How To Measure End Tidal Co2 Etco2? Essential Guide

End tidal CO2 is the concentration of carbon dioxide in the air you breathe out at the very end of an exhaled breath. It is measured with a device called a capnograph, which displays a number (EtCO2) and a waveform (capnogram). You can measure it two ways: mainstream, where a sensor sits directly on the breathing tube, or sidestream, where a small pump pulls exhaled air to a separate sensor.

That short answer covers the core. What follows explains how each method works, what the numbers mean, and where the readings can mislead you.

What Is End Tidal CO2 and Why Does It Matter?

End tidal CO2 reflects how well carbon dioxide is moving from your blood into your lungs and out of your body. It is a window into three things at once: your metabolism, your circulation, and your breathing.

Here is the physiology. Cells produce CO2 as they make energy. That CO2 travels in the blood to the lungs, crosses into the air sacs, and leaves when you exhale. The capnograph measures the CO2 at the end of that exhaled breath, when the air coming out is richest in CO2 from the deepest part of the lungs.

In healthy people, EtCO2 usually runs close to arterial CO2 (PaCO2), often within a few points of it. That relationship is why clinicians use EtCO2 as a noninvasive stand-in for a blood gas in the right situations. It is not a perfect match, and the gap between the two can widen with certain lung and circulation problems. But for monitoring trends over time, it is fast, continuous, and does not require a needle.

EtCO2 matters most when a patient is sedated, on a ventilator, or undergoing a procedure where breathing can change quickly. A falling number can be an early warning. A sudden drop to near zero can mean the breathing tube is not in the airway at all.

How To Measure End Tidal CO2 EtCO2: The Two Main Methods

There are two established ways to measure EtCO2, and each has real tradeoffs. The choice usually comes down to the patient, the setting, and what the clinician needs to watch.

Mainstream Capnometry

In mainstream measurement, a sensor sits directly in line with the breathing circuit, right at the airway. The CO2 is measured as the gas flows past the sensor. Because the sample is read immediately, the reading is essentially real-time.

Mainstream sensors are heavier, which can matter in small patients or when the tube needs to stay stable. They also add a bit of dead space to the circuit. In exchange, you get a fast, accurate waveform with no sampling tube to clog.

Sidestream Capnometry

In sidestream measurement, a thin tube pulls a small sample of exhaled air away from the patient to a sensor elsewhere. This keeps the airway connection light and lets the monitor sit wherever it is convenient.

The tradeoff is a slight delay, because the gas has to travel to the sensor. Water vapor and secretions can collect in the sampling line and cause errors if it is not managed. Sidestream is common in non-intubated patients, in intensive care, and in settings where a lightweight connection matters.

Colorimetric Detectors

A third option, the colorimetric detector, is a single-use device that changes color based on CO2. It does not give a number or a waveform. It gives a yes-or-no answer about whether CO2 is present. It is used mainly to confirm tube placement in emergencies, not for continuous monitoring.

How Do You Read a Capnogram Waveform?

The waveform tells you more than the number alone. A normal capnogram has a distinct shape, and changes in that shape point to specific problems.

A normal breath produces a curve that rises sharply, forms a fairly flat plateau, then drops back to zero as the next breath begins. The highest point of that plateau is the end tidal value.

When the plateau slopes upward instead of staying flat, it can suggest that the lungs are not emptying evenly. When the whole curve is lower than expected, it can point to reduced blood flow to the lungs, poor sampling, or a leak. When the curve suddenly disappears, that is a red flag for a disconnected circuit or a tube that has moved out of the airway.

Reading the shape alongside the number is what separates good monitoring from just watching a digit. The number alone can look reassuring while the waveform shows a problem brewing.

What Do the Numbers Mean?

A normal EtCO2 in a healthy resting adult is generally in the range of 35 to 45 mmHg. This mirrors the normal range for arterial CO2.

Values outside that range are not automatically a problem, but they point in a direction worth understanding.

• Low EtCO2 (below normal): often means the person is breathing too fast or too deep, or that less blood is reaching the lungs. It can also reflect a sampling problem.
• High EtCO2 (above normal): often means breathing is too slow or too shallow, or that the body is producing more CO2 than it is clearing.
• Sudden drop to near zero: a strong signal that the airway or circuit is no longer connected, or that the tube is not in the windpipe.

One important caveat: EtCO2 is a trend tool more than a single snapshot. A single reading can be thrown off by the patient’s position, a leak, or a blocked sampling line. Watching how the number moves over time is usually more useful than reacting to one value.

Mainstream vs Sidestream: How Do They Compare?

Both methods measure the same thing. The differences are practical, and they matter depending on the situation.

Neither method is universally better. A clinician picks based on the patient’s size, whether they are intubated, and what the monitor needs to show.

What Can Throw Off an EtCO2 Reading?

EtCO2 is only as good as the signal reaching the sensor. Several things can distort it.

Leaks in the breathing circuit let room air mix with exhaled air and pull the number down. A blocked or wet sampling line in sidestream systems can delay or flatten the reading. Low blood flow to the lungs, as happens in shock or cardiac arrest, lowers EtCO2 even when the lungs are working fine, because less CO2 is being delivered to them in the first place.

Body temperature and metabolism also shift CO2 production. A person with a high fever produces more CO2. So does someone with a high metabolic rate. These factors change the number without any problem in the airway or lungs.

This is the non-obvious part: a low EtCO2 does not always mean fast breathing. During cardiac arrest, a very low EtCO2 reflects poor circulation, and it can rise when chest compressions are effective. The number is telling you about blood flow, not just breathing.

Why Monitoring Trends Beats Watching One Number

A single EtCO2 value is a snapshot. The trend is the story.

During sedation, a slowly climbing EtCO2 can signal that breathing is becoming too shallow before oxygen levels drop. During a procedure, a sudden fall can flag a problem in seconds. During resuscitation, changes in EtCO2 can reflect whether circulation is improving.

This is why the waveform and the trend are read together, not in isolation. The device gives a number, but the meaning comes from context: what the patient is doing, what changed, and how fast.