Frequency-dependent effects of diltiazem on the atrioventricular node during experimental atrial fibrillation

Noninvasive Assessment of Atrioventricular Nodal Function: Effect of Rate-Control Drugs during Atrial Fibrillation

BACKGROUND

During atrial fibrillation (AF), conventional electrophysiological techniques for assessment of refractory period or conduction velocity of the atrioventricular (AV) node cannot be used. We aimed at evaluating changes in AV nodal properties during administration of tecadenoson and esmolol using a novel ECG-based method.

METHODS

Fourteen patients (age 58 ± 8 years, 10 men) with AF were randomly assigned to either 75 or 300 μg intravenous tecadenoson. After tecadenoson wash-out, patients received esmolol continuously (100 μg/kg per min for 10 mins, then 50 μg/kg per min for 50 mins). Atrial fibrillatory rate (AFR) and heart rate (HR) were assessed in 15-min segments. Using the novel method, we assessed the absolute refractory periods of the slow and fast pathways (aRPs and aRPf) of the AV node to produce an estimate of the functional refractory period.

RESULTS

During esmolol infusion, AFR and HR were significantly decreased and the absolute refractory period was significantly prolonged in both pathways (aRPs: 387 ± 73 vs 409 ± 62 ms, P < 0.05; aRPf: 490 ± 80 vs 529 ± 58 ms, P < 0.05). During both tecadenoson doses, HR decreased significantly and AFR was unchanged. Both aRPs and aRPf were prolonged for a 75 μg dose (aRPs: 322 ± 97 vs 476 ± 75 ms, P < 0.05; aRPf: 456 ± 102 vs 512 ± 55 ms, P < 0.05) whereas a trend toward prolongation was observed for a 300 μg dose.

CONCLUSIONS

The estimated parameters reflect expected changes in AV nodal properties, i.e., slower conduction through the AV node for tecadenoson and prolongation of the AV node refractory period for esmolol. Thus, the proposed approach may be used to assess drug effects on the AV node in AF patients.

Rate-dependent AV nodal refractoriness: a new functional framework based on concurrent effects of basic and pretest cycle length

The atrioventricular (AV) node filters atrial impulses. Underlying rate-dependent refractory properties are assessed with the effective (ERPN; longest nonconducted atrial cycle length) and functional (FRPN; shortest His bundle cycle) refractory period determined with premature protocols at different basic rates. Fast rates prolong ERPN and shorten FRPN, but these effects vary with subjects, age, and species. We propose that these opposite and variable effects reflect the net sum of concurrent cumulative and noncumulative effects associated with basic (BCL) and pretest cycle length (PTCL), respectively. To test this hypothesis, we assessed selective and combined effects of five BCL (S1S1) and six PTCL (S1S2) on ERPN, FRPN, and their subintervals (ERPN = A2H2 + H2A3 and FRPN = H2A3 + A3H3, where A is atrium and H is His bundle) with S1S2S3 protocols in six rabbit heart preparations. At control BCL, PTCL shortenings prolonged ERPN (113 ± 12 vs. 101 ± 14 ms, P < 0.01) as a net result of prolonged A2H2 and curtailed H2A3. At control PTCL, BCL shortenings increased ERPN (127 ± 20 vs. 101 ± 14 ms, P < 0.01) by prolonging A2H2. FRPN did not vary with BCL but decreased (163 ± 6 vs. 175 ± 10 ms, P < 0.01) with PTCL that curtailed H2A3. Equal BCL and PTCL shortenings as in standard protocols prolonged ERPN but left FRPN unchanged. Notably, ERPN and FRPN significantly correlated through their H2A3 subinterval. In conclusion, BCL and PTCL are both important determinants of AV nodal refractoriness and together account for rate-induced changes in ERPN and FRPN observed during standard premature protocols. ERPN and FRPN are related variables. Similar functional rules may govern nodal refractory behavior during supraventricular tachyarrhythmias.

Re-examining the clinical safety and roles of calcium antagonists in cardiovascular medicine

Despite recent analyses questioning the safety of calcium antagonists, evidence and clinical practice strongly support a major role for these drugs in the management of many cardiovascular diseases such as arrhythmia, vascular spasm, hypertension, diastolic dysfunction, stable angina, and myocardial infarction. These agents are a heterogeneous class of drugs with each formulation possessing unique properties and clinical applications. This article presents a review of the available literature and discusses the recommended use of various calcium antagonists in the treatment of diseases of the heart and vascular system.

Pharmacologic control of heart rate in atrial fibrillation

Atrial fibrillation is associated with a resting heart rate in excess of age-matched subjects in sinus rhythm, and there is an additional steep rise in rate during exertion. This article reviews the factors responsible for this tachycardia, the pharmacologic agents commonly used for heart rate control, and the effects of atrial antiarrhythmic agents on the heart rate during paroxysmal atrial fibrillation.

Newer developments in the management of atrial fibrillation

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia encountered in clinical practice. Unlike reentrant supraventricular tachycardia and malignant ventricular tachyarrhythmias, for which highly effective and safe nonpharmacologic therapies are available, the treatment of AF remains controversial and often problematic. Whereas electrical cardioversion restores sinus rhythm in most patients with AF, the maintenance of sinus rhythm often requires membrane-active antiarrhythmic drugs that may increase mortality by inducing ventricular proarrhythmia. The control of ventricular response rate, often associated with oral anticoagulation to prevent thromboembolic complications, is an alternative strategy in AF management. The relative efficacy and risks of these strategies and their respective role in different patient subgroups remain to be established. This article focuses on newer developments in the management of AF, including prospects for improved methods to maintain sinus rhythm, newer approaches to rate control, controversies regarding the use of oral anticoagulation, and novel nonpharmacologic therapies. These newer developments may lead over the next 10 years to a revolution in the management of AF as profound as that produced over the last 10 years by nonpharmacologic therapy of other arrhythmias.

Effects of diltiazem on concealed atrioventricular nodal conduction in relation to ventricular response during atrial fibrillation in anesthetized dogs

By means of a new quantitative index for concealed conduction, we evaluated the effects of diltiazem on atrioventricular (AV) node concealment and correlated this index with the variability of the ventricular response during atrial fibrillation in 16 anesthetized mongrel dogs. After determination of the atrial effective refractory period (ERP), AV nodal ERP (AVNERP), concealment zone, and concealment index (AVNERP of blocked atrial extrasystole/AVNERP of conducted atrial extrasystole), the R-R intervals during atrial fibrillation induced by electrical stimulation were measured. Both low (0.1 mg/kg) and medium (0.2 to 0.4 mg/kg) doses of diltiazem prolonged the AVNERP and increased the mean R-R interval during atrial fibrillation. Only medium doses of diltiazem increased the degree of concealed conduction in the AV node and accentuated the variability of R-R intervals. There was a good positive correlation between the variability of the ventricular response during atrial fibrillation and the concealment index. In conclusion, medium doses of diltiazem are more effective in reducing heart rate during atrial fibrillation than a low dose. However, medium doses also increase the degree of concealed conduction in the AV node and enhance the irregularity of the ventricular response during atrial fibrillation. Measurement of the concealment index is useful for quantitating the degree of concealed conduction in the AV node, which is actually an important determinant of the ventricular response during atrial fibrillation.

Rate-dependence of class III actions in the heart

The pharmacodynamics of many antiarrhythmic drugs are altered by heart rate. The ability of sodium channel blockers to decrease conduction velocity (class I action) is more pronounced with rapid heart rates. Drugs with class III action increase action potential duration and refractoriness in the heart. Most drugs with class III actions, currently being developed, produce their action by blocking one or several of the potassium channels responsible for repolarization. In vitro and in vivo studies have shown that their ability to increase repolarization time is less pronounced, or even disappears, at rapid pacing or heart rates. This so called 'inverse' rate-dependence of class III action is a characteristic of all drugs currently used in man except amiodarone, for which prolongation of repolarization time persists to a limited extent with rapid heart rates. It has been suggested that one possible mechanism of the inverse rate-dependence of class III action is related to the preferential binding of drugs to the potassium channels in the closed, polarized state. An inverse rate-dependence of class III action has also been found on prolongation of refractoriness. However, preliminary studies suggest that the positive inotropism of class III drugs not only persists but may increase with rapid heart rates. The clinical consequences of this phenomenon remain unclear, especially in view of the fact that the rate-dependence of class III action on dispersion of repolarization has not been specifically studied and that class III actions tend to decrease in ischemic tissues. However, the increase of action prolongation at slow heart rates may contribute to the bradycardia-dependent development of torsades de pointes arrhythmias.(ABSTRACT TRUNCATED AT 250 WORDS)

Redefining the role of digoxin in the treatment of atrial fibrillation

Atrial fibrillation (AF) encompasses a variety of discrete clinical syndromes, including paroxysmal, chronic, acute, and postoperative. Digoxin, long considered the mainstay of therapy for rate control in all types of AF, appears to have only modest electrophysiologic effects, which are mediated primarily by the autonomic nervous system. Digoxin has less potency than the calcium antagonists or beta-blocking drugs with respect to atrioventricular nodal blockade. Although less potent than calcium antagonists or beta-blocking drugs on the atrioventricular node, digoxin provides positive inotropic support, whereas the other 2 agents can suppress left ventricular function. Thus, digoxin is the agent of choice in patients with AF in the setting of significant left ventricular dysfunction. However, in the absence of left ventricular dysfunction, digoxin should be considered second-line therapy for the treatment of all AF syndromes.

Role of calcium antagonists for heart rate control in atrial fibrillation

Atrial fibrillation is one of the most common symptomatic sustained arrhythmias seen in clinical practice. Many patients with atrial fibrillation and a ventricular response greater than 120 beats/min will experience cardiac symptoms. In the past, control of heart rate in these patients consisted of administration of intravenous digoxin, but this often proved to be ineffective or limited by toxicity. Recently, intravenous beta blockers such as esmolol have been used to slow the ventricular rate during atrial arrhythmias, but in some studies their use has been limited by hypotension. Alternatively, a bolus of an intravenous calcium antagonist, e.g., diltiazem or verapamil, may be administered to achieve acute slowing of the ventricular response. An intravenous bolus of diltiazem or verapamil may be effective, but use of either may be limited by its short duration of action and the inability to administer repeated boluses to tightly control or "fine tune" the heart rate. However, a new bolus plus maintenance infusion technique with diltiazem has shown promise in initial studies. It appears that in the future, continuous infusion techniques with intravenous calcium antagonists will be available that provide safe and effective sustained control of the ventricular response during atrial arrhythmias.