Diltiazem IV to Oral Dose Conversion Formula Remains a Medical Mystery

What’s the conversion from an intravenous (IV) diltiazem infusion to an oral dosage? That’s a question I’ve been asked a few times over the years.

Despite the accepted practice and dogma dictating diltiazem IV to PO conversion, there’s little, if any, evidence to explain why the formula seems to work.

The conversion comes in handy when a patient with atrial fibrillation has been rate controlled after a bolus of diltiazem and is on a diltiazem drip, but now needs conversion to an oral formulation. The hospital has a policy stating a patient on a diltiazem drip must go to a cluster or ICU bed, but the patient could go to a general medical floor, or even get discharged home.

The formula that can be used to estimate the total daily oral dose from an IV diltiazem drip is as follows:

Oral dose = {IV drip rate (in mg/hr) x 3 + 3}x10

The standard rates for diltiazem generally convert as follows:

· 5 mg/hr to 180 mg/day

· 7.5 mg/hr to 260 mg/day

· 10 mg/hr to 330 mg/day

· 15 mg/hr to 480 mg/day

The total daily dose should be divided q6h. The immediate-release dosage form must be used initially, and rounding will be necessary because it’s only available as 30 mg, 60 mg, 90 mg, and 120 mg.

When actually making the switch, give the first oral dose about 1 hour before you plan to stop the drip. After the first hour, slowly titrate down the diltiazem drip by 2.5 mg/hr increments until 0. By then, the diltiazem should have time to be absorbed and distributed (time to Tmax is approximately 1 to 1.5 hours).

Despite being cited on some pharmacy practice resource sites, references to the source of this formula are absent.

Over the years, I’ve sought the initial description of how these figures came to be. I’ve also attempted (and failed) to derive the formula from the pharmacokinetic parameters of diltiazem, but multiplying the infusion rate in mg/hour by 3, then adding 3, and then multiplying that by 10 appears random. Meanwhile, conducting literature searches through PubMed and Google Scholar has yielded no helpful results, and pharmacology and pharmacokinetic textbooks similarly don’t provide any guidance.

While the conversion formula has anecdotal clinical effectiveness, it should be taken into consideration that it could be nothing more than a ballpark guess. With all the time and effort I’ve invested in debunking medical myths and dogmas, this one has stumped me the most.

Perhaps someone else has found some evidence or explanation in their investigation?