Observations on Digitalis Intoxication-A Plea

The development of positive inotropic agents for chronic heart failure: how have we gone astray?

Because cardiac contractility is impaired in chronic heart failure, many pharmacologic agents have been developed to increase the contractile state of the failing heart. These drugs produce impressive hemodynamic effects, but long-term therapy has failed to produce clinical benefits and has increased mortality in treated patients. This experience has led many physicians to suggest that positive inotropic therapy be abandoned as a therapeutic approach for heart failure. However, recent studies suggest that the efficacy and safety of many (if not all) positive inotropic drugs can be greatly enhanced by reducing the dose of these drugs. The importance of dose is dramatically illustrated by the results of trials with vesnarinone, which decreases mortality when used in low doses but increases mortality when administered in doses only twice as large. Although low doses of positive inotropic drugs may be clinically superior to high doses, it is not clear that these low doses exert significant inotropic effects. All positive inotropic drugs exert actions on the circulation in addition to stimulating the heart, and these ancillary properties may be particularly important at low doses of these drugs. Low doses of milrinone and pimobendan may act primarily to dilate peripheral blood vessels; low doses of digitalis may exert only neurohormonal effects, and low doses of vesnarinone may act as an antiarrhythmic agent. If the noninotropic actions of low doses account for the therapeutic benefits of these drugs, then the positive inotropic effects seen at high doses may be primarily responsible for their adverse effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Radioimmunoassay of serum digoxin in relation to digoxin intoxication

Serum digoxin estimations were done in 98 patients receiving digoxin for heart failure of varied aetiology. Digoxin toxicity or the lack of it was determined on the basin of established electrocardiographic criteria. Fifty-two patients were classified as 'toxic' and 46 as 'non-toxic'. The difference is the mean digoxin levels between the two groups was highly significant (P less than 0.001). The mean serum digoxin level in 'non-toxic' patients was slightly higher than that found by other investigators. Fairly good correlations have been noted between different dosage schedules and various rhythm disturbances. Death was attributed to digoxin toxicity in only 2 patients who showed electrocardiographic evidence of intoxication at the time of death.

Digitalis toxicity: epidemiology and clinical use of serum concentration measurements

Despite continuing advances in understanding of the basic pharmacology of the cardiac glycosides, digitalis intoxication remains a common clinical problem. Physician education programs and increasing use of serum or plasma concentration data have, however, been shown to be capable of substantially reducing the incidence of digitalis toxicity. Methodologic progress and availability of commercial radioimmunoassay kits have placed measurement of clinically relevant serum or plasma cardiac glycoside concentrations within the capability of most well equipped clinical laboratories. Extensive experience with serum digitalis levels now provides a basis for ongoing examination of the role of these measurements in clinical practice. Results of studies to date demonstrate that mean serum digoxin and digitoxin levels are significantly higher in patients with electrocardiographic evidence of toxicity compared with patients without such evidence. It must be emphasized, however, that because of overlap in serum digitalis levels between these two groups, sole dependence on these levels for established of a diagnosis of digitalis toxicity is not warranted. Multiple factors influence individual responses to cardiac glycosides, and serum concentration data must be interpreted in the over-all clinical context. Type and extent of underlying heart disease are important determinants of the clinical response to any given dose or concentration of cardiac glycoside. Knowledge of the serum digitalis concentration is likely to be helpful in the setting of suspected digitalis intoxication in the absence of an adequate history, or in the presence of fluctuating renal function, overt or suspected malabsorption, or uncertain bioavailability. More generally, such measurements may prove useful whenever an unanticipated response to digitalis is encountered, whether it be suspected toxicity or the absence of an expected therapeutic effect.

Potassium canrenoate in the treatment of long-term digoxin-induced arrhythmias in conscious dogs

The effects of potassium canrenoate on arrhythmias induced by long-term progressive digoxin toxicity were studied in eight conscious beagle dogs. Sinus bradycardia and sinoatrial block, as well as atrioventricular (A-V) conduction disturbances, were consistently alleviated by administration of potassium canrenoate. Premature supraventricular (including junctional) and ventricular depolarizations as well as ventricular tachycardias were also suppressed. Although potassium canrenoate always terminated the digitalis-induced arrhythmias, it usually converted the rhythm to sinus arrhythmia rather than to normal sinus rhythm. Equimolar sodium canrenoate, but not potassium chloride, had similar reversal effects on arrhythmias induced by long-term digoxin intoxication. These data indicate that canrenoate, a diuretic agent with reported positive inotropic effects, may be useful in the treatment of digitalis-induced arrhythmias in man.

Digoxin metabolism in the elderly

Tritiated digoxin (0.5 mg) was given intravenously to five elderly men (mean age, 77 years) and nine young men (mean age, 27 years). The elderly were not in congestive heart failure. The serum creatinines of the old and young were not different. However, the creatinine clearance averaged 56 ml/min/1.73 m 2 in the old and 122 ml/min/1.73 m 2 in the young ( P <0.001). Digoxin clearance averaged 53 ml/min/1.73 m 2 in the old and 83 ml/min/1.73 m 2 in the young ( P <0.001). The blood concentrations of tritiated digoxin were significantly higher in the elderly throughout the study ( P <0.05). When blood digoxin concentrations were corrected for body surface area, there was no significant difference between the two groups during the first 24 hr, but thereafter the concentrations in the elderly were higher.

The same dose of digoxin resulted in higher blood concentrations and longer blood half-life in the elderly. This is due to the smaller body size and a diminished urinary excretion of digoxin in the elderly.