What is the normal deadspace to tidal volume ratio (Vd/Vt)?

What is the normal deadspace to tidal volume ratio (Vd/Vt)?

• A. 60-80%
• B. 0-5%
• C. 40-60%
• D. 20-40%

Answer: (D)

The main idea is that a portion of each breath never participates in gas exchange because it fills the airways or reaches nonperfused areas—the dead space. The dead-space to tidal-volume ratio (VD/VT) measures how much of the breath is wasted in that way. In a healthy person, this fraction is typically about one-fifth to two-fifths of the breath. That corresponds to roughly 0.2 to 0.4, or 20–40%. For example, with a typical tidal volume around 500 mL, about 150 mL is dead space, giving a VD/VT near 0.3. Values outside this range suggest unusually high or low dead space, which isn’t normal. So the normal range is 20–40%.

The main idea is that a portion of each breath never participates in gas exchange because it fills the airways or reaches nonperfused areas—the dead space. The dead-space to tidal-volume ratio (VD/VT) measures how much of the breath is wasted in that way. In a healthy person, this fraction is typically about one-fifth to two-fifths of the breath. That corresponds to roughly 0.2 to 0.4, or 20–40%. For example, with a typical tidal volume around 500 mL, about 150 mL is dead space, giving a VD/VT near 0.3. Values outside this range suggest unusually high or low dead space, which isn’t normal. So the normal range is 20–40%.