Difference between SpO2 vs. ETCO2: Their Roles in Respiration and Blood Oxygenation


Both of the measurements oxygen saturation (SpO2) and end-tidal carbon dioxide (ETCO2) relate to breathing, but they differ as widely in and out. In fact, SpO2 tells how much oxygen circulates in your blood, while ETCO2 tells how much carbon dioxide you expel when you breathe.

They require different kinds of equipment to measure. Only one can typically be measured in the home setting using pulse oximetry. Combined with VO2 max, though, they provide a well-rounded respiratory picture that helps clinicians determine treatment and monitor warning signs of respiratory failure.


SpO2 Basics

If you run a 5K race and then get a pulse oximetry test to measure your SpO2, you’ll probably find your SpO2 level dropped relatively low. It may read about 90 percent. You’ll feel like you’re out of breath and might want some oxygen.

Normally, a healthy individual has a pulse oximetry reading of 95 percent to 100 percent. If SpO2 drops below 90 percent, you may need to go to the hospital.


ETCO2 Basics

If you hold your breath and then measure your ETCO2 using a capnometer or a small tube inserted into an oxygen mask, you will obtain a reading of 0. Since you don’t breathe out any carbon dioxide while holding your breath, the capnometer doesn’t pick up any carbon dioxide.

Similar to how pulse oximetry is done for SpO2, healthy individuals share similar readings with ETCO2. A healthy individual has a reading of 35 to 45 mmHg.

Because both measurements have a normal reading, medical practitioners can use them in a clinical setting to monitor breathing. During surgeries, medical personnel typically monitor both levels.


Oxygen Saturation (SpO2) Defined

The term oxygen saturation, or SpO2, refers to a measurement expressed as a percentage of the blood’s oxygen-carrying hemoglobin relative to the hemoglobin not carrying oxygen. To function properly, the human body requires at least 90 percent oxygen in the blood. Pulse oximetry measures oxygenation or arterial blood oxygen saturation.


The Normal Amount of Oxygen in Blood

The typical SpO2 reading for a healthy individual ranges between 95 and 100 percent. A pulse oximetry test reveals a lot about a person’s health since readings of below 90 percent signal respiratory distress. During sleep, the normal SpO2 reaches 90 percent. During workouts, it may drop momentarily, sometimes below 90 percent.


Abnormal Oxygen Levels: Hypoxemia and Hypoxia

When the blood’s level of oxygen falls too low for an extended period, the individual can develop hypoxemia. The most obvious symptom of this, cyanosis, results in the skin appearing blue. If it goes untreated, hypoxemia can turn into hypoxia. In hypoxia, body tissues also experience low levels of oxygen, and this can permanently damage body tissues. It can lead to heart damage, brain damage, or death.


How the Body Maintains Normal SpO2 levels

In normal breathing, the lungs disperse inhaled oxygen, which binds to hemoglobin. The hemoglobin circulates through your body. During workouts, the body’s need for blood oxygen increases. If you’ve ever observed a football player breathing from an oxygen mask between plays, he did so to raise the level of oxygen in his blood and promote alveolar ventilation.


Pulse Oximetry: Measuring Arterial Blood Oxygen Saturation

Pulse oximetry measures the level of oxygen in the blood using a device called a pulse oximeter. This device is slipped onto the finger, usually the index finger, or the toe. Others attach it to the head or onto the foot. 

Pulse oximetry devices vary for the age of the person and their medical condition. A pulse oximeter used in fetuses during the birthing process or pediatric patients  differs in design from one used to take adults’ pulse oximetry levels.

You can use an adult pulse oximeter for pediatric patients if their finger goes all the way to the end of the probe. But using an adult oximeter on children may result in an inaccurate reading.

If medical personnel diagnose a patient with carbon monoxide poisoning, adjust the reading of the pulse oximetry device as the excess carbon dioxide tricks the oximeter into overestimating arterial oxygenation in the patient.

Some fitness trackers measure SpO2 using a built-in oximeter. This measurement appears in the standard readout of vital signs on devices, such as Fitbit and Apple Watch. When you work out, knowing how much oxygen circulates in your blood can help you determine whether you’re hitting the right intensity. If your pulse oximetry reading hits 90 percent or lower and remains in that level, you should proceed immediately to the emergency room. Your oxygen saturation levels have dropped dangerously low.

Fitness trackers don’t provide exact enough readings for medical use, such as alveolar ventilation, due to pulse oximeter limitations in their design. If your doctor told you to monitor your SpO2 readings, use an actual pulse oximetry device for that purpose. Medical devices don’t have the pulse oximeter limitations that fitness trackers and smartwatches do.


End-Tidal Carbon Dioxide (ETCO2) Defined

Capnography monitoring measures carbon monoxide expelled. Unlike pulse oximetry, you can’t measure your ETCO2 at home or during a gym workout. It requires advanced medical machinery to measure. Capnography is the process used to measure your ETCO2 level. It results in a waveform called a capnogram, which displays on an LED or touch screen on the machine used. Capnography monitoring measures carbon monoxide expelled.

While the value of the CO2 exhaled tops the list of data gleaned from this test, it also records your respiratory rate and breathing pattern and depth. The measurement comes close to alveolar CO2 but isn’t the same thing.

In a nutshell, shallow breathing results in lower readings. Deep breathing results in higher readings. With breathing changes detected immediately, medical personnel can register respiratory distress and immediately address it to treat the patient by administering supplemental oxygen. The ETCO2 measurement tells the doctor if the patient becomes apneic, hypoventilates, or hyperventilates. As a real-time statistic, this measurement provides a clear picture of ventilation, whether natural or using supplemental oxygen.


Uses of ETCO2

Clinicians use end-tidal carbon dioxide or ETCO2 at three distinct points in the treatment process:

  • Emergency medicine
  • Intensive care wards
  • Surgery
  • Breathing retraining

Let’s consider how each applies to medical treatment and provides alerts of potential respiratory distress.

Emergency Medicine

When a patient goes into the emergency room, their end-tidal CO2 readings contribute to the full picture of how well they’re breathing and processing oxygen and carbon dioxide.

Intensive Care Wards

In an intensive care unit (ICU), the capnometer can substitute for blood gas tests. Clinicians frequently use it to monitor patients on assisted ventilation for signs of respiratory distress. Its real-time readings and waveform on the monitor offer up-to-date information that offers a moment-to-moment measurement of carbon dioxide, unlike an ABG test that gets run when the patient undergoes the admitting process.


In a surgical setting, medical personnel monitor ETCO2, especially with regard to patient response to anesthesia. A spike or reduction in this reading can signal respiratory distress. It offers an option to register respiratory distress when the patient can’t report a problem.

Breathing Retraining

The ETCO2 measurement provides a doctor with information on a patient’s progress in learning

normalized breathing. The respiratory therapist can also teach the patient to use the measurement as a biofeedback tool.


Biofeedback and Better Breathing

Respiratory therapists aim to retrain individuals with diseases like COPD to breathe better and

conserve energy so they can better manage normal daily tasks without experiencing shortness of breath. When a human pays attention to their breathing, it changes their breathing patterns. This aids in retraining a person to breathe with more control.

Over-breathing causes CO2 deficiency. This causes shortness of breath or worsens it, and there is a lengthy list of other physical and mental reactions, ranging from anxiety and emotional outbursts to chest pain and feelings of suffocation.

Athletes can use breathing retraining to learn how best to control breathing and modulate their breathing. A combination of readings of SpO2 and ETCO2 can provide the feedback necessary to begin learning how and when to alter breathing patterns to avoid sports-induced hyperventilation.


Using the Two Measurements

Typically, a patient can perform pulse oximetry easily at home. Pulse oximeters cost less than $25 each, making them an affordable fitness device. Many fitness trackers include the sensors needed to take these data measurements.

We find the opposite true for ETCO2. Although a few lower-priced machines cost about $200, most capnometers cost more than $2,000. No wonder this data remains a favorite in clinical settings but not outside them. We carry a full line of mainstream and sidestream capnography sensors. 

Although a capnometer does get used for biofeedback, that typically occurs in a clinical setting when a respiratory therapist uses a pulse oximeter and capnometer to monitor a patient’s breathing. Few individuals outside the medical field know and understand capnography.

Fitness trackers and at-home devices do not monitor ETCO2. Integrating a screen capable of displaying a capnography waveform will require a larger screen. You can take fairly accurate pulse oximetry readings with a fitness tracker or smartwatch, though.


In Conclusion

While SpO2 and end-tidal carbon dioxide (ETCO2) differ, both provide clinicians with important information that helps recognize or signal respiratory failure. Each indicator offers a peek at how the body handles the breathing process. If the blood isn’t receiving enough oxygen, the SpO2 alerts medical personnel, and if the body isn’t expelling enough carbon dioxide, the ETCO2 readings indicate that.

Pulse oximetry measures oxygenation. Capnography monitoring measures carbon monoxide expelled. Both measurements offer important data on breathing, with changes detected immediately.

Applied together, especially with arterial blood oxygen saturation, these indicators form a full picture of the patient’s overall respiratory health. The plethora of fitness trackers and smartwatches that use sensors to take pulse oximetry reading make it easy to integrate into at-home athletic training.

The opposite remains true for ETCO2, which doesn’t appear on any of the fitness trackers. Unlike pulse oximetry, this test requires more expensive, advanced devices that range in cost from $200 to $2,000 and can display a large capnography waveform. A respiratory therapist may integrate these devices and measurements into their program for retraining the breathing of those with a COPD diagnosis or who participate in athletics.