Capnometry

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Capnography curve (capnogram)

Capnometry ( Greek καπνός kapnos "smoke" and μέτρον metron "measure") is a medical method to measure and monitor the content of carbon dioxide (CO 2 ) in the exhaled air of a patient . Devices that only provide pure numerical values ​​are called capnometers . Capnographs also show the associated curve. The introduction of capnometry, together with pulse oximetry (measurement of oxygen saturation in the blood), has led to a significant reduction in complications in ventilated patients.

Areas of application

Main flow measurement: measuring cuvette and measuring probe for capnometry, separated

Capnometers or capnographs are an integral part of medical monitoring , especially of anesthesia machines in anesthesia , where they measure the carbon dioxide content of the exhaled air. Anesthesia machines are designed as anesthesia circuit parts in which the ventilation air is passed through a carbon dioxide absorber after leaving the patient and only gases that are actually used are added again. This gas recycling reduces the consumption of anesthetic gases considerably, which is a contribution to environmental protection and also saves costs. However, monitoring the amount of carbon dioxide in the air during inhalation is important here. In addition, the concentrations of other gases such as oxygen , nitrous oxide and gaseous anesthetics are monitored in anesthesia machines .

Capnometers are also used to monitor ventilated patients in intensive care units and in rescue services . The provision of a measuring device for the end-tidal carbon dioxide (etCO 2 ) on all rescue vehicles is mandatory here (DIN EN 1789: 2007). The devices are often permanently installed in the ventilator , medical monitor or defibrillator , but there are also smaller, portable devices specially designed for monitoring during transport. In intensive transport and rescue services, the capnography in ventilated patients is considered standard.

Modern ultrasound-based spirometers are also able to determine the course of the CO 2 . The characteristic shape of the expiratory CO 2 concentration allows conclusions to be drawn about the lung structure and distribution disorders. For example, pulmonary emphysema can be detected at an early stage .

Capnometry is also important in the long-term use of oxygen in therapy and in sleep medicine.

Measurement method

Main current measurement: measuring cuvette and measuring probe, plugged together

Depending on the measurement technology, a distinction is made between the main flow, the bypass or the side flow method. In most devices, the measurement is based on the principle of infrared spectroscopy , which was described by Luft in 1943 .

  • With the main flow method, a measuring cuvette is inserted into the hose system, through which the infrared light absorption is continuously determined. The advantages are that the entire air volume is measured and that no volume losses occur. The system-related disadvantage is the measuring cuvette with the detector, which increases the weight of the tube system close to the endotracheal tube and thus represents an increased risk of extubation. In addition, the measuring cuvette must be heated so that no condensation water interferes with the measurement. To avoid this interference, the measuring cuvette can be placed between the HME filter and the ventilator so that it remains dry.
  • With the sidestream method , a small amount of air is constantly sucked out and fed through a thin hose to the detector, where the measurement is then carried out. The advantage is that the weight close to the patient remains low. Disadvantages are that the measurement is delayed (until the air is sucked to the evaluation unit) and measurements of the tidal volume are falsified if the air is not subsequently fed back into the system.

Statements of the measured value

Capnographic display on an anesthesia monitor

The capnometry measures the percentage or the partial pressure of carbon dioxide. The normal range for the end-expiratory carbon dioxide partial pressure is 33 to 43 mmHg or, if the measured value is given as a concentration, 4.3 to 5.7% by volume. The conversion between the two units depends on the ambient air pressure, which the device therefore also measures. The adjustment for patient transport, for example in rescue helicopters or ambulance aircraft , then happens automatically. As a rule of thumb, the specification in mmHg is approximately seven times the specification in% by volume. The decisive factor is the proportion of carbon dioxide at the end of exhalation, when the breathing gas is no longer mixed with CO 2 -free dead space . (end-tidal CO 2 or etCO 2 ) This measured value corresponds approximately to the arterial carbon dioxide partial pressure (p a CO 2 ), as determined in a blood gas analysis , with a small difference of about 3–5 mmHg in the case of healthy lungs . This difference is explained by an admixture of venous blood with arterialized blood. It increases if the exhalation of carbon dioxide is made more difficult by a disease of the lungs or if there is insufficient blood flow to the lungs, as is the case with a pulmonary embolism , for example . Based on the carbon dioxide concentration of the exhaled air, since with constant carbon dioxide production, the partial pressure or concentration of carbon dioxide (in arterial blood and in the exhaled air) is inversely proportional to the alveolar ventilation , ventilation can therefore be adapted relatively well to the patient. In addition, blood gas analyzes should be carried out because, in addition to the carbon dioxide partial pressure, they also determine the oxygen partial pressure, the pH value of the blood and other metabolic parameters.

It is also possible to identify at an early stage whether an endotracheal tube or a tracheal cannula is correctly positioned, misplaced or disconnected, whether there are leaks in the hose system and whether the minute ventilation corresponds to the patient's needs. The metabolic status of the patient can also be assessed. The last point enables early intervention in the event of certain complications such as malignant hyperthermia or gives an indication of the effectiveness of resuscitation . The body temperature or the depth of anesthesia have an influence on the measured value. In ventilated patients with increased intracranial pressure , capnometry is a useful addition to monitoring, since the cerebral blood flow and thus the intracranial pressure correlate with the p a CO 2 . This makes it easier to carry out moderate hyperventilation treatment.

See also

Web links

Commons : Capnometry  - collection of images, videos and audio files
  • Edgar Voigt, Jens Pelikan: CO 2 measurement in ventilation. Drägerwerk AG & Co. KGa, Lübeck 2015. ( PDF file; 1.01 MB ).

literature

  • JE Thompson, MB Jaffe: Capnographic waveforms in the mechanically ventilated patient. In: Respir Care. 50 (1), Jan 2005, pp. 100-108. Review. PMID 15636648 ( pdf, 230 kB )

Individual evidence

  1. ↑ JH Tinker, DL Dull, RA Caplan, RJ Ward, FW Cheney: Role of monitoring devices in prevention of anesthetic mishaps: a closed claims analysis. In: Anesthesiology. 71 (4), Oct 1989, pp. 541-546. PMID 2508510
  2. ↑ S3- guideline for multiple trauma / treatment of seriously injured persons of the DGU . In: AWMF online (as of 07/2011)
  3. ↑ Andrea C. Imhof: Ultrasound spirometry / capnography as a method to investigate interdisciplinary differences in lung function in jumping, military and distance horses . University thesis. Berlin 2001.
  4. ↑ D. Bösch, C.-P. Criée: pulmonary function test . Springer Medizin Verlag, 2009, ISBN 978-3-540-88038-7 .
  5. ↑ a b c Lothar Ullrich, Dietmar Stolecki, Matthias Grünewald: Thiemes intensive care and anesthesia with DVD . Georg Thieme Verlag, 2006, ISBN 3-13-130910-5 , p. 92 f .
  6. ↑ a b c d Wolfgang Oczenski: Breathing and breathing aids: breathing physiology and ventilation technology . Georg Thieme Verlag, 2008, ISBN 978-3-13-137698-5 , p. 371 ff .
  7. ↑ KF Luft: About a method of registering gas analysis with the help of the absorption of ultrared rays without spectral decomposition. In: Z. techn. Physics. 24, 1943, pp. 97ff.
  8. ↑ Thomas Ziegenfuß: Emergency Medicine . Springer, Heidelberg 2007, ISBN 978-3-540-48633-6 , pp. 36 ff .
  9. ↑ Thomas Pasch, S. Krayer, HR Brunner: Definition and parameters of acute respiratory insufficiency: ventilation, gas exchange, respiratory mechanics. In: J. Kilian, H. Benzer, FW Ahnefeld (ed.): Basic principles of ventilation. Springer, Berlin a. a. 1991, ISBN 3-540-53078-9 . (2nd, unchanged edition, ibid. 1994, ISBN 3-540-57904-4 , pp. 93–108; here: pp. 95–97)
  10. ↑ Michael Fresenius, Michael Heck: Repetitorium Intensivmedizin . Springer, Heidelberg 2006, ISBN 3-540-72279-3 , pp. 23 f .