Capnometry in the intensive care unit (literature review)

Abstract

Capnometry/capnography is a method of measuring and displaying the concentration of carbon dioxide (CO2) in respiratory gases. Most often, this term involves measuring the partial pressure of carbon dioxide (PCO2) at the end of exhalation (end-tidal CO2, ETCO2). The curve formed during capnography is called a capnogram, where the inspiratory and expiratory segments are distinguished. The main determinants of ETCO2 are CO2 production, cardiac output, pulmonary perfusion, and alveolar ventilation. Normally, ETCO2 is 2–5 mm Hg lower than arterial blood PCO2. This gradient increases when the ventilation-perfusion ratio is impaired, for example, in pulmonary embolism or pulmonary hypoperfusion during cardiac arrest. The shape of the capnogram, in turn, depends on the concentration of CO2 in the alveoli and the pattern of their emptying, as well as on cardiac output. This allows the use of capnography to register bronchospasm, CO2 circulation in the circuit, spontaneous breathing attempts, etc. In Ukraine, capnometry and capnography are routinely used in the anesthesia during laparoscopic surgeries, which require strict control of ETCO2 due to the insufflation of CO2 into the abdominal cavity and its absorption by the peritoneum. However, the spectrum of capnometry application is actually much wider than just laparoscopic surgery. In the intensive care unit, it includes but is not limited to the following indications: confirmation of the endotracheal tube position during tracheal intubation; monitoring the respiratory circuit integrity, including while turning and transporting a patient; evaluation of the cardiopulmonary resuscitation effectiveness; evaluation of the mask ventilation effectiveness; breathing monitoring during procedural sedation; monitoring during mechanical ventilation; auxiliary monitoring of hemodynamics. This literature review discusses the advantages, disadvantages and methods of using capnometry/capnography for each of the listed indications in detail.