Calculate the VD/VT ratio for a patient who has a PaCO2 of 40 mm Hg and a PECO2 of 30 mm Hg
A. 1.0
B. 0.75
C. 0.5
D. 0.25
?ANS: D
A. Incorrect response: See explanation D.
B. Incorrect response: See explanation D.
C. Incorrect response: See explanation D.
D. Correct response: To calculate the VD/VT ratio knowing the PaCO2 and the PCO2, one needs to use the modified Bohr equation, which is
VD/VT = (PaCO2 – PCO2) ? PaCO2
Therefore, based on a PaCO2 of 40 mm Hg and a PECO2 of 30 mm Hg:
VD/VT = (40 mm Hg – 30 mm Hg) ? 40 mm Hg
= 0.25
A normal VD/VT ratio is about 0.30.
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