quiet respiratory depression. Studies comparing the two monitors in patients undergoing endoscopy procedures found that capnography detected respiratory depression significantly earlier than pulse oximetry. By the time oxygen saturation drops, the problem has been building for a while. Capnography catches it at the start.
Capnography During CPR
One of the most impactful uses of capnography is during cardiac arrest. When someone’s heart stops, the amount of CO2 reaching the lungs plummets. The EtCO2 reading during chest compressions serves as a real-time report card on CPR quality. If the reading drops below 10 mmHg in an intubated patient, it signals that compressions are not generating enough blood flow and need to be deeper or faster.
Capnography also provides one of the earliest signs that the heart has restarted. When circulation spontaneously returns, a surge of CO2-rich blood suddenly reaches the lungs, and the EtCO2 value jumps abruptly toward the normal range of 35 to 40 mmHg. This spike often appears on the monitor before a pulse can even be felt, giving the resuscitation team an immediate signal that their efforts are working.
Where Capnographs Are Used
Capnography is standard equipment in operating rooms, where confirming that a breathing tube is correctly placed in the airway (and not the esophagus) is one of its most critical functions. If the tube is in the wrong place, no CO2 waveform appears, and the team knows immediately to reposition it.
Beyond surgery, capnographs are used in intensive care units to monitor ventilated patients, in emergency departments to assess breathing status, during procedural sedation in endoscopy suites or dental offices, and increasingly in ambulances. Portable, handheld capnographs have made the technology accessible well outside the hospital, allowing paramedics to verify tube placement and monitor ventilation in the field.