For drugs with first order kinetics the time required to achieve steady state levels can be predicted from
Ans. is 'b' i.e., Half-life Steady stateplasma concentration After 5 t1/2, nearly complete drug is eliminated from the body o If a constant dose of drug is repeated before 5 V/2, the concentration in blood will be higher, because some remnant of the previous dose will be present in the body. o So, when a drug is infused at a constant rate the plasma concentration rises until a state is reached at which the rate of administration of drug to the body is exactly equal to the rate of elimination. o This is called the steady state ---> when it is attained the amount of drug in the body remains constant i.e. the plasma concentration is on a plateau ---> plateau principal of drug accumulation. o If a drug is administered by a constant rate the time to reach steady state depends only on the t1/2. o With passage of each V/2 period of time, the plasma concentration rises by half the difference between the current concentration and the ultimate steady state (100%) concentration. o For all practical purposes, after 5 t1/2 the amount of drug in the body will be constant and the plasma concentration will be at a plateau. o For example, if a drug with t1/2 4 hours and plasma concentration 100 mg/ml is given at a constant rate ie every 4 hrs. 1st dose --> at 0 hrs 2nd dose --> at 4 hrs 3rd dose at 8 hrs 4th dose --> at 12 hrs-------------------- so on. o After 4 hrs (1 t1/2) the drug concentration --> 50 mg/ml. o After 8 hrs (2 t1/2) 75 mg/ml (50 of 1 st dose + 25 of 2" dose) o After 12 hrs (3 t'/2) --> 87.5 mg/ml (50 of 3" dose + 25 of 2" dose + 12.5 of 1' dose) o After 16 hrs (4 t1/2) --> 93.75 (50 of 4th dose + 25 of 3rd dose + 12.5 of 2' dose + 6.25 of 1" dose) o After 20 hrs (5 t1/2) --> 96.875 (50 of 5th dose + 25 of 4th dose + 12.5 of 3rd dose + 6.25 of 2' dose + 3.125 of V dose) o After 24 hrs (6 t1/2) --> 96.875 (50 of 6th dose + 25 of 5th dose + 12.5 of 4th dose + 6.25 fo 3rd dose + 3.125 of 2' dose + nil from 1st dose, after 5 t1/2 nearly complete drug elimination occurs & first dose is eliminated after 5 VA). o So, after 5 t1/2 plasma concentration will be constant.
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