[Another type of interaction between blood levels of digitalis and anti-arrhythmic drugs: digoxin and amiodarone. Experience with long-term therapy].

In clinical Arrhythmology it is often necessary to associate digitalis and antiarrhythmic agents. This calls for study of possible interaction between the employed drugs. We found a statistically significant correlation between digitalis and amiodarone plasma level in patients on long term treatment with both drugs. A statistically significant linear correlation between plasma amiodarone level and digoxin (0.25 mg/day) or beta-methyldigoxin (0.20 mg/day) was documented in 33 patients. 23 patients had been treated with these drugs for paraxysmal reciprocating supraventricular tachycardia since an average of 52 months (computerized follow-up). (Amiodarone average weekly dose was 1078 +/- 168 mg after a loading dose of 12 gm given over one month). 10 patients were on chronic treatment with higher weekly doses of amiodarone (average dose 2380 +/- 731 mg per week). Thyroid function tests (T4; T3; T3UP; TSH; rT3) were checked in every patients. Further studies are warranted to understand the mechanism of the interaction between amiodarone and digitalis. As a clinical implication we point out that amiodarone-digoxin (or betamethyldigoxin) interaction in our patients has neither resulted in over-therapeutic plasma level nor in signs of digitalis toxicity.