The QT interval: a predictor of the plasma and myocardial concentrations of amiodarone

A study was performed to assess whether plasma and myocardial concentrations of amiodarone correlated with changes on the surface electrocardiogram. Nine patients--seven with angina and two with paroxysmal ventricular tachycardia--were treated with oral amiodarone (200-400 mg daily) for at least nine months before undergoing cardiac surgery. QT intervals were measured from lead II of the surface electrocardiograms recorded before amiodarone treatment and immediately before surgery. Patients with prominent U waves after taking amiodarone were excluded from the study. Plasma and myocardial samples were collected at the beginning of the surgical procedure for estimating plasma and myocardial concentrations using the high performance liquid chromatographic technique. Amiodarone caused a significant lengthening of the QTc interval. There was a good correlation between plasma and myocardial concentrations, and both correlated well with the percentage increase in the QTc interval. Although there was a strong correlation between the dosage given (mg/kg/day) and both plasma and myocardial concentrations, the correlation with the percentage increase in the QTc interval was weaker but still highly significant. Despite previous reports to the contrary, the findings indicate that the plasma concentration of amiodarone does correlate well with the myocardial concentration. The degree of lengthening of the QTc interval may be used clinically to estimate the myocardial concentration of amiodarone.