Capnography tells you about alveolar ventilation?

• A. It measures CO2 in exhaled gas, indicating ventilation adequacy and aiding tube placement verification.
• B. It measures tidal volume.
• C. It measures respiratory rate only.
• D. It measures oxygen saturation.

Answer: (C)

Capnography gives you real-time insight into alveolar ventilation by measuring the amount of carbon dioxide in exhaled gas. The CO2 level you see, especially the end-tidal CO2 value, reflects how effectively the patient is exchanging air in the alveoli. A normal or expected ETCO2 indicates adequate ventilation, while rising, falling, or absent ETCO2 can signal changes in ventilation, apnea, or issues with airway placement. This is why capnography is also a valuable tool for confirming endotracheal tube placement—the characteristic waveform and persistent ETCO2 after intubation demonstrate that CO2-rich air is reaching the lungs.

It’s true that you can infer respiratory rate from the capnography waveform because breaths create a repeating CO2 pattern, but the essential purpose of capnography is to assess CO2 elimination and ventilation per breath, not to measure tidal volume or oxygen saturation. Oxygen saturation is monitored by pulse oximetry, not capnography, and tidal volume is assessed by other devices and measurements.