A 56-year-old woman with a 75-pack-year history of smoking cigarettes visits her physician in Mumbai because of shoness of breath. The physician sends her to a pulmonary function laboratory for a complete workup. The table below shows pulmonary volumes and capacities obtained using simple spirometry and helium washout techniques. Lung volumes Amount Functional residual capacity 4.5L Inspiratory reserve volume 1.5L Inspiratory capacity 2.0L Vital capacity 3.0L What is the residual volume of this patient?
A 56-year-old woman with a 75-pack-year history of smoking cigarettes visits her physician in Mumbai because of shoness of breath. The physician sends her to a pulmonary function laboratory for a complete workup. The table below shows pulmonary volumes and capacities obtained using simple spirometry and helium washout techniques. Lung volumes Amount Functional residual capacity 4.5L Inspiratory reserve volume 1.5L Inspiratory capacity 2.0L Vital capacity 3.0L What is the residual volume of this patient?
💡 Explanation
**Core Concept**
The question requires understanding of lung volumes and capacities, specifically how to calculate residual volume using the given pulmonary function test results. **Residual volume (RV)** is the volume of air left in the lungs after a maximal exhalation.
**Why the Correct Answer is Right**
To find the **residual volume**, we can use the formula: RV = Functional Residual Capacity (FRC) - Expiratory Reserve Volume (ERV). However, since ERV is not directly given, we can also use the relationship between lung volumes and capacities. Given that Vital Capacity (VC) = Inspiratory Reserve Volume (IRV) + Inspiratory Capacity (IC) - since IC = IRV + Tidal Volume (TV) and VC = IRV + TV + ERV, we can derive ERV from the given data. Knowing that FRC = RV + ERV and VC = IRV + TV + ERV, and given that IC = 2.0L and IRV = 1.5L, we can calculate TV as IC - IRV = 0.5L. Then, knowing VC = 3.0L and IRV = 1.5L, we find ERV as VC - IRV - TV = 3.0L - 1.5L - 0.5L = 1.0L. Finally, using FRC = 4.5L, we calculate RV as FRC - ERV = 4.5L - 1.0L = 3.5L.
**Why Each Wrong Option is Incorrect**
**Option A:** This choice would be incorrect if it does not match our calculated RV.
**Option B:** Similarly, if this does not align with our calculation, it is incorrect.
**Option C:** If this option does not equal 3.5L based on our calculations, it is wrong.
**Option D:** Any option not reflecting our calculated residual volume is incorrect.
**Clinical Pearl / High-Yield Fact**
In clinical practice, an increased **residual volume** is often seen in obstructive lung diseases such as chronic obstructive pulmonary disease (COPD), where airway obstruction prevents full exhalation, trapping more air in the lungs.
**Correct Answer:** 3.5L
✓ Correct Answer: D. 3.5 L
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