Therapeutic drug monitoring.... digoxin & theophylline

[jay.d]

Hello, I've got a pharmacy practise exam coming up soon and part of it is TDM of digoxin and theophylline. I've managed to commit to memory loading dose, maintenance dose equations etc however a couple of things are niggling me...

1) Loading dose should be split into 3 doses 1) 50% 2) 25% 3) 25% the lecture notes supplied back this up however there is one example (with supplied answers) whereby 2 doses are given which equates to 1) 500mcg 2) 250mcg.

So my first question is how do I decide when it is necesary to give 2 doses or 3 to load up?

My next question is regarding theophylline, In the example given the patient suffered from COPD and was taking a course of erythromycin, both of which reduce clearance by 20% each....upon working out the clearance (Cl = 0.04 x IBW x factor) I decided that the final calculation should be:

Cl = 0.04 x IBW x 0.8 x 0.8

However the answer given just multiplied by 0.8 once. I'm not happy with that since they are two totally different factors independant of each other.....

So my next question is: Am I right? should I have multiplied by 0.8 (accounting for COPD) and then again by 0.8 (accounting for erythromycin), or was the model anwer correct?

Answers on a postcard!

Thanks.

[Nik]

Not come across three times a day loading doses for digoxin in practice, occasionally seen twice daily eg 500mcg each time but usually give all in one go. In the example you've given above, it may be easier to administer dig as 500mcg then 250 mcg compared to 375mcg then 187.5 then 187.5. Reduces risk of error and saves nursing time.
For the theophylline q how would the fact that a patient suffers from COPD affect clearance of theophylline ?

[jay.d]

Not come across three times a day loading doses for digoxin in practice, occasionally seen twice daily eg 500mcg each time but usually give all in one go. In the example you've given above, it may be easier to administer dig as 500mcg then 250 mcg compared to 375mcg then 187.5 then 187.5. Reduces risk of error and saves nursing time.
For the theophylline q how would the fact that a patient suffers from COPD affect clearance of theophylline ?

Thanks Nik, I suspected as such, so for practicality and/or reduction of error... makes sense.

As for theophylline apparently severe COPD reduces theophylline clearance by 20% (cirrhosis 60%, CHF 50%)

[Nik]

I know plasma conc of theophylline can be affected by HF and liver disease but never come across dose adjustment based on the fact that a patient has COPD. Will have to look into it a bit further.

[Nik]

Found some info to suggest advanced stages of COPD may reduce clearance. Original post didn't mention anything about severe copd, just "a patient suffering with copd", so in that situation I wouldn't multiply by 0.8 again.

[crit care]

in practice i have never had to calculate clearance of theophylline. usually we do blood levels and see what they come back as from the lab..usual range is 10-20. Then adjust dose and rate accordingly depending on th levels. follows first order kinetics

As for digoxin, i have usually seen the loading dose given in 2 doses...500mcg then a further 250 or 500 depending on renal function and clinical symptoms

[jay.d]

Found some info to suggest advanced stages of COPD may reduce clearance. Original post didn't mention anything about severe copd, just "a patient suffering with copd", so in that situation I wouldn't multiply by 0.8 again.

My bad, I was trying to illustrate how 2 different factors could affect clearance but alas I picked a bad example (or just didnt write down the example properly). So had the patient sever COPD and was currently taking a course of erythromycin the calculation would be:

Cl = 0.04 IBW x 0.8 x 0.8?

[Nik]

Well yes the calculation would probably then be correct, but I'd agree with critcare in that in practice dose adjustment based on plasma levels would be more practical.

[jay.d]

Thanks for clearing it up, for all intent and purposes practicality is out of the window for the 2 hours between 9:30 and 11:30am tomorrow

[Nik]

Well good luck with the exam.