A student performs spirometry on a subject using a simple water-seal spirometer. Which of the following sets of lung volumes or capacities CANNOT be determined from this tracing alone?

• A Tidal volume and inspiratory reserve volume
• B Expiratory reserve volume and inspiratory capacity
• C Inspiratory capacity and vital capacity
• D Functional residual capacity, residual volume and total lung capacity ✓

Explanation

Spirometry records only volumes that exchange with the atmosphere. Residual volume remains in the lungs after maximal expiration and never appears in the tracing, so any capacity containing it, namely FRC, RV and TLC, requires an indirect method such as helium dilution, nitrogen washout or body plethysmography. All other listed combinations consist purely of measurable volumes. This distinction between direct and indirect measurement of lung volumes is a repeatedly examined fact.

Reference: Ganong's Review of Medical Physiology, 27th ed.

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