E-DRUG Digitalis toxicity
>If you ask yourself: is my patient digitalis toxic?
>What do you think about the utility of measurement of serum digoxin
>concentration to diagnose digitalis toxicity?
This issue was dealt with briefly by the RxConsult discussion group early
in particular in two posts that said:
"Dose adjustment should be on the basis of clinical response. If the
person is well controlled and without signs of toxicity, then the dose is
probably OK. I would not empirically adjust the dose in this case just
because the books say so.
Levels are really only useful for detecting or confirming toxicity. In an
80+ yr old 250mg does sound like a high dose (depending on the person's
renal function and body weight), so if levels have never been done I would
probably like to see them done."
".....measuring serum levels for digoxin is not always necessary. It is
nonetheless useful to determine toxicity, compliance, etc. It is also
important to remember that the normal therapeutic range is a reference
range, and some patients may require greater levels to achieve a clinical
effect, and similiarly there are those who may become toxic despite being
within the reference range. Thus an 80 year old, who may have exceptional
renal function and heavy body weight may still require up to 250 mcg/day to
achieve clinical effect.
Having said that, and since it may not always be practical to measure
levels, nor do we want to take blood from everybody who walks through the
front door, experience and "guess-timations" can usually detect at risk
patients. Given that digoxin is a drug which is predominantly cleared by
the kidneys, a quick calculation of creatinine clearance can provide a VERY
crude quick "guess-timate" of digoxin dose (for people who can never
remember PK equations nor have the time to do the calcs on every patient on
dig). Assumming that a normal healthy adult male of 70kg is going to
usually require 250 - 312.5 mcg/day, a patient with half renal function
would typically require about 125-187.5 mcg/day, and a patient with next to
nothing renal function about 62.5 mcg/day."
Alternatively (to be more precise), you can try calculating a loading dose
using 10 - 15 mcg/kg and then calculate the % of that dose which is
eliminated in 24 hours = [14 + (CrCL/5)]. So, for a bloke of 70 kg with a
CrCL of say 50 mL/min, the loading dose would be about 700 to 1050 mcg, and
the maintenance dose approximately 168 (187.5) - 252 (250) mcg/day.
As you can see, neither of my crude methods is ideal (though the second
method fairs a little better!), but with experience you will be able to
make sensible decisions as to when a serum level would be useful. When in
doubt, there is nothing better than a serum level to confirm. Finally, it
is important to emphasise an often neglected point and that is to "treat
the patient not the numbers". If the patient derives a clinical benefit and
shows no signs of toxicity, then it doesn't matter what the levels are (and
vice versa) - from memory, the derivation of the reference range is rather
Hope this helps. Thanks to those who provided the original posts to RxConsult.
Mark Dunn B Pharm MPS MACPP JP
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