Effect of lidocaine on escape rate in patients with complete atrioventricular block: B. Proximal His bundle block

Structure, function and clinical relevance of the cardiac conduction system, including the atrioventricular ring and outflow tract tissues

It is now over 100years since the discovery of the cardiac conduction system, consisting of three main parts, the sinus node, the atrioventricular node and the His-Purkinje system. The system is vital for the initiation and coordination of the heartbeat. Over the last decade, immense strides have been made in our understanding of the cardiac conduction system and these recent developments are reviewed here. It has been shown that the system has a unique embryological origin, distinct from that of the working myocardium, and is more extensive than originally thought with additional structures: atrioventricular rings, a third node (so called retroaortic node) and pulmonary and aortic sleeves. It has been shown that the expression of ion channels, intracellular Ca(2+)-handling proteins and gap junction channels in the system is specialised (different from that in the ordinary working myocardium), but appropriate to explain the functioning of the system, although there is continued debate concerning the ionic basis of pacemaking. We are beginning to understand the mechanisms (fibrosis and remodelling of ion channels and related proteins) responsible for dysfunction of the system (bradycardia, heart block and bundle branch block) associated with atrial fibrillation and heart failure and even athletic training. Equally, we are beginning to appreciate how naturally occurring mutations in ion channels cause congenital cardiac conduction system dysfunction. Finally, current therapies, the status of a new therapeutic strategy (use of a specific heart rate lowering drug) and a potential new therapeutic strategy (biopacemaking) are reviewed.

Physiology of the escape rhythm after radiofrequency atrioventricular junctional ablation

The physiology of the escape rhythm (ER) and its response to pharmacological modulation under varying autonomic conditions were studied 48 patients undergoing radiofrequency ablation of the atrioventricular junction (AVJ) for refractory atrial fibrillation. The QRS morphology and cycle length (CL) of the baseline ER were measured 15 minutes postablation. The CL of the ER was measured in response to doses of isoproterenol, atropine, adenosine, lidocaine, and verapamil. The ER QRS was narrow (QRS < 120 ms) in 20 patients and wide (QRS > 120 ms) in 28 patients. Of the 28 patients with wide QRS ER, 11 patients had a new bundle branch block (8 patients new right bundle branch block [RBBB] and 2 patients new left bundle branch block [LBBB]). The ERCL was similar in both narrow and wide ERs (1,593 +/- 376 ms and 1,516 +/- 296 ms, P = 0.44). In 23 patients receiving isoproterenol infusion, the ER CL decreased with increasing doses from 1 mcg/min to 2 mcg/min (1,378 +/- 200 to 1,240 +/- 229 ms, P < 0.001), but did not decrease further at 3 mcg/min (1,201 +/- 192 ms, P = 0.48 vs 2 mg/min). Seven patients received 0.02 mg/kg of atropine, and ER decreased significantly (1,572 +/- 408 ms to 1,319 +/- 333 ms, P = 0.028). In 30 patients who received intravenous boluses of adenosine (6-18 mg), the ER did not change significantly. In 28 patients who received 150 mg of lidocaine, the ER increased from 1,462 +/- 286 ms to 1,715 +/- 467 ms (P < 0.001), and one patient developed transient asystole. Nineteen patients received 7.5 mg of verapamil, and the ER did not change (1,488 +/- 313 ms to 1,513 +/- 666 ms, P = 0.80). There was no significant difference in response to isoproterenol, adenosine, lidocaine, or verapamil between the patients with wide and narrow QRS ERs. We conclude that patients may have stable ERs immediately following AVJ ablation even when a wide complex ER results. The ER is responsive to sympathetic stimulation and vagal blockade. The ER is prolonged after lidocaine but not after verapamil, suggesting response to sodium but not to calcium channel blockade. These data are consistent with an ER originating in the distal compact AV node or proximal His bundle.

The influence of elevated Mg2+ concentrations on cardiac electrophysiologic parameters

This study was performed to evaluate the direct effects of magnesium ions on cardiac conduction velocities and on refractoriness using a modified Langendorff perfusion system and a special ECG recording and stimulation technique. An increase of the MG2+ concentration to 2.3 mM had no detectable effects. Higher concentrations of up to 4.6 mM decreased the spontaneous sinus rate and the conduction velocity through the AV node and the His bundle in a concentration-dependent manner. During stimulation with premature beats, in the presence of 4.6 mM of Mg2+, the effective and relative refractory periods (ERP, RRP) of sinoatrial, AV nodal, and His bundle conduction and the ERP of the ventricular myocardium were significantly prolonged, whereas the ERP of the atrial myocardium remained unaffected. The refractoriness of sinoatrial, AV nodal, and His bundle conduction, and of the ventricular myocardium, were also prolonged during pacing with a stepwise increased pacing rate. These observations suggest a marked rate-dependent inhibitory effect of magnesium on the refractoriness of the His-bundle conduction and of the ventricular myocardium, which might be an important factor in the antiarrhythmic effectiveness of magnesium in ventricular tachyarrhythmias.

Lidocaine-induced second-degree mobitz type II heart block

Lidocaine-induced atrioventricular (AV) conduction disturbances are an ill-defined phenomenon. Electrophysiological studies in both animal and human subjects have shown that therapeutic doses of lidocaine have no significant effect on AV nodal and His-Purkinje conduction time in the presence of intact AV conduction. Sporadic reports of accelerated AV conduction or complete heart block following lidocaine administration have been published. One case of Mobitz type II heart block has been reported in a patient with a prolonged QTc interval (0.61 sec) who was also receiving prenylamine. Electrophysiological studies designed to evaluate lidocaine's effects on AV conduction present conflicting observations. We report a case of Mobitz type II heart block following therapeutic doses of lidocaine in the absence of acute myocardial infarction or concomitant cardioactive drug administration. This case, in conjunction with other reported data, suggests the occurrence of lidocaine-induced AV block to be unpredictable. Although its occurrence may be infrequent, the severity of these reactions warrants careful selection and monitoring of patients who are to receive lidocaine.

The effects of overdrive pacing and lidocaine on atrioventricular junctional rhythm in man: the role of abnormal automaticity

The effects of overdrive pacing and lidocaine were studied in 22 patients with atrioventricular (AV) junctional rhythms. Based on the responses to cardiac pacing and lidocaine, patients were divided into two groups. AV junctional rhythms in group I (17 patients) were suppressed by overdrive pacing, and their rates were decreased by lidocaine. Lidocaine also prolonged the junctional recovery time in these patients. AV junctional rhythms in group II (five patients) were not suppressed by overdrive pacing. In contrast, the rate increased after overdrive pacing. Lidocaine did not alter the basic cycle lengths or the recovery times of the AV junctional rhythms in this group of patients. The data suggest that AV junctional rhythms in group I were caused by normal automaticity, while those in group II were probably due to abnormal automaticity.

Pharmacologic treatment of cardiac arrhythmias: 25 years of progress

This review of practical and theoretical advances in antiarrhythmic drug therapy consists of four parts. Part 1, on clinical applications, compares the approaches to treatment 25 years ago with those of today, examines the current status of antiarrhythmic drugs used 25 years ago, reports on drugs approved for clinical use during the past 25 years, reviews new experimental drugs and suggests an approach to classification of antiarrhythmic drugs. Part 2 summarizes the contributions of cellular electrophysiology to the understanding of drug action, with emphasis on the drug-induced block of the voltage- and time-dependent properties of the rapid sodium channel. The subsequent section contains a brief discussion of the impact made by the new knowledge and the new diagnostic technology on the contemporary practices. The main conclusions are 1) that the more rational approach to treatment has benefited proportionately more patients with supraventricular than with ventricular arrhythmias, and 2) that new advances have made it possible to design successful treatments for certain patients with problems that could not be resolved in the past.