Fri12November0729PM 6

A pharmaceutical company has developed a new drug. It obeys a first order, single compartment elimination model. It has a rate constant of 0.02 s-1.

Following an intravenous bolus dose, which of the following values best approximates to the percentage remaining in plasma in 50 seconds?

(Please select 1 option)

1% Incorrect answer selected

63%

7%

37% This is the correct answer

14%

Explanation

A first order, single compartment elimination model is described by a negative exponential curve.

C = C0e-kt

Where C is concentration at time t, C0 is concentration at time 0, e is Euler's number, k is the rate constant and t is time.

The time constant tau (t) is the time taken for an exponential to fall to 37% of original value an alternative defiinition is the time it would have taken for a negative exponential process to complete, were the initial rate of change to be maintained throughout.

However, the crux of this question is to remember that the time constant (t) is the inverse of the rate constant (k)

k = 0.02 s-1

Tau (t) = 1/k = 1/0.02 = 50 s

Therefore 1 time constant has elapsed and the plasma concentration has fallen by 63% (37% remains) .

Reference:

Peck TE, Hill SA, Williams M. Pharmacology for Anaesthesia and Intensive Care. 3rd ed. Edinburgh: Cambridge University Press; 2008.

Answer Statistics

1

47%

2

13%

3

6%

4

24%

5

13%

Times answered: 250