Digoxin and the Exercise Electrocardiogram

Abstract

To the Editor: Tonkon and associates1Tonkon MJ Lee G DeMaria AN et al.Effects of digitalis on the exercise electrocardiogram in normal adult subjects.Chest. 1977; 72: 714-718Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar have confirmed our findings2Adair RF Hellerstein HK White LW Digoxin induced exercise ECG changes in young men: ST-T walk through phenomenon above 80% max heart rate (abstract).Circulation. 1972; 46: II-11Google Scholar that the effects of digoxin on the exercise electrocardiogram disappear at near maximal heart rates. This constitutes a new type of “walk-through” phenomenon, and physicians supervising exercise tests should be alert to the possible role of digitalis when “abnormal” ST-segment displacement disappears as exercise continues and heart rate increases. In our study, which has not been published in detail, the effects of oral therapy with digoxin were evaluated in 11 normal young men during multistage bicycle exercise according to a protocol described elsewhere.3Hellerstein HK Hornsten TR Assessing and preparing the patient for return to a meaningful, productive life.J Rehabil. 1966; 32: 48-52PubMed Google Scholar Each subject was tested while receiving a daily dose of 0, 0.125, 0.25, and 0.5 mg of digoxin in a double-blind manner. Modified X and Y bipolar leads were used. The response of the S-T segment to exercise was analyzed using two sets of criteria, Bruce's STB criterion (mean ST-segment displacement of 1.0 mm or more at 50 to 69 msec after the nadir of the S wave) and the criteria of Lester et al.4Lester FM Sheffield LT Reeves TJ Electrocardiographic changes in clinically normal older men following near maximal and maximal exercise.Circulation. 1987; 36: 5-14Crossref Scopus (45) Google Scholar Therapy with digoxin had no effect on the heart rate or blood pressure at rest or on the maximal achieved heart rate, systolic blood pressure, or estimated myocardial uptake of oxygen during exercise. The exercise ECG became abnormal in two of 11 subjects receiving 0.125 mg of digoxin daily, in six of ten subjects receiving 0.25 mg of digoxin daily, and in eight of 11 subjects receiving 0.5 mg daily. ST-segment displacement due to therapy with digoxin increased with workload and heart rate up to 65 to 75 percent of the maximal heart rate but then disappeared as exercise continued at the same or a higher workload and as near maximal heart rates were attained (Fig 1). No test showed abnormal ST-segment displacement at 90 percent or more of the maximal heart rates. The study by Tonkon et al1Tonkon MJ Lee G DeMaria AN et al.Effects of digitalis on the exercise electrocardiogram in normal adult subjects.Chest. 1977; 72: 714-718Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar and our study reached similar conclusions, despite different methods and types of subjects, and tend to reinforce each other. Tonkon et al1Tonkon MJ Lee G DeMaria AN et al.Effects of digitalis on the exercise electrocardiogram in normal adult subjects.Chest. 1977; 72: 714-718Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar used a treadmill, studied only one dosage of digoxin, included women and older subjects, and correlated the electrocardiographic findings with levels of digoxin in the serum. We used a bicycle ergometer and found this “walk-through” effect at each of three different dosages of digoxin. Both groups of investigators agreed that therapy with digoxin did not affect any measured hemodynamic variable. Why does digoxin-induced ST-segment displacement disappear at near maximal heart rates? We suggest that a rise in the serum level of potassium due to strenuous exercise5Kilburn KH Muscular origin of elevated plasma potassium during exercise.J Appl Physiol. 1966; 21: 675-678PubMed Google Scholar could compensate for digoxin-induced depletion of myocardial potassium or could interfere with digoxin-induced inhibition of microsomal sodium-potassium adenosine triphosphatase. Further investigation of this phenomenon may help to elucidate the intracellular effects of glycosides of digitalis.