Capnography: CO₂ Measurement Values, Curves and Trends

Capnography: Definition
Capnography describes the continuous measurement of end-tidal CO₂ (etCO₂) in exhaled gas; WEINMANN ventilators perform this using infrared spectroscopy and the sidestream method. CO₂ concentration in exhaled air is the most important parameter when monitoring ventilation treatment. This makes it possible to rule out reliably an incorrectly-positioned tracheal tube and to monitor the success of ventilation.
The capnography option from WEINMANN is available in the MEDUMAT Standard² ventilator.
Advantages of using capnography
- More extensive monitoring of CO₂ measured values
- Improved diagnosis of lung dysfunction - as in the case of asthma, for example
- Reliable monitoring of tracheal tube position following intubation
- Support for determining ROSC during resuscitation
- Continuous measurement and display of the etCO₂ value
What is etCO₂?

End-tidal carbon dioxide (etCO₂) is the component of exhaled air determined by capnography. etCO₂ concentration is measured at the end of expiration when the respiratory gas is no longer mixed with CO₂-free dead space volume.
End-tidal CO₂ concentration depends both on production of CO₂ and on elimination of CO₂. The latter is affected by lung ventilation and by perfusion, i.e. the passage of blood through the heart and lungs. The settings on the ventilator, such as frequency and tidal volume, also play a key role in ventilation.
How does etCO₂ correlate with paCO₂?
The partial pressure of carbon dioxide (paCO₂) is determined by blood gas analysis in arterial blood. It indicates the proportion of carbon dioxide pressure in relation to total gas pressure within the respiratory air. The normal range for the end-expiratory partial pressure of CO₂ is between 33 mmHg and 43 mmHg. This corresponds to an etCO₂ concentration of between 4.3% by volume and 5.7% by volume.
The hyperbolic CO₂ binding curve reflects the dependency of etCO₂ concentration on partial pressure of CO₂: The higher the partial pressure of CO₂ in the blood, the higher the etCO₂ concentration is too.
How is etCO₂ obtained?
During CO₂ measurement, sample gas is passed through an optical measuring section. It is then filtered for different wavelengths. The results are converted with the aid of microprocessors and shown in the display in the form of numbers and curves.
Capnometry/capnography can measure the CO₂ content of expiration air in two ways.
1. Mainstream method
In the mainstream method, a measuring cuvette between the endotracheal tube and the Y-piece measures the absorption of infrared light.
Advantages:
There is no time delay in measurement and measurement encompasses the entire volume of air.
Disadvantages:
The additional measuring cuvette between the patient valve and the tracheal tube results in an increased dead space volume when the mainstream method is used. This is particularly relevant when ventilating young children or infants.
What is more, the measuring cuvette always has to be heated to 39 °C to stop condensation interfering with the measurement and thus to prevent an invalid measurement.
The mainstream method furthermore involves an increased risk of disconnection due to the additional measuring cuvette.
2. Sidestream method
In the sidestream method, a small amount of air is continuously extracted and passed to the detector where the measurement is taken.
Advantages:
The sidestream method can also be used on non-intubated patients via a nasal cannula with a CO₂ extraction line. In addition, the sidestream method offers the major advantage that no additional measuring cuvette is required. The dead space volume of the breathing circuit is thus significantly reduced and the patient end of the ventilation hose is also lighter in weight.
Disadvantages:
etCO₂ measurement is slightly delayed in the sidestream method, making it somewhat slower than the mainstream measurement method.
Indications: Areas of application for CO₂ measurement
Capnography is used for monitoring in anesthesia and intensive care medicine, as well as in EMS and intensive care transport operations. CO₂ measurement is specifically recommended in the following cases:
- For monitoring and controlling ventilation
- For improved diagnosis of lung dysfunction - as in the case of asthma or COPD, for example
- The Deutsche Gesellschaft für Anästhesiologie und Intensivmedizin (DGAI - German Society for Anesthesiology and Intensive Care Medicine) recommends capnography in its guidelines for checking tracheal tube position.2
- The guidelines of the European Resuscitation Council (ERC) recommend capnography during resuscitation for monitoring chest compressions and detecting ROSC.3
- The Deutscher Rat für Wiederbelebung [German Council for Resuscitation] recommends CO₂ measurement for checking tracheal tube position.4
Hypocapnia: low partial pressure of carbon dioxide

Hypocapnia describes the condition in which the partial pressure of carbon dioxide in arterial blood falls below the normal range. There are various causes of hypocapnia.
- Absolute hyperventilation: Increased exhalation of CO₂ occurs.
- Low body temperature: Respiratory rate and depth fall, metabolic processes slow down. This leads to reduced production of CO₂.
- Shock: The body begins centralization, constricting peripheral blood vessels so that less blood circulates. Decreased blood flow to the extremities results in the tissues producing less carbon dioxide.
As CO₂ concentration affects acid/alkaline balance, hyperventilation can lead to the pH in arterial blood rising.2 As carbon dioxide is acid, increased exhalation of CO₂ may result in respiratory alkalosis; this may manifest itself in symptoms such as confusion, dizziness, and cramps.3
Hypercapnia: Elevated partial pressure of carbon dioxide

In hypercapnia, there is an increased level of CO₂ in the blood. The most common cause of this is hypoventilation.
In hypoventilation, shallow breathing may not provide adequate CO₂ for gas exchange in the lungs. This means not enough CO₂ will be removed from the body, so it will accumulate in the blood, leading to an elevated partial pressure of carbon dioxide.
Reduced exhalation of CO₂ may result in respiratory acidosis which in turn may lead to headaches, anxiety, dizziness, and confusion.4 In addition, if respiratory failure persists, the patient may develop CO₂ narcosis, leading to a depressed level of consciousness and in extreme cases, to loss of consciousness.5
CO₂ measurement in ventilation
Discharge of CO₂ should be regulated during ventilation in such a way that the pH of the blood remains within the normal range or returns to the target range. paCO₂ value is indirectly proportional to alveolar ventilation (VTalv/min) that describes the volume actually reaching the alveoli. This volume results from the difference between tidal volume (Vt) and dead space ventilation.
An increase in minute volume accordingly leads to a drop in the partial pressure of CO₂ and vice versa. The minute volume supplied should therefore be increased in patients with elevated etCO₂ values.
This can be effected by increasing ventilation frequency, modifying the volume in volume-controlled ventilation or raising the upper pressure limit in pressure-controlled ventilation. In this case, a higher top limit leads to the lungs being inflated more, allowing more volume to flow in.
If excessively low etCO₂ values are found during capnography, patients should be ventilated less. To achieve this, ventilation frequency can be reduced, the upper pressure level reduced in pressure-controlled ventilation or the volume reduced in volume-controlled ventilation.
Individual modifications must be coordinated with the patient’s condition as well as their respiratory system compliance and airway resistance. Modifications should be made in small increments due to the time taken for etCO₂ values to react.1,6
Analyzing CO₂ measured values in ventilation
CO2 measurement delivers valuable information for assessing ventilation and allows airway management to be monitored accurately.
A sudden drop in end-tidal carbon dioxide to almost zero may indicate disconnection of the ventilation system, dislocation of the tracheal tube or complete obstruction of the tube by the accumulation of secretions. In the event of such changes, check immediately that the tracheal tube is positioned correctly and that the airway is not completely blocked. These capnography results might also reflect accidental extubation, intubation of the esophagus or a ventilator malfunction.
A drop to low etCO₂ values above zero may indicate various problems including leaks in the ventilation system, partial misplacing of the tracheal tube or partial occlusion of the airway. A tracheal tube in the hypopharynx may also trigger this development.
An exponential fall in etCO₂ value during ventilation may be caused by a sudden drop in blood pressure or a cardiopulmonary bypass. In more serious cases, however, this change may indicate more severe conditions such as pulmonary embolism, significant blood loss or cardiac arrest, all requiring immediate medical intervention.
Consistently low etCO₂ values during ventilation may indicate hyperventilation due to excessively high minute volume or low body temperature. Conversely, consistently elevated values may be caused by respiratory depression due to medication, metabolic alkalosis or inadequate minute ventilation.
Comprehensive monitoring using capnography allows accurate analysis of the etCO₂ parameter, supports the adaptation of ventilation therapy and contributes to optimizing patient care.7
CO₂ measurement during resuscitation
CO2 measurement is playing an increasingly important role in resuscitation: On the one hand, CO₂ measurement can deliver important information that may have a key impact on resuscitation care. On the other hand, the results of CO2 measurement must always be examined critically, because chest compressions (typically 100-120/min) cause high-frequency air movements in the vicinity of the opening of the patient connection of the ventilation hose; these movements may lead to false low measured values being recorded.
However, the measured value is frequently sufficiently meaningful to allow important conclusions to be drawn:
- Has the patient been intubated correctly? If so, this should be detectable from a rise in CO2 measured value after a few mechanical breaths.
- Is the quality of chest compressions sufficient? As gas exchange in the lungs is significantly affected by the depth and frequency of compressions, the measured value for CO2 during resuscitation is an important indicator of the effectiveness of those chest compressions. In addition, the value measured for CO2 allows you to determine whether the patient is being ventilated at the correct frequency – avoiding episodes of hyperventilation.
- Has the patient returned to spontaneous circulation? As soon as the heart starts pumping again independently, this is immediately apparent from a sharp rise in the measured value for CO2. This is ideally seen in the CO2 curve.
- If the CO2 measured value is very low for an extended period, the assessment should include whether resuscitation efforts are still delivering the desired patient benefit. In this case, it is very important to understand that CO2 measurement is just one of a number of parameters to be considered.
Capnography at WEINMANN
WEINMANN uses the sidestream method that extracts a gas sample via the ventilation hose and analyzes it.
WEINMANN’s MEDUMAT Standard² ventilator not only displays CO₂ value in numerical form, it also displays curves (capnography) and trends for monitoring ventilation.
Choose between a MEDUMAT Standard² with or without capnography. If you select the former, you need the appropriate breathing circuit to allow CO₂ measurement to be performed during ventilation.
This means you can monitor ventilation treatment more effectively and receive additional support for CPR and RSI.
Independently of ongoing ventilation, you can also perform capnography with MEDUMAT Standard² during oxygen inhalation to check the level of carbon dioxide in the respiratory gas.
1https://www.thieme-connect.de/products/ebooks/lookinside/10.1055/b-0034-20966
3https://cprguidelines.eu/assets/posters/Leitlinien-kompakt_08.11.2021.pdf
4 https://leitlinien.dgk.org/files/2022_kommentar_kardiopulmonale_reanimation_druck.pdf
7 https://www.draeger.com/Content/Documents/Products/co2-measurement-bk-gesamt-9097449-de.pdf