The effect of intravenous digoxin on the occurrence of ventricular tachyarrhythmias in acute myocardial infarction in man

Acute hemodynamic effects of digoxin alone or in combination with other vasoactive agents in patients with congestive heart failure

Although digitalis preparations have been in use for greater than 200 years, it is only within the last 2 decades that the central hemodynamic and neurohumoral effects occurring over several hours following intravenous administration of digoxin have been investigated in patients with congestive heart failure (CHF). Although digoxin has been shown to stimulate myocardial contractility in tissue preparations, its positive inotropic activity does not consistently translate into improvements in hemodynamic measurements in humans. Digoxin given intravenously results in increased cardiac index and decreased heart rate, left ventricular filling pressure, and right atrial pressure, as well as in acute attenuation of neurohumoral abnormalities, in patients with chronic CHF who have abnormal baseline hemodynamic measurements. Unlike other drugs with positive inotropic activity, however, digoxin does not influence hemodynamics in normal volunteers or in CHF patients in whom hemodynamics have been normalized with other therapies. These differing effects may be related to the drug's diverse peripheral vascular effects in CHF patients in whom vasodilation may occur in comparison with those that occur in normal subjects in whom the peripheral vasoconstrictor effects may prevent the inotropic effects of the drug from being translated into an increase in cardiac output. The hemodynamic effects of digoxin in patients with chronic CHF due primarily to diastolic dysfunction have not been fully investigated. Intravenous digoxin produces hemodynamic effects in patients with CHF associated with acute myocardial infarction, but these changes are small compared with those resulting from the administration of dobutamine. Digoxin does not appear to influence hemodynamic measurements in patients with right ventricular dysfunction unless concomitant left ventricular failure is present. In patients with chronic left ventricular dysfunction, the hemodynamic effects of intravenous digoxin and vasodilators are enhanced when these agents are given in combination.

Risk factors and manifestations of digoxin toxicity in the elderly

The incidence of digoxin toxicity increases with age, largely because the two most common conditions that benefit from use of digoxin, congestive heart failure and atrial fibrillation, are markedly more prevalent in old age. Whether the elderly are more sensitive to the effects of digoxin because of age per se is unclear. However, several other factors render the elderly more susceptible to digoxin toxicity. These include an age-related decline in renal function and a decrease in volume of digoxin distribution. There is also an increase in the number of comorbid conditions, including cardiovascular and chronic obstructive pulmonary disease, which heighten susceptibility to digoxin toxicity. Moreover, treatment of these diseases with such interactive medications as quinidine and calcium channel blockers may increase the serum level of digoxin. Similarly, such electrolyte imbalances as hypokalemia and hypomagnesemia occur more frequently in the elderly as a result of diuretic therapy. However, recent data suggest that manifestations of digoxin toxicity among younger and older patients do not differ. Similar incidences of cardiac toxicity, gastrointestinal toxicity, and altered mental status are found in both patient populations. Treatment of digitalis toxicity in the elderly is the same as for younger patients. Response rates to Digibind are not diminished in the elderly.

How to digitalize and to maintain optimal digoxin levels in congestive heart failure

Due to the narrow therapeutic-to-toxic ratio of digoxin, numerous studies have been done to assess the optimal digoxin level in patients with congestive heart failure. A digoxin level of 0.7-1.5 ng/mL (or 0.9-2.0 nMol/L) is generally considered optimal, but even at these levels toxicity may occur in certain clinical situations such as severe pulmonary disease or when electrolyte or metabolic disturbances are present. The optimal daily maintenance dose of digoxin depends on the preparation given and can be calculated by the equation of Jelliffe, which is largely based on the creatinine clearance of the patient. The daily digoxin dose must also be adjusted to take into consideration disease processes or concomitant drug therapy that can alter the volume of distribution, the biotransformation, or the excretion of the drug.

Congestive heart failure

A review of the epidemiology, pathophysiology, and treatment of congestive heart failure is presented, with particular attention given to newer modalities of therapy.

Facilitation of lethal ventricular arrhythmias by therapeutic digoxin in conscious post infarction dogs

The proarrhythmic potential of digoxin, administered in a therapeutic dosage regimen, was evaluated in conscious dogs in the subacute phase of myocardial infarction. In this evaluation, digoxin (0.0125 mg/kg/day intravenously) or vehicle were administered to conscious dogs for periods of 5 to 7 days, commencing 4 to 5 days after anterior myocardial infarction. Before treatment, programmed ventricular stimulation failed to initiate ventricular tachycardia in 26 post infarction dogs. After treatment, programmed stimulation initiated ventricular tachyarrhythmias in only 1 of 13 digoxin-treated dogs (1.36 +/- 0.17 ng/ml serum digoxin) and in 0 of 13 vehicle-treated dogs. However, the incidences of early ventricular fibrilation (4 of 10 digoxin vs 0 of 12 vehicle; p less than 0.05) and of 24-hour mortality (6 of 10 digoxin vs 2 of 12 vehicle; p less than 0.05) occurring in response to the development of posterolateral ischemia in the presence of previous anterior myocardial infarction was significantly greater in digoxin-treated (1.47 +/- 0.19 ng/ml serum digoxin) than in vehicle-treated animals. These findings suggest an enhanced susceptibility toward the development of ischemia-related lethal arrhythmias in the presence of therapeutic digoxin serum concentrations early after myocardial infarction, which is not predicted by programmed ventricular stimulation testing.

Association of digitalis therapy with mortality in survivors of acute myocardial infarction: observations in the Beta-Blocker Heart Attack Trial

The influence of digitalis therapy on survivors of acute myocardial infarction was examined in the placebo-treated patients from the Beta-Blocker Heart Attack Trial (BHAT). Two hundred fifty (13%) of the 1,921 placebo-treated patients were receiving digitalis at the time of randomization. Patients receiving digitalis differed from those not receiving digitalis in such baseline characteristics as age, prior history of heart failure, prior myocardial infarction and angina pectoris. They also experienced a higher proportion of in-hospital complications including pulmonary edema, persistent hypotension, atrial fibrillation and heart failure in addition to a greater prevalence of complex ventricular premature beats. The total mortality rate over a mean 25 month follow-up period for digitalis-treated patients was 20.4% compared with 8.2% for patients not receiving digitalis; the odds ratio was 2.87 (p less than 0.05). When the mortality rates were adjusted for heart failure and ventricular premature beat complexity, patients receiving digitalis again demonstrated a higher mortality rate, although the adjusted odds ratio was now lower (1.70). When the patients receiving or not receiving digitalis were compared by a multiple logistic regression analysis adjusting for 17 independent variables predictive of mortality, the use of digitalis was no longer independently predictive of total mortality (adjusted odds ratio 1.07). These data indicate that patients receiving digitalis had more extensive cardiovascular disease and greater morbidity than patients not receiving digitalis. Their subsequent higher mortality rate was probably related to these factors rather than to digitalis therapy.

Factors affecting tolerance to digitalis

A review of factors altering the safety margin between a therapeutic and a toxic dose of digitalis includes the consideration of: clinical conditions to which digitalis action may be undesirable, allergy and hypersensitivity to digitalis, physiologic factors modifying tolerance to digitalis, factors that change the amount of digitalis in the body, nervous and metabolic factors modifying tolerance to digitalis, modifications of digitalis tolerance produced by the status of the myocardium, and modifications of digitalis tolerance produced by diseases of other organs. The problems related to digitalis toxicity are more common than those of resistance to treatment. The most important factors contributing to decreased tolerance and risk of toxicity are: heart disease, poor renal function, hypokalemia and hypothyroidism. The roles of impaired liver function, chronic lung disease, acid-base disturbances, anesthesia, autonomic imbalance, calcium and magnesium are less important and less well established.

Enhancement of triggered activity in ischemic Purkinje fibers by ouabain: a mechanism of increased susceptibility to digitalis toxicity in myocardial infarction

Enhanced susceptibility to toxic arrhythmias by digitalis administration has been reported in clinical and experimental myocardial infarction. To investigate the mechanism responsible for this phenomenon, the effects of superfusion with normal Tyrode's solution and superfusion with Tyrode's solution containing 4 X 10(-8)M of ouabain in ischemic Purkinje fibers were compared. Ischemic Purkinje fibers of small endocardial preparations from 1 day old myocardial infarcts in 18 dogs were used for the study. During control conditions, these endocardial preparations demonstrated delayed afterdepolarizations and triggered activity. Superfusion with normal Tyrode's solution resulted in a gradual increase in maximal diastolic potential and action potential amplitude, a decrease in delayed afterdepolarizations amplitude and slowing and termination of triggered activity. Superfusion for 90 minutes with Tyrode's solution containing ouabain resulted in: 1) an increase in the magnitude of delayed afterdepolarizations in preparations demonstrating subthreshold delayed afterdepolarizations, 2) sustainment of triggered activity in preparations showing nonsustained triggered activity, and 3) shortening of cycle lengths of the triggered activity in preparations demonstrating sustained triggered activity before superfusion with ouabain. These effects occurred despite the gradual increase in maximal diastolic potential and action potential amplitude. Superfusion of normal Purkinje fibers with Tyrode's solution containing 4 X 10(-8)M of ouabain for 90 minutes did not result in delayed afterdepolarizations or triggered activity. Thus, ouabain at a concentration that has no toxic effect on normal Purkinje fibers may enhance arrhythmias in ischemic Purkinje fibers by increasing the magnitude of delayed afterdepolarizations and enhancing triggered activity.

Ischemic-induced alterations in cardiac sensitivity to digitalis

Intravenous infusion of acetylstrophanthidin to 6 dogs, after a 60 min left anterior descending coronary artery occlusion, was associated with a 43.0 +/- 10.5% decrease in the dose of digitalis needed to produce ventricular arrhythmias as compared to the pre-ischemic dose (97.5 +/- 8.0 microgram/kg). Reperfusion of the ischemic region for 2 h after a 90 min occlusion resulted in a 54.4 +/- 6.7% decrease in the arrhythmogenic dose. Direct intracoronary infusions of digitalis into the ischemic region, after a 90 min coronary occlusion followed by 2 h of reperfusion, was associated with a 47.7 +/- 6.4% decrease in the dose of digitalis needed to produce arrhythmias. The pre-ischemic (control) arrhythmogenic dose of digitalis via the intracoronary infusion method was 1.5 +/- 0.3 microgram/kg (mean +/- S.E.M. of 7 dogs). Sodium pump activity, estimated from the ouabain-sensitive 86Rb uptake in sodium-loaded ventricular slices, was significantly higher in slices obtained from the ischemic regions (6.84 +/- 0.30 nmoles 86Rb/mg dry wt. (mean +/- S.E.M.), than from the non-ischemic regions (3.43 +/- 0.64 nmoles 86Rb/mg dry wt.). Sensitivity of the sodium pump activity to the inhibitory effect of ouabain also was increased in the ischemic regions as indicated by a shift in the log dose--response curve to the left. Thus, it appears that there is an increase in myocardial sensitivity to the toxic effect of digitalis after temporary ischemia and it appears to be related to an increase in the sensitivity of the Na+,K+-ATPase or sodium pump to the inhibitory effect of digitalis.

Current considerations in digoxin usage

Basic considerations in biotransformation and pharmacodynamics are presented as a basis for understanding clinical usage. The role of polarity in determining a given glycoside's duration of action and extent of biotransformation is emphasized. The pharmacokinetics are summarized emphasizing the fact that digoxin is not completely absorbed by oral administration. The important relationship of serum digoxin levels to myocardial content and apparently to myocardial response is reviewed. This relationship and the development of precise methods for measurement of digoxin in serum provide the clinician with accurate means to assess myocardial tolerance for digoxin under diverse clinical circumstances. This review includes discussion of methods of digitalization, appropriate use of serum levels, apparent and real resistance to digoxin, and apparent and real sensitivity to digoxin. The limitations of serum levels as a precise guide to toxicity are analyzed. Finally, new developments in use of immunologic therapy for digoxin intoxication are presented.