A comparative analysis of antegrade and retrograde conduction patterns in man

Evaluation of the input site and characteristics of the antegrade fast pathway based on three-dimensional bi-atrial stimulus-ventricle mapping

Purpose

Previous studies examined the right atrial (RA) input site of the antegrade fast pathway (AFp) (AFpI). However, the left atrial (LA) input to the atrioventricular (AV) node has not been extensively evaluated. In this study, we created three-dimensional (3-D) bi-atrial stimulus-ventricle (St-V) maps and analyzed the input site and characteristics of the AFp in both the RA and LA.

Methods

Forty-four patients diagnosed with atrial fibrillation or WPW syndrome were included in this study. Three-dimensional bi-atrial St-V mapping was performed using an electroanatomical mapping system. Sites exhibiting the minimal St-V interval (MinSt-V) were defined as AFpIs and were classified into seven segments, four in the RA (F, S, M, and I) and three in the LA (M1, M2, and M3). By combining the MinSt-V in the RA and LA, the AFpIs were classified into three types: RA, LA, and bi-atrial (BA) types. The clinical and electrophysiological characteristics were compared.

Results

AFpIs were most frequently observed at site S in the RA (34%) and M2 in the LA (50%), and the BA type was the most common (57%). AFpIs in the LA were recognized in 75% of the patients. There were no clinical or electrophysiological indicators for predicting AFpI sites.

Conclusions

Three-dimensional bi-atrial St-V maps could classify AFpIs in both the RA and LA. AFpIs in the LA were frequently recognized. There were no significant clinical or electrophysiological indicators for predicting AFpI sites, and 3-D bi-atrial St-V mapping was the only method to reveal the precise AFp input site.

Programmed His Bundle Pacing: A Novel Maneuver for the Diagnosis of His Bundle Capture

BACKGROUND

During permanent nonselective His bundle (ns-HB) pacing, it is crucial to confirm HB capture/exclude that only right ventricle (RV) myocardial septal pacing is present. Because the effective refractory period (ERP) of the working myocardium is different than the ERP of the HB, we hypothesized that it should be possible to differentiate ns-HB capture from RV myocardial capture using programmed extrastimulus technique.

METHODS

In consecutive patients during HB pacemaker implantation, programmed HB pacing was delivered from the screwed-in HB pacing lead. Premature beats were introduced at 10-ms steps during intrinsic rhythm and also after a drive train of 600 ms. The longest coupling interval that resulted in an abrupt change of QRS morphology was considered equal to ERP of HB or RV myocardium.

RESULTS

Programmed HB pacing was performed from 50 different sites in 32 patients. In 34 of 36 cases of ns-HB pacing, the RV myocardial ERP was shorter than HB ERP (271.8±38 versus 353.0±30 ms; P<0.0001). Programmed HB pacing using a drive train resulted in a typical abrupt change of paced QRS morphology: from ns-HB to RV myocardial QRS (34 of 36 cases) or to selective HB QRS (2 of 36 cases). Programmed HB pacing delivered during native conduction resulted in obtaining selective HB QRS in 20 of 34 and RV myocardial QRS in 14 of 34 of the ns-HB cases. In RV myocardial-only pacing cases (false ns-HB pacing, n=14), such responses were not observed-the QRS morphology remained stable. Therefore, the programmed HB pacing correctly diagnosed all ns-HB cases and all RV myocardial pacing cases.

CONCLUSIONS

A novel maneuver for the diagnosis of HB capture, based on the differences in ERP between HB and myocardium, was formulated, assessed, and found as diagnostically valuable. This method is unique in enabling to visualize selective HB QRS in patients with otherwise obligatory ns-HB pacing (RV myocardial capture threshold <HB capture threshold).

Human His-Purkinje System: Normal Electrophysiologic Behavior

The His-Purkinje system (HPS) plays a significant role in human pathophysiology, but knowledge is scattered. This article highlights some of the relevant concepts, phenomena, and mechanisms; clarifies, expands, confirms, or modifies commonly encountered clinical events; and adds new information, which is often available but obscure. Also included are the essentials of HPS anatomy and physiology. It is important to abandon inaccurate concepts that are still taught and occasionally appear in text books.

Recovery of Ventriculo-Atrial Conduction after Adrenaline in Patients Implanted with Pacemakers

BACKGROUND

Ventriculo-atrial (VA) conduction can have negative consequences for patients with implanted pacemakers and defibrillators. There is concern whether impaired VA conduction could recover during stressful situations. Although the influence of isoproterenol and atropine are well established, the effect of adrenaline has not been studied systematically. The objective of this study was to determine if adrenaline can facilitate recovery of VA conduction in patients implanted with pacemakers.

METHODS

A prospective study was conducted on 61 consecutive patients during a 4-month period (April-July 2014). The presence of VA conduction was assessed during the pacemaker implantation procedure. In case of an impaired VA conduction, adrenaline infusio was used as a stress surrogate to test conduction recovery.

RESULTS

The indications for pacemaker implantation were: sinus node dysfunction in 18 patients, atrioventricular (AV) block in 40 patients, binodal dysfunction (sinus node+ AV node) in two patients and other (carotid sinus syndrome) in one patient. In the basal state, 15/61 (24.6%) presented spontaneous VA conduction and 46/61 (75.4%) had no VA conduction. After administration of adrenaline, there was VA conduction recovery in 5/46 (10.9%) patients.

CONCLUSIONS

Adrenaline infusion produced recovery of VA conduction in 10.9% of patients with absent VA conduction in a basal state. Recovery of VA conduction during physiological or pathological stresses could be responsible for the pacemaker syndrome, PMT episodes, or certain implantable cardiac defibrillator detection issues.

His bundle activates faster than ventricular myocardium during prolonged ventricular fibrillation

Background

The Purkinje fiber system has recently been implicated as an important driver of the rapid activation rate during long duration ventricular fibrillation (VF>2 minutes). The goal of this study is to determine whether this activity propagates to or occurs in the proximal specialized conduction system during VF as well.

Methods and Results

An 8×8 array with 300 µm spaced electrodes was placed over the His bundles of isolated, perfused rabbit hearts (n = 12). Ventricular myocardial (VM) and His activations were differentiated by calculating Laplacian recordings from unipolar signals. Activation rates of the VM and His bundle were compared and the His bundle conduction velocity was measured during perfused VF followed by 8 minutes of unperfused VF. During perfused VF the average VM activation rate of 11.04 activations/sec was significantly higher than the His bundle activation rate of 6.88 activations/sec (p<0.05). However from 3–8 minutes of unperfused VF the His system activation rate (6.16, 5.53, 5.14, 5.22, 6.00, and 4.62 activations/sec significantly faster than the rate of the VM (4.67, 3.63, 2.94, 2.24, 3.45, and 2.31 activations/sec) (p<0.05). The conduction velocity of the His system immediately decreased to 94% of the sinus rate during perfused VF then gradually decreased to 67% of sinus rhythm conduction at 8 minutes of unperfused VF.

Conclusion

During prolonged VF the activation rate of the His bundle is faster than that of the VM. This suggests that the proximal conduction system, like the distal Purkinje system, may be an important driver during long duration VF and may be a target for interventional therapy.

Prolonged transient atrial electrical silence following termination of chronic atrial tachyarrhythmias

INTRODUCTION

Atrial standstill is a rare heterogeneous arrhythmia characterized by electrical and mechanical standstill and electrical inexcitability. A long-lasting progressive form is seen with cardiac and neuromuscular diseases, and a familial or idiopathic form may have a genetic basis. A transient form was described secondary to drug intoxication, electrolyte imbalance, cardiac inflammation, and ischemia.

METHODS

We investigated three patients with long-standing atrial tachyarrhythmia (AT) (atrial flutter in two, and focal atrial tachycardia in one). All patients underwent a complete electrophysiological study with mapping of right and left atrial activity and radiofrequency ablation (RF Abl) of AT.

RESULTS

Following RF Abl of AT, all three patients manifested transient atrial electrical silence in the absence of known reversible causes. Atrial electrical silence was observed when, following AT termination, an escape atrioventricular (AV) junctional rhythm (in two patients) and an escape VVI pacemaker rhythm (in one patient) showed transient ventriculo-atrial (VA) conduction block (up to 30 seconds). A dominant sinus rhythm was observed to return 30 minutes, 90 minutes, and 12 hours, respectively, in the three patients. Two patients received a dual chamber pacemaker and a decision was made not to upgrade the patient with VVI pacemaker.

DISCUSSION AND CONCLUSIONS

The present report expands the spectrum of the syndrome of atrial standstill and raises interesting questions regarding possible electrophysiologic mechanism(s) of prolonged post overdrive atrial standstill. The report suggests that chronic overdrive of sinus and subsidiary atrial pacemakers may result in calcium overloading of cardiac cells, which is known to cause suppression of pacemaker activity as well as increased intracellular resistance. These mechanisms can possibly result in either prolonged suppression of sinus and atrial pacemaker activity and/or pacemaker exit block.

Participation of a concealed atriohisian tract in the reentrant circuit of the slow–fast type of atrioventricular nodal reentrant tachycardia

BACKGROUND

The retrograde fast pathway in typical atrioventricular nodal reentrant tachycardia (AVNRT) exhibits marked variation in its electrophysiologic properties.

OBJECTIVE

The purpose of this study was to characterize the retrograde fast pathway and localize the lower turnaround site of the reentrant circuit in typical AVNRT.

METHODS

Seventy-four patients with typical AVNRT were divided into two groups according to the response of the retrograde fast pathway to intravenous administration of adenosine triphosphate (ATP) during ventricular pacing: ATP-S [n = 47 (63.5%)] with and ATP-R without [n = 27 (36.5%)] His-atrial (H-A) block. H-A intervals were measured from the most proximal His-bundle electrogram to the earliest atrial activation during the tachycardia (HAt) and entrainment pacing from the parahisian right ventricular region (HAe). It was postulated that the HAt was the difference in conduction time between the lower common pathway (x) and retrograde fast pathway (y) (HAt = y - x), whereas HAe was the sum of the two (HAe = y + x). Hence, x = (HAe-HAt)/2. x >0 suggested the presence of a lower common pathway, whereas x <0 suggested the absence of a lower common pathway and lower turnaround site within the His bundle.

RESULTS

x was significantly smaller in ATP-R than ATP-S (-6 +/- 5 vs 4 +/- 4 ms, P <.05) and was <0 in 23 (85%) of 27 ATP-R patients. The maximal increment in H-A interval during ventricular pacing was significantly longer in ATP-S than ATP-R (35 +/- 33 vs 2 +/- 2 ms, P <.05).

CONCLUSION

A concealed atriohisian tract totally bypassing the atrioventricular node constituted the retrograde fast pathway in one third of all typical AVNRT cases.

Role of Compact Node and Posterior Extension in Direction-Dependent Changes in Atrioventricular Nodal Function in Rabbit

INTRODUCTION

AV nodal conduction properties differ in the anterograde versus the retrograde direction. The underlying substrate remains unclear. We propose that direction-dependent changes in AV nodal function are the net result of those occurring in the slow and fast pathways.

METHODS AND RESULTS

Anterograde and retrograde AV nodal properties were determined with a premature protocol before and after posterior extension (slow pathway) ablation, and before and after upper compact node (fast pathway) ablation. Each ablation was performed in a different group of six rabbit heart preparations. In control, nodal minimum conduction time (NCTmin) and effective refractory period (ERPN) typically were longer, and maximum conduction time (NCTmax) was shorter in the retrograde compared to the anterograde direction. Posterior extension ablation prolonged anterograde ERPN from 91 +/- 10 ms to 141 +/- 15 ms (P < 0.01) and shortened NCTmax from 150 +/- 13 ms to 82 +/- 7 ms (P < 0.01) but did not affect retrograde conduction. Thus, the posterior extension normally contributes to the anterograde but not retrograde recovery curve. Compact node ablation prolonged anterograde conduction (NCTmin increased from 57 +/- 2 ms to 73 +/- 7 ms, P < 0.01) but did not alter ERPN and NCTmax. This ablation abolished retrograde conduction in two preparations and resulted in retrograde slow pathway conduction in four, the latter being interrupted by posterior extension ablation. Thus, the compact node accounts for the baseline of the recovery curve in both directions. Ablation of the compact node results in anterograde slow pathway conduction over the entire cycle length range and may result in retrograde slow pathway conduction.

CONCLUSION

Direction-dependent properties of the AV node arise from those of the compact node-based fast pathway and posterior extension-based slow pathway. Normal AV node has bidirectional dual pathways.

Atrioventricular nodal reentry tachycardia in patients with sinus node dysfunction: electrophysiologic characteristics, clinical presentation, and results of slow pathway ablation

INTRODUCTION

Sinus node dysfunction (SND) is frequently associated with impaired AV conduction. This study investigated the electrophysiologic properties of dual AV nodal pathways in patients suffering from both SND and AV nodal reentrant tachycardia (AVNRT).

METHODS AND RESULTS

Two groups of patients with slow-fast AVNRT underwent invasive electrophysiologic testing and catheter ablation of the slow pathway. Group A comprised 10 patients with SND (age 70 +/- 8 years). Group B included 10 age-matched patients without SND (age 69 +/- 7 years; P = NS) who served as controls. Patients of group A exhibited prolongation of the anterograde Wenckebach cycle lengths (WBCLs) of both the fast pathway (559 +/- 96 vs 361 +/- 38 msec; P < 0.01) and the slow pathway (409 +/- 57 vs 339 +/- 32 ms; P < 0.01). However, the delta between the WBCLs of the fast and the slow pathways was larger in patients of group A (150 +/- 80 vs 22 +/- 20 msec; P < 0.01). Retrograde fast pathway conduction was well preserved with no difference in WBCLs (356 +/- 42 vs 330 +/- 47 msec; P = NS). Cycle lengths of AVNRT were longer in group A (468 +/- 46 vs 363 +/- 37 msec; P < 0.01). Clinically, all patients of group A suffered from multiple episodes of AVNRT per week, which was not the case in any patient of group B (P < 0.01). Catheter ablation of the slow pathway eliminated AVNRT in all patients without complications.

CONCLUSIONS

Patients with AVNRT and SND exhibit characteristic electrophysiologic alterations of both AV nodal pathways. Clinically, this results in significantly more frequent episodes of tachycardia. Slow pathway ablation appears to be safe and effective in these patients.

Clinical and electrophysiologic characteristics and long-term efficacy of slow-pathway catheter ablation in patients with spontaneous supraventricular tachycardia and dual atrioventricular node pathways without inducible tachycardia

OBJECTIVES

We sought to investigate the long-term efficacy of slow-pathway catheter ablation in patients with spontaneous, documented paroxysmal supraventricular tachycardia (PSVT) and dual atrioventricular (AV) node pathways but without inducible tachycardia.

BACKGROUND

The lack of reproduction of clinical PSVT by programmed electrical stimulation, which is not uncommon in AV node reentrant tachycardia (AVNRT), is a dilemma in making the decision of the therapeutic end point of radiofrequency catheter ablation.

METHODS

Twenty-seven patients (group A) with documented but noninducible PSVT and with dual AV node pathways were prospectively studied. Programmed electrical stimulation could induce a single AV node echo beat in 12 patients, double echo beats in 4 patients and none in 11 patients at baseline or during isoproterenol infusion. Of the patients in group A, 16 underwent slow-pathway catheter ablation and 11 did not. The clinical and electrophysiologic characteristics of the 27 patients were compared with those of patients with dual AV node pathways and inducible AVNRT (group B, n = 55) and patients with dual AV node pathways alone without clinical PSVT (group C, n = 47).

RESULTS

During 23+/-13 months of follow-up, none of the 16 patients with slow-pathway catheter ablation had recurrence of PSVT. However, 7 of the 11 patients without ablation had PSVT recurrence at 13+/-14 months of follow-up (p < 0.03 by Kaplan-Meier analysis). Compared with groups B and C, group A consisted predominantly of men who had better retrograde AV node conduction and a narrower zone for anterograde slow-pathway conduction.

CONCLUSIONS

Slow-pathway catheter ablation is highly effective in eliminating spontaneous PSVT in which the tachycardia is not inducible despite the presence of dual AV node pathways.

Electrophysiologic properties of the atrioventricular node in pediatric patients

OBJECTIVES

The purpose of this study was to characterize anterograde and retrograde properties of the atrioventricular (AV) node in children and to determine the presence of ventriculoatrial (VA) conduction and dual AV node pathways.

BACKGROUND

Although AV node reentry is common in adults, it accounts for 13% of pediatric supraventricular tachycardia (SVT). The age-related changes in the AV node with development are poorly understood. The incidence of dual AV node pathways and VA conduction in the pediatric population is unknown.

METHODS

Electrophysiologic studies were performed in 79 patients with normal hearts and no evidence of AV node arrhythmias. Patients were classified into two groups by age: group I = 49 patients (0.39 to 12.8 years old, mean [+/- SD] age 8.5 +/- 3.6); group II = 30 patients (13.4 to 20.0 years old, mean age 15.6 +/- 1.8).

RESULTS

There was a significant difference (p < 0.05) in the cycle length (CL) at which anterograde AV block occurred between group I (305 +/- 63 ms) and group II (350 +/- 91 ms). Sixty-one percent of children had VA conduction with no age-related differences. There was no significant difference in the mean CL of retrograde VA block (360 ms). The incidence of dual AV node pathways in group I was 15% and 44% in group II (p < 0.05).

CONCLUSIONS

These findings suggest that AV node electrophysiology undergoes maturational changes. The increase in AV node reentrant tachycardia in adults may relate to changes in the relative refractoriness and conduction of the AV node or to differences in autonomic input into the AV node that allow dual pathway physiology to progress to SVT.

Effects of verapamil, propranolol, and procainamide on adenosine-induced negative dromotropism in human beings

Adenosine, verapamil, propranolol, and procainamide are widely used antiarrhythmic drugs. The interactions among them are still not known in human beings. Adenosine-induced negative dromotropic effects were assessed by rapid bolus injection of adenosine during constant high right atrial pacing in each patient. The initial dose of adenosine was 0.5 mg and was followed by a stepwise increment of 0.5 mg until atrioventricular (AV) nodal block occurred. The negative dromotropic actions of adenosine were examined in the control state and in the following three protocols in three groups of patients: (1) In 12 patients (group 1), intravenous verapamil, 0.15 mg/kg, was given; (2) In 12 patients (group 2), intravenous propranolol, 0.1 mg/kg, was given; and (3) in 10 patients (group 3), intravenous procainamide, 15 mg/kg, was given. The dose-response curves of adenosine on AV nodal conduction were almost identical in the control state and after verapamil, propranolol, or procainamide injection. However, verapamil, in contrast to propranolol, significantly reduced the dose of adenosine required to produce AV nodal block, from 4.4 +/- 0.7 mg to 2.7 +/- 0.3 mg (p < 0.01). Of note, procainamide exerted no significant effects on adenosine-induced negative dromotropism on AV nodal conduction or AV nodal block. In conclusion, the negative dromotropic effects of adenosine are preserved and independent even in the presence of verapamil, propranolol, or procainamide. Both verapamil and propranolol can exhibit additive effects with adenosine in prolonging AV nodal conduction time; however, only verapamil can reduce the dose of adenosine required to produce AV nodal block. This finding indicates that the dose of adenosine may be reduced for patients who have already been treated with verapamil.

Reproducibility of electrophysiological measurements in cardiac transplant recipients

The clinical usefulness of certain electrophysiological measurements, particularly those of sinus node function, is limited by variation in autonomic tone resulting in poor reproducibility. The denervated transplanted heart is not susceptible to direct autonomic control and, therefore, electrophysiological measurements may be more reproducible in this group. To our knowledge, this hypothesis has not previously been systematically evaluated. Ten adult recipients underwent serial electrophysiological studies between 10-18 days after cardiac transplantation. Five studies were performed at 2-hour intervals during a single day, between 9:00 a.m. and 5:00 p.m. Spontaneous cycle length (SCL) was recorded. Sinus node recovery time (SNRT), sinoatrial conduction time (SACT), and atrioventricular (AV) Wenckebach cycle length were measured using standard techniques. The effective refractory periods of the complete AV conducting system (AVERP), atrium (AERP), and ventricle (VERP) were measured. Corrected maximal SNRT was normal in all subjects. Mean coefficients of variation (Cv) for SCL, corrected maximal SNRT, and SACT were 2.8%, 7.4%, and 3.5%, respectively. AVERP was less than AERP in seven subjects, limiting further analysis. The mean Cv for AV Wenckebach cycle length was 2.1%. The mean coefficients of variation for AERP were 3.6% and 3.7%, and for VERP 3% and 3.3%, at 600- and 400-ms drive cycle lengths, respectively. Previous studies report much greater variation in innervated subjects particularly of indices of sinus node function. Thus, the reproducibility of electrophysiological measurements of sinus and AV node function in the transplanted heart is better than in normal subjects. This may have important implications for the reliability of electrophysiological testing in transplant recipients.

Atrioventricular nodal conduction and refractoriness

The AV node is ideally suited to act as a weigh station between atrium and ventricle. It is believed that the latter function is achieved by virtue of nodal conduction being dependent on slow channel conduction probably carried by calcium currents. The AV nodal structure allows for encouragement of decremental conduction but also allows for two or more pathways or collection of cells which may trigger reentrant arrhythmias.

Physiological substrate for antidromic reciprocating tachycardia. Prerequisite characteristics of the accessory pathway and atrioventricular conduction system

BACKGROUND

Although the anatomic "substrate" for the occurrence of antidromic reciprocating tachycardia (ART) has been previously examined, the underlying physiological substrate for this unusual arrhythmia in patients with the Wolff-Parkinson-White syndrome has not been thoroughly characterized.

METHODS AND RESULTS

The electrophysiological properties of the accessory pathway and normal ventriculoatrial conduction system in 30 patients with ART and a single accessory pathway were compared with those observed in a control group of 36 patients without this arrhythmia to elucidate the critical physiological substrate essential for the development and maintenance of ART. Inducible ART had a mean cycle length of 286 +/- 31 msec. The average retrograde ventriculoatrial conduction system effective refractory period in ART patients was significantly less than that in the control group (244 +/- 32 versus 291 +/- 46 msec, p = 0.0002). All of the ART patients showed retrograde conduction over the normal conduction system at cycle lengths of less than or equal to 360 msec; 23 had 1:1 conduction to less than or equal to 300 msec, and 16 showed 1:1 propagation at cycle lengths of less than or equal to 260 msec. The shortest cycle length accompanied by 1:1 retrograde propagation over the normal conduction system in patients with ART was also significantly less than that observed in the control group (274 +/- 39 versus 347 +/- 73 msec, p less than 0.001). The accessory pathway anterograde ERP in ART patients with 1:1 retrograde conduction over the normal ventriculoatrial conduction system at cycle lengths of less than or equal to 360 was significantly less than that seen in comparable control patients (247 +/- 23 versus 284 +/- 56 msec, p = 0.001), and the accessory pathway location was significantly further from the atrioventricular node in 21 patients with ART undergoing surgery than that in 22 operated control patients (3.8 +/- 0.8 versus 2.9 +/- 0.8 mapping units, p = 0.0025) who also had retrograde ventriculoatrial conduction to cycle lengths of less than or equal to 360 msec. No significant differences in anterograde atrioventricular conduction system properties, retrograde accessory pathway refractoriness, or shortest ventricular pacing cycle lengths maintaining 1:1 conduction via the accessory pathway were observed between groups.

CONCLUSIONS

This quantitative characterization of the properties of conduction and refractoriness of both the accessory pathway and ventriculoatrial conduction system and the relation between these characteristics and the accessory pathway location in ART patients provides additional insight into the prerequisites for the initiation and maintenance of this rhythm disturbance.

Electrophysiologic characteristics of manifest and latent retrograde conduction in dogs

Atrioventricular (AV) nodal reentry requires intact retrograde ventriculoatrial (VA) conduction. The purpose of this study was to assess the contribution of various pacing and pharmacologic techniques to uncover VA conduction during apparent unidirectional VA block, and to evaluate the role of several biologic and electrophysiologic factors in concealment of retrograde conduction. Forty healthy dogs underwent catheter-electrophysiologic studies of AV and VA conduction. Group I (20 animals) had intact VA conduction. Group II (six animals) had VA dissociation with ventricular pacing initiated during sinus rhythm, but the presence of VA conduction was confirmed by isoproterenol infusion or by premature ventricular stimulation. In group III (14 animals), the above techniques failed to uncover VA conduction. Eight of 14 group III animals underwent thoracotomy and crushing or freezing of the sinoatrial (SA) node. Ventricular pacing initiated during sinus standstill was associated with 1:1 VA conduction in each experiment. VA conduction time and retrograde Wenckebach cycle length, both in the baseline state and during isoproterenol infusion, were significantly longer in the eight animals in group III than in those in group I. Age, gender, weight, breed, sinus cycle length, and anterograde AV conduction properties were not significantly different between groups I, II, and III. The data suggest that (1) in normal dogs, complete unidirectional VA block probably does not exist; (2) in the presence of anterograde input to the AV node, even sophisticated pacing and pharmacologic maneuvers may fail to uncover the presence of VA conduction; (3) although anterograde input is essential for concealment of VA conduction, the phenomenon is more closely associated with depressed retrograde than with anterograde AV nodal characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of intravenous isoproterenol in the electrophysiologic induction of atrioventricular node reentrant tachycardia in patients with dual atrioventricular node pathways

To assess the role of intravenous isoproterenol for the facilitation of electrophysiologic induction of atrioventricular (AV) node reentrant tachycardia, 20 patients with dual AV node pathways who lacked inducible AV node reentrant tachycardia at control study had a constant isoproterenol infusion administered and underwent repeat study. Six (30%) of 20 patients (group I) had inducible AV node reentrant tachycardia during isoproterenol infusion whereas the other 14 (70%) patients (group II) did not. Paroxysmal supraventricular tachycardia was clinically documented in all 6 group I patients compared to 3 (21%) of 14 group II patients (p = 0.002). The sensitivity and specificity of isoproterenol-facilitated induction of AV node reentrant tachycardia were 67 and 100%, respectively. The isoproterenol-facilitated induction of sustained AV node reentry was mediated by resolution of the weak link in anterograde slow pathway in 2 (33%) patients, in retrograde fast pathway in 3 (50%) and in both anterograde slow and retrograde fast pathways in 1 (17%) patient. Four group I patients were given intravenous propranolol, 0.2 mg/kg body weight, and had complete suppression of isoproterenol-facilitated induction of AV node reentry. Thus, intravenous isoproterenol is a rather sensitive and highly specific adjunct to electrophysiologic induction of AV node reentrant tachycardia in patients with dual AV node pathways but without inducible sustained AV node reentry.

Electrophysiological determinants of antidromic reentry induced during atrial extrastimulation. Insights from a pacing model of Wolff-Parkinson-White syndrome

The electrophysiology of antidromic reentry, a less common phenomenon than orthodromic reentry, remains a poorly understood aspect of the Wolff-Parkinson-White (WPW) syndrome. We used a pacing model of ventricular preexcitation in patients without WPW, so that electrophysiological events in the normal pathway during atrial extrastimulation (A1-A2 technique) could be precisely delineated without the obscuring effect of an actual accessory pathway. Ventricular preexcitation was simulated by an A1-V1 sequential basic drive with A2-V2 extrastimulation at progressively shorter A1-A2 (equal to V1-V2) coupling intervals. At each coupling interval tested within the zone of atrioventricular (A-V) nodal effective refractory period (since anterograde block of A2 was considered mandatory for manifestation of antidromic reentry), responses were assessed after A2 alone (method I), V2 alone (method II), and A2 plus V2 (method III, the complete preexcitation model). The entire pacing protocol was performed at two A-V intervals, short (50 msec) and long (150-180 msec), thereby simulating different proximities between the A pacing site and "accessory pathway" location. Of 47 consecutive unmedicated patients screened for the study protocol, 38 failed to meet minimal prerequisites for possible initiation of antidromic reentry because of failure in 18 (38% of total) to achieve anterograde A-V nodal block of A2, even though 1:1 ventriculoatrial conduction to cycle lengths less than or equal to 500 msec (less than or equal to 400 msec in 12) was present; and poor or absent ventriculoatrial conduction in the others. The nine remaining candidates underwent the full pacing protocol. Antidromic reentry (retrograde atrial response following V2 in method III) was observed in only two cases (4% of total), and both were associated with retrograde His-Purkinje system delays (documented by method II) occurring in tandem with a long A-V interval, thereby allowing for completion of retrograde A-V nodal recovery after penetration by A2. Indeed, such a prolonged recovery time prevented initiation of antidromic reentry in six of the nine patients (proven by intact ventriculoatrial conduction in method II). Retrograde A-V nodal block of V2, independent of A2, prevented an antidromic echo in one case. Findings in our model help to clarify the various factors, including specific anterograde and retrograde A-V nodal properties; anatomic relation between the accessory and normal pathways; and the retrograde His-Purkinje system delays, that must prevail in a concerted fashion to permit the initiation of antidromic reentry during the A1-A2 technique in patients with the WPW syndrome.

The excitable gap in atrioventricular nodal reentrant tachycardia. Characterization with ventricular extrastimuli and pharmacologic intervention

Our purpose was to characterize the excitable gap during atrioventricular nodal reentrant tachycardia (AVNRT) to elucidate the electrophysiologic substrate of this clinically familiar microreentrant arrhythmia. Accordingly, in 11 patients with classic slow-fast AVNRT (mean cycle length, 342 +/- 41 msec), a single ventricular extrastimulus (V2) was periodically delivered after a spontaneous tachycardia beat (V1) until ventricular refractoriness was reached. With this technique, an excitable gap was considered present when atrial preexcitation of at least 20 msec could be achieved along with tachycardia resetting (noncompensatory pause after V2). The range of V1V2 intervals that resulted in atrial preexcitation constituted the preexcitation zone. Five patients (45%) showed evidence of an excitable gap at baseline, with a maximal atrial preexcitation achievable of 33 +/- 6 msec, representing 9 +/- 1% of the tachycardia cycle length. Verapamil was then administered to all 11 patients with the purpose of slowing the anterograde tachycardia wavefront before arrival of V2. This resulted in widening of the preexcitation zone in three patients by a mean of 50 +/- 37 msec, with a corresponding increase in maximal atrial preexcitation to 70 +/- 32 msec, or 16 +/- 4% of AVNRT cycle length, and the appearance of atrial preexcitation in two patients who lacked it during baseline. In the remaining six patients, AVNRT was not sustained after verapamil or was too unstable for evaluation. During baseline, V2A2 conduction time increased by only 5 +/- 3 msec throughout the preexcitation zone, with values at the outer border unchanged after verapamil, implying a fully excitable gap in the retrograde limb. In all patients with a preexcitation zone, AVNRT was consistently reset by V2, both at baseline and after verapamil, with a "flat" but mainly "increasing" response pattern as V1V2 was shortened. Hence, a significant number of patients with AVNRT have evidence of an excitable gap whose demonstrability can be facilitated by pharmacologic intervention; documentation of an increasing resetting response pattern, most apparent after verapamil, provides new evidence for a reentrant mechanism in AVNRT; and while not definitively proven, the presence of a fully excitable gap during AVNRT is most consistent with a microreentry circuit that incorporates an anatomic obstacle.

Electrophysiologic spectrum of concealed intranodal conduction during atrial rate acceleration in a model of 2:1 atrioventricular block

Concealed anterograde penetration of the atrioventricular (AV) node has been used to explain a wide variety of electrocardiographic findings. The effects of atrial rate acceleration on this phenomenon remain undefined. To examine the dynamic interrelations between conducted and nonconducted beats at different atrial rates, a unique atrial pacing protocol of functional 2:1 AV block was used in 10 patients. The pacing protocol involved abrupt transitions from 2:1 to 1:1 AV conduction and enabled quantification of conduction delay produced by nonpropagated impulses over extremes of atrial rate. Stable 2:1 AV conduction was maintained over a mean range of atrial paced cycle lengths of 289 +/- 29.6 to 223 +/- 33.0 msec, respectively. The mean AV conduction time during 2:1 and corresponding 1:1 drives at the longest atrial paced rates were 169 +/- 33.5 and 136.5 +/- 26.9 msec, respectively--revealing a significant effect of nonpropagated impulses on subsequent conduction. Surprisingly, at the shortest atrial paced rates, the mean AV conduction times were 191.5 +/- 31.8 and 161.0 +/- 23.3 msec, respectively. The lack of significant changes in conduction time between 2:1 and 1:1 drives at the extremes of atrial rate (32.5 vs. 30 msec, p = NS) suggests that the effect of concealed conduction is "fixed" and independent of rate. Clinical implications and postulated electrophysiologic mechanisms are discussed.

Sustained bundle branch reentry as a mechanism of clinical tachycardia

The incidence of sustained bundle branch reentrant (BBR) tachycardia as a clinical or induced arrhythmia or both continues to be underreported. At our institution, BBR has been the underlying mechanism of sustained monomorphic ventricular tachycardia in approximately 6% of patients, whereas mechanisms unrelated to BBR were the cause in the rest. Data gathered from 20 consecutive patients showed electrophysiologic characteristics that suggest this possibility. These include induction of sustained monomorphic tachycardia with typical left or right bundle branch block morphology or both and atrioventricular dissociation or ventriculoatrial block. On intracardiac electrograms, all previously published criteria for BBR were fulfilled, and in addition, whenever there was a change in the cycle length of tachycardia, the His to His cycle length variation produced similar changes in ventricular activation during subsequent complexes with no relation to the preceding ventricular activation cycles. Compared with patients with ventricular tachycardia due to mechanisms unrelated to BBR, patients with BBR had frequent combination of nonspecific intraventricular conduction defects and prolonged HV intervals (100% vs. 11%, p less than 0.001). When this combination was associated with a tachycardia showing a left bundle branch block pattern, BBR accounted for the majority compared with mechanisms unrelated to BBR (73% vs. 27%, p less than 0.01). The above finding in patients with dilated cardiomyopathy should raise the suspicion of sustained BBR because dilated cardiomyopathy was observed in 95% of the patients with BBR. Twelve of the 20 patients were treated with antiarrhythmic agents, and the other eight were managed by selective catheter ablation of the right bundle branch with electrical energy. Our data suggest that sustained BBR is not an uncommon mechanism of tachycardia; it can be induced readily in the laboratory and is amendable to catheter ablation by the very nature of its circuit. The clinical and electrophysiologic features outlined in this study should enable one to correctly diagnose this important arrhythmia.

Atrial sensing to augment ventricular tachycardia detection by the automatic implantable cardioverter defibrillator: a utility study

The triggering of automatic implantable cardioverter defibrillator (AICD) discharges by supraventricular tachycardias, despite the presence of a probability density function algorithm, remains a limitation of an otherwise highly effective device. We systematically investigated the diagnostic utility which theroretically could derive form the addition of atrial sensing capability to the AICD in 25 patients with 30 inducible sustained monomorphic ventricular tachycardias (VTs) at clinically relevant rates (greater than or equal to 150 beats/min). Patients were included only if they were not taking medication capable of depressing ventriculoatrial (VA) conduction for at least 5 half-lives prior to electrophysiological testing. We tested the simple criterion for VT that ventricular cycle length (CL) be shorter than the atrial CL (not met in sinus or most other supraventricular tachycardias). Mean VT CL was 283 +/- 47 ms (range 210 to 370). In 25 (83%) VTs, the VT criterion was consistently satisfied. Of the five cases in which the criterion was not met, 1:1 VA conduction during VT was present in four, three of which initially manifested 2:1 VA conduction lasting from 14 to 28 s and therefore would have transiently fulfilled the VT criterion. The remaining patient who failed to satisfy the VT criterion had ongoing atrial flutter during a relatively slower sustained VT, but this circumstance could be recognized because of the varying AV interval. The absence of 1:1 VA conduction at CLS less than or equal to 400 ms during ventricular pacing accurately predicted the absence of 1:1 VA conduction during VT in 95% of patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrioventricular nodal conduction and refractoriness following abrupt changes in cycle length

The properties of the atrioventricular (AV) nodal conduction and effective refractory period in man are generally evaluated at a constant basic cycle length (CL) and, in most cases, they demonstrate an inverse relationship to the drive cycle. The response of AV node to abrupt change in CL is less defined. We therefore studied the effects of abrupt changes in CL on AV nodal conduction time and refractoriness in 18 patients. AV nodal conduction time, and effective and functional refractory periods were measured during: (1) a constant long CL, (2) a constant short CL, and (3) after an abrupt increase in CL just prior to the introduction of extrastimuli. In 10 of the 18 patients a constant long CL of 600 ms, a constant short CL of 400 ms and a sudden short-to-long change in CL (400 to 600 ms) were tested. AV nodal conduction times (A2H2) were measured at the shortest and longest comparable A1A2 intervals. The mean value of the shortest A2H2 intervals for constant CL of 600 ms was 144 +/- 18 ms; for a constant CL of 400 ms it was 162 +/- 17 ms; after a sudden short-to-long change in CL (400 to 600 ms) it was 142 +/- 14 ms. The mean value of the longest A2H2 intervals at a constant CL of 600 ms was 185 +/- 18 ms; at a constant CL of 400 ms it was 236 +/- 26 ms (p less than 0.01) and after a short-to-long change in CL (400 to 600 ms) 199 +/- 21 ms. AV nodal effective refractory periods measured at the same three CLs had mean values of 279 +/- 13 ms; 300 +/- 15 ms and 294 +/- 13 ms, respectively. Similar results were obtained when other CLs such as 700 to 900, 500 to 900, and 400 to 700 ms were tested. The data suggest that after abrupt short-to-long changes in CL, AV nodal function curves shift from long constant CL toward short constant CL as the coupling intervals decrease, indicating a cumulative pattern. Although the return to baseline conduction time after the fast basic rate is known to be slow, the limitation of this effect to the very early premature beat in the human has not been reported previously.

Reassessment of AV and VA conduction and AV junctional reentry in the normal dog heart: the role of altered autonomic tone

The effects of isoproterenol, atropine, and metoprolol on atrioventricular (AV) and ventriculoatrial (VA) conduction were studied in 30 normal dogs under pentobarbital anesthesia using percutaneously introduced catheters. The inducibility of AV junctional reentry was also assessed before and after drug administration. Resting AV conduction was normal in all dogs, but VA conduction was present in only 57%. Isoproterenol facilitated both antegrade and retrograde conduction, with a preferential effect on retrograde conduction. VA conduction was demonstrated after isoproterenol in 91% of dogs. After testing all drugs, VA conduction was demonstrable in at least one study in 97% of dogs. Atropine had less effect than isoproterenol, suggesting that basal vagal tone was not high in this model. Dual AV nodal pathways were detectable in the antegrade direction in four (13%) dogs, and in the retrograde direction in an additional four (13%) dogs. Single AV junctional echoes were inducible with atrial stimulation in one dog with dual antegrade pathways, but were inducible with ventricular stimulation in at least one study in 83% of dogs with intact retrograde conduction. Sustained AV junctional reentry was never induced before or after drug administration. In conclusion, VA electrical continuity is almost always intact in the normal dog, but its demonstration is significantly modified by the autonomic nervous system. Isoproterenol has preferential effects on retrograde conduction and may have selective influence on distal AV nodal conduction. Twenty-six percent of normal dogs have evidence of dual AV nodal pathways. Single AV junctional echoes are inducible with ventricular stimulation in the majority of dogs and are a normal finding. Sustained AV junctional reentry is not inducible in the normal intact dog heart.

Modulation of conduction and refractoriness in atrioventricular junctional reentrant circuit. Effect on reentry initiated by atrial extrastimulus

The importance of activation sequence of an atrioventricular junctional reentrant (AVJRe) circuit, before delivery of an extrastimulus, has received little attention in studies concerned with clinical tachycardias. In this study a change in activation sequence was accomplished using bidirectional activation (V-A sequential pacing) during the basic drive (V1A1-V1A1). It was noted that, compared with an atrial extrastimulus (A2) after an atrial drive (A1-A1), earlier activation (by V1 impulse of the V1A1-V1A1 drive) consistently improved conduction, or decreased refractoriness, or both, in the anterograde as well as the retrograde pathway of the AVJRe circuit. In all patients, five with AV nodal reentry and six with Wolff-Parkinson-White syndrome, reentrant tachycardia could be prevented during V-A sequential pacing. In four of eleven patients, reentry was prevented despite achieving the so-called critical atrioventricular nodal delays that had previously caused reentry during control study. This finding suggested that conduction delay necessary for reentry was related to the site of block, which in turn was affected by V-A sequential pacing. We concluded that changing the activation sequence during basic drive modulates conduction and refractoriness in AVJRe circuits, and allows the study of a wide range of electrophysical factors that prevent or permit reentry.

Effect of concealed anterograde impulse penetration on retrograde atrioventricular nodal conduction in man

The manner in which concealed anterograde impulse penetration may affect retrograde atrioventricular nodal conduction was studied systematically in 12 patients with intact ventriculoatrial (VA) conduction. After the last beat of a basic atrial drive (A1), an extrastimulus (A2) was introduced 20 msec inside the effective refractory period of the atrioventricular node. A ventricular extrastimulus (Vp) was then introduced at a progressively shorter A1Vp coupling interval both in the presence (method I) and absence (method II) of A2. In two patients, Vp was never conducted retrogradely to the atria with method I despite the presence of VA conduction during method II. In the remaining 10 patients, the VpAp interval was longer with method I vs method II; moreover, retrograde block of Vp ultimately occurred at a mean A2Vp coupling interval of 359 +/- 153 msec (range 190 to 540 msec) during method I despite the persistence of VA conduction during method II at comparable A1Vp coupling intervals. Before onset of retrograde block in method I, the VpAp curve took one of the following three forms: (1) crescendo, a progressively increasing VpAp interval; (2) flat, a constant VpAp interval, (four cases); or (3) discontinuous, a marked jump in the VpAp interval before the onset of retrograde block (two cases). Our findings may serve to elucidate some poorly understood electrophysiologic phenomena.

Effects of upright posture on anterograde and retrograde atrioventricular conduction in patients with coronary artery disease, mitral valve prolapse or no structural heart disease

To assess the effects of posture on anterograde and retrograde atrioventricular conduction, electrophysiologic testing was performed in 25 patients in both the supine and 45 degrees upright positions on a tilt table. Retrograde conduction was present during ventricular pacing in 17 patients in the supine position; all 17 continued to manifest retrograde conduction in the upright position. In all patients with absent retrograde conduction while supine, retrograde conduction could not be demonstrated while upright. Upright posture significantly (p less than 0.05) shortened the sinus cycle length (from 808 +/- 34 to 678 +/- 26 ms, mean +/- standard error of the mean), AH interval during sinus rhythm (78 +/- 6 to 69 +/- 6 ms), and AH interval during atrial pacing at cycle length 500 ms (123 +/- 13 to 91 +/- 9 ms). Total atrioventricular conduction time during atrial pacing shortened significantly (from 169 +/- 13 to 136 +/- 10 ms), as did ventriculoatrial conduction time during ventricular pacing (from 192 +/- 9 to 178 +/- 7 ms). Upright posture also significantly shortened both anterograde block cycle length (390 +/- 20 to 328 +/- 17 ms) and retrograde block cycle length (466 +/- 27 to 354 +/- 18 ms). However, the effect of upright posture on retrograde block cycle length was significantly greater than on anterograde block cycle length: a 21% decrease retrograde vs a 14% decrease anterograde (p less than 0.05). These effects may produce clinically important changes in characteristics of arrhythmias that depend on the properties of anterograde and retrograde conduction.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac and skeletal muscle abnormalities in cardiomyopathy: comparison of patients with ventricular tachycardia or congestive heart failure

Results of cardiac muscle and skeletal muscle biopsies were compared in 22 patients with cardiomyopathy; 11 patients presented with symptoms secondary to ventricular tachycardia (Group 1) and 11 had symptoms of severe congestive heart failure (Group 2). No patient had structural or ischemic cardiac disease. In Group 1 patients, hemodynamic abnormalities were subtle, but invasive study demonstrated dilated cardiomyopathy in two patients and restrictive cardiomyopathy in nine. In Group 2, eight patients had dilated cardiomyopathy and three had restrictive cardiomyopathy. Cardiac biopsy results were abnormal in all 22 patients and the abnormalities were similar for the two groups. Cardiac histologic study revealed a spectrum of abnormalities including fibrosis, dilated sarcoplasmic reticulum, increased numbers of intercalated discs and mitochondrial abnormalities. Histologic abnormalities of skeletal muscle were similar in each group, consisting of endomysial fibrosis and increased lipid deposits. Slightly more than half of the Group 1 and Group 2 patients also had a low concentration of skeletal muscle long chain acylcarnitine. These data demonstrate that abnormalities of both cardiac and skeletal muscle are common in patients with cardiomyopathy; abnormalities are similar whether initial symptoms are due to ventricular tachycardia or congestive heart failure. It is suggested that these patients with cardiomyopathy may have a generalized myopathy.

Atrioventricular nodal reentrant tachycardia: studies on upper and lower 'common pathways'

Electrophysiologic studies were performed in 28 patients with documented atrioventricular (AV) nodal reentrant supraventricular tachycardia (SVT) to investigate the presence of AV nodal tissue situated between the tachycardia circuit and both the atrium (upper common pathway, UCP) and the His bundle (lower common pathway, LCP). All patients demonstrated a 1:1 AV relationship during SVT. The study protocol consisted of atrial then ventricular pacing at the SVT cycle length. UCPs were manifested in eight of 28 (29%) patients by either antegrade AV Wenckebach (six patients) or a paced atrium-His (AH) interval exceeding the AH in SVT (two patients, differences 5 and 9 msec). LCPs were manifested in 21 of 28 (75%) patients by either retrograde Wenckebach periodicity (two patients) or a paced HA interval exceeding the HA in SVT (19 patients, mean difference 25 +/- 20 msec). By these criteria, eight patients (29%) had evidence for both UCPs and LCPs. UCPs were more likely than LCPs to be manifested by Wenckebach criteria (p less than .05). Thus the AV nodal reentrant SVT circuit appears to be intranodal and is frequently surrounded by AV nodal tissue (UCP and LCP), antegrade and retrograde conduction properties of these common pathways are discordant in some cases, and conduction properties of UCP tissue differ from those of LCP tissue. These findings may have relevance in that the UCP or LCP may limit the ability of premature extrastimuli to penetrate the circuit to initiate or terminate AV nodal SVT.

Electrophysiologic mechanisms of orthodromic tachycardia initiation during ventricular pacing in the Wolff-Parkinson-White syndrome

Orthodromic tachycardia is the most common arrhythmia in patients with Wolff-Parkinson-White syndrome. It is often initiated during incremental ventricular pacing that requires the onset of retrograde block along the normal pathway (that is, atrioventricular [AV] node-His-Purkinje system) with concomitant retrograde atrial activation by way of the accessory pathway. However, the site of retrograde block, that is, the AV node versus the His-Purkinje system, during incremental ventricular pacing and, hence, the mechanism of orthodromic tachycardia initiation have not been systematically elucidated. The mechanisms of orthodromic tachycardia induction were studied in 17 patients with Wolff-Parkinson-White syndrome using a specially designed pacing protocol. A beat by beat analysis indicated that the retrograde His-Purkinje system block was the most common initiating mechanism of orthodromic tachycardia in 14 of the 17 cases. In two cases, AV node block preceded the onset of orthodromic tachycardia, whereas the data in the remaining case suggested that both mechanisms were operative but at different pacing cycle lengths. The orthodromic tachycardia induction with His-Purkinje system block occurred within the first two cycles in most cases. When orthodromic tachycardia initiation was delayed beyond the first two cycles of the ventricular train it represented either a 2:1 block in the His-Purkinje system; a linking phenomenon in the His-Purkinje system; or a block in the AV node. These data have methodologic, mechanistic and therapeutic implications for patients with the Wolff-Parkinson-White syndrome.

Functional characteristics of human macro-reentry: a study of "pre-excited" circuits by extrastimulus method

The effect of improved conduction in areas of delay was tested during macro-reentry within the His-Purkinje system, in an attempt to separate the role of conduction delay from that of prematurity of the extrastimulus as the key determinant of reentry. Using the right ventricular extrastimulus technique (S1S2 method), both the critical His-Purkinje system delays and the zone of S1S2 intervals causing His-Purkinje system reentry were determined. Then, using a previously described technique of atrioventricular (AV) sequential pacing during the basic drive, the potential site of His-Purkinje system conduction delay was (anterogradely) excited earlier (pre-excitation), as compared with the control S1S2 method. This produced a decrease in retrograde His-Purkinje system delay (S2H2), as compared with the same S1S2 interval during the control method. Changing the degree of pre-excitation at each S1S2 interval allowed for determination of the critical (or shortest) S2H2 delay necessary for His-Purkinje system reentry at each coupling interval. Of importance was the observation that the critical delay was not specific for each case but varied with the prematurity of S2. For example, the critical S2H2 delay required for reentry was actually less at shorter S1S2 intervals as compared with longer S1S2 intervals (from 206 +/- 25 to 187 +/- 20 ms, p less than 0.01). These data suggest that manifestation of reentry is a complex interplay between degree of prematurity and conduction delay. The so-called critical conduction delay can be readily modified by altering the site of block, which in turn may be dependent on prematurity of the extrastimulus.

Effect of pharmacologic autonomic blockade on ventriculoatrial conduction

To determine the influence of autonomic tone on retrograde ventriculoatrial (VA) conduction, incremental atrial and ventricular pacing was performed before and after pharmacologic autonomic blockade in 28 patients. VA conduction during ventricular pacing was demonstrated, with highest frequency in patients capable of 1:1 atrioventricular (AV) conduction at atrial paced cycle lengths of 300 ms or less (7 of 7, 100%). In subjects with 1:1 AV conduction at minimum cycle lengths of 300 to 500 ms, 14 of 21 (67%) demonstrated VA conduction in the control state; however, only 12 of 21 (57%) did so after autonomic blockade. The lowest frequency was observed in those capable of 1:1 AV conduction at minimum cycle lengths of 505 ms or more before and after autonomic blockade (2 of 7, [29%], p less than or equal to 0.02 compared with values in the first group). No change in the mean minimum ventricular paced cycle length at which 1:1 VA conduction could be maintained was demonstrated after autonomic blockade. In individual subjects, incremental change in this cycle length after autonomic blockade correlated positively with the corresponding change in minimum atrial cycle length at which 1:1 AV conduction could be maintained (r = 0.62, p less than 0.005), and was concordant in direction in 18 of 21. In conclusion, the sympathetic and parasympathetic modulation of VA conduction is balanced and concordant in direction to the effect on AV nodal conduction.

Effect of sudden rate acceleration on the human His-Purkinje system: adaptation of refractoriness in a dampened oscillatory pattern

Although the refractoriness of the human His-Purkinje system (HPS) during constant-cycle length pacing appears to be closely related to the cycle length of the stimulation, the mode of adaptation of this refractoriness with sudden rate acceleration is not well understood. A systematic evaluation of this adaptation was performed in 14 patients with normal QRS durations and HV intervals referred for electrophysiologic evaluation. The relative refractory period of the HPS (HPS-RRP) was evaluated by the extrastimulus (S2) method during a constant ventricular drive (S1) having a cycle length as close to sinus rhythm as possible. An accelerated train of 6 ventricular beats (S1') was then added to the constant drive and the HPS-RRP of each successive beat of this train was similarly determined. Mean S1 cycle length was 750 +/- 164 msec (range 600 to 1000). Mean S1' cycle length was 475 +/- 55 msec (range 400 to 600). The HPS-RRP of each successive beat of the accelerated train was significantly shorter than that during the S1 drive and behaved in a dampened oscillatory fashion alternating from a lower value on the odd-numbered beats to a higher value on the even-numbered beats. In contrast, the effective and relative refractory periods of the ventricular myocardium during the accelerated train behaved in a cumulative manner, decreasing progressively with the first 2 beats of the train before reaching a plateau value. In conclusion, the data reported here present a new and intriguing picture of the mode of adaptation of the HPS refractoriness to sudden rate acceleration. At least in the range of the cycle lengths used in this study, the refractoriness of the HPS behaves in a dampened oscillatory manner that is radically different from the behavior of the ventricular myocardial refractoriness.

Sympathetic and periodic vagal influences on antegrade and retrograde conduction through the canine atrioventricular node

Concurrent stimulation of the parasympathetic and sympathetic branches of the autonomic nervous system causes a diminished sympathetic response at high levels of vagal activity. This "accentuated antagonism" has been demonstrated for cardiac chronotropic, inotropic, and dromotropic responses. The effect on conduction was demonstrated with tonic stimulation of the vagus nerve. However, normally the vagus nerve fires periodically at certain times in the cardiac cycle. Thus, we have studied whether a similar interaction exists in the modulation of atrioventricular condition when short bursts of vagal stimulation were placed in various portions of the cardiac cycle. Anesthetized open-chest mongrel dogs were instrumented for stimulation of the cervical vagi and stellate ganglia when the heart was paced. We determined the relationship between cardiac cycle length, direction of action potential propagation, and levels of sympathetic and vagal activation and their effects on atrioventricular conduction times. All of the factors investigated, namely levels of vagal and stellate stimulation, pacing intervals, and direction of propagation of action potentials, affected atrioventricular conduction times. Furthermore, the vagal effect was greater at short cardiac cycle lengths. When bursts of vagal stimulation were timed to result in maximal or minimal prolongation of atrioventricular conduction, no significant effects of sympathetic-parasympathetic interaction on atrioventricular conduction times were apparent. However, an analysis of the differences in prolongation of atrioventricular conduction with periodic vagal stimulation revealed that a significant sympathetic-vagal interaction existed for these differences. Thus, autonomic neurotransmitters differentially affect cardiac conduction times depending on time of application of the stimulus.

Comparison of ramp and stepwise incremental pacing in assessment of antegrade and retrograde conduction

Conventional assessment of antegrade (AV) and retrograde (VA) conduction involves stepwise increments in pacing rates until block in conduction is observed. This study was designed to establish the comparative characteristics of ramp pacing, in which the rate is continuously and smoothly incremented until block occurs. Two hundred and ten patients participated in portions of this study. Stepwise pacing was performed in 10 beat/minute steps, with the rate held for at least 15 seconds at each step; if marked prolongation or variability in conduction was observed, the rate was held constant for up to 60 seconds to allow for accommodation. With ramp pacing, the rate was gradually increased at a steady 2-4 beats/minute/second. Whenever possible, both stepwise and ramp pacing were performed for assessment of both antegrade and retrograde conduction. All patients had conducted sinus rhythm as their baseline mechanism. Antegrade conduction was similar using incremental stepwise and ramp pacing. The AH interval at a cycle length (CL) of 500 ms, the maximum AH increment, the cycle length at AV block were all remarkably similar (p = NS). Assessment of retrograde conduction produced similar results, with insignificant differences between maximum conducted VA intervals, and cycle length at VA block using the two pacing techniques. Ramp pacing provides a useful and rapid alternative to conventional stepwise incremental pacing in the assessment of antegrade and retrograde conduction in patients using both normal and accessory pathways. Ramp pacing was better tolerated, and some correlations between antegrade and retrograde conduction were stronger with the ramp pacing technique.

Unidirectional retrograde atrioventricular nodal block in man: determinants of reversibility by vagal antagonism

The mechanism of unidirectional retrograde atrioventricular (AV) nodal block remains largely unknown. In this study, factors determining the reversal of the unidirectional block by atropine were evaluated in 12 patients who had no demonstrable ventriculoatrial (VA) conduction during ventricular pacing. Six patients demonstrated 1:1 VA conduction after atropine (group I), while the remaining six patients continued to show VA block (group II). During the control study there was no significant difference in the sinus cycle length and AH interval between the two groups. The percent decrease in sinus cycle length after atropine was also similar in groups I and II (i.e., 23 +/- 12 and 26 +/- 6, respectively). The effect on antegrade AV nodal conduction (i.e., the percent decrease in AH interval), however, was significantly greater in group I (24 +/- 9) as compared to group II (9 +/- 5) (p less than 0.004). The onset of VA conduction appeared to correlate with the improvement of antegrade conduction. The ratio of these two effects of atropine (i.e., percent decrease in AH interval to percent decrease in sinus cycle length) was higher when VA conduction was first demonstrated in group I (2.3 +/- 1.1) than at the maximal effect of atropine (1.2 +/- 0.3), reflecting a relatively greater decrease in sinus cycle length. Three of six group I patients redeveloped VA block at maximal effect of atropine. The results suggest a functional and dynamic nature of the unidirectional AV nodal block, possibly caused by vagal influence exaggerating the well-known directional asymmetry of AV nodal conduction in man.(ABSTRACT TRUNCATED AT 250 WORDS)