In patients who are mechanically ventilated, Vd/Vt can be as high as what percentage?

• A. 20-40%
• B. 4-5%
• C. 60%
• D. 10-25%

Answer: (C)

Vd/Vt represents the portion of each breath that does not reach gas-exchange units. In healthy adults it’s about 20–40%, reflecting anatomical plus some physiologic dead space. When patients are mechanically ventilated, especially with lung injury or severe V/Q mismatch, physiologic dead space can become markedly elevated. In the most extreme cases, it can approach about 60% of the tidal volume, meaning only roughly 40% of each breath effectively ventilates the alveoli. That’s why the 60% option is the best answer: it reflects the upper range you can see in ventilated patients with significant dead space. Normal values (20–40%) and small elevations like 10–25% or 4–5% don’t capture the potential to reach the higher end in disease, and 60% represents that maximum practical upper limit.