Effects of digoxin on sinus nodal function before and after vagal blockade in patients with sinus nodal dysfunction: a clue to the mechanisms of the action of digitalis on the sinus node

Diagnosis and practical management of digoxin toxicity: a narrative review and consensus

There are currently no universally accepted guidelines for the management of digoxin toxicity. In the absence of clinical practice guidelines, a set of consensus recommendations for management of digoxin toxicity in the clinical setting were developed through a modified Delphi approach. The recommendations highlight the importance of early recognition of signs of potentially life-threatening toxicity that requires immediate treatment with digoxin-specific antibodies. The consensus identifies a straightforward approach to dosing immune antibody fragments according to the presence or absence of signs of life-threatening toxicity. Supportive measures and management of specific signs of toxicity are also covered.

Drug Interactions Affecting Antiarrhythmic Drug Use

Antiarrhythmic drugs (AAD) play an important role in the management of arrhythmias. Drug interactions involving AAD are common in clinical practice. As AADs have a narrow therapeutic window, both pharmacokinetic as well as pharmacodynamic interactions involving AAD can result in serious adverse drug reactions ranging from arrhythmia recurrence, failure of device-based therapy, and heart failure, to death. Pharmacokinetic drug interactions frequently involve the inhibition of key metabolic pathways, resulting in accumulation of a substrate drug. Additionally, over the past 2 decades, the P-gp (permeability glycoprotein) has been increasingly cited as a significant source of drug interactions. Pharmacodynamic drug interactions involving AADs commonly involve additive QT prolongation. Amiodarone, quinidine, and dofetilide are AADs with numerous and clinically significant drug interactions. Recent studies have also demonstrated increased morbidity and mortality with the use of digoxin and other AAD which interact with P-gp. QT prolongation is an important pharmacodynamic interaction involving mainly Vaughan-Williams class III AAD as many commonly used drug classes, such as macrolide antibiotics, fluoroquinolone antibiotics, antipsychotics, and antiemetics prolong the QT interval. Whenever possible, serious drug-drug interactions involving AAD should be avoided. If unavoidable, patients will require closer monitoring and the concomitant use of interacting agents should be minimized. Increasing awareness of drug interactions among clinicians will significantly improve patient safety for patients with arrhythmias.

Mutation of the Na+/K+-ATPase Atp1a1a.1 causes QT interval prolongation and bradycardia in zebrafish

The genetic underpinnings that orchestrate the vertebrate heart rate are not fully understood yet, but of high clinical importance, since diseases of cardiac impulse formation and propagation are common and severe human arrhythmias. To identify novel regulators of the vertebrate heart rate, we deciphered the pathogenesis of the bradycardia in the homozygous zebrafish mutant hiphop (hip) and identified a missense-mutation (N851K) in Na⁺/K⁺-ATPase α1-subunit (atp1a1a.1). N851K affects zebrafish Na⁺/K⁺-ATPase ion transport capacity, as revealed by in vitro pump current measurements. Inhibition of the Na⁺/K⁺-ATPase in vivo indicates that hip rather acts as a hypomorph than being a null allele. Consequently, reduced Na⁺/K⁺-ATPase function leads to prolonged QT interval and refractoriness in the hip mutant heart, as shown by electrocardiogram and in vivo electrical stimulation experiments. We here demonstrate for the first time that Na⁺/K⁺-ATPase plays an essential role in heart rate regulation by prolonging myocardial repolarization.

Evaluation of the Toxicity of Adonis Aestivalis in Calves

Toxicosis of Adonis aestivalis is well documented in horses, but little is known of its toxicity in cattle. A. aestivalis (summer pheasant's eye) was collected over multiple years, under different growing conditions, and at various stages of maturity, dried, and administered to calves to evaluate the toxicity of A. aestivalis in cattle. Four 300-lb Holstein, and 2 90-lb, preruminating Jersey calves were administered 1% body weight of ground A. aestivalis via a stomach tube and monitored for clinical signs for 2 weeks and 1 week, respectively. The Holstein calves were then fed 0.2 to 1% body weight A. aestivalis daily for 4 to 5 weeks. The Holstein calves had transient, mild cardiac abnormalities during the feeding trial. Mild, transient gastrointestinal and cardiac signs were noted in the preruminating calves. No gross or microscopic lesions were seen on necropsies performed at the end of the study. Based on the results of this study, cattle do not appear to be as susceptible to toxicosis from A. aestivalis as other species, such as horses and pigs.

Case files of the medical toxicology fellowship of the California poison control system-San Francisco: calcium plus digoxin-more taboo than toxic?

A 68 year old woman with a history of diabetes mellitus presented to the emergency department after experiencing several days of nausea and vomiting. Recorded vital signs included: blood pressure 98/32 mm Hg, heart rate 69 bpm, "normal" respirations, and no fever. Her initial ECG revealed peaked T waves and regular rhythm with no visible P waves. Initial serum electrolytes were notable for a potassium level of 7.7 mmol/L, and a creatinine level of 9.6 mg/dL (849 micromol/L). She was unable to provide a list of her current medications or other medical history. There was no old chart immediately available for comparison.

[Cognitive impairment induced by digoxin intake in patients older than 65 years]

Cognitive impairment or clinical signs of dementia in an old patient who receives digoxin, should suggest a digitalis intoxication. Symptoms can be present although a normal digoxin serum concentration. It is recommended to stop the treatment to obtain a regression of dementia symptoms.

Absence of interactive effects of trans-1,2-cyclohexanediol, a major metabolite of the side-chain of candesartan cilexetil, on digoxin-induced arrhythmias in dogs

trans-1,2-Cyclohexanediol, the major metabolite of the cilexetil moiety of candesartan cilexetil (CC), has been reported to have potent pro-arrhythmic effects in dogs with congestive heart failure (CHF), especially when co-administered with digoxin. To verify this and to clarify the clinical relevance and the underlying mechanisms, a series of in vivo and in vitro experiments was conducted. When CC up to 300 mg/kg was administered orally to intact dogs, no changes in the electrocardiograms (ECG) or the required cumulative doses of ouabain to induce ventricular arrhythmias were observed. In dogs with CHF, intravenous bolus administration of trans-1,2-cyclohexanediol at 4 mg/kg followed by continuous infusion at 0.1 mg x kg(-)(1) x min(-)(1) had no effects on the ECG parameters, the type, incidence, and onset time of digoxin-induced arrhythmias or the metabolism of digoxin. In an in vitro experiment using isolated guinea pig papillary muscle, trans-1,2-cyclohexanediol (1 - 100 micromol/L) showed no effects on any parameter of the action potentials. Because no effects were observed in these experiments where the exposure levels of trans-1,2-cyclohexanediol were extremely high compared to those in humans given the maximum therapeutic dose of CC, it is unlikely that CC would induce arrhythmias in clinical use even in patients treated with cardiac glycosides.

Drug effects on the sinus node. A clinical perspective

The direct and indirect effects of drugs on the sinoatrial (SA) node are described in relation to basic cellular mechanisms and clinical applications. The effects of the different classes of antiarrhythmic agents are considered in terms of their direct depressant and excitatory actions. Indirect actions, particularly autonomic effects, are also discussed. Clinical aspects of the pharmacologic management of disorders of sinus node function such as the bradycardia-tachycardia syndrome, inappropriate sinus tachycardia, sinus bradycardia, and the identification of sinus node dysfunction by drug effects are considered in detail.

Reappraisal of atrioventricular junctional pacemaker automaticity in the sick sinus syndrome. Clinical significance and the role of autonomic chronotropic influences

Postpacing impulse recovery times of the junctional tissue (junctional automaticity) were determined by atrial or ventricular overdrive pacing in 27 patients with dysfunction of the sinus node. The maximum junctional recovery time (MJRT) could be measured in 22 patients and ranged from 1,630 to 9,730 ms (mean 3,860 +/- 2,077); the maximum corrected junctional recovery time (MJRTc) could be measured in 18 patients and ranged from 140 to 5,986 ms (mean 2,089 +/- 1,529). Autonomic influence on the JRTs was evaluated by intravenous administration of atropine (1.5 mg) alone or in combination with propranolol (5 to 6 mg). Of the seven patients in whom MJRTc and/or MJRT could be measured before and after drug intervention, the JRTs shortened in four and prolonged in three after combination of atropine and propranolol. Atropine alone shortened MJRT in all eight patients studied. Our data reveal that both vagal and catecholamine-dependent factors (especially vagal over-activity) are operative in the escape mechanism of the junctional tissue.

Clinical types of proarrhythmic response to antiarrhythmic drugs

The problem of drug-induced or drug-aggravated cardiac arrhythmias has been recognized for many years. Digitalis glycosides and class I, II, III, or IV antiarrhythmic drugs can cause severe sinus node dysfunction or atrioventricular block. Digitalis can cause a variety of supraventricular arrhythmias; atrial tachycardia with block and nonparoxysmal atrioventricular junctional tachycardia are the most characteristic. Recognition that class IA and class III antiarrhythmic drugs can aggravate arrhythmias or cause new arrhythmias in patients being treated for potentially malignant or malignant ventricular arrhythmias has intensified the interest in proarrhythmia in recent years. Several characteristic types of proarrhythmic response have been described. Torsades de pointes (multiform) ventricular tachycardia (VT) accompanied by prolongation of the QT interval can be caused by class IA and class III antiarrhythmic drugs as well as other drugs that bind to the membrane sodium channels of ventricular cells, for example, tricyclic antidepressants. Uniform VT in patients with malignant ventricular arrhythmias and poor left ventricular function is a characteristic proarrhythmic response to class IC antiarrhythmic drugs. Bidirectional VT or accelerated idioventricular rhythm are characteristic of digitalis toxicity. More difficult to establish as proarrhythmic responses are increased frequency of ventricular premature depolarizations and increased ease of inducing VT with programmed ventricular stimulation.

Clinical effects of digoxin on sinus node and atrioventricular node function after pharmacologic autonomic blockade

The effects of digoxin on sinus node and atrioventricular (AV) node function were studied in 18 patients (mean age 53.6 years) with normal intrinsic heart rates. Electrophysiologic testing was performed both during basal state and after autonomic blockade with propranolol and atropine. Full digitalization was achieved by intravenous administration of digoxin (0.02 mg/kg) given in three divided doses over a 24-hour period. The following day, after a basal recording, autonomic blockade was again induced and the study was repeated. During basal state, digoxin significantly prolonged the sinus cycle length (SCL) (p less than 0.01) and the AH interval (p less than 0.01). However, when the intrinsic sinus node functions were compared (i.e., the values obtained after autonomic blockade), digoxin did not produce significant changes in intrinsic SCL, corrected sinus node recovery time, and sinoatrial conduction time. No significant changes were noted even in the intrinsic AH interval and AV nodal refractory periods. These findings suggest that: (1) intravenous administration of digoxin in therapeutic doses does not produce any depression of the intrinsic functions of the sinus node and AV node; and (2) the depressant effects induced by digoxin during basal state appear to be mediated through the autonomic nervous system.

Influence of digoxin on sinus node function after pharmacologic autonomic blockade

The effect of iv digoxin on normal sinus node function was studied after pharmacologic autonomic blockade (AB) in ten patients. Sinus cycle length (SCL), sinus node recovery time (SNRT) and sinoatrial conduction time (SACT) were determined before and after AB with propranolol (0.2 mg/kg body weight) and atropine sulfate (0.04 mg/kg body weight) iv, and 15 min, 30 min, and 45 min after 1 mg iv digoxin. AB resulted in a significant decrease (P less than 0.01) in SCL (916 +/- 158 to 716 +/- 120 ms), in SNRT (1,229 +/- 221 to 871 +/- 190 ms), and in SACT (79 +/- 34 to 44 +/- 10 ms). Fifteen minutes after iv digoxin there was no significant change observed in SCL (716 +/- 120 to 708 +/- 92 ms), in SNRT (871 +/- 190 to 864 +/- 148 ms), or in SACT (44 +/- 10 to 46 +/- 15 ms). Similar results were obtained 30 min after digoxin administration. It is concluded that a single therapeutic dose of digoxin has no direct effect on electrophysiologic parameters of normal intrinsic sinus node function.

Comparative electrophysiologic effects of digoxin in the nonsedated chronically instrumented puppy

The electrophysiologic effects of acute but not chronic administration of cardiac glycosides have been studied. Nineteen chronically instrumented beagle puppies underwent 2-week courses of parenteral digoxin in three dosage regimens: six received digoxin, 0.04 mg/kg/day; seven received 0.03 mg/kg/day; and 11 received 0.02 mg/kg/day. Mean serum concentrations were 3.2 ng/ml, 1.3 ng/ml, and 1.0 ng/ml, respectively. Significant electrophysiologic effects on sinus node function were produced only by the highest dose. Atrioventricular node conduction was significantly delayed among animals receiving both high and middle dosages. All three regimens significantly effected atrioventricular specialized conduction system functional refractory periods. Atropine decreased digoxin-induced effects on all measured parameters but totally eliminated the digoxin effect on the corrected sinus node recovery time.

Intracardiac electrophysiologic study of intravenous diltiazem and combined diltiazem-digoxin in patients

Fifteen patients without sinoatrial (SA) or atrioventricular (AV) node dysfunction underwent electrophysiologic study (EPS) before and after intravenous diltiazem: 0.20 mg/kg bolus followed by 0.0007 mg/kg/min infusion (seven patients) or 0.25 mg/kg bolus followed by 0.0012 mg/kg/min infusion (eight patients). In six patients intravenous digoxin (0.018 mg/kg) was given and 45 minutes later EPS was repeated while the diltiazem infusion continued. Diltiazem prolonged sinus cycle length (+7%, p less than 0.01), lengthened AH conduction time (+22% in constant rate atrial paced rhythm, p less than 0.001), prolonged AV node functional and effective refractory periods (+6%, p less than 0.01 and +16%, p less than 0.05, respectively), lengthened AV node Wenckebach cycle length (+13%, p less than 0.001), shortened atrial functional refractory period (-3%, p less than 0.05), and reduced mean arterial pressure (-8%, p less than 0.005 in constant rate atrial paced rhythm). Subsequently, intravenous digoxin further prolonged sinus cycle length (+12%, p less than 0.05), AH conduction time (+17%, p less than 0.05), AV node Wenckebach cycle length (+9%, p less than 0.05), and AV node functional refractory period (+7%, p less than 0.05), shortened atrial effective refractory period (-7%, p less than 0.05) and ventricular effective refractory period (-6%, p less than 0.05), and increased systolic arterial pressure (+6%, p less than 0.05). Diltiazem and digoxin have additive depressant effects on SA and AV node function without significant adverse effects.

Effects of digitalis on the human sick sinus node after pharmacologic autonomic blockade

To study the effects of digitalis on the sinus node and the mechanisms involved, 16 patients with the sick sinus syndrome had electrophysiologic assessment of sinus nodal function during (1) control study, (2) after pharmacologic autonomic blockade with propranolol (0.2 mg/kg body weight and atropine sulfate 0.04 mg/kg intravenously), and (3) 10 minutes after 0.01 mg/kg of intravenous ouabain. The study was completed within 30 minutes of pharmacologic autonomic blockade. During the control study 50 percent of patients had an abnormal corrected sinus nodal recovery time or abnormal sinoatrial conduction time, or both. The effects of ouabain on sinus nodal function were compared with those after pharmacologic autonomic blockade. Ouabain significantly increased both intrinsic sinus cycle length (ouabain 975 +/- 194 ms [mean +/- standard deviation]; autonomic blockade 1,025 +/- 218 ms, probability [p] less than 0.001) and corrected sinus nodal recovery time (ouabain 615 +/- 503 ms; autonomic blockade 575 +/- 536 ms, p less than 0.05). In contrast there was no significant change in sinoatrial conduction time after ouabain (ouabain 141 +/- 56 ms; autonomic blockade 132 +/- 45 ms; difference not significant). The effects of ouabain were similar in patients with both normal and abnormal sinus nodal function. These findings suggest that (1) digitalis in therapeutic doses has a depressant effect on intrinsic sinus nodal automaticity in patients with normal as well as abnormal sinus nodal function; (2) digitalis has no significant effects on sinoatrial conduction; and (3) the effects of digitalis on sinus nodal automaticity are primary and independent of its vagal and antiadrenergic effects.

Automaticity of subsidiary pacemakers of patients with dysfunction of the sinus node

Automaticity (postpacing impulse recovery time) of the sinus node or subsidiary pacemakers (or both) was determined by intracardiac recordings and atrial overdrive pacing in three patients with disease of the sinus node. Their electrocardiographic manifestations included sinus bradycardia, sinoatrial block, sinus arrest, atrioventricular junctional rhythm, and atrioventricular dissociation. One patient, who initially had syncope, demonstrated prolonged time for recovery of the sinus node (maximum corrected value, 3,485 msec) and impaired atrioventricular junctional automaticity (maximum corrected junctional recovery time, 1,460 msec). Of the remaining two patients without syncopal attacks, one had an adequate low atrial or junctional escape mechanism following cessation of pacing. The maximum low atrial escape interval was 1,660 msec, and the maximum junctional recovery time was 1,770 msec (corrected value, 140 msec). The third patient disclosed a prolonged maximum corrected recovery time for the sinus node (870 msec), while the maximum junctional recovery time was only 1,810 msec (corrected value, 160 msec). We conclude that automaticity of the subsidiary pacemakers in disease of the sinus node can be determined in the presence of atrioventricular dissociation or total sinus arrest, and that determination of the recovery time of the subsidiary pacemakers may be useful in identifying symptomatic patients with sinus nodal dysfunction.

Electrophysiologic effect of digitalis on sinoatrial nodal function in children

Despite the prevalence of digitalis usage in children, the electrophysiologic effects of digitalis on sinoatrial (S-A) nodal function is unknown in this age group. The purpose of this study was to determine the effect of digitalis on sinoatrial conduction time as well as on S-A nodal automatically. Ten subjects (mean age 10.5 years) underwent electrophysiologic assessment of S-A nodal function before and 30 minutes after administration of ouabain (0.01 mg/kg). Total S-A conduction time increased in each subject and the mean value after ouabain (182 msec +/- 13 standard errors of the mean [SEM]) was significantly higher (P less than 0.01) than before (149 msec +/- 11). The sinus cycle length was variable after ouabain (P greater than 0.1). The corrected sinus nodal recovery time also was variable (P greater than 0.1), decreasing substantially in three subjects. Mechanisms of the effect of digitalis on the S-A node and atrium are proposed and discussed. It is concluded that digitalis prolongs the S-A conduction time in children with normal S-A nodal function. By prolonging the S-A conduction time, digitalis may artifactually shorten corrected S-A nodal recovery time in some patients.