Relation between serum digoxin concentration and the electrocardiogram

The relation between serum digoxin concentration and the electrocardiogram was assessed by correlating computerized measurements of electrocardiographic parameters (PR, QRS, QT and QTc intervals, ST segment, and T‐wave amplitude) with the serum digoxin concentration in 97 patients on digoxin maintenance therapy and in 40 nondigitalized control subjects. None of the patients had unstable ischemic heart disease, electrolyte disorders, medication known to influence the ST segment, and/or the presence of a bundle‐branch block or ventricular hypertrophy. We found a trend toward lengthened PR interval and shortened QT and QTc intervals in digitalized versus nondigitalized patients. Increasing serum digoxin concentrations were associated with progressive depression of the ST segment and decreased T‐wave amplitude (p<0.001). A normal ST segment in four leads (I, aVF, V₅, V₆) excluded the presence of a serum digoxin concentration > 1.3 ng/ml in our patients, whereas severe ST‐segment depression with a J point of > 100 m̈V was a strong indicator for the presence of a serum digoxin concentration >2.0 ng/ml in our selected patient population (specificity 99%, sensitivity 30%, predictive accuracy 85%). We conclude that computerized electrocardiographic analysis of the ST segment may provide clinically useful information for the management of selected patients on digitalis therapy and may therefore increase the diagnostic yield of the electrocardiogram in predicting the presence of higher serum digoxin concentrations in a small but significant percentage of patients.