What Is the Normal End-Tidal CO2 Range?

In a healthy adult breathing normally at rest, end-tidal CO2 (EtCO2) falls between 35 and 45 mmHg. That range mirrors what is happening in the bloodstream, since EtCO2 tracks the concentration of carbon dioxide at the very end of an exhaled breath, the point at which the gas most closely reflects what is leaving the alveoli. But the number is deceptively simple. It shifts with breathing rate, heart function, body temperature, altitude, and dozens of clinical scenarios, which is exactly why clinicians watch it so closely.

What EtCO2 Actually Reflects

Carbon dioxide is a waste product of metabolism. Your cells produce it constantly, and the blood carries it to the lungs, where it diffuses into the air sacs and gets exhaled. The concentration of CO2 at the tail end of a breath captures a snapshot of three overlapping systems at once: how fast your body is producing CO2, how well your blood is delivering it to the lungs, and how effectively your lungs are ventilating. A change in any one of those three can nudge EtCO2 up or down, even when the other two are perfectly fine. That triple dependency is what makes the number so useful in critical care and so tricky to interpret in isolation.

EtCO2 is measured by a device called a capnograph, which uses infrared light to detect CO2 in exhaled gas. The reading is typically displayed as a number in mmHg and as a waveform, the capnogram, which traces the CO2 concentration across each breath cycle. The waveform has distinct phases corresponding to early expiration (dead-space gas with almost no CO2), a rapid rise as alveolar gas arrives, and a plateau that represents the richest alveolar gas. The peak of that plateau is the EtCO2 value.

How Breathing Rate Moves the Number

Breathing faster and deeper than normal, the pattern called hyperventilation, is one of the quickest ways to drive EtCO2 down. You are washing CO2 out of the lungs faster than the body can produce it. In a study of paced hyperventilation, rapid and deep breathing dropped EtCO2 by about 35% compared to a normal baseline.¹ Even modest changes in breathing pace can matter. Paced breathing at a slow rate without careful instruction lowered EtCO2 by roughly 5 mmHg, enough to push a person from the middle of the normal range toward its lower edge.²

Hypoventilation, the opposite pattern of slow or shallow breaths, lets CO2 accumulate and pushes EtCO2 above 45 mmHg. This is commonly seen in people who are sedated, in patients with severe obesity, or in anyone whose respiratory drive is depressed. The practical takeaway is that an abnormal EtCO2 reading does not automatically mean something is wrong with the lungs or the heart. Sometimes the person is simply breathing too fast or too slowly.

The Circulatory Connection

Because blood is what delivers CO2 to the lungs, anything that changes cardiac output changes how much CO2 reaches the alveoli per breath. In a well-known animal model, a strong logarithmic relationship was found between cardiac output and EtCO2, meaning that as blood flow to the lungs dropped, EtCO2 fell sharply.³ Separate work confirmed a high linear correlation between the two, and showed that when cardiac output became critically low, venous CO2 climbed while EtCO2 paradoxically fell, because the lungs were no longer receiving enough blood to exhale the gas.⁴

This relationship is why a sudden drop in EtCO2 during surgery or in an ICU can signal a catastrophic event like a pulmonary embolism or cardiac arrest. The lungs are still ventilating, but blood has stopped arriving with CO2 to exhale. In that context, a plummeting number is not about breathing at all. It is a red flag about circulation.

Metabolism and Body Temperature

When your metabolic rate climbs, so does CO2 production. Fever, sepsis, thyroid storms, and even the absorption of CO2 during certain surgeries can push EtCO2 upward. During laparoscopic procedures where the abdomen is inflated with carbon dioxide gas, EtCO2 has been shown to peak around 30 minutes into insufflation, reaching mean values in the low-to-mid 40s.⁵ The anesthesiologist compensates by increasing the ventilator rate, which illustrates how EtCO2 serves as a real-time feedback dial in the operating room.

There is a wrinkle, though: in spontaneously breathing subjects, rising body temperature can trigger faster breathing, which washes out CO2 and temporarily lowers EtCO2 even while CO2 production is climbing. Research on experimental hyperthermia found that EtCO2 was a reliable marker of increasing metabolism only when the subject was on a mechanical ventilator; in spontaneously breathing animals, the reflex tachypnea masked the metabolic rise.⁶ This is a good example of how the same number can mean different things depending on the clinical context.

The Gap Between Arterial CO2 and EtCO2

In a perfect lung, the CO2 in the last bit of exhaled gas would match the CO2 dissolved in arterial blood. In reality, EtCO2 runs a few mmHg lower than arterial CO2 (PaCO2) in most people, because a small fraction of alveoli are ventilated but poorly perfused, diluting the exhaled CO2 with gas that never picked up much carbon dioxide. This arterial-to-end-tidal CO2 gradient is typically 2 to 5 mmHg in healthy lungs.

The gradient widens in lung diseases that create ventilation-perfusion mismatch, like COPD, pulmonary embolism, or acute respiratory distress syndrome. Modeling work has confirmed that factors such as anatomical dead space, pulmonary shunt fraction, and CO2 production rate each independently influence the size of this gap.⁷ In patients with severe hypercapnia placed on noninvasive ventilation, the gap between PaCO2 and EtCO2 narrowed over the course of treatment, and EtCO2 correlated strongly with arterial values by the end of the observation period.⁸ The clinical lesson: when the gradient is small, EtCO2 tracks PaCO2 well and you can trust the number as a proxy for what is in the blood. When lung disease widens the gradient, EtCO2 underestimates the true arterial CO2, sometimes substantially.

EtCO2 During Cardiopulmonary Resuscitation

One of the most dramatic uses of EtCO2 monitoring is during CPR. Because the number depends on blood flow through the lungs, it becomes a real-time gauge of how effectively chest compressions are generating circulation. A higher EtCO2 during CPR generally means better blood flow, which translates into a better chance of getting the heart restarted. In a multicenter study of pediatric cardiac arrest, children whose average EtCO2 during CPR reached 20 mmHg or above had a survival-to-discharge rate of about 65%, compared with roughly 48% for those whose values stayed lower.⁹

When a patient’s heart restarts spontaneously during resuscitation, EtCO2 typically surges as the restored cardiac output suddenly floods the lungs with CO2-laden blood. Research on adult cardiac arrest patients found that those who ultimately achieved return of spontaneous circulation had noticeably higher EtCO2 values starting as early as five minutes into resuscitation.¹⁰ The question of whether a very low EtCO2 should lead teams to stop CPR is more controversial. One pre-hospital study cautioned that an initial EtCO2 at or below about 10 mmHg should not be used as a hard cutoff for abandoning resuscitation efforts, because some patients with readings that low still achieved return of circulation.¹¹

Traumatic Brain Injury and the Danger Zone Below 35

In patients with suspected severe traumatic brain injury, keeping CO2 within the normal range takes on special urgency. Low CO2 causes blood vessels in the brain to constrict, reducing cerebral blood flow at a time when the injured brain desperately needs oxygen. A large study of prehospital TBI patients found an L-shaped relationship between EtCO2 and 30-day mortality: values below 35 mmHg were associated with sharply increased odds of dying, with an odds ratio of about 1.9 for hypocapnia. Interestingly, readings above 45 mmHg did not show a significant association with worse outcomes.¹² The practical message for paramedics and emergency physicians is clear: aggressive hyperventilation, once a standard response to head trauma, can be harmful. Keeping EtCO2 within the 35 to 45 mmHg window is now a target in most prehospital TBI protocols.

Altitude Changes Everything

If you have ever hiked at high elevation and felt dizzy or short of breath, part of what is happening involves your EtCO2 falling. Lower oxygen at altitude triggers faster, deeper breathing, which blows off extra CO2. Research on mountaineers found a strong negative correlation between EtCO2 and altitude, and the degree of EtCO2 decline also correlated with the development of acute mountain sickness symptoms.¹³ At extreme elevations, EtCO2 values in the low 20s are not unusual and do not indicate disease. They are a normal physiological response. This means the standard 35-to-45 range applies at sea level and becomes progressively less relevant the higher you go.

Catching Trouble During Sedation

Outside the operating room, capnography has gained traction in emergency departments and procedure suites where patients receive sedation for things like fracture reduction, endoscopy, or dental work. The danger during sedation is respiratory depression: the patient breathes too slowly or stops briefly, and CO2 rises or the waveform disappears. Pulse oximetry, which measures blood oxygen, has been the traditional safety monitor, but it lags behind the actual event. In a pediatric sedation study, capnography detected all episodes of apnea and about 77% of hypoventilation episodes that pulse oximetry missed entirely. Capnography flagged problems a median of 35 seconds earlier than pulse oximetry did.¹⁴ Those 35 seconds can be the difference between a gentle verbal reminder to breathe and a full airway rescue.

Mainstream Versus Sidestream Monitors

There are two main types of capnography devices. Mainstream sensors sit directly in the breathing circuit, right at the airway, and measure CO2 as it passes through. Sidestream devices draw a small sample of exhaled gas through a thin tube to an analyzer located away from the patient. Both are widely used, and for the headline EtCO2 number, they agree well. A comparison study found that EtCO2 values and physiological dead-space measurements from the two techniques correlated excellently across different breathing rates.¹⁵ Where they diverge is in the finer details of the waveform shape, particularly measurements related to the early phase of expiration, where sidestream sampling introduces a small transport delay and mixing effect. For routine monitoring, both types give you reliable EtCO2 readings. For advanced waveform analysis, mainstream is considered the reference standard.

What the Waveform Shape Tells You

The EtCO2 number gets the most attention, but experienced clinicians also read the shape of the capnogram waveform. A normal capnogram has a sharp upstroke, a flat alveolar plateau, and a crisp drop back to baseline at the start of the next breath. Distortions in that shape carry diagnostic information. In patients with obstructive airway disease, for example, the plateau takes on an upward slope rather than staying flat, because different lung regions empty at different rates and the slowest regions, which are the most obstructed, deliver their CO2 last.

A case study tracking an asthma exacerbation over several days found that the slope of the plateau improved before the absolute EtCO2 value came down, suggesting the waveform shape can be a more sensitive early marker of treatment response.¹⁶ The waveform also helps confirm endotracheal tube placement. A properly placed tube produces a consistent rectangular waveform with each breath; a tube accidentally placed in the esophagus produces little to no waveform at all. Work on capnogram phases has formalized these shapes into numbered segments, with the alveolar plateau designated Phase III and the transition between inspiration and expiration carefully defined.¹⁷

EtCO2 During Exercise

During incremental exercise testing, the pattern of EtCO2 change over time can reveal problems with lung blood flow that are not obvious at rest. In healthy people, EtCO2 rises modestly during light-to-moderate exercise as increased cardiac output delivers more CO2 to well-ventilated lungs. It then stabilizes or begins to fall at high intensities as ventilation outpaces CO2 delivery. In patients with chronic thromboembolic pulmonary hypertension, this normal rise-then-plateau pattern is disrupted. Some show an abrupt EtCO2 drop at the very start of exercise or a slow, progressive decline throughout, reflecting impaired blood flow through obstructed pulmonary vessels.¹⁸ These patterns can help distinguish between different causes of exercise intolerance when other tests are ambiguous.

For recreational athletes wearing consumer-grade fitness monitors, EtCO2 tracking remains uncommon, but the underlying physiology explains some familiar sensations. The lightheadedness you feel after a sprint, for instance, is partly driven by a temporary drop in CO2 from heavy breathing, which constricts cerebral blood vessels just long enough to make you dizzy. It resolves on its own as your breathing rate settles and CO2 climbs back into its resting range.