Capnography

Capnography (Greek καπνός Kapnos "smoke" and μέτρον metron " measure " ) is a medical procedure to measure the content of the exhaled air of carbon dioxide (CO2 ) of a patient and monitor. Devices that provide only the pure numeric values ​​, hot Capnometer. Capnographs also provide the associated curve represents the introduction of capnography has performed together with pulse oximetry (measurement of oxygen saturation in the blood) to a significant reduction of complications in mechanically ventilated patients.

Areas of application

Capnometer or capnography are an integral part of the medical surveillance (monitoring), especially of anesthesia equipment in anesthesia, where they measure the carbon dioxide content of the expired air. Anesthesia devices are constructed as parts of the anesthesia circuit, in which the ventilation air is passed after exiting the patient through a Kohlenstoffdioxidabsorber and only actually spent gases are again added. This gas recycling reduces the use of anesthetic gases significantly, which is a contribution to the environment and also saves costs. However, monitoring of the carbon dioxide in the air during inhalation is important here. In addition, anesthesia machines in the concentrations of other gases such as oxygen, nitrous oxide and the gaseous anesthetic can be monitored.

Capnometer are also used in the monitoring of ventilated patients in intensive care units and in the emergency service, there, this technology sets but only by relatively slow. Often the devices in the ventilator, medical monitor or defibrillator are built in, but there are also smaller, portable equipment specially designed for monitoring during transport. In intensive care transports and rescue service capnometry is considered in ventilated patients as a standard.

Modern ultrasound-based spirometer are also to determine the course of CO2 in the situation. The characteristic shape of expiratory CO2 concentration allows conclusions on lung structure and distribution problems. For example, can be detected at an early stage emphysema hereby.

In the long term use of oxygen in the therapy as well as in sleep medicine capnometry has its importance.

Measurement methods

Depending on the measurement technique is different from the main stream - by- stream or even side-stream processing. The measurement is based in most equipment on the principle of infra-red spectroscopy, which was described in 1943 by air.

• The main current method, a sample cell is placed in the tube system through which the infrared light absorption is continuously detected. Advantages are that the total amount of air is measured and that no volume loss. Systemic disadvantage is the sample cell to the detector, which increases the weight of the tubing close to the endotracheal tube, and so represents an increased Extubationsgefahr. In addition, the sample cell must be heated to prevent condensation disturbs the measurement. To avoid this error, the sample cell between HME filter and ventilator can be attached so that it stays dry.

• The side-stream process, a small amount of air is continuously drawn off and passed through a thin tube to the detector, where the measurement is carried out then. The advantage is that it allows the weight close to the patient is minimal. Disadvantages are that the measurement is made is delayed (up to the air is drawn for evaluation ) and measurement of the tidal volume to be distorted, if the air is not supplied, then the system again.

Statements of the measured value

The normal range for the end-tidal carbon dioxide partial pressure is 33 to 43 mm Hg or if the measured value is specified as a concentration at 4.3 to 5.7 vol - %. The conversion between two units depends on the ambient air pressure that the device therefore also measures. The adjustment in patients transports approximately in rescue helicopters and ambulance aircraft is done automatically. As a rule of thumb, that the statement in mmHg is about seven times the indication in % by volume. Decisive is the proportion of carbon dioxide at the end of exhalation when the breathing gas is not mixed with CO2-free dead space. ( end-tidal CO2 or ETCO2 ) This value corresponds to the healthy lungs approximately, with a small difference of about 3-5 mmHg, arterial carbon dioxide partial pressure ( PaCO2 ), as determined in a blood gas analysis. This difference can be explained by an admixture of venous blood in the arterialized. It increases as the exhalation of carbon dioxide is complicated by a disorder of the lungs or if there is a decreased blood flow to the lung, as is the case with a pulmonary embolism. Based on the carbon dioxide concentration of the air exhaled by the ventilator can therefore relatively well adapted to the patient. In addition, blood gas analysis should be performed, since it also the oxygen partial pressure, the pH of the blood and other metabolic parameters determine the next Kohlenstoffdioxidpartialdruck.

Also, early on, whether an endotracheal tube or a tracheal cannula is properly installed or is diskonnektiert if leaks are present in the hose system and whether the respiratory minute volume corresponds to the needs of the patient. The metabolic status of the patient can be assessed. The last point allows early intervention with certain complications such as malignant hyperthermia or a guide value for the effectiveness of resuscitation. The body temperature, or the depth of anesthesia have an influence on the measured value. In mechanically ventilated patients with increased intracranial pressure, the capnography is a useful addition to monitoring because of the cerebral circulation and thus the intracranial pressure with the paCO2 correlate. The implementation of a moderate hyperventilation treatment is made easier with it.