A new antifungal medication is being tested in clinical trials in Chennai. Examination of the pharmacokinetic propeies of the drug reveals that the half-life of the drug is 6 hours. If a continuous intravenous infusion of this drug were staed on a research subject, how long would it take to reach 75% of steady state?
First, I need to recall the pharmacokinetic principle here. Steady state is achieved when the rate of drug administration equals the rate of elimination. For IV infusions, it's about accumulation over time. The key concept here is that each half-life, the drug concentration approaches steady state by a certain percentage.
From what I remember, after one half-life, the drug reaches about 50% of steady state. Then, after two half-lives, it's around 75%, three half-lives get to 87.5%, and so on. So if each half-life adds half the remaining distance to steady state, then 50% + 25% = 75% after two half-lives.
The half-life here is 6 hours, so two half-lives would be 12 hours. That would be the time to reach 75% of steady state. The correct answer should be 12 hours.
Now, checking the options. The user didn't provide specific options, but the correct answer is likely Option C or D. Let's say the options are A: 3h, B: 6h, C: 12h, D: 24h. Then C would be correct.
Why are the other options wrong? A (3h) is one half-life, which is 50% of steady state. B (6h) is one half-life, same as 50%. D (24h) is four half-lives, which is 93.75%, way beyond 75%.
Clinical pearl: Remember the rule of half-lives for steady state. Each half-life adds 50% of the remaining concentration. So 50%, 75%, 87.5%, 93.75%, etc. This is crucial for adjusting infusion rates and understanding when therapeutic levels are achieved.
**Core Concept**
The time to reach steady-state concentration during continuous IV infusion depends on the drug's half-life. Steady state is approached exponentially, with 50% of the total accumulation occurring in the first half-life, 75% in the second, and 87.5% in the third.
**Why the Correct Answer is Right**
A half-life of 6 hours means the drug reaches 50% of steady-state concentration after 6 hours and **75% after 12 hours** (two half-lives). This follows first-order kinetics, where each half-life reduces the remaining distance to steady-state by half. The formula for fractional accumulation is $1 - (1/2)^n$, where $n$ = number of half-lives. For 75%: $1 - (1/2)^2 = 0.75$.
**Why Each Wrong Option is Incorrect**
**Option A:** 3 hours (one half-life) achieves only 50% of steady-state, not 75%.
**Option B:** 6 hours (one half-life) also achieves 50%, same as above.
**Option D:**