etco2: The Vital Sign You Should Be Watching
Pulse oximetry tells you about oxygenation. Capnography tells you much more.
In the prehospital environment, we don't have the luxury of a complete laboratory workup or unlimited diagnostic testing.
That's why waveform capnography is such a powerful tool.
Capnography provides a continuous, real-time measurement of exhaled carbon dioxide (CO₂). It can provide information about ventilation, perfusion, metabolism, airway placement, and response to treatment.
The key is learning to use the number, waveform, and trend together.
ETCO₂ vs. SpO₂
SpO₂ primarily tells you about oxygenation.
ETCO₂ gives you information about ventilation and can also provide clues about perfusion and metabolism.
This distinction matters.
A patient can have a normal SpO₂ while their ventilation is deteriorating—particularly when supplemental oxygen is being administered.
Capnography can identify changes in ventilation much sooner than waiting for oxygen saturation to fall.
Think of it this way:
SpO₂ asks: "Are they oxygenating?"
ETCO₂ asks: "Are they ventilating—and what might their circulation be telling us?"
Airway Confirmation
One of the most important uses of continuous waveform capnography is confirming and monitoring advanced airway placement.
After placing an endotracheal tube or supraglottic airway, waveform capnography provides objective evidence that exhaled CO₂ is reaching the monitoring device.
But don't make the mistake of thinking:
"I saw CO₂ once, so I'm done."
The airway can become displaced.
The patient can deteriorate.
The tube can become obstructed.
The connection can come apart.
Continuous waveform capnography allows you to monitor the airway throughout patient care and transport.
A sudden loss or significant change in the waveform should make you stop and reassess.
Capnography During Cardiac Arrest
Capnography becomes especially valuable during cardiac arrest.
Continuous ETCO₂ can help you evaluate:
Quality of CPR
Advanced airway placement
Changes in perfusion
Return of spontaneous circulation (ROSC)
Deterioration after ROSC
During CPR, ETCO₂ provides a continuous indicator that can help you assess whether compressions are generating effective circulation.
A sudden, sustained increase in ETCO₂ can be an early clue that ROSC has occurred—prompting you to immediately reassess the patient for a pulse and other signs of circulation.
Likewise, persistently low or falling values should make you reconsider the quality of CPR and the patient's overall perfusion.
The Waveform Can Tell You More Than the Number
Don't become obsessed with the number on the monitor.
Look at the waveform.
The shape of the capnogram can provide important information about what is happening with the patient's ventilation.
For example, patients experiencing significant bronchospasm from conditions such as asthma or COPD may develop a characteristic "shark-fin" waveform caused by obstructed exhalation.
As bronchospasm improves with treatment, the waveform may improve as well.
This gives you something incredibly valuable in the field:
Real-time feedback on whether your treatment is working.
Capnography and Ventilation
ETCO₂ can also help guide ventilation.
If a patient is being ventilated manually or mechanically, capnography gives you immediate feedback about changes in ventilation.
Instead of simply watching the chest rise and fall, you're getting information about what is actually happening with exhaled CO₂.
This becomes particularly important after intubation.
A patient can have a perfectly placed tube and still be ventilated poorly.
The tube being in the trachea doesn't mean you're ventilating the patient correctly.
Capnography helps you continuously evaluate that process.
Capnography and Sepsis
Here's where capnography becomes especially interesting for the prehospital provider.
ETCO₂ can provide clues about a patient's perfusion status—and that can be valuable when evaluating a patient with suspected sepsis.
In sepsis, profound physiologic changes can occur, including vasodilation, impaired circulation, and altered cellular metabolism.
As perfusion deteriorates, the amount of CO₂ delivered to the lungs can decrease, potentially resulting in a lower ETCO₂.
A low ETCO₂ in a patient with suspected infection, abnormal vital signs, and other signs of poor perfusion should increase your concern for significant illness.
But here's the important part:
ETCO₂ does not diagnose sepsis.
You shouldn't look at an ETCO₂ of 25 mmHg and say, "This patient is septic."
Instead, use it as another piece of information.
Ask:
Does the patient have a suspected infection?
Are they tachycardic?
Are they hypotensive or trending toward hypotension?
Is their mental status changing?
Are they showing signs of poor perfusion?
Is their ETCO₂ unexpectedly low?
Is the ETCO₂ changing as I treat the patient?
Trend matters.
If your interventions improve perfusion and the patient's ETCO₂ rises along with other signs of clinical improvement, that may provide additional evidence that your resuscitation is having an effect.
Capnography and Internal Hemorrhage
Now consider the patient with occult or internal hemorrhage.
Maybe there's no obvious external bleeding.
The patient simply looks sick.
They're pale.
They're tachycardic.
Their blood pressure may still be "normal."
And their ETCO₂ is unexpectedly low.
Why might that happen?
Remember where CO₂ comes from:
Cells produce CO₂ → blood transports it → the lungs exhale it.
If significant blood loss reduces circulating volume and cardiac output, less CO₂ may be transported to the lungs.
The result can be a decrease in ETCO₂.
This is why a falling ETCO₂ can be concerning in a patient who may be experiencing hemorrhagic shock.
And here's the important prehospital lesson:
Don't wait for hypotension to recognize shock.
A patient can compensate for significant blood loss before their blood pressure finally falls.
If you have a patient with a mechanism or presentation concerning for internal bleeding and you see:
Tachycardia + altered mental status + cool/pale skin + poor peripheral perfusion + falling ETCO₂
you should be thinking seriously about circulatory compromise, even if the blood pressure hasn't collapsed yet.
Again, ETCO₂ cannot tell you:
"This patient is bleeding internally."
But it can provide another physiologic clue that something is wrong.
Watch the Trend
This is one of the most important concepts with capnography.
Don't just look at the ETCO₂ value once.
Watch what happens over time.
For example:
ETCO₂ 34 → 30 → 26
That's a different clinical story than:
ETCO₂ 26 → 30 → 34
The first trend may indicate worsening perfusion or ventilation.
The second may indicate improvement.
The number doesn't exist in isolation.
The trend tells the story.
Don't Ignore Perfusion
ETCO₂ isn't just about the lungs.
CO₂ is produced by cellular metabolism and transported through the circulation to the lungs, where it is exhaled.
That means ETCO₂ is influenced by more than ventilation.
It is also affected by circulation and metabolism.
A significant decrease in ETCO₂ can therefore be a warning sign of worsening perfusion in the appropriate clinical context.
This can be particularly useful when assessing patients with:
Shock
Sepsis
Hemorrhage
Cardiac arrest
Cardiogenic states
Other causes of decreased cardiac output
But remember:
ETCO₂ is a piece of the puzzle—not the entire puzzle.
Always interpret the number and waveform alongside the patient's clinical presentation, blood pressure, respiratory status, SpO₂, mental status, skin signs, pulse quality, and other available data.
A Better Way to Think About Capnography
Don't think of capnography as:
"That thing I put on the tube."
Think of it as a continuous window into the patient's physiology.
Ask yourself:
Is the patient ventilating?
Is the airway still in place?
Is my treatment improving their ventilation?
Are my compressions generating circulation?
Did ROSC occur?
Could this patient be developing shock?
Could a low or falling ETCO₂ support my concern for sepsis or hemorrhage?
Has the waveform changed?
These questions turn capnography from a monitor you attach to a patient into a clinical tool you actively use.
The Bottom Line
Capnography gives prehospital providers something incredibly valuable:
Real-time information about what's happening inside the patient.
Use it to:
Confirm the airway.
Monitor ventilation.
Evaluate CPR.
Recognize possible ROSC.
Trend perfusion.
Identify changes that may support concern for shock, sepsis, or hemorrhage.
Evaluate treatment response.
And most importantly:
Don't just look at the ETCO₂ number. Look at the waveform, watch the trend, and connect what you see to what's happening with your patient.
A low ETCO₂ doesn't tell you exactly what's wrong.
But when you combine it with the patient's history, physical exam, vital signs, and clinical presentation, it may give you an important clue that your patient is becoming critically ill before the rest of the picture becomes obvious.
Capnography isn't just another number on the monitor. It's another set of eyes on your patient.
