EtCO2: Definition, Types and Indications of Capnography for the Measurement of Carbon Dioxide

Capnography is the numerical measurement and visualization of end-expiratory carbon dioxide.

End-tidal capnography (EtCO2) refers to the graphical measurement of the partial pressure of carbon dioxide (in mm Hg) during expiration (i.e., end-tidal carbon dioxide [End tidal capnography, pressure positive end-expiration]).

First established in the 1930s, the clinical use of end-tidal capnography measurement became accessible in the 1950s with the production and distribution of capnography monitors.

End-tidal capnography monitoring has become a key component in advancing patient safety within anesthesiology with continuous technological advancements.

The American Society of Anesthesiology (SAA) has endorsed Tidal Capnography as a standard of care for General and Moderate Anesthesia or Deep Procedural Sedation.

Studies have shown that during cardiac arrest, end-tidal capnography values ​​of more than 10-20 mmHg are associated with the return of spontaneous circulation (ROSC).

Consequently, other specialties, including critical care and emergency medicine, more frequently implement end-tidal capnography monitoring, although using this technology to save lives outside of the operating room remains inadequate.


Types of capnograph

There are two types of capnograph, “sidestream” and “mainstream.”

In the “mainstream” technique, the sampling window is in the fan circuit and measures carbon dioxide, while in the “sidestream,” the gas analyzer is outside the fan circuit.

The gas analyzer uses infrared radiation, mass or Raman spectra, and a photoelectric spectrum technology in both types. The flow measurement equipment is used in volumetric capnography.

The colorimetric carbon dioxide detector is an example of the main form.

These devices have a pH-sensitive indicator, which changes color on inspiration and expiration.

These color changes are in response to changes in carbon dioxide concentration.

In the presence of a small amount of carbon dioxide, the device has a base color, which gradually changes with increasing carbon dioxide concentration.

A normal capnograph has a square wave pattern, beginning in the inspiratory phase (peak expiratory carbon dioxide (PECO2) = 0 mmHg) and continuing through the expiratory grade.

ase 0 (inspiratory phase) occurs suddenly with an inspiration.

The expiration phase includes three phases:

Phase I (latency phase): the onset of expiration represents the anatomical dead space of the respiratory tract and cannot be discerned from the anterior inspiratory phase (PECO2 = 0 mmHg).

Phase II: a very rapid increase in the expiratory carbon dioxide peak, representing the exhalation of mixed air.

Phase III (Plateau phase): reflects the alveolar expiratory flow (a slight increase in peak expiratory carbon dioxide), which occurs at the height of end-expiratory carbon dioxide (ETCO2).

In this phase, the maximum expiratory carbon dioxide is close to the tension of alveolar carbon dioxide (PACO2).

Emergency physicians are always looking for a reliable, non-invasive instrument to detect life-threatening conditions in patients.

One of the methods suggested recently in the emergency department is capnography, or the End of monitoring the expiration of the tides.

This study aimed to review the applications of tidal expiration monitoring completion in the emergency department.


According to the American Heart Association (AAC), continuous waveform capnography, combined with clinical evaluation, is “the most reliable method of confirming and monitoring the correct placement of an endotracheal tube.”

Capnography can also be used to ensure ventilation with supraglottic devices and confirm that a spontaneously ventilating patient is indeed breathing (for example, through a face mask or nasal cannula sampling).

More generally, end-tidal capnography is used in the following settings:

  • General anesthesia.
  • Procedural sedation, including sedation with monitored anesthesia.
  • Ventilation analysis (e.g., in the intensive care unit [ICU]).
  • Cardiac arrest to confirm tracheal intubation and the adequacy of chest compressions.

Diagnostic use

Capnography provides information on carbon dioxide production, pulmonary (lung) perfusion, alveolar ventilation, breathing patterns, and carbon dioxide removal from the anesthesia breathing circuit and the ventilator.

The shape of the curve is affected by some forms of lung disease; In general, there are obstructive conditions such as bronchitis, emphysema, and asthma. The mixture of gases within the lung is affected.

Conditions such as pulmonary embolism and congenital heart disease, which affect pulmonary perfusion, do not, by themselves, affect the shape of the curve, but they do significantly affect the ratio of expired carbon dioxide to carbon dioxide in arterial blood.

Capnography can also measure carbon dioxide production, a measure of metabolism.

Increased carbon dioxide production is seen during fever and chills. Reduced production is seen during anesthesia and hypothermia.

Use in anesthesia

There is an interaction between two components during anesthesia: the patient and the anesthesia delivery device (which is usually a breathing circuit and a ventilator).

The critical connection between the two components is an endotracheal tube or a mask, and carbon dioxide is typically monitored at this junction.

Capnography directly reflects the removal of carbon dioxide from the lungs to the anesthesia device.

Indirectly, it reflects carbon dioxide production by the tissues and the circulatory transport of carbon dioxide to the lungs.

When expired carbon dioxide is related to dead volume rather than time, the area under the curve represents the volume of carbon dioxide in respiration.

And therefore, over a minute, this method can produce the removal of carbon dioxide per minute, an essential measure of metabolism.

Sudden changes in carbon dioxide elimination during lung or heart surgery generally involve significant changes in cardiorespiratory function.

Capnography is more effective than clinical judgment alone in the early detection of adverse respiratory events, such as hypoventilation, esophageal intubation, and circuit disconnection; in this way, injury to the patient is avoided.

During procedures performed under sedation, capnography provides more helpful information, for example, on the frequency and regularity of ventilation, than pulse oximetry.

Capnography provides a rapid and reliable method to detect life-threatening conditions (poor position of the tracheal tubes, unsuspected ventilatory failure, circulatory failure, and faulty breathing circuits) and avoid potentially irreversible patient injury.

Capnography and pulse oximetry together could have helped prevent 93% of preventable anesthesia mishaps, according to a closed SAA (American Society of Anesthesiology) claims study.

In emergency medical services

Capnography is increasingly being used by emergency medical services personnel to assist in their evaluation and treatment of patients in the prehospital setting.

These uses include checking and monitoring the position of an endotracheal tube or an airway insertion device for blind insertion.

A properly placed tube in the trachea protects the patient’s airway and allows the paramedic to breathe for the patient. A misplaced line in the esophagus will lead to the patient’s death if it is not detected.

A March 2005 study in the Annals of Emergency Medicine compared field intubations using continuous capnography to confirm intubations versus no use.

It showed zero unrecognized misplaced intubations in the control group versus 23% misplaced tubes in the unmonitored group.

The American Heart Association affirmed the importance of using capnography to verify probe placement in its 2005 cardiopulmonary resuscitation and EKG guides.

The American Heart Association also points out in its new guidelines that capnography indirectly measures cardiac output.

It can also be used to monitor the effectiveness of CPR and as an early indication of the return of spontaneous circulation (ROSC).

Studies have shown that when a person performing CPR becomes tired, the patient’s final tidal carbon dioxide (ETCO2, the level of carbon dioxide released at the End of expiration) drops and increases when a new rescuer takes over.

Other studies have shown that when a patient experiences a return of spontaneous circulation, the first indication is usually a sudden increase in carbon dioxide from the end tide as the circulating current washes untransported carbon dioxide from the tissues.

Similarly, a sudden drop in carbon dioxide at the End of the tide may indicate that the patient has lost pulses, and it may be necessary to initiate cardiopulmonary resuscitation.

Paramedics are now also beginning to monitor the carbon dioxide status of the end tide in non-intubated patients by using a special nasal cannula that collects the carbon dioxide.

A high final carbon dioxide reading in a patient with altered mental status or severe respiratory distress may indicate hypoventilation and a possible need for intubation.

Low carbon dioxide readings in patients may indicate hyperventilation.

Capnography because it provides a breath-by-breath measurement of a patient’s ventilation.

You can quickly reveal a worsening trend in a patient’s condition by providing paramedics with an early warning system about a patient’s respiratory status.

Clinical studies are expected on the uses of capnography in asthma, congestive heart failure, diabetes, circulatory shock, pulmonary embolism, acidosis, and other conditions, with possible implications for the prehospital use of capnography.

Use by registered nurses

Registered nurses use capnography in critical care settings to determine if a nasogastric tube used for feeding has been placed in the esophagus instead of the trachea.

Typically, a patient will cough or choke if the tube is out of place, but most patients in critical care settings are sedated or in a coma.

If a nasogastric tube is accidentally placed into the trachea instead of the esophagus, the feeding from the box will enter the lungs, which is a life-threatening situation.

Cardiopulmonary resuscitation

The balance between carbon dioxide production, supply, and removal can be monitored by final capnography.

In the case of cardiopulmonary arrest, the cardiac output is reduced to zero, and therefore the transport of carbon dioxide from the tissues to the lungs cannot occur.

The end-tide capnometry of an artificially ventilated patient would show, after several wash breaths, a flat-tide program with zero-end tide capnography during the arrest.

Once chest compressions are started, the blood circulation will supply carbon dioxide back to the lungs, and the final tidal program will rise and fall with each breath as the carbon dioxide is vented.

End-expiratory carbon dioxide levels of 20 mm Hg or more indicate adequate chest compressions during cardiopulmonary resuscitation.

And failing to reach a level of at least 10 mm Hg after 20 minutes of cardiopulmonary resuscitation (CPR) can help decide to end resuscitation efforts.

Complications prevention

Continuous monitoring of tidal capnography can provide an early warning of impending hypoxemia.

Several studies have shown that respiratory depression is detected through end-tidal capnography 30-60 seconds before it is seen through oxygen saturation.

Airway assessment

Confirmation of endotracheal intubation is vital in airway management in the emergency department, whereas there is no definitive diagnostic tool to verify correct intubation in emergency rooms.

Recently, capnography was used as the gold standard to confirm the correct placement of the endotracheal tube.

The end-expiratory carbon dioxide colorimetric is a safe, reliable, simple, and portable tool for determining correct endotracheal tube placement in hemodynamically stable patients and is very useful when a capnograph is not available.

However, when patients are ventilated with a bag or mask or consume carbonated beverages or antacids, this can cause a false positive result but generally indicates the actual impact after six breaths.

Baking soda leads to a higher end-expiratory carbon dioxide level for 5 to 10 minutes.

During cardiac arrest, which leads to a decrease in pulmonary-tissue carbon dioxide transport, capnography can show correct and incorrect intubation (false negative).

Procedural sedation and analgesia

Capnography is an effective method for diagnosing early respiratory depression and airway disorders, especially during sedation, reducing severe complications.

Capnography provided more safety in monitoring patients during sedation.

The prescription of oxygen does not affect the respiratory function parameters evaluated by capnography.

It shows poor airway function earlier than any other device, 5 to 240 seconds earlier than pulse oximetry.

Capnography is more sensitive than clinical evaluation in diagnosing respiratory dysfunction; for example, in many cases where apnea was experienced during sedation, apnea was not recognized by clinicians, but capnography was able to identify it.

Obstructive lung disease

In obstructive airway diseases, hypoventilation can cause shortness of breath and hypercarbia.

There is a relationship between tidal carbon dioxide and partial arterial carbon dioxide (PaCO2) in patients with acute asthma in the emergency department.

Capnography is the dynamic monitoring of patients with acute respiratory conditions, such as asthma, chronic obstructive pulmonary disease, bronchiolitis, and heart failure.

Bronchospasm is associated with a prolonged expiratory phase (E1, E2, E3) on the program in patients with obstructive diseases such as chronic obstructive pulmonary disease.

Changes in end-expiratory carbon dioxide and expiratory phase slope were correlated with forced expiratory volume (E1, E2, E3) in 1 second (FEV1) and peak expiratory flow rate (PEFR). English).

An end-expiratory carbon dioxide is an indispensable tool for assessing the severity of obstructive respiratory disease in the emergency department.

End-expiratory carbon dioxide is higher in patients with an exacerbation of chronic obstructive pulmonary disease admitted to the hospital than those discharged from the emergency department.

Pulmonary embolism

In thromboembolism, end-expiratory carbon dioxide is significantly lower than usual due to reduced pulmonary perfusion and increased alveolar dead space, which reduces the amount of carbon dioxide exhaled from the lungs.

So the pressure of venous carbon dioxide (PvCO2) increases, and all these changes lead to an increase in the arterial end-expiratory carbon dioxide gradient.

This helps to diagnose pulmonary embolism, incredibly silent pulmonary embolism, correctly. Volumetric capnography is used to monitor thrombolysis in sizeable pulmonary embolisms.

The mean value of carbon dioxide of the final tide and the decrease in the partial pressure of carbon dioxide / partial pressure of oxygen during 30 seconds correlate with the clinical probability or rule out pulmonary embolism.

Heart failure

The rapid differentiation of heart failure as the cause of dyspnea from other respiratory causes is significant in choosing an appropriate therapy.

Distinguishing chronic obstructive pulmonary disease/asthma exacerbation and acute heart failure is sometimes very difficult, primarily when the two exist together, and treatment decisions in this situation are very complex.

End-expiratory carbon dioxide in patients with cardiac causes is very different from patients with respiratory distress due to obstructive reasons.

An end-expiratory carbon dioxide level> 37 mmHg was not observed in any patient with heart failure. However, an end-expiratory carbon dioxide level> 37 mmHg is slightly sensitive to diagnosing chronic obstructive pulmonary disease/asthma.

The end-expiratory carbon dioxide level during cardiopulmonary exercise tests in patients with heart failure has a high predictive value for cardiac events.

The N-terminal pro-brain natriuretic peptide on the quantitative capnography side is very useful in diagnosing and treating patients with acute dyspnea (respiratory or cardiac causes) in the emergency department.

The widespread use of quantitative capnography can be beneficial in the daily work of emergency physicians.


Hypotensive shock is a clinical feature of many diseases and is associated with a high mortality rate in the emergency department.

Emergency physicians continually strive to find new ways to diagnose shock early to begin treatment as soon as possible.

Capnography is considered a simple and non-invasive method to detect and estimate shock intensity in the early stage.

End-expiratory carbon dioxide is known to decrease in volume-related hypotensive states.

End-tidal carbon dioxide correlates with blood pressure, serum lactate, and base excess.

In early-stage shock linked to reduced cardiac output, the amount of carbon dioxide from the late tide decreases significantly.

This is due to decreased blood flow in the pulmonary artery during reduced cardiac output, disrupting the perfusion rate of ventilation.

With increasing bypass, the final tidal carbon dioxide level decreases, while the partial pressure of oxygen does not change.

With the decrease in blood pressure, the carbon dioxide drops from the end tide, and the partial pressure of carbon dioxide from the end tide of oxygen increases.

There is a correlation between dehydration and the amount of sodium bicarbonate and carbon dioxide from the end tide. The carbon dioxide from the end tide can be used as a simple and non-invasive indicator to determine dehydration.

Metabolic disorder

Carbon dioxide (CO2) is one of the end products of metabolism. It is transferred to the lungs through the bloodstream and transmitted through the respiratory system, whereby the exhalation of carbon dioxide reflects the body’s metabolic state.

End-tidal carbon dioxide is a quick, inexpensive, and non-invasive indicator for estimating the amount of bicarbonate (HCO3) and partial pressure of oxygen in critical and emergencies.

Due to the direct connection between end-tidal carbon dioxide and bicarbonate, end-tidal carbon dioxide is a predictor of metabolic acidosis and mortality. Thus the capnograph as a metabolic acidosis screening tool is very useful in the emergency department.

Tidal carbon dioxide can be recommended as a non-invasive method for determining metabolic acidosis and can be used to detect early metabolic acidosis in spontaneously breathing patients.

However, arterial blood gas should be the gold standard for diagnosis and treatment management.

Diabetic Keto Acidosis (DKA)

Patients with diabetes mellitus are at increased risk of major and disabling complications, one of the most important being diabetic ketoacidosis.

The direct linear relationship between tidal carbon dioxide and bicarbonate helps predict acidosis.

It was shown that there is no diagnosis of diabetic ketoacidosis when the end-expiratory carbon dioxide is 36, and there is a diagnosis of diabetic ketoacidosis when the end-expiratory carbon dioxide l ≤ 29.

Carbon dioxide from the 30 to 35 end tide is considered the cut-off point, making it clinically valuable for diagnosing acidosis.

Furthermore, low partial pressure of the oxygen level correlates with an increased risk of brain edema in children with diabetic ketoacidosis.

Therefore, capnography can be used to identify individuals at high risk for cerebral edema based on the relationship between end-tidal carbon dioxide and oxygen partial pressure.

When the patient’s glucose is above 550 mg/dl, end-tidal carbon dioxide is a valuable tool to rule out diabetic ketoacidosis.


Among children with diarrhea and vomiting, tidal carbon dioxide independently correlates with serum bicarbonate concentration.

This is a non-invasive index to measure the severity of acidosis in patients with gastroenteritis.

End-tidal carbon dioxide can be used to estimate bicarbonate in many emergency situations.


Tidal carbon dioxide cannot be used to rule out serious injury in patients who meet the criteria for trauma care.

End-tidal carbon dioxide ≤30 mmHg may be associated with an increased risk of severe traumatic injury.

There is an inverse relationship between prehospital tidal carbon dioxide and traumatic mortality rates.

Therefore, carbon dioxide from the end tide can enhance triage and assist emergency medical service personnel in planning the transfer of patients to the appropriate trauma center.

Low tide carbon dioxide strongly correlates with shock in trauma patients and suggests the severity of the patient’s condition in the first 6 hours of admission.


End-tidal carbon dioxide is used in the emergency department as an indicator for measurement in many clinical situations.

Capnography is a non-invasive and accurate method of measuring end-tidal carbon dioxide and can help emergency physicians in some critical situations.

While this is not used in many emergencies and is not routinely used in the emergency department.

Its application increases in many emergencies, such as patients undergoing mechanical ventilation, procedural sedation and analgesia, lung disease, heart failure, shock, metabolic disorder, and trauma.

This means that capnography should be considered an essential tool in the emergency department; however, more research is needed to evaluate its application in specific diseases and clinical conditions.