Capnography: CO₂ Measurement Values, Curves and Trends

The capnography in use

Capnography: Definition

Capnography describes the continuous measurement of the end-tidal CO2 (etCO2) in exhaled gas, which WEINMANN performs using infrared spectroscopy and the sidestream method. The CO₂ concentration in the exhaled air is the most important parameter when monitoring ventilation treatment. This makes it possible to more reliably rule out tube malposition, and to monitor the success of ventilation therapy.

The capnography option is available for the WEINMANN MEDUMAT Standard² ventilator.

Advantages of using capnography

  • More extensive ventilation monitoring
  • Improved diagnosis of lung dysfunction, such as asthma
  • Reliable monitoring of the tube position after intubation
  • Support for determining an ROSC during resuscitation
  • Continuous measurement and display of the etCO₂ value

What is #etCO₂#?

Curve chart with CO2 curve

End-tidal carbon dioxide (etCO₂) is the component of exhaled air measured by capnography. The etCO₂ concentration is measured at the end of expiration, when the respiratory gas is no longer mixed with CO₂-free dead space volume.

How does etCO₂ correlate with paCO₂?

The partial pressure of carbon dioxide (paCO₂) is determined in arterial blood by blood gas analysis. It indicates the proportion of carbon dioxide in the total pressure within the respiratory air. The normal range for end-expiratory CO₂ partial pressure is between 33 mmHg and 43 mmHg. This corresponds to an etCO₂ concentration of between 4.3 and 5.7% by volume.

The hyperbolic CO₂ binding curve reflects the dependence of the etCO₂ concentration on the CO₂ partial pressure: the higher the CO₂ partial pressure in the blood, the higher the etCO₂ concentration.

How is etCO₂ obtained?

During the CO₂ measurement, sample gas is passed through an optical measurement section. It is then filtered for different wavelengths. The results are converted with the help of microprocessors, and shown numerically and graphically on the display.

The CO₂ content of expired air can be measured in two ways in the course of capnometry/capnography:

1. Mainstream method

In the mainstream method, a measuring cuvette between the endotracheal tube and the Y-piece measures the absorption of infrared light.


There is no time delay in measurement, and the measurement encompasses the entire volume of air.


The additional measuring cuvette between the patient valve and the tube results in an increased dead space volume when applying the mainstream method. This is particularly relevant when ventilating infants or young children.

Moreover, the measuring cuvette must be constantly heated to 39 °C in order to prevent the measurement from being affected by condensation, and so avoid incorrect measurements.

The mainstream method also poses an increased risk of disconnection due to the additional measuring cuvette.

2. Sidestream method

In the sidestream method, a small amount of air is permanently extracted and directed to the detector, where the measurement is taken.


The sidestream method can also be applied to non-intubated patients via a nasal cannula with a CO₂ suction line. In addition, the sidestream method offers the major advantage that no additional measuring cuvette is required. Consequently, the dead space volume of the breathing circuit is significantly reduced, and the ventilation hose is also lighter in weight at the patient end.


In the sidestream method, etCO₂ measurement is slightly delayed, meaning it is 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 transports. CO₂ measurement is specifically recommended in the following cases:

  • For monitoring and control of ventilation
  • For improved diagnosis of lung dysfunction, such as asthma or COPD
  • For tube position control1
  • For checking the position of the endotracheal tube and the quality of CPR during advanced resuscitation2
  • For resuscitation to monitor HDM and detect ROSC3

Hypocapnia: low partial pressure of carbon dioxide

CO2 curve on curve chart

Hypocapnia describes the condition in which the partial pressure of carbon dioxide in the arterial blood falls below the normal range. There are various causes of hypocapnia:

  • Absolute hyperventilation: Increased CO₂ exhalation occurs.
  • Low body temperature: Respiratory rate and depth decrease, and metabolic processes slow down. This leads to reduced production of CO₂.
  • Shock: The body begins centralization, in which peripheral blood vessels are constricted and less blood circulates. Decreased blood flow to the extremities results in less carbon dioxide production in the tissues.

Hypercapnia: increased partial pressure of carbon dioxide

Curve chart showing CO2 curve

In hypercapnia, there is an increased etCO₂ content in the blood. The most common cause of this is hypoventilation.

In hypoventilation, not enough CO₂ can be exhaled because breathing is too shallow or the airways are obstructed (such as in cases of COPD). This results in an increased partial pressure of carbon dioxide in the blood, with simultaneously lower oxygen uptake.

Capnography from WEINMANN

Capnography with MEDUMAT Transport

WEINMANN uses the sidestream method, which extracts a gas sample via the ventilation hose and evaluates it.

WEINMANN’s MEDUMAT Standard² ventilator not only shows the CO₂ value numerically but also displays curves (capnography) and trends.

MEDUMAT Standard²

MEDUMAT Standard² features capnography using the sidestream method. Choose between a MEDUMAT Standard² with or without capnography. Therefore you need the appropriate breathing circuit, which enables the CO₂ measurement to be performed during ventilation.

If capnography is integrated in the MEDUMAT Standard², the end-tidal CO₂ level will be displayed as a measured value, a curve, and a trend over a longer period. This allows you to better monitor ventilation therapy, and provides further support for CPR and RSI modes. In addition, you can carry out capnography with MEDUMAT Standard² independently of ongoing ventilation during oxygen inhalation to check the carbon dioxide content in the respiratory gas.