Therapeutic drug monitoring.... digoxin & theophylline

[bobbin]

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

Is it the case that lower doses of theophylline are generally used nowadays, and so blood levels are performed less frequently than in the past?

[crit care]

for oral theophylline that may be the case, but i'm coming from a place where they use continuous 24hr infusions of aminophylline where daily levels are done. Probably out in the community then yes levels are done less routinely (if at all)

[Nik]

Have very rarely even seen IV aminophylline where I work - just goes to show the difference in practice between trusts. Nor have I come across routine monitoring unless pts exhibit signs of toxicity or non-compliance or if starting oral theophylline for the first time.

[crit care]

i don't think its a trust thing, probably more areas where you work. we use iv aminophylline in ED, intensive care and sometimes admissions wards for patients with severe asthma/COPD!

we do levels if people are on oral, can show compliance and if they are toxic or not.

[_Rob_]

It is an interesting topic as there is some suggestion that what we would normally consider sub-optimal dosing (i.e. those giving a plasma level <10) may still be effective. Therefore lower doses might give the patient some benefit with a reduced level of risk. However, I have also seen the revese; a patient with chronic supra-therapeutic levels but when we tried to reduce them, became breathless again. Ultimately, we had no option (even after toughing it out for sometime) but to put the dose back-up and accept the levels high (around 27ish if i remeber correctly).

Will be honest, we still use lots of IV aminophylline however, we do like to dabble in IV salbutamol and SC terbutaline for a little variation.

Whilst on the topic to TDM and drugs with narrow therapeutic windoze, anyone noticed what is happening to the Phenytoin loading doses in the BNF?? Couple of years ago it went from 15mg/kg to 18mg/kg...now in the 61st Ed its gone to 20mg/kg!

[Nik]

Because patients never seem to get weighed on admission we use the 20mg/kg dose as an approx if we need to load someone with phenytoin.

[crit care]

yeah i've noticed that....but we usually give a loading dose of 1g in most cases and a further 500mg on top if needed.....although we say if pt <65kg use 18mg/kg for loading dose if in status.

iv salbutamol we use bits of....not seen it that often in practice! often used alongside ketamine as well.

[Nik]

Have only come across iv salbutamol with glucose+insulin for hyperkalaemia. Not common practcice though because they usually just repeat the insulin/glu combo