capnography description and normal capnography use:-
- The amount of carbon dioxide in the exhaled gases can be measured using a capnograph.
- The capnography passes infrared light through the gases to be analyzed.
- The carbon dioxide in the gases absorbs the infrared light in proportion to its concentration.
- This absorption is measured and the carbon dioxide concentration is displayed according to a nomogram.
- When the carbon dioxide concentration is displayed against time, a capnogram is obtained.
- The normal capnogram has four different phases.
- The phases I, II, and III represent expiration and phase IV, inspiration.
- The carbon dioxide concentration at the end of expiration is called the end-tidal carbon dioxide concentration (ETCO₂, normal range 35-40 mm Hg) which represents the alveolar carbon dioxide concentration.
- Normally, the arterial CO₂ concentration is nearly equal to this end-tidal CO₂ concentration.
Carbon dioxide is excreted from the body only through one organ in the body, i.e. lungs.
A normal capnogram obtained from the exhaled gases helps identify the correct position of the endotracheal tube.
If the capnograph registers zero levels of carbon dioxide, the endotracheal tube is malpositioned as in oesophageal intubation or accidental extubation. Artificial ventilator disconnection also results in zero ETCO₂ levels.
The ETCO₂ levels may also reduce to near-zero levels if no carbon dioxide returns to the lungs from the tissues as in profound hypotension and an impending cardiac arrest may be suspected.
However, the ETCO₂ will register zero if the patient is in cardiac arrest since there is no circulation and no CO₂ is brought to the lungs. In such situations, a capnograph cannot be used to identify the correct position of the endotracheal tube.
However, a capnograph is extremely useful in assessing the effectiveness of CPR. If the ETCO₂ is > 15 mm Hg during CPR, the chances of successful resuscitation are higher.
When there is an increase in dead space ventilation the arterial to end-tidal CO₂ concentration gradient increases (normal: 0–5 mm Hg).
What is the normal range for ETCO2?