CLT and Vd (Cont'd)

CLT

Vd = k =

0.693

t 1/2

Different drugs may have different CLT and similar Vd. This makes the elimination rate constant and the half life of these drugs to be different.

Example:

Erythromycin

CLT = 0.24 L/hr/kg

Vd = 0.6 L/kg

Isoniazid

CLT = 0.36 L/hr/kg

Vd = 0.6 L/kg

Erythromycin:
k = CLT/Vd = 0.4 hr-1

Isoniazid:
k = CLT/Vd = 0.6 hr-1

Erythromycin and isoniazid have similar Vd, however erythromycin has lower CLT. The lower CLT makes erythromycin plasma concentration decreases at a rate slower than that of isoniazid ( smaller k and longer half life).

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