Slow channel calcium antagonists in the treatment of supraventricular tachycardia

Tetrandrine: a new ligand to block voltage-dependent Ca2+ and Ca(+)-activated K+ channels

Extensive pharmacological investigations on tetrandrine, one of the traditional medicinal alkaloids, are reviewed. Tetrandrine has been used clinically in China for centuries in the treatment of many diseases. A recent series of studies has revealed major mechanisms underlying its multiple pharmacological and therapeutic actions. One of the most interesting discoveries is that tetrandrine is a new kind blocker of the voltage-activated, L-type Ca2+ channel in a variety of excitable cells, such as cardiac, GH3 anterior pituitary and neuroblastoma cells, as well as in rat neurohypophysial nerve terminals. Although tetrandrine does not belong to any of the three classical Ca2+ channel blocker groups, electrophysiological and radioligand binding studies show that tetrandrine is an L-type Ca2+ channel blocker with its binding site located at the benzothiazepine receptor on the alpha 1-subunit of the channel. In addition, tetrandrine is a blocker of the voltage-dependent T-type Ca2+ channel. It is clear that tetrandrine's actions in the treatment of cardiovascular diseases, including hypertension and supraventricular arrhythmia, are due primarily to its blocking of voltage-activated L-type and T-type Ca2+ channels. Furthermore, this alkaloid is a potent blocker of the Ca(2+)-activated K+ (K(Ca)) channels of neurohypophysial nerve terminals. The blocking kinetics of tetrandrine on the K(Ca) channel is quite different from that of typical K(Ca) channel blockers such as tetraethylammonium and Ba2+. Although the clinical role of tetrandrine as a blocker of the K(Ca) channels is unclear, it is a promising ligand for the study of K(Ca) channel function.

The pharmacologic treatment of atrial fibrillation

The pharmacologic treatment of atrial fibrillation (AF) is aimed at controlling the ventricular response, restoring sinus rhythm, and preventing or delaying relapses. In the control of ventricular response, digitalis maintains a primary role when the arrhythmia is accompanied by heart failure. In ischemic, hypertensive, and degenerative (whose number is increasing at present) cardiopathies without evident ventricular dilatation, treatments with calcium antagonists (such as verapamil, gallopamil, or diltiazem) or beta-blocking agents must be preferred. In order to control the ventricular response in patients with chronic AF during physical activity, the association of digitalis with beta-blocking agents or calcium antagonists seems to provide satisfactory results. The drugs of the IC class, especially flecainide, represent a certain therapeutical progress in the restoration of sinus rhythm in the treatment of paroxysmal atrial fibrillation affecting subjects without evident alterations of ventricular function, particularly in subjects with Wolff-Parkinson-White syndrome, with forms of vagal origin, or with atrial fibrillation alone. A therapeutic combination of digitalis and quinidine may produce resolution of the arrhythmia in the presence of altered ventricular function or when AF is of an uncertain onset. In patients with hypertensive, ischemic, and/or degenerative cardiopathy without evident ventricular or advanced heart failure, the verapamil-quinidine association may also be effective and even quicker. The combination of drugs of the I and III class for restoration of the sinus rhythm in particularly resistant forms of AF without evident structural heart alterations is promising but must be verified in a greater number of patients. In the prevention of relapses amiodarone appears to have the widest spectrum of advantages from an electrophysiologic point of view; however, because of its many side effects, amiodarone represents a late therapeutical choice. The promising results obtained with flecainide are disputed by the results of the CAST, which limit the possibilities of using this drug to a low number of cases (W.P.W. syndrome, AF of vagal origin, atrial fibrillation alone). In the past, quinidine and disopyramide have been the drugs most widely used in the prophylaxis of AF. These drugs have a similar efficacy, and both of them provided some positive results. However, because of untoward side effects (especially for quinidine) during chronic treatment, the use of these drugs has been questioned. Perhaps in the majority of patients, the less dangerous therapeutic choice after the termination of the fibrillation is a combination of drugs slowly down AV node activity (digitalis or calcium antagonists and beta blockers) with class IA antiarrhythmics.

Antiarrhythmic actions of diltiazem during experimental atrioventricular reentrant tachycardias. Importance of use-dependent calcium channel-blocking properties

The purpose of this study was to determine if the known frequency-dependent effects of diltiazem on inward calcium current result in selective actions during supraventricular tachycardia. These effects were evaluated by use of an experimental model of orthodromic atrioventricular reentrant tachycardia (AVRT). AVRT was induced in 15 dogs over a wide range of retrograde conduction times before and after two doses of diltiazem. Diltiazem produced a tachycardia-related suppression of atrioventricular nodal conduction resulting in greater efficacy for faster than for slower AVRTs. The degree of slowing for tachycardias that remained inducible after diltiazem administration was greater for AVRTs with a rapid initial rate (dose 1, 29%; dose 2, 40%) than for slower AVRTs (dose 1, 11%, p less than 0.01; dose 2, 18%, p less than 0.001). Rate-dependent AVRT slowing occurred because of a time-dependent phase of AH interval prolongation after the onset of tachycardia, which was observed only after diltiazem administration. to further clarify the mechanism of diltiazem's selective actions against faster tachycardias, its effects on the minimum pathway for reentry, or wavelength, were examined in four dogs. The ratio of refractory period to revolution time (RP/RT), an index of wavelength, was measured for each AVRT before and after diltiazem administration. Diltiazem increased the positive slope of the relation between RP/RT and the AVRT rate threefold compared with control (p less than 0.05). This rate-dependent effect prevented AVRT when RP/RT became greater than unity. In conclusion, rate-dependent atrioventricular node depression by diltiazem results in greater tachycardia slowing and higher rates of termination during atrioventricular reentrant tachycardias with faster initial rates and shorter retrograde conduction intervals.

Calcium channel antagonists: Part VI: Clinical pharmacokinetics of first and second-generation agents

A survey of the pharmacokinetic properties of the three prototypical calcium antagonist agents shows that they have in common a very high rate of hepatic first-pass metabolism with, in the case of verapamil and diltiazem, the formation of an active metabolite that affects the dose during chronic therapy. Therefore, the major factor altering the pharmacokinetic properties and the dose of the drug required is the capacity of the liver to metabolize the drug, which in turn depends on the hepatic blood flow and the activity of the hepatic metabolizing systems. Hence liver disease, a low cardiac output, and coadministration of certain drugs inducing or inhibiting the hepatic enzymes, all indirectly affect the pharmacokinetic properties of the calcium antagonists. There are also other potential drug interactions of a kinetic or dynamic nature that may arise. In general, renal disease has little effect on the pharmacokinetics of calcium antagonists.

Efficacy and safety of intravenous diltiazem for treatment of atrial fibrillation and atrial flutter. The Diltiazem-Atrial Fibrillation/Flutter Study Group

This study evaluates the effectiveness and safety of intravenous diltiazem for the treatment of atrial fibrillation and atrial flutter. A double-blind, parallel, randomized, placebo-controlled protocol was used, and 6 large, urban hospitals, both university-affiliated and private, participated. The study involved 113 patients with atrial fibrillation or flutter, a ventricular rate greater than or equal to 120 beats/min and systolic blood pressure greater than or equal to 90 mm Hg without severe heart failure. The dose of intravenous diltiazem (or identical placebo) was 0.25 mg/kg/2 minutes followed 15 minutes later by 0.35 mg/kg/2 minutes if the first dose was tolerated but ineffective. If a patient did not respond, the code was broken and the patient was allowed to receive open-label diltiazem if placebo had been given. Of 56 patients, 42 (75%) randomized to receive diltiazem responded to 0.25 mg/kg and 10 of 14 responded to 0.35 mg/kg, for a total response rate of 52 of 56 patients (93%), whereas 7 of 57 patients (12%) responded to placebo (p less than 0.001). After the double-blind protocol, 49 of the 57 patients who received placebo were then given diltiazem; 47 of 49 responded, for an overall response rate of 99 of 105 patients (94%) with diltiazem. The median time from the start of drug infusion to the maximal decrease in heart rate was 4.3 minutes. Side effects occurred in 14 patients, 7 of whom had asymptomatic hypotension not requiring intervention. Thus, intravenous diltiazem was rapidly effective for slowing the ventricular response in most patients with atrial fibrillation or atrial flutter. Blood pressure decreased slightly. Side effects were mild.

Effects of halothane and calcium entry blockers on atrioventricular conduction-a comparative study of verapamil, diltiazem, and nifedipine

The effects of halothane on AV nodal function were evaluated in dogs with verapamil, diltiazem, or nifedipine during atrial pacing using the technique of His-bundle electrocardiography. Fifty-one mongrel dogs were divided into six groups. Anesthesia was induced with ketamine 100 mg im. and thiamylal 25 mg/kg iv. The animals were intubated and mechanically ventilated at normocapneic levels. Anesthesia was maintained with 50% nitrous-oxide in oxygen with pancuronium 2 mg im. Dogs in groups I, III, and V were anesthetized with 0.8% halothane and 50% nitrous-oxide in oxygen. We observed interactions between halothane and intravenous administration of either verapamil 0.1 mg/kg, diltiazem 0.15 mg/kg, or nifedipine 0.01 mg/kg respectively. Dogs in groups II, IV, and VI were administered either verapamil, diltiazem, or nifedipine iv without halothane. There were prolongations of sinus cycle length (SCL) (414 +/- 10 to 542 +/- 19 msec.), atrium-His (AH) interval (73 +/- 3 to 97 +/- 5 msec.), and functional refractory period (FRP) of the AV-node (227 +/- 5 to 260 +/- 5 msec.) in halothane anesthesia in groups I, III, and V. There were more prolongations of these variables after iv administration of verapamil (SCL; 617 +/- 35, AH; 118 +/- 7, FRP of the AV node; 311 +/- 4) and diltiazem (SCL; 554 +/- 19, AH; 118 +/- 12, FRP of the AV node; 283 +/- 12) but no prolongations after nifedipine (SCL; 533 +/- 19, AH; 99 +/- 8, FRP of the AV node; 272 +/- 9). Comparing effects of calcium entry blockers with and without halothane in groups I and II, III and IV, or V and VI, there were additive depressing effects of halothane with either verapamil or diltiazem on AV nodal function. And there is a difference between the effects of nifedipine on SCL with and without halothane.

Calcium channel antagonists. Part III: Use and comparative efficacy in hypertension and supraventricular arrhythmias. Minor indications

The major antihypertensive mechanism of calcium antagonists is by decreasing the systemic vascular resistance, modified by the counter-regulatory responses of the baroreflexes and the renin-angiotensin-aldosterone system. In severe hypertension, the concept that calcium overload of the vascular myocyte could precipitate or aggravate peripheral vasoconstriction provides a logical basis for the use of these agents as first choice therapy; nifedipine, especially, has been well tested. As monotherapy for mild to moderate hypertension each of the three first-generation agents compares well with beta-blockers. Calcium antagonists may have a special role in the therapy of certain patient groups (elderly, black) or in those subjects whose life style involves intense physical or mental exertion (hemodynamics better maintained than with beta-blockade) or in patients with early end-organ damage such as left ventricular hypertrophy or renal insufficiency. However, the goal blood pressure may not be reached during monotherapy so that drug combinations may be required. Further indications for these compounds are as follows. Verapamil and diltiazem are frequently used in supraventricular tachycardias including acute and chronic atrial fibrillation. In the arrhythmias of the Wolff-Parkinson-White syndrome, there is the potential danger of provocation of anterograde conduction. Further indications for calcium antagonists, still under evaluation, include congestive heart failure (controversial), hypertrophic cardiomyopathy (verapamil), primary pulmonary hypertension (high doses required), Raynaud's phenomenon (nifedipine and diltiazem effective), peripheral vascular disease (proof not yet documented), cerebral insufficiency and subarachnoid hemorrhage (nimodipine promising), migraine, exertional bronchospasm, renal disease, atherosclerosis (experimental), and primary aldosteronism (nifedipine inhibits aldosterone release). Second-generation agents include dihydropyridines, such as nitrendipine, nicardipine, felodipine, amlodipine, nisoldipine, nimodipine, and isradipine. From these will be selected agents that are longer acting and provide higher vascular selectivity. New preparations of existing agents include slow-release formulations of nifedipine, verapamil, and diltiazem. Minor side effects include those caused by vasodilation (flushing and headaches), constipation (verapamil), and ankle edema. Serious side effects are rare and result from improper use of these agents, as when intravenous verapamil is given to patients with sinus or atrioventricular nodal depression from drugs or disease, or nifedipine to patients with aortic stenosis. The potential of a marked negative inotropic effect is usually offset by afterload reduction, especially in the case of nifedipine. Yet caution is required when calcium antagonists, especially verapamil, are given to patients with myocardial failure unless caused by hypertensive heart disease. Drug interactions of calcium antagonists occur with other cardiovascular agents such as alpha-adrenergic blockers, beta-adrenergic blockers, digoxin, quinidine, and disopyramide.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium channel blockers

The calcium channel blockers initially were approved for the treatment of classical and variant angina pectoris. Recent studies indicate that these agents also are useful in such diverse conditions as pulmonary and systemic hypertension, hypertrophic cardiomyopathy, arrhythmias, asthma, Raynaud's syndrome, esophageal spasm, myometrial hyperactivity, cerebral arterial spasm, and migraine.

Calcium channel blockers in emergency medicine

Diltiazem, nifedipine, and verapamil inhibit calcium entry into cells via different mechanisms with different pharmacologies. They display different relative effects on different cardiovascular functions, a complex interplay of direct actions and adrenergic reflexes. Peripheral arterial vasorelaxation causes adrenergic reflex activity which opposes their direct negative chronotropic, dromotropic, inotropic, and hypotensive actions. Verapamil's most potent activity is electrophysiologic, and nifedipine's effects are hemodynamic; diltiazem acts like a less-potent combination of verapamil and nifedipine. All three drugs are efficacious in angina. These three drugs may not be interchangeable in all patients, but individualization of therapy is possible. Future indications for calcium channel blocker therapy may include hypertrophic cardiomyopathy, cerebral vasospasm, migraine headaches, pulmonary hypertension, asthma, esophageal spasm, intestinal ischemia, Raynaud's phenomenon, dysmenorrhea, and premature labor.

Cardiac arrhythmias. Emergency management

Cardiac arrhythmias often call for emergency treatment, and often it is the primary care physician who must respond quickly and effectively. In this article, Dr Brent reviews the various types of arrhythmias and the methods available for dealing with them on an emergency basis. According to one peer reviewer, this article contains "the information that the clinician should have at the tips of his or her fingers at all times."