Capnography monitor is probably the most useful monitor in contemporary anesthesia practice. Capnography provides information on pulmonary function (eg, dead space, airway obstruction), gas exchange (eg, ventilation-perfusion mismatch), metabolism (eg, hypermetabolic versus hypometabolic states), and cardiovascular function (eg, reduced response to myocardial dysfunction, pulmonary embolism or a left-to-right shunt). Nonetheless, its most useful functions are confirming proper endotracheal tube placement, detecting airway obstruction or disconnection, and identifying anesthesia-ventilator breathing circuit failures (eg, valve failure, exhausted CO2 absorber, leak).
Capnography monitors continuously display the CO2 concentration of the gas in the airway. The partial pressure of CO2 at the end of normal expiration is a reflection of the gas leaving the alveoli and is an estimate of the partial pressure of alveolar CO2 (PACO2). When ventilation and perfusion are well matched, PACO2 approximates arterial PCO2 (PaCO2), so PaCO2 is equal to PACO2 plus PETCO2. The presence of periodically exhaled CO2 can be used to confirm airway patency, verify endotracheal intubation position, and verify adequacy of lung ventilation. In addition, decreased cardiac output due to hypovolemia or cardiac insufficiency can lead to decreased pulmonary perfusion. This causes an increase in alveolar dead space, which dilutes PETCO2. Animal studies have shown that a 20% reduction in cardiac output results in a 15% reduction in PETCO2.
In patients with endotracheal intubation and mechanical ventilation, changes in the shape of the capnogram displayed by capnography often provide clues to changes in lung pathology and malfunction of the ventilation equipment. For example, a staircase pattern in stage II may indicate continuous lung emptying, which may occur in mainstem partial bronchial obstruction. An upwardly sloping platform during expiration is a classic sign of poor ventilation with late emptying of the alveolar space and elevated PCO2, which can occur with expiratory obstruction at the level of the smaller airways, such as chronic obstructive pulmonary disease (COPD), bronchospasm, and Other forms of ventilation-perfusion mismatch. Pulmonary embolism is another cause of decreased end-tidal CO2.