Measurement of tissue-bound amiodarone and its metabolites by computed tomography

Amiodarone is strongly tissue-bound and serum levels are a poor guide to therapeutic efficacy. The electrocardiographic measure of the QT interval corrected for heart rate (QTc) is a better guide but is unhelpful in patients with bundle branch block or U-waves on the electrocardiogram. Myocardial amiodarone levels are the most accurate guide but are not easy to obtain. There is, however, a relationship between myocardial concentration and hepatic concentration of amiodarone and its metabolites. Since amiodarone contains iodine, and there is hepatic uptake, the increased hepatic attenuation from single slice computed tomography was compared with serum levels and the electrocardiographic QTc in 12 patients before and during amiodarone therapy. Hepatic attenuation increased by a mean value of 18.25 HU over a 12 month study period. This increase correlated well with increased QTc (r = 0.83) and with serum amiodarone levels (r = 0.89), but less well with serum desethyl amiodarone levels (r = 0.43). An iodine-containing phantom was used to construct a curve of attenuation against iodine concentration in mol/l. Thus an indirect measurement of amiodarone concentration in g/l wet weight of liver could be determined.