Effect of amiodarone in the Wolff-Parkinson-White syndrome. A clinical and electrophysiological study

Six patients with Wolff-Parkinson-White (WPW) syndrome were given long-term treatment with amiodarone. Symptomatic relief was obtained in all. Tolerance to the drug was good. Reversible corneal changes appeared after some weeks' treatment in five patients. No thyroid side-effects were noticed. Prior to treatment, dual atrioventricular (AV) conduction was demonstrated on His bundle electrograms in all six patients. Recordings were made at varied heart rates, using atrial and ventricular pacing. Reciprocating tachycardia was readily provoked by properly timed extra stimuli in all patients. When amiodarone treatment had become clinically effective, a second comparative study was made in four patients after 26--85 days' treatment. Amiodarone reduced heart rate and second degree AV block appeared at a lower atrial pacing rate. It increased the refractory periods of right atrium, AV node, and the accessory pathway in proportion to the duration of treatment. Induction of tachycardia was effectively prevented by the drug. It appears that amiodarone in chronic treatment has a predictable and unique depressant action on cardiac conduction, supporting the opinion that this compound, despite side-effects, has an important role to play in the treatment of refractory arrhythmias in patients with the WPW syndrome.

Electrophysiological methods in assessing cardiac effects of the tricyclic antidepressant imipramine

The tricyclic antidepressant agent imipramine was tested intravenously in 8 healthy individuals with respect to its effect on sinus recovery time, intervals in His bundle electrograms and the duration of the repolarization in the right atrium and ventricle judged from the refractoriness and monophasic action potential duration. Sinus recovery time was unchanged after the drug. The interval between the His bundle deflection and the start of the QRS complex in the standard lead showed no consistent changes but increased to pathological values in two individuals. The duration of the repolarization decreased in the atrium--an effect which could be arrhythmia-provoking or arrhythmia-protecting. The duration increased at the ventricular level. This effect could explain the antiarrhythmic effect on ventricular ectopics observed in other investigations.

Inotropic, chronotropic and dromotropic effects of prenalterol, a new cardiostimulant drug

Prenalterol (H 133/22) is an adrenergic beta-receptor stimulator which unlike isoproterenol is both orally active and has a long duration of action. The inotropic, chronotropic and dromotropic effects of prenalterol were investigated in pentobarbital anaesthetized dogs. Prenalterol was found to increase left ventricular maximum dP/dt in a dose-dependent manner up to a dose level of 50 micrograms/kg. Additional doses resulted in only small further increases in cardiac contractility. Electrophysiological studies were done to compare the effects of prenalterol and isoproterenol on sinus node and ventricular pacemaker function. Complete AV block was produced by electrocauterization of the His bundle. Prenalterol accelerated both ventricular and sinus node pacemakers at doses up to 50 micrograms/kg. The increase in ventricular rate was greater than the increase in sinus rate following supramaximal inotropic doses of prenalterol yet no ventricular extrasystoles were ever observed. Similar acceleration of the ventricular and sinus node pacemakers were observed by infusion of isoproterenol. Prenalterol and isoproterenol accelerated AV nodal conduction and ventricular conduction but had little effect upon His-Purkinje conduction. Sinus node reset time was abbreviated as was the functional and effective refractory periods of the atrium and AV nodes.

Electrophysiological studies with prenalterol

Electrophysiological studies with prenalterol in 19 patients (6 women, 13 men, 5 with sinus node disease, 4 with AV node disease, 7 with double node disease, 2 with conduction disturbance below His bundle, 1 normal) showed that sinus node function (heart rate, sinus node recovery time) is uniformly improved by this beta-stimulator. Also AV conduction is significantly and uniformly improved (shortening of AH interval and of the functional refractory period of AV conduction). There is no or little influence on intra-atrial conduction and on conduction below the His bundle. However, spontaneous depolarisation in His-Purkinje fibers--as tested in patients with complete AV block and ventricular demand pacemaker--is increased through beta-stimulation with prenalterol as reflected by shorter escape intervals and higher frequency escape rhythm. Prenalterol may be of clinical use in patients with cardiomyopathies who developed bradycardia under digitalisation or patients with severe bradyarrhythmia either with or without digitalis. It might also be useful in rare emergency situations, when complete pacemaker failure develops.

Electropharmacologic effect of a standard dose of intravenous procainamide in patients with sustained ventricular tachycardia

BACKGROUND

Patients with inducible sustained ventricular tachycardia (VT) sometimes receive intravenous procainamide during electrophysiologic testing. Unfortunately, the responses to intravenous and subsequent oral drug therapy are variable and may be discordant.

HYPOTHESIS

It was the aim of this study to determine whether this variability might be explained by heterogeneity in the electropharmacologic response, even in a homogeneous population.

METHODS

We studied 42 patients who had spontaneous malignant ventricular arrhythmia and were inducible to sustained monomorphous VT during electrophysiologic testing. Each received 15 mg/kg of intravenous procainamide followed by a 2 mg/min infusion. Serum levels were drawn immediately following programmed stimulation. The mean procainamide level was 6.7 +/- 1.4 mcg/ml with an N-acetyl procainamide level of 1.0 +/- 0.5 mcg/ml. The 14 procainamide responders (5 of whom were noninducible and 9 whose VT cycle length increased > 100 ms) and the 28 nonresponders had similar procainamide and NAPA levels (6.5 +/- 1.4 vs. 6.7 +/- 1.4 mcg/ml).

RESULTS

There was no significant difference in baseline clinical parameters, His to ventricular electrogram (HV) interval, effective refractory period, or VT cycle length. Prolongation of the effective refractory period and infra His conduction time occurred to a similar extent in responders and nonresponders.

CONCLUSION

We conclude that procainamide has a consistent dose-response relationship with respect to refractoriness and conduction in patients with malignant arrhythmias. However, acute antiarrhythmic efficacy of procainamide cannot be predicted by clinical factors, drug levels, or drug-induced changes in common electrophysiologic parameters.

Analysis of proarrhythmic potential of antipsychotics risperidone and olanzapine in anesthetized dogs

In vitro electrophysiological studies have shown that second-generation antipsychotic drugs risperidone and olanzapine inhibit rapidly activating delayed rectifier K(+) currents and prolong action potential duration of the isolated ventricular myocardium. In this study, we analyzed in vivo cardiohemodynamic and electrophysiological profiles of risperidone and olanzapine using the halothane-anesthetized canine model to clarify their proarrhythmic potential. A clinically relevant dose of risperidone (0.03 mg/kg, i.v.) did not affect the ventricular repolarization process, whereas the supra-therapeutic doses (0.3 and 3 mg/kg, i.v.) prolonged the duration of monophasic action potential of the ventricle. Furthermore, the terminal repolarization period, an index of extent of electrical vulnerability, was prolonged after the supra-therapeutic doses. In contrast, therapeutic to supra-therapeutic doses of olanzapine (0.03-3 mg/kg, i.v.) hardly affected the ventricular repolarization process. Therefore, more caution has to be paid on the use of risperidone than olanzapine for patients with risks of the elevated plasma concentration.

[Amiodaron effects on atrioventricular conduction]

AIM

To assess short- and long-term effects of amiodaron on atrioventricular conduction (AVC) of the heart.

MATERIAL AND METHODS

Amiodaron was given to 477 patients (mean age 48.7 +/- 0.7 years) with various arrhythmias caused, as a rule by coronary heart disease. A mean saturation dose was 809.4 +/- 13.4 mg/day, a mean maintanence dose--263.5 +/- 5.4 mg/day. Mean follow-up 20.85 +/- 1.2 months. The frequency of AVC disorders was estimated at regular Holter monitoring.

RESULTS

In saturation, atrioventricular block of the first degree arose in 17.2% patients, of the second and third degree--only in one patient. AVC disorders disappeared at reduction of amiodaron dose. Only in one patient the drug was discontinued because of recurrent block of the third degree and bradycardia.

CONCLUSION

In saturation, ECG records lengthening of P-R intervals by 18.9%, on the average. Maintenance therapy increases the length of P-R interval by 8.1%, on the average.

Progressive facilitation of antegrade conduction via an accessory pathway in a patient with Wolff-Parkinson-White syndrome and permanent atrial fibrillation

The case of a 64-year-old man with Wolff-Parkinson-White syndrome and permanent atrial fibrillation (AF) is reported. The patient was admitted due to electrocardiographic feature of AF with rapid conduction over the left-sided accessory pathway. Administration of pirmenol effectively suppressed the ventricular response via an accessory pathway. A transesophageal echocardiography detected an uncertain thrombus in the left atrial appendage. During the 33-month follow-up period, the ventricular response via an accessory pathway was progressively facilitated. Radiofrequency catheter ablation using a transseptal approach was performed during AF, resulting in complete elimination of the antegrade accessory pathway conduction.

Cardiomyocyte apoptosis in cocaine-induced myocarditis with involvement of bundle of His and left bundle branch

Cocaine abuse can cause degenerative and inflammatory alterations of the myocardium. Cocaine-induced myocarditis has been previously described, but mostly in chronic cocaine addicts. Among several pathogenetic mechanisms of cocaine-related myocardial damage, the myocardial adrenergic stress and cardiomyocytes apoptosis have been recently proposed. We report an unusual case of myocarditis involving the bundle of His and left bundle branch and cardiac myocytes apoptosis in otherwise healthy occasional cocaine abuser.

Effects of acute systemic endothelin receptor blockade on cardiac electrophysiology in vivo

BQ-123, a selective endothelin-A receptor antagonist, has been demonstrated to suppress arrhythmias. However, the role of physiologic levels of endogenous endothelin-1 (ET-1) with respect to electrophysiologic properties of the heart is unknown. BQ-123 (0.45, 0.9, 1.8, 3.6, 7.2, and 14.4 microg/kg/min; n = 10) or saline (control, n = 5) was administered IV for 15 minutes of continuous-rate infusion at incremental doses to anesthetized normal pigs. BQ-123 had no effect on PR and QT interval, QRS duration, intraatrial and AV nodal conduction time as well as the atrial, AV nodal, and ventricular effective refractory periods. As compared with baseline, BQ-123 at 7.2 and 14.4 microg/kg/min caused an increase in heart rate (99 +/- 17 versus 110 +/- 14 and 118 +/- 14 bpm, respectively; P < 0.05), shortened sinus node recovery time (818 +/- 165 versus 641 +/- 69 and 609 +/- 74 milliseconds, respectively; P < 0.05) and decreased mean arterial pressure at 14.4 microg/kg/min (95 +/- 18 versus 80 +/- 11 mm Hg; P < 0.05). We conclude that in the normal pig, physiologic levels of ET-1 have no effect on conduction properties of atrial, AV nodal, or Purkinje fibers. However, antagonism of ET-1 by BQ-123 unmasks the effect of ET-1 on maintenance of vasomotor tone, which in turn may affect heart rate and sinus node automaticity in the intact pig.

The antipsychotic and antiemetic drug prochlorperazine delays the ventricular repolarization of the in situ canine heart

Electropharmacological effect of the antipsychotic and antiemetic drug prochlorperazine was assessed using the halothane-anesthetized in vivo canine model (n = 5). Up to 10 times higher than the clinically relevant doses of prochlorperazine (< or = 3 mg/kg, i.v.) did not induce cardiohemodynamic collapse in the model. Meanwhile, clinically relevant to supratherapeutic doses (0.3 - 3 mg/kg, i.v.) prolonged the ventricular repolarization period in a dose-related and reverse-use dependent manner that could become proarrhythmic substrates. Thus, caution has to be paid on the use of prochlorperazine particularly for patients with risks of the elevated plasma drug concentration, compromised cardiac repolarization, and/or frequent ventricular premature beats.

Structure–affinity relationships of 5 ′ -aromatic ethers and 5 ′ -aromatic sulfides as partial A 1 adenosine agonists, potential supraventricular anti-arrhythmic agents

Atrial fibrillation (AF) is the most commonly encountered sustained clinical arrhythmia with an estimated 2.3 million cases in the US (2001). A(1) adenosine receptor agonists can slow the electrical impulse propagation through the atrioventricular (AV) node (i.e., negative dromotropic effect) resulting in prolongation of the stimulus-to-His bundle (S-H) interval to potentially reduce ventricular rate. Compounds that are full agonists of the A(1) adenosine receptor can cause high grade AV block. Therefore, it is envisioned that a compound that is a partial agonist of the A(1) adenosine receptor could avoid this deleterious effect. 5(') Phenyl sulfides (e.g., 17, EC(50)=1.26 microM) and phenyl ethers (e.g., 28, EC(50)=0.2 microM) are partial agonists with respect to their AV nodal effects in guinea pig isolated hearts. Additional affinity, GTPgammaS binding data suggesting partial activity of the A(1) adenosine receptor, and PK results for 5(') modified adenosine derivatives are shown.

Effects of intravenous nifekalant, a class III antiarrhythmic drug, on atrial vulnerability parameters in patients with paroxysmal atrial fibrillation

Nifekalant, a class III antiarrhythmic drug, has been shown to suppress ventricular tachyarrhythmias, but its effects on AF are unclear. The aim of this study was to clarify the effects of nifekalant on the atrial vulnerability parameters in patients with paroxysmal AF. The study included 18 patients with paroxysmal AF who underwent electrophysiological study before and after intravenous infusion of nifekalant. The atrial electrophysiological parameters including the atrial effective refractory period (AERP), maximum intraatrial conduction delay, and wavelength index, calculated as the ratio of AERP to the maximum conduction delay, were quantitatively measured at baseline and during nifekalant infusion. The mean AERP was significantly prolonged from 214 +/- 27 ms at baseline to 242 +/- 39 ms after nifekalant (P < 0.001). Although earlier studies have shown that nifekalant does not affect the atrial conduction time, the mean maximum conduction delay of the study patients was significantly prolonged from 59 +/- 19 ms at baseline to 72 +/- 28 ms after nifekalant (P = 0.015). There was no significant difference in the wavelength index at baseline (4.1 +/- 1.7) and after nifekalant (4.1 +/- 2.5). However, when the differences of AERP and wavelength index were defined as each parameter during nifekalant infusion minus that at baseline, the difference of AERP showed a direct positive correlation with that of the wavelength index (P = 0.013). In conclusion, nifekalant may be effective in the prevention of AF due to prolongation of the AERP. However, in those patients who have a lesser degree of prolongation of the AERP by nifekalant, the wavelength index tended to be decreased, suggesting that the drug might augment the propensity for AF.

Supraventricular tachycardia in children

The mechanisms causing different supraventricular tachycardias can be identified with the aid of the 12-lead ECG using Tipple's approach. The main aims of this retrospective study were to use the 12-lead ECG to determine the underlying mechanisms of supraventricular arrhythmias and to evaluate the effectiveness of the treatment modalities used. Forty-one patients were included in the study. The main findings were: nine of the 41 patients had atrial tachycardias while junctional tachycardia occurred in 32/41 of our patients. The underlying mechanisms causing the junctional tachycardias were: AVNRT (n = 21), AVRT (n = 10) and JET (n = 1). Of the 10 patients presenting with AVRT, eight were less than one year old. AVNRT occurred more often in the older age group (>1 year of age). Fifteen of the 41 patients had spontaneous cessation of their supraventricular tachycardia. The drug most commonly used during the acute and long-term phases was digoxin. Amiodarone was used in six patients with an 80% success rate. In the early 80s verapamil was used in five patients with a 100% success rate. It is important to note that verapamil is no longer used in children due to its side effects. Lately, adenosine phosphate is the drug of choice in most supraventricular tachycardias. The management of supraventricular tachycardias in paediatric practice is mainly based on clinical studies and individual experience. Care must therefore be taken to choose medication regimens that are likely to be effective with the minimum risk of potentiating abnormal haemodynamics or conduction.

Effects of mexiletine on the canine model of sparfloxacin-induced long QT syndrome

Potential utility of mexiletine for the treatment of sparfloxacin-induced long QT syndrome was assessed using the in vivo halothane-anesthetized canine model. At 30 min after the administration of a supratherapeutic dose of sparfloxacin (30 mg/kg, i.v.), the mean blood pressure and heart rate decreased, whereas repolarization process and effective refractory period of the ventricular muscle were significantly prolonged. Additional administration of a clinically recommended dose of mexiletine (3 mg/kg, i.v.) at this time point increased the mean blood pressure, suppressed ventricular contraction, delayed atrioventricular as well as intraventricular conduction, and shortened repolarization process and effective refractory period. The extent of abbreviation of the repolarization was more prominent than that of the refractoriness, indicating that mexiletine could decrease the electrical vulnerability of the heart during sparfloxacin overdose. Thus, mexiletine may become a promising pharmacological strategy against the drug-induced long QT syndrome.

In vivo canine cardiac electrophysiologic profile of 1,4-benzodiazepine IKs blockers

Previous cardiac electrophysiologic studies of blockers of the slowly activating delayed rectifier (IKs) current have focused primarily on ventricular repolarization. This report summarizes an extensive in vivo cardiac electrophysiologic profile of four 1,4-benzodiazepine IKs blocker analogues (L-761334, L-763540, L-761710, and L-768673) in dogs. At 3.0 mg/kg intravenously, all four analogues elicited 14.5%-21.4% increases in ventricular refractoriness and 19.2%-22.6% increases in QTc interval. Concomitant 11.1%-13.5% increases in atrial refractoriness were noted with all four analogues. Decreases in sinus heart rate of 8.4%-17.3% were noted with all four compounds. No effects on atrial, His Purkinje, ventricular conduction or atrial and ventricular excitation were observed. One analogue, L-761710, significantly delayed atrioventricular (AV) nodal conduction (40.7+/-17.4% increase in atrial-to-His interval) and increased the AV conduction system functional refractory period 19.9+/-6.2%. The lack of effect of the other three 1,4-benzodiazepine IKs blockers on AV nodal function at dosages producing comparable effects on atrial and ventricular refractoriness suggest that the AV nodal effects of L-761710 were unrelated to IKs blockade. These findings indicate IKs plays important roles in both atrial and ventricular refractoriness as well as pacemaker function in the dog heart, suggesting potential utility for IKs blockers in the treatment of atrial and ventricular arrhythmias.

Constitutional rho-kinase regulates atrioventricular nodal conduction and ventricular repolarization of the canine heart

Given the limited information, physiological roles of Rho-kinase in the cardiac conduction system and ventricular repolarization process were assessed in comparison with those in the coronary vascular tone. A specific Rho-kinase inhibitor Y-27632 was administered to the nutrient coronary artery of the canine isolated, blood-perfused atrioventricular node preparation under the monitoring of the ventricular monophasic action potentials. Administration of Y-27632 moderately suppressed the atrioventricular nodal conduction, slightly but significantly accelerated the repolarization process, and potently increased the coronary blood flow, whereas it hardly affected the intraventricular conduction. The estimated concentrations of Y-27632 causing the currently observed effects were enough to inhibit Rho-kinase. These results suggest that constitutional Rho-kinase functions to moderately facilitate the atrioventricular nodal conduction, slightly delay ventricular repolarization process, and significantly increase the coronary vascular tone.

Free radicals potentiate the negative dromotropic effect of adenosine in guinea pig isolated heart

OBJECTIVE

Adenosine is released during myocardial ischaemia and delays atrioventricular nodal (AV) conduction. We hypothesized that free radicals present during reperfusion potentiate the negative dromotropic effect of adenosine on the AV node.

METHODS AND RESULTS

Guinea pig hearts were prepared using the Langendorff technique, paced (200 beats/min), and instrumented to measure the atrium-to-His bundle (A-H) interval, an index of AV nodal conduction time. Adenosine (2 microM) prolonged the A-H interval by 5.7 +/- 0.5 ms from a control value of 35.7 +/- 1.3 ms. (n = 10, P < 0.05). In the absence of adenosine, the superoxide (O2-) generator pyrogallol (20 microM) did not affect the A-H interval (0.7 +/- 0.2 ms prolongation, n = 10). However, concurrent infusion of adenosine (2 microM) and pyrogallol (20 microM) lengthened the A-H interval by 11.0 +/- 0.8 ms from control (n = 10, P < 0.001). This A-H interval prolongation was reversed by cyclopentyl-1,3-dipropylxanthine (100 nM), a selective A1-adenosine receptor antagonist (P<0.001, n = 5). Similarly, A-H interval prolongation was decreased to 4.3 +/- 0.4 ms when NG-methyl-L-arginine (100 microM), a nitric oxide (NO) synthase inhibitor, was infused (n = 4). The superoxide scavenger superoxide dismutase (200 U/ml) also diminished the A-H interval prolongation to 7.1 +/- 0.6 ms (n = 4, P < 0.001). Ba2+ ( 100 microM), a blocker of the adenosine-induced inward potassium current (I(K,ADO)), did not significantly affect this potentiation (13.0 +/- 0.8 and 10.8 +/- 0.7 ms greater than control A-H interval in the absence and presence of Ba2+, respectively, n = 4).

CONCLUSIONS

Superoxides and adenosine delay AV nodal conduction in a synergistic manner via a NO-dependent mechanism involving an I(K,ADO)-independent component. This phenomenon may contribute to the genesis of reperfusion arrhythmias.

Dual rate-dependent cardiac electrophysiologic effects of haloperidol: slowing of intraventricular conduction and lengthening of repolarization

Treatment with the neuroleptic agent haloperidol is sometimes associated with serious cardiac arrhythmias. The proarrhythmic potential of haloperidol may be linked to the drug's rate-dependent modulation of cardiac impulse conduction and repolarization. Herein these heart rate-dependent electrophysiologic actions of haloperidol are investigated in vivo. In anesthetized guinea pigs, haloperidol (0.02 mg/kg/min intravenously) produced significant rate-dependent slowing of intraventricular conduction. On abruptly changing the driving cycle length from 500 ms to 300 ms, conduction slowing rapidly reached a new steady state with a rate constant of 0.80 per beat +/- 0.07. The time course of recovery from conduction slowing on interruption of rapid pacing at a cycle length of 250 ms was well described by two time constants, tau(rec1) = 18.9 ms +/- 8.0 and tau(rec2) = 141.8 ms +/- 87.1, suggesting rapid dissociation of the drug from the Na+ channel. During prolonged stimulation, conduction slowing had a biphasic dependence on heart rate: for each 10-bpm increment in heart rate, conduction slowing increased by 7.9% at rates <220 bpm and by 17% at rates >220 bpm. At all tested cycle lengths, haloperidol caused a significant lengthening of Q(T) intervals, which was inversely dependent on heart rate. Numeric analysis suggested that the excessive increase in conduction slowing at rates >220 bpm was due to the drug's Q(T)-prolonging effect, indicating that, at short cycle lengths, the impulses encroached on the refractory period. Thus, in vivo, haloperidol slows intracardiac conduction with rapid on/off kinetics, comparable to the class I antiarrhythmic agent lidocaine. The Q(T) prolongation by haloperidol may lead to an excessive conduction slowing at high heart rates.

Refractory adenosine-sensitive congenital His bundle tachycardia: response to calcium-channel blockers

The congenital form of His bundle tachycardia is an uncommon pediatric arrhythmia. We report the case of a 7-year-old child with tachycardiomyopathy. The incessant arrhythmia, detected in infancy, was resistant to amiodarone and beta-blockers. During electrophysiologic study, the tachycardia converted to sinus rhythm with intravenous adenosine and diltiazem. Subsequently, the child is maintaining sinus rhythm on oral verapamil. Calcium-channel blockers should be considered for the treatment of this arrhythmia, which is often resistant to multiple antiarrhythmic drugs.

Age-related differences in the direct cardiac effects of cisapride: narrower safety range in the hearts of young rabbits

Although cisapride is widely used to treat gastrointestinal motility disorders, it has been associated with QT prolongation, torsades de pointes, and cardiac arrest. Only in children, however, has atrioventricular (AV) block after cisapride been reported. This study used Langendorff perfusion to determine the direct effects of cisapride (0.03, 0.1, 0.3, and 1 microM) on the conduction properties of neonatal (<7 d) and adult (>3 mo) rabbit hearts. At a clinically relevant dose (0.03 microM), cisapride slowed the recovery of the His-Purkinje system. At 0.1 microM, the refractoriness of the His-Purkinje system and conduction through this system were prolonged. Corrected QT intervals and the ventricular refractory period were also lengthened. These parameters were significantly more prolonged in neonates than in adults. The level of AV block at rapid atrial pacing shifted from the AV node to the His-Purkinje system, with an ED(50) of 0.06 and 0.52 microM in the neonate and the adult, respectively. In the neonate, cisapride even resulted in infranodal AV block rhythm (ED50 = 0.12 microM), but this was not the case in the adult. Polymorphic ventricular tachycardia after cisapride was induced in one in seven neonates (14%;, 0.1 microM) and in one in seven adults (14%; 0.03 microM). It is concluded that cisapride may affect the refractoriness of cardiac tissue and that the His-Purkinje system seems to be the most sensitive. In neonatal hearts, this modification may, in fact, progress to infranodal AV block. Such susceptibility to cisapride strongly indicates that the therapeutic safety range used for the young heart should be narrowed.

[Comparative study of electrophysiological mechanisms of anti-arrhythmia III class agents, cardiocyclide, nibentan, and sotalol during experimental myocardial infarction and sympathetic stimulation]

Electrophysiological mechanisms of the action of cardiocyclide, nibentan, and sotalol--antiarrhythmic agents of class III--was studied in dogs with experimental myocardial infarction induced by a two-step occlusion of the coronary artery. Cardiocyclide exhibited the properties typical of the class III antiarrhythmics by prolonging the ventricular repolarization and increasing the effective refractory periods in the atrium and ventricles. The degree of manifestation of these antiarrhythmic effects of cardiocyclide is independent of the induced heart rate, which is related to the ability of this drug to block the slow activation component (IKs) of the delayed rectified potassium current. Nibentan elongates the QT interval and increases the effective atrial and ventricular refractory periods, but the effect was dependent of the stimulation frequency. Sotalol, which also exhibited the properties of a class III antiarrhythmogen possessing beta-blocking activity, produced more pronounced inhibiting action upon the sinus node function and conduction (in comparison with the analogous effects of cardiocyclide). This is probably related to the ability of sotalol to block the cardiac adrenoreceptors. The effect of sotalol is also frequency-dependent, which is related to the blocking of rapid activating component (IKr) of) of the delayed rectified potassium current. On the background of isoproterenol infusion, cardiocyclide completely retained the electrophysiological and antiarrhythmic effects. The efficacy of nibentan and sotalol with respect to the repolarization and refractoriness significantly decrease under the conditions of sympathetic nervous system activation. The ability of sotalol to suppress the sinus node function and conduction on the background of isoproterenol infusion is retained.

[Electrophysiological effects of ethacizine in experimental myocardial infarction and stimulation of sympathetic heart innervation]

The electrophysiological effect of ethacizine, an antiarrhythmic agent of class IC, was studied in dogs with experimental myocardial infarction under the beta-adrenergic stimulation conditions. It was found that ethacizine (i) slows down the atrioventricular conduction and the AV-node conduction; (ii) increases the time of the sinus node function recovery; and (iii) increases the effective atrioventricular refractory periods. Under the conditions of isoproterenol-induced stimulation of the beta-adrenergic structures, some of the ethacizine effects (e.g., increased refractoriness) completely disappeared and the other (slowed down conduction) decreased. As the stimulation frequency was increased, the effect of ethacizine upon the conduction exhibited a decrease. Upon the isoproterenol injection, dependence of the QRS complex on the forced rhythm frequency disappeared.

Cardiac electrophysiologic and hemodynamic effects of sildenafil, a PDE5 inhibitor, in anesthetized dogs

A recent in vitro study demonstrated that supratherapeutic concentrations of sildenafil, a phosphodiesterase type 5 (PDE5) inhibitor, blocked I(Kr) and prolonged cardiac repolarization. This study assessed the in vivo cardiohemodynamic and electrophysiologic effects of sildenafil using a halothane-anesthetized, closed-chest canine model (n = 5) to bridge the gap between basic observation and clinical experience. Intravenous administration of sildenafil citrate in doses of 0.03, 0.3, and 3.0 mg/kg for 10 min, which provided sub-to supratherapeutic plasma drug concentrations, did not affect the monophasic action potential duration or effective refractory period of the right ventricle during the sinus rhythm as well as the ventricular pacing at the cycle length of 400 and 300 ms. However, sildenafil decreased the total peripheral resistance, simultaneously inducing positive chronotropic and inotropic effects at the top dose, which gave plasma concentrations at least 10 times higher than the therapeutic range. This cardiohemodynamic profile of sildenafil can be largely explained by reflex sympathetic activation associated with its vasodilator effect. Meanwhile, the lack of prolongation of the ventricular repolarization phase at the therapeutically relevant to moderately supratherapeutic sildenafil concentrations supports the earlier clinical studies that indicate that sildenafil has no effect on electrocardiogram.

Effect of carvedilol on atrioventricular conduction in the ischemic heart

We compared the effects of carvedilol on atrial-His and His-ventricular conduction with those of propranolol in isolated rat hearts. Hearts were perfused retrograde, and atrial-His and His-ventricular intervals were measured. The effective doses that increased conduction times by 25% were 10(-6) M for atrial-His and 3x10(-6) M for His-ventricular for propranolol, and 8x10(-8) M for atrial-His and 10(-8) M for His-ventricular for carvedilol. Prazosin did not affect the atrial-His and His-ventricular intervals. After ischemia-reperfusion, atrial-His and His-ventricular intervals increased to a greater extent with 10(-6) M carvedilol. To determine the direct membrane effect, we examined the transmembrane action potential in guinea pig papillary muscle. Both drugs decreased the maximum upstroke velocity equally. Our data indicate that carvedilol had a greater effect on atrioventricular conduction in the setting of ischemia-reperfusion than did propranolol. This effect of carvedilol was not due to its alpha-adrenoceptor blocking property or to a direct membrane effect.

QT-prolonging effects of sparfloxacin, a fluoroquinolone antibiotic, assessed in the in vivo canine model with monophasic action potential monitoring

Sparfloxacin, a fluoroquinolone antibacterial agent, prolongs cardiac repolarization, which may predispose to torsades de pointes. This study was designed to assess simultaneously the hemodynamic and electrophysiologic effects of sparfloxacin using the halothane-anesthetized, closed-chest in vivo canine model (n = 6). Sparfloxacin was intravenously administered in the following two doses with a pause of 20 min, a clinically relevant dose of 3.0 mg/kg/10 min and a 10 times higher dose of 30 mg/kg/10 min. After the low dose of sparfloxacin, cardiac output increased, heart rate decreased, and ventricular repolarization and refractory periods were prolonged. After the high dose, cardiac output increased, whereas heart rate and mean blood pressure decreased, and ventricular repolarization and effective refractory periods were prolonged. The increment was greater in repolarization than in refractoriness, indicating an increase of electrical vulnerability. Because sparfloxacin prolonged repolarization in a reverse use-dependent manner, its negative chronotropic effect may have potentiated the QT prolongation. Left ventricle preload, left ventricular contraction, and AV nodal as well as intraventricular conduction were minimally affected. These results suggest that caution should be used when administering sparfloxacin to patients having risk factors for QT prolongation.

A comparison of calcitonin gene-related peptide effects on coronary flow and cardiac conduction system in the guinea pig

The purpose of this study was to examine the effects of calcitonin gene-related peptide (CGRP) on coronary flow and various parts of cardiac conduction system (CCS) in the isolated guinea pig heart by recording the cardiac electrogram and His bundle activity simultaneously. The coronary flow (CF) was measured and the right atrium was paced at incremental frequencies periodically. Perfusing heart with CGRP at the concentrations of 3 30 nmol/L increased CF strikingly and dose-dependently. CGRP (30 nmol/L) caused a moderate and significant increase in the spontaneous heart rate, but failed to influence the atrioventricular nodal and His bundle-Purkinje fiber conduction time and the longest atrial pacing cycle length inducing 3 2 Wenckebach and 2 1 AV nodal conduction. The results indicate that CGRP in the guinea pig heart might act mainly on coronary vessels and then the sinus node, but did not affect the conductivity and refractoriness of the AV conduction system.

His-Purkinje system reentry as a proarrhythmic effect of flecainide

We report a case of tachycardia due to reentry within the His-Purkinje system (HPS) occurring after introduction of flecainide. The patient presented with a mild mitral regurgitation and normal left ventricular function. He had incomplete left bundle branch block with left-axis deviation. At the electrophysiology study, a prolonged HV interval was observed at baseline, and the tachycardia could be reproduced after ajmaline infusion. Six months after interruption of flecainide, the patient remains free of arrhythmia recurrence. The authors emphasize that proarrhythmic effects of flecainide may include reentry within the HPS in patients with underlying HPS disease.

Interaction between autonomic tone and the negative chronotropic effect of adenosine in humans

Prior studies have demonstrated that sympathetic tone may influence the effects of adenosine on His-Purkinje automaticity, and that enhanced vagal tone may influence its effects on the sinus node. However, the interaction between autonomic tone and the effects of adenosine on the sinus node in humans remains unknown. Therefore, this study was designed to investigate the interaction between different states of autonomic tone and the bradycardiac response of the sinus node to adenosine. In 11 patients without structural heart disease who underwent a clinically indicated electrophysiology procedure, the sinus cycle length was measured before and after a 12-mg bolus of adenosine in the baseline state, during an infusion of 2 mcg/min of isoproterenol, after the administration of 0.2 mg/kg of propranolol, and again after the administration of 0.04 mg/kg of atropine. Adenosine significantly lengthened the sinus cycle length in the baseline state (760 +/- 165 vs 909 +/- 188 ms, P < 0.05), during isoproterenol infusion (516 +/- 67 vs 766 +/- 146 ms, P < 0.05), after propranolol (850 +/- 153 vs 914 +/- 143 ms, P < 0.05) and after the combination of propranolol and atropine (662 +/- 76 vs 801 +/- 121 ms, P < 0.05). The degree of lengthening in sinus cycle length was significantly greater (P < 0.05) during isoproterenol infusion (253 +/- 157 ms, or 51% +/- 40%) than in the baseline state (149 +/- 85 ms, or 20% +/- 12%), after propranolol (68 +/- 53 ms, or 8% +/- 8%), and after propranolol and atropine (140 +/- 110 ms, or 21% +/- 18%). The negative chronotropic effect of adenosine is influenced by autonomic tone. The effect of adenosine on the sinus node is accentuated by beta-adrenergic stimulation and unaffected by beta-adrenergic blockade or combined beta-adrenergic and cholinergic blockade.

Assessment of atrioventricular nodal physiology in the mouse

Transgenic mice are increasingly being utilized for understanding cardiac electrophysiologic abnormalities. However, little is known about the normal atrioventricular nodal and infraHisian physiology in the mouse because of the prior inability to record a His-bundle deflection. We present the first comprehensive examination of the murine atrioventricular nodal and His-Purkinje systems employing His-bundle recordings. Normal, healthy, male C57BL/6J mice (n = 48) underwent an in vivo electrophysiology study using a 2 F octapolar electrode catheter. Effective refractory periods were determined during premature atrial and ventricular stimulation. The PR interval measured 44 +/- 6 ms with a mean sinus cycle length of 185 +/- 42 ms. Baseline AH intervals were 36 +/- 5 ms and HV intervals were 10 +/- 2 ms. At a pacing cycle length of 140 ms the atrioventricular nodal effective refractory period (AVNERP) and atrial effective refractory period (AERP) were 86 +/- 19 ms and 57 +/- 17 ms, respectively. The mean AV Wenckebach and 2:1 paced cycle length were 103 +/- 14 ms and 84 +/- 13 ms, respectively. Premature atrial stimulation curves were asymptotic without discontinuity. A subset of nine mice was studied after administration of isoproterenol. The sinus cycle length, AVNERP and AERP decreased significantly from baseline measurements. This method establishes a practical and feasible technique to record in vivo His-bundle electrograms in the mouse to assess atrioventricular nodal and infraHisian physiology. Use of this model will allow for the examination of abnormalities of atrioventricular nodal and infraHisian conduction in transgenic murine models.

[Post-resuscitation neurosomatic deviations and the feasibility of their correction with a drug of metabolic action]

Circulation in white rat males was stopped for 10 minutes by ligation of the intrathoracic vascular bundle of the heart. Proxipin in a dose 10 or 20 mg/kg was injected i.p. after restoration of effective cardiac activity and the next 3 days. Proxipin improved resuscitation outcomes, relieved symptoms of CNS excitability 4-6 months after resuscitation, changed adaptation to stress agents, protected against risk-factors of atherogenesis. However, proxipin-induced inhibition of the motor activity suppressed the free-choice ability to find food.

Modulation of AV nodal and Hisian conduction by changes in extracellular space

Previous studies have demonstrated that the extracellular space (ECS) component of the atrioventricular (AV) node and His bundle region is larger than the ECS in adjacent contractile myocardium. The potential physiological significance of this observation was examined in a canine blood-perfused AV nodal preparation. Mannitol, an ECS osmotic expander, was infused directly into either the AV node or His bundle region. This resulted in a significant dose-dependent increase in the AV nodal or His-ventricular conduction time and in the AV nodal effective refractory period. Mannitol infusion eventually resulted in Wenckebach block (n = 6), which reversed with mannitol washout. The ratio of AV nodal to left ventricular ECS in tissue frozen immediately on the development of heart block (n = 8) was significantly higher in the region of block (4.53 +/- 0.61) compared with that in control preparations (2.23 +/- 0.35, n = 6, P < 0.01) and donor dog hearts (2.45 +/- 0.18, n = 11, P < 0.01) not exposed to mannitol. With lower mannitol rates (10% of total blood flow), AV nodal conduction times increased by 5-10% and the AV node became supersensitive to adenosine, acetylcholine, and carbachol, but not to norepinephrine. We conclude that mannitol-induced changes in AV node and His bundle ECS markedly alter conduction system electrophysiology and the sensitivity of conductive tissues to purinergic and cholinergic agonists.

Retrograde fast pathway ablation for atrioventricular nodal reentry associated with markedly prolonged PR intervals

Three patients with typical atrioventricular nodal reentrant tachycardia (AVNRT) and markedly prolonged PR intervals (>300 ms) without dual pathway physiology at baseline or during isoproterenol infusion underwent successful fast pathway ablation and remained asymptomatic without recurrent AVNRT, atrioventricular block, or symptomatic bradycardia for a mean of 19 months. In patients with recurrent AVNRT and markedly prolonged PR intervals, selective ablation of the retrograde fast pathway can eliminate AVNRT without further impairment of anterograde atrioventricular nodal function.

Pharmacological stress testing of the His-Purkinje system in patients with bifascicular block

This literature review, based mainly on the English-language literature, focuses on pharmacological stress testing of the His-Purkinje system as part of an invasive electrophysiological study. The main target group for this investigation is patients with bifascicular block and syncope in which intermittent high grade AV block is suspected. Several drugs have been used for this purpose, mainly Class I antiarrhythmic agents such as ajmaline, procainamide, disopyramide, and flecainide. Most studies, unfortunately, suffer from limited patient numbers, lack of adequate control groups, and/or adequate follow-up. The sensitivity of the disopyramide stress test has been shown to be 75%-100% for prediction of impending high grade AV block. The specificity was > 90%. Studies on procainamide have shown a sensitivity of 60% but the specificity has not been assessed. There are no studies allowing a strict comparison of the diagnostic value of pharmacological provocation with different drugs. Based on the similarities of the electrophysiological effects on the His-Purkinje system of the above Class I agents, it is reasonable to assume that all of them might be of diagnostic value in the present clinical context, provided atrial and ventricular stimulation after drug is included in the protocol.

Effects of adenosine on retrograde refractoriness of accessory atrioventricular connections

Ventricular premature stimuli were used to demonstrate adenosine-mediated decreases in the retrograde refractoriness of accessory atrioventricular connections. This response is consistent with the concept that accessory atrioventricular connections have electrophysiologic properties that are similar to those of atrial myocardium.

Unusual locations for adenosine-sensitive accessory atrioventricular pathways with decremental conduction

INTRODUCTION

Accessory AV pathways with decremental conduction are uncommon and, in particular, are thought not to occur at the anterior portion of the mitral annulus.

METHODS AND RESULTS

This report describes successful catheter ablation in three patients with accessory AV pathways that were adenosine sensitive and showed decremental conduction properties. The pathways were located at the anteroseptal, anteroparaseptal, and anterolateral aspects of the mitral annulus.

CONCLUSION

Accessory pathways with decremental conduction do occur anywhere around the mitral annulus, even in the area of fibrous continuity between the aortic leaflet of the mitral valve and the aortic valve itself.

Effects of volatile anesthetics on atrial and AV nodal electrophysiological properties in guinea pig isolated perfused heart

BACKGROUND

Knowledge of the anesthetic effects on atrial and atrioventricular (AV) nodal electrophysiologic properties is fundamental to understand the modulatory role of anesthetics on the pathogenesis of supraventricular tachycardias, and to individualize the perioperative management of patients with supraventricular tachycardias or AV nodal conduction disturbances. Therefore the authors studied the effects of three commonly used volatile anesthetics on the electrophysiologic properties of the atrium and AV node.

METHODS

The concentration-dependent electrophysiologic effects of halothane, isoflurane, and desflurane (0-2 minimum alveolar concentration [MAC]) were studied in guinea pig Langendorff-perfused hearts fit with instruments to simultaneously measure atrial and AV nodal conduction times and atrial monophasic action potential duration. Atrial and AV nodal effective refractory periods were measured simultaneously using a computer-assisted premature stimulation protocol. The concentrations of anesthetics in the gas phase were monitored by an infrared gas analyzer.

RESULTS

Volatile anesthetics caused markedly different concentration-dependent effects on atrial conduction, repolarization, and refractoriness, and on AV nodal function. At equianesthetic concentrations, halothane depressed atrial conduction the most, whereas desflurane caused the greatest shortening of atrial monophasic action potential duration. Halothane had no significant effect on atrial refractoriness, whereas at 2 MAC desflurane significantly shortened and isoflurane significantly prolonged atrial effective refractory periods by 18.1+/-13.5% and 13.2+/-14.7%, respectively. On an equi-MAC basis, the rank order of potency for the anesthetics to prolong AV nodal conduction time and AV nodal ERP was halothane > desflurane > isoflurane.

CONCLUSION

The different electrophysiologic effects of volatile anesthetics in the atrium and AV node suggest that these agents may modulate atrial dysrhythmogenesis in distinctly different ways.

The effects of intrinsic nitric oxide on cardiac neural regulation in cats

In this study, we aimed to elucidate the effects of intrinsic nitric oxide (NO) on cardiac neural regulation. Twenty-two cats were anesthetized with 1.5% isoflurane and allocated to Group I (intact; n = 7), Group D (denervated baroreceptors and vagi; n = 8), or Group B (autonomic blockade with i.v. hexamethonium, propranolol, and atropine; n = 7). Cardiac sympathetic nerve activity (CSNA), mean arterial pressure (MAP), sinus heart rate (HR), and A-H and H-V intervals during pacing (150 bpm) were measured before and after i.v. administration of a NO synthase inhibitor, NG-nitro-L-arginine (L-NNA, 30 mg/kg) and after reversal with an excessive dose of L-arginine (300 mg/kg), before and during intermittent electrical stimulation of the posterior hypothalamus. L-NNA significantly increased MAP in Groups I and B, but not in Group D. L-NNA significantly decreased HR and lengthened A-H in Group I, but not in other groups. L-arginine further decreased HR and lengthened A-H unexpectedly. The reasons for these findings could not be determined in this study. L-NNA did not change CSNA. Hypothalamic stimulation did not potentiate L-NNA-induced changes in CSNA, hemodynamic variables, and atrioventricular conduction. In conclusion, intrinsic NO may modulate atrioventricular conduction and sinus rate through a vagal cholinergic, rather than a nonautonomic mechanism.

IMPLICATIONS

Elucidating the roles of intrinsic nitric oxide (NO) on cardiac neural regulation is important. In intact, vagotomized, and baroreceptor-denervated or pharmacologically autonomic blockaded cats, an NO synthesis inhibitor was administered, and atrioventricular conduction and cardiac sympathetic neural discharge were measured. The results suggest a vagal cholinergic mechanism of intrinsic NO.

Effects of 2,3-butanedione monoxime on atrial-atrioventricular nodal conduction in isolated rabbit heart

INTRODUCTION

2,3-Butanedione monoxime (BDM) has been found to reversibly block cardiac contraction, without blocking electrical conduction. This study characterizes the dose-dependent effects of BDM on the conduction through the atrioventricular node (AVN) of rabbit heart.

METHODS AND RESULTS

Thirteen isolated atrial-AVN preparations were used in control, during and after exposure to 5, 10, and 20 mM BDM. Anterograde and retrograde pacing protocols were used to obtain the Wenckebach cycle length, effective and functional refractory periods of the AVN, index of AVN conduction delay (the area under the AVN conduction curve), as well as index of intra-atrial conduction delay between the AVN inputs. Compared to control, 5 and 10 mM BDM produced either shortening or no effect on all of the above parameters except a slight (6% and 14%, respectively) increase in the intra-atrial delay. At 20 mM, BDM produced a further increase in the intra-atrial delay (up to 50%) as well as in the retrograde AVN conduction delay (up to 16%), while the characteristics of the anterograde conduction were still improved. The effects of perfusion with BDM on these parameters were reversible after washout.

CONCLUSIONS

Aside from its known effect as an electromechanical uncoupler, BDM reversibly altered some of the electrical responses of the AVN. Most of these alterations, however, did not impede but rather improved AVN conduction. Since a dose of 10 mM is sufficient to fully eliminate undesirable motion, BDM should be considered a safe and valuable tool in AVN studies in vitro requiring a mechanically quiescent preparation.

Influence of duration of rapid ventricular pacing on ventricular refractoriness in the presence of propafenone and lidocaine

Propafenone and lidocaine have a rate dependent negative dromotropic effect on intraventricular conduction. We investigated the use dependent actions of propafenone and lidocaine on intraventricular conduction in isolated guinea pig hearts perfused by the method of Langendorff. Of primary interest was how the number of stimuli of the conditioning train (S1) might influence the ventricular effective refractory period (VERP) when refractoriness is assessed at a high pacing rate. Propafenone (0.3 microM) and lidocaine (50 microM) caused a comparable prolongation of the intraventricular conduction time during sinus rhythm. During ventricular pacing in the presence of propafenone an abrupt decrease of the pacing cycle length (220 to 120 ms) resulted in an initial peak of rate dependent prolongation of the QRS interval that subsequently decreased to a stable steady-state level. Lidocaine also induced a rate dependent increase of the intraventricular conduction time up to a steady-state level. The time constant, characterizing the changes of the intraventricular conduction time after shortening the ventricular pacing cycle length from 220 to 120 ms was significantly (P < 0.01) longer in the presence of propafenone (tau = 31 +/- 4 beats; mean +/- SEM; n = 11) than for lidocaine (tau = 3 +/- 1; n = 10). Both drugs caused the greatest increase of the VERP when the number of conditioning stimuli (S1, interstimulus interval = 120 ms) was in the range of their respective time constant. However, when the number of conditioning stimuli was further increased, VERP progressively diminished. These effects may be explained by a shortening of the action potential during high rates that results in a decreased binding of propafenone to Na+ channels and by the direct shortening of repolarization period by lidocaine (Class IB drug).

Clinical electrophysiologic effects of a single high oral dose of amiodarone

Several recent reports have described the antiarrhythmic effects of a single high oral dose of amiodarone but clinical electrophysiologic effects have not been reported. The present study was performed to assess electrophysiologic effects in 12 patients. After baseline electrophysiologic studies (EPS) patients were administered a single oral dose of 30 mg/kg of amiodarone. EPS was repeated 7.5 +/- 0.5 hours later. Plasma levels of amiodarone and its metabolite desethylamiodarone were determined at the time of the second EPS, Holter monitoring was performed for 24 hours after amiodarone administration. Amiodarone significantly increased the following parameters: corrected QT interval (+4.5%), functional refractory period of the right atrium (+7%); AH interval (+12.3%), effective refractory period of the atrioventricular node (+18.5%), and cycle length of Wenckebach block (+8.4%). These effects were not correlated with plasma levels of amiodarone and desethylamiodarone. Holter monitoring detected no significant bradycardia or arrhythmia. These findings indicate that the effects of a single high oral dose of amiodarone are the same as those known to be induced by acute intravenous administration.

Changes in atrioventricular conduction properties with refractory-period modulation

Dofetilide, clofilium, and risotilide, three drugs known to prolong cardiac action potentials and refractory periods, were studied by using a perfused isolated rabbit heart preparation with intermittent premature pacing and bipolar surface electrograms. The rate-related effects of these drugs on atrioventricular (AV) conduction were tested by pacing at a long (400 ms) and a short (250 ms) basic cycle length (BCL). All three drugs increased refractory periods in a concentration-dependent manner in most segments of the AV axis. The maximal atrio-His (AH) conduction interval (AHmax) and delta AH (AHmax - AHmin) produced by premature pacing was decreased by the highest concentration of each drug at the 400-ms BCL, whereas only clofilium reduced AHmax and delta AH at the 250-ms BCL. Changes in delta AH correlated best with changes in the atrial functional refractory period. The His-Purkinje system conduction interval (HV), represented by delta HV, was unaffected by any drug at either BCL. These results show that if atrial or nodal refractory periods are increased sufficiently, AHmax but not AHmin was decreased at the 400-ms BCL. Because dofetilide and risotilide did not affect AHmax at the 250-ms BCL, these drugs may be less effective at preventing AV nodal reentrant tachycardias than a drug such as clofilium that displays less rate dependency.

Is vagal innervation to the atrioventricular node impaired after radiofrequency ablation of the slow atrioventricular nodal pathway?

To assess the potentially adverse effects of RF catheter ablation (RFCA) of the slow AV nodal pathway on the parasympathetic innervation to the AV node in patients with AV nodal reentrant tachycardia (AVNRT), AV nodal conduction was evaluated following vagal stimulation by means of a phenylephrine bolus injection (200 micrograms) before and after RFCA in ten patients (mean age, 37 +/- 14 years). Nine patients with AV reentrant tachycardia (AVRT) due to a left free wall accessory pathway served as a control group (mean age of 37 +/- 12 years). Whereas no prolongation of the AH interval was observed in the AVNRT group following the phenylephrine bolus during sinus rhythm, despite a significant slowing in sinus rate, phenylephrine administration in AVRT patients was associated with both slowing of the sinus rate and prolongation of the AH interval. Following successful RFCA, the same responses were observed. To delineate the indirect effect of heart rate on AV conduction in response to the phenylephrine bolus, the AH interval was also measured during fixed atrial pacing. A marked prolongation of the AH interval occurred in both groups following phenylephrine administration. This prolongation was biphasic in 50% of AVNRT patients before ablation, suggesting a predominant effect of vagal stimulation on the fast AV nodal pathway. RFCA was associated with disappearance of discontinuous AV conduction in all but one patient with AVNRT. Vagal stimulation caused the same amount of AH interval prolongation as before RFCA in both study groups. In conclusion, patients with AVNRT have a preserved modulation of AV nodal conduction in response to vagal stimulation during sinus rhythm. In addition, vagal stimulation seems to exert a predominant effect on the fast AV nodal pathway. RFCA of the slow AV nodal pathway in patients with AVNRT does not cause detectable damage to the vagal innervation to the AV node.

Cardiac electrophysiological effects of falipamil in the conscious dog: comparison with alinidine

We studied the cardiac electrophysiological effects of falipamil, a specific bradycardic agent, in conscious dogs, in comparison with those of alinidine. Sinus rate, corrected sinus recovery time, and Wenckebach point were measured in six intact dogs. Atrial rate, ventricular rate, and atrial effective refractory period were measured in six atrioventricular-blocked dogs. In both groups, blood pressure was also monitored. Each dog received, with at least a three-day interval, falipamil (hydrochloride) and alinidine (hydrobromide) in four successive intravenous injections, 30 min apart, at 0.5, 0.5, 1, and 2 mg kg-1. Falipamil increased sinus rate and atrial rate, but decreased ventricular rate, whereas alinidine decreased sinus rate and ventricular rate, but increased atrial rate. Falipamil shortened corrected sinus recovery time and increased Wenckebach point, whereas alinidine lengthened corrected sinus recovery time and decreased Wenckebach point. Falipamil and alinidine increased atrial effective refractory period. Neither falipamil nor alinidine modified mean blood pressure in either group. Overall, these results show that (a) falipamil exhibits effects on the electrical activity of the heart, reflecting the predominant direct vagolytic effect of this drug, (b) alinidine exhibits effects reflecting the marked antiarrhythmic potential of this agent, and (c) thus indicate that two drugs with almost identical specific bradycardic properties can produce quite different electrophysiological effects in the conscious dog.

Developmental effects of d-sotalol on anterograde and retrograde atrioventricular conduction in the rabbit

INTRODUCTION

These experiments investigate the developmental effects of d-sotalol on standard electrophysiologic parameters of anterograde and retrograde AV conduction in the rabbit.

METHODS AND RESULTS

Using bipolar electrograms and standard pacing techniques, the effects of graded concentrations of d-sotalol on anterograde and retrograde conduction in mature and immature perfused rabbit hearts were compared. Also, a quantitative assessment of the drug's effects on a rate-dependent property of anterograde AV node (AVN) conduction, termed the "recovery process," was compared in mature and immature rabbit hearts. The main developmental electrophysiologic findings of this investigation are: (1) in both the mature and immature rabbit heart, d-sotalol increases the anterograde conduction time and prolongs refractoriness of the AVN, yet the minimal concentrations of d-sotalol that produce these changes are lower in the neonate; (2) d-sotalol increases the anterograde refractory period of the His-Purkinje system in both age groups, but increases anterograde infra-Hisian conduction only in the neonate; (3) 1 x 10-4 M d-sotalol significantly changes the time constant of the AVN recovery process in the neonate, but not in the adult; (4) for retrograde conduction, slow conduction through the AVN (HAmax) and infra-Hisian region (VHmax) are increased by d-sotalol in the neonate, but not in the adult.

CONCLUSIONS

The findings of this study illustrate that d-sotalol has different effects on parameters of the developing AV conduction system. This implies that there may be maturational changes in the ionic currents that are responsible for anterograde and retrograde AVN and His-Purkinje conduction.

The effects of dobutamine and phenylephrine on atrioventricular conduction during combined use of halothane and thoracic epidural lidocaine

The purpose of this study was to measure cardiac sympathetic nerve activity (CSNA) and atrioventricular (AV) conduction and to test the effects of dobutamine (DOB) and phenylephrine (PHE) on AV conduction during combined use of halothane and thoracic epidural lidocaine. Cats were anesthetized with 1 % halothane and an epidural catheter was inserted through T-9 laminectomy. His bundle and atrial electrocardiograms were obtained and atrial electric stimulation was performed using quadripolar catheter electrodes. Cats underwent left thoracotomy, and CSNA was recorded directly from the left ventrolateral or ventromedial nerve. In addition to sinus cycle length (SCL) measurement during spontaneous beating, the functional refractory period (FRP) of the atrioventricular node (AV node), effective refractory period (ERP) of the atrium, atrium-His (A-H) intervals were determined just before and 10, 20, and 30 min after epidural administration of 1% lidocaine (0.2 mL/kg) in Group C. DOB 5 micrograms/kg/min (Group DOB) and PHE 0.5-1.0 micrograms/kg/min (Group PHE) were intravenously administered from 12 to 22 min after epidural lidocaine. CSNA and mean arterial pressure (MAP) were markedly decreased and SCL, FRP of AV node, ERP of atrium and A-H interval were significantly prolonged after epidural lidocaine. MAP increased to baseline level during DOB or PHE infusion. Worsening of cardiac electrophysiological variables was improved with DOB infusion, but did not change with PHE infusion. We conclude that thoracic epidural lidocaine during halothane anesthesia almost eliminates CSNA, and thereby attenuates sinus node automaticity and AV node function. DOB restored normal cardiac electrophysiological variables, and therefore is preferable to phenylephrine as a pressor drug.

[Effects of anesthesia with inhalation anesthetics on excitation, conduction and refractory parameters of the heart. Experiments in dogs]

Cardiac excitation, conduction, and refractory parameters of the heart were investigated in 40 beagle dogs to obtain basic values under the influence of the following 9 inhalation anaesthetic regimes using HIS-bundle electrography and programmed stimulation: 1. halothane; 2. enflurane; 3. isoflurane; 4. halothane+nitrous oxide; 4. enflurane+nitrous oxide; 6. isoflurane+nitrous oxide; 7. alfentanil/midazolam/succinylcholine+nitrous oxide; 8. fentanyl/midazolam/pancuronium bromide+nitrous oxide; 9. propofol+nitrous oxide. It could be shown that inhalation anaesthetics under laboratory conditions exert some influences on the observed cardiac parameters, although they are not as strong as those of intravenous anaesthetics. Therefore their effects should be considered in experiments with other substances or in cardiovascular results.