Why can total lung capacity (TLC) not be measured by spirometry?

• A. It includes vital capacity
• B. It includes residual volume (RV), which can't be measured
• C. It requires a blood test
• D. It is affected by heart rate

Answer: (B)

Total lung capacity (TLC) cannot be measured by spirometry primarily because it includes the residual volume (RV), which is the volume of air that remains in the lungs after a maximal exhalation. Spirometry is a method that measures various lung volumes, except for RV, since this volume cannot be exhaled during a standard breathing maneuver. Therefore, any measurement or calculation of TLC using spirometry would necessarily be incomplete without accounting for this residual volume.

While TLC does include vital capacity (VC), the key reason for not being able to measure it via spirometry lies in the inclusion of RV, which is not accessible for measurement using this technique. Additionally, TLC does not involve any aspect of a blood test or heart rate in its calculation or measurement. Thus, the inability to measure RV is what fundamentally limits the assessment of TLC through spirometric methods.