DIGITALIS INTOXICATION FOLLOWING CONVERSION TO SINUS RHYTHM

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.

Digoxin and the susceptibility of the canine heart to countershock-induced arrhythmia

This study investigated the effects of therapeutic and subtoxic doses of digoxin on the risk of ventricular tachycardia (VT) after graded, transthoracic shocks in anesthetized dogs. A series of direct current shocks (5, 10, 25, 50, 75, 100, 150 and 200 J) was delivered to 33 normal dogs and 6 dogs with a healed (32 +/- 7 days) myocardial infarct (MI). In 10 untreated dogs, the duration of post-shock VT was highly reproducible when 3 separate series of shocks were delivered at 2-hour intervals. In 6 normal dogs treated with oral digoxin (0.5 mg/day for 5 to 7 days), a series of shocks delivered before and during treatment (serum levels 1.5 +/- 0.5 ng/ml) resulted in the same duration of post-shock VT. In 18 normal and 6 dogs with MI, a series of shocks was given before and 90 minutes after a therapeutic dose of digoxin (0.05 mg/kg intravenously). At this dose of digitalis (serum level 2.5 +/- 1.0 ng/ml), there was no difference in the duration of post-shock VT in either normal dogs or dogs with MI. A third series of shocks was given after achieving subtoxic digitalization with additional intravenous digoxin (0.01 mg/kg) every 30 minutes until a premature ventricular stimulus evoked a repetitive ventricular response. The subtoxic doses of digitalis (serum levels 13.9 +/- 4.7 ng/ml) increased the duration of post-shock VT in both normal dogs (100%) and dogs with MI (700%) (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Absence of cardioversion-induced ventricular arrhythmias in patients with therapeutic digoxin levels

To determine the incidence of cardioversion-induced ventricular arrhythmias in patients with therapeutic serum levels of digoxin, 19 patients (average age [+/- standard deviation] 61 +/- 12 years) undergoing elective direct current cardioversion for atrial fibrillation were studied. Only patients with therapeutic serum digoxin levels (range 0.5 to 1.9 ng/ml; mean 1.1 +/- 0.5) at the time of cardioversion were included. Patients with acute myocardial ischemia or unstable angina, serious electrolyte disturbance or those requiring class I antiarrhythmic agents for control of ventricular or supraventricular arrhythmias were excluded. Ambulatory electrocardiograms were recorded for 24 hours before and 6 hours after cardioversion. No patient developed malignant ventricular arrhythmias (ventricular triplets or tachycardia) in the immediate 3 hour period after cardioversion. Furthermore, there were no significant (p less than 0.05) differences in the frequency of ventricular premature beats or couplets before and after cardioversion. To determine whether the level of serum digoxin or the strength of the applied shock had a significant effect on the development of postcardioversion arrhythmias, the change in frequency of single premature ventricular beats after cardioversion was compared with the serum digoxin level (ng/ml) and the applied energy level (joules) by means of linear regression analysis. There was no significant (p less than 0.05) relation between these variables. These findings suggest that patients with therapeutic serum levels of digoxin may safely undergo cardioversion without the concomitant use of class I antiarrhythmic agents.

Serum digoxin concentrations in atrial fibrillation: a review

Studies evaluating the relationship of serum digoxin concentrations (SDCs) with pharmacodynamic effects in atrial fibrillation have important limitations. In general, a poor correlation is found between SDC and ventricular rate, but this is understandable, considering the many other factors that affect conduction through the atrioventricular node. The ventricular rate, although a clinically important and easily monitored parameter, may not always be a good measure of digoxin effect. In certain patients, signs and symptoms of toxicity may develop before the desired decrease in heart rate. The SDC may provide, in many cases, information that cannot be obtained solely from the clinical response, but is of great relevance to therapeutic decision making.

Cardioversion and defibrillation

The use of electrical energy for the immediate treatment of atrial and ventricular arrhythmias is practical and easily applied. The method, though simple, is the most effective method for terminating cardiac arrhythmias and is associated with only a low risk if properly employed. In symptomatic patients, the utilization of cardioversion reduces patient discomfort and complications which may occur while awaiting pharmacologic reversion of arrhythmia. At present, transthoracic defibrillation is the only practical method for terminating VF. Despite the safety of electrical reversion, proper precautions are necessary to prevent complications. In particular, the discharge of excessive energies, especially in the presence of digitalis toxicity, promises grave and life-threatening consequences. The use of antiarrhythmic medications is not supplanted by cardioversion and defibrillation. Rather, ongoing drug therapy is frequently necessary to prevent recurrence of arrhythmia.

Effect of quinidine on plasma concentration and renal clearance of digoxin. A clinically important drug interaction

1 Thirty patients on maintenance digoxin therapy and admitted for cardioversion of atrial fibrillation were closely monitored with regard to plasma levels of digoxin and quinidine. 2 Seventeen of these patients were kept on maintenance digoxin therapy. After an initial lag period of 6 to 18 h after the addition of quinidine their digoxin levels started to increase and had increased by between 20 and 330% after 3 days on quinidine. Side-effects attributed to the raised digoxin concentration occurred in 6 of these patients. 3 As studied in 5 of these 17 patients the renal clearance of digoxin decreased markedly when quinidine was added to the therapy. There was also a slight but significant reduction in creatinine clearance (n = 4). 4 In 13 patients digoxin was discontinued 36 h prior to the first quinidine dose. Also in these patients digoxin plasma levels increased significantly. 5 It is concluded that quinidine causes an unpredictably large increase in plasma digoxin and that this effect is probably at least initially to a large part due to a redistribution of digoxin in the body. The relative contributions of re-distribution and impaired renal clearance of digoxin to the increase in digoxin steady-state levels are presently unknown. 6 It is recommended that close monitoring of digoxin concentration and appropriate reduction of the maintenance dose is undertaken when quinidine is to be given to patients on digitalis therapy.

Assessment of electrical conversion of supraventricular arrhythmias

Direct current shock therapy in 244 cases of atrial fibrillation, fifty cases of paroxysmal supraventricular tachycardia, and eighteen cases of atrial flutter has been assessed. The administration of intravenous diazepam (Valium) allowed direct current shock therapy without general anaesthesia and tracheal intubation.

Seventy percent of cases of atrial fibrillation, 96% of cases of paroxysmal supraventricular tachycardia, and 94% of cases of atrial flutter reverted to sinus rhythm. The 190 patients first treated were followed up after the attempt at conversion until relapse of the arrhythmia or for a minimum of 2 years. The best results were obtained in arrhythmia of thyrotoxic aetiology. In patients with ischaemic/hypertensive heart disease and rheumatic heart disease the results were disappointing.

The success rate with direct current shock therapy is inversely related to the cardiothoracic index, and with increase in heart size there is also deterioration in the long-term results. Only one of the patients who had a cardiothoracic index of more than 55% remained in sinus rhythm at the end of 2 years.

The success rate decreases with increasing duration of the preceding arrhythmia, particularly when it had been present for more than 1 year, and the stability of the attained sinus rhythm also decreases with increasing duration of the preceding arrhythmia.

Direct current shock therapy is the method of choice in most cases of atrial flutter. Direct current shock therapy is seldom indicated in paroxysmal supraventricular tachycardia except in cases with underlying serious heart disease. Direct current shock therapy is contra-indicated in digitalis-induced arrhythmias.

Plasma digoxin concentrations in patients with atrial fibrillation

Plasma digoxin concentrations were measured by radioimmunoassay in 116 patients with atrial fibrillation on long-term oral treatment with the drug, and in 23 patients with digoxin toxicity. The mean concentrations were 1.4 ng./ml. and 3.1 ng./ml., respectively. Though an overlap occurred between the therapeutic and toxic ranges, toxicity is unlikely to occur below a level of 2 ng./ml. Plasma concentration showed a poor correlation with resting heart rate during atrial fibrillation. In patients with good renal function, however, a significant correlation was found between oral dose and plasma concentration. No evidence was obtained for increased sensitivity to therapeutic concentrations of the drug in elderly subjects, but the doses required to achieve these concentrations tended to be less than in younger patients.

Direct current shock and digitalis. A clinical and experimental study

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Electrical reversion of cardiac arrhythmias

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