Capnography measures which of the following?

Capnography is a monitoring tool primarily used to measure the concentration of carbon dioxide (CO2) in exhaled air. This technique provides valuable information about respiratory function and can help assess the patient's ventilation status. When using capnography, the focus is on the measurement of expired CO2 because it reflects the metabolic activity of tissues and how well CO2 is being eliminated by the lungs during exhalation.

The measurement of expired CO2 is particularly significant in various clinical settings, such as during anesthesia or in critical care, as it can help monitor a patient’s respiratory status and detect conditions like hypoventilation or hyperventilation. This is why the answer specifying expired CO2 is correct; it directly relates to the fundamental purpose of capnography in tracking real-time changes in carbon dioxide levels in the breath.

The other options mentioned do not pertain to capnography measurements. For instance, expired O2, inhaled CO2, and exhaled nitrogen are not what capnography evaluates, as the technology is specifically designed to assess the levels of carbon dioxide in expired air rather than oxygen or nitrogen components.