Which factor increases the volume of distribution for hydrophilic drugs during pregnancy?

Which factor increases the volume of distribution for hydrophilic drugs during pregnancy?

• A. Decreased total body water
• B. Increased total body water
• C. Increased albumin
• D. Decreased tissue perfusion

Answer: (B)

Hydrophilic drugs distribute mainly in body water, especially extracellular fluid. During pregnancy, total body water expands significantly due to plasma volume and extracellular fluid increases. That bigger distribution space means more of the drug can spread beyond the plasma into tissues and fluids, raising the volume of distribution. Because Vd = dose divided by plasma concentration, a larger Vd for hydrophilic drugs makes a given dose produce a lower plasma concentration, which is why dosing may need adjustment. Decreased total body water would shrink the distribution space and lower Vd. Increased albumin would tend to keep more drug bound in the plasma (reducing free drug available to distribute into tissues, particularly for protein-bound drugs), and decreased tissue perfusion would limit distribution into tissues—neither of which increases Vd for hydrophilic drugs like the increased total body water does.

Hydrophilic drugs distribute mainly in body water, especially extracellular fluid. During pregnancy, total body water expands significantly due to plasma volume and extracellular fluid increases. That bigger distribution space means more of the drug can spread beyond the plasma into tissues and fluids, raising the volume of distribution. Because Vd = dose divided by plasma concentration, a larger Vd for hydrophilic drugs makes a given dose produce a lower plasma concentration, which is why dosing may need adjustment.

Decreased total body water would shrink the distribution space and lower Vd. Increased albumin would tend to keep more drug bound in the plasma (reducing free drug available to distribute into tissues, particularly for protein-bound drugs), and decreased tissue perfusion would limit distribution into tissues—neither of which increases Vd for hydrophilic drugs like the increased total body water does.