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Skip to 0 minutes and 11 seconds So moving on to step number three Find the volume diffusion based on the first IV load 120 mg over one hour And this is the equation that we can use where T or small T is one hour. So we have K, 0.231 we have T is one hour. we have peak concentration of eleven point two after the first loading and we knew the infusion rate was a hundred twenty million per hour so solve for only unknown here which is V and a V turns out to be nine point five liter for gentamicin Calculation step number four find the patient’s clearance for gentamicin.

Skip to 1 minute and 5 seconds We know clearance is K times V You ought to have remember this formula or equation by now So clearance is equal to K times V is equal to 0.231 times nine point five and that would give us an answer of two point two liter per hour and that’s gentamicin clearance for this particular patient. So again that is a formula to memorize if you don’t memorize many other equations. Calculation step number five

Skip to 1 minute and 45 seconds calculate the dosing interval to constrain gentamicin concentration00:01:50.920 –> 00:01:55.560 between the peak of eight and a trough of one.

Skip to 1 minute and 56 seconds So remember this equation defines the tau or the dosing interval based on peak concentration and trough concentration. So we have here k point two three one the peak concentration is eight as decided by the infection specialist and the trough concentration is one with infusion time at one hour and as a result tau turn out to be 10 hours. We’ve run it up to 12 hours. Okay continue on to calculation step number 6 Find the appropriate infusion rate at the above calculated dosing interval

Skip to 2 minutes and 46 seconds by this equation: the peak concentration is equal to k zero which is the zero order in infusion rate times 1 minus e to the minus K cap T divided by K elimination rate constant times the volume tribution times 1 minus e to the minus K tau. Here the peak concentration is 8 mg/L As decided by the infection specialist we know K is point 231 and the infusion time is 1 hour volume distribution is 9.5 and the Tau based on our previous calculation is 12 hours and therefore the zero the infusion rate turn out to be 79.7 mg We’ll rounded it up to 80 milligram Q12 hour.

Skip to 3 minutes and 49 seconds So here we realize that the initial maintenance infusion at 80 million QAh was undesirable because it could lead to toxicity here I have one additional question if this patient was obese, then would the dosage of recommendation be any different? The answer is not really. Because here we use the patient specific data from the two plasma concentration that we obtained. So this is the power of individualized medicine or individualized dosing regimen by applying patience specific kinetic information We are able to take advantage of the individualized dosing regimen

A case for Gentamycin : 2

Continuing from the previous step, Prof. Lee illustrates the other questions this time.

First, we can calculate volume of distribution (V) since we already have elimination constant rate (k), infusion time (t), peak concentration (C peak), and loading dose (K0).

Secondly, we can derive the clearance from the equation : CL= k * V.

Following that, knowing peak and trough concentration, we will be able to calculate the suitable dosing interval, and find out the appropriate infusion rate.

Finally, due to the individualized dosing regimen, we do not have to alter our dosage recommendation for obese patients.

If you have any questions or thoughts, please share them below.

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Pharmacokinetics: Drug Dosing in Renal Disease

Taipei Medical University