Para-Hisian entrainment: a novel pacing maneuver to differentiate orthodromic atrioventricular reentrant tachycardia from atrioventricular nodal reentrant tachycardia

INTRODUCTION: Para-Hisian pacing during sinus rhythm can help to identify the presence of an accessory pathway (AP). In this maneuver, the retrograde activation time and pattern are compared during capture and loss-of-capture of the His bundle while pacing from a para-Hisian position. However, identification of a retrograde AP does not necessitate that it is operative during the tachycardia of interest; conversely, slowly conducting or "distant" bypass tracts may not be identified. We evaluated the utility of entrainment or resetting of tachycardias from the para-Hisian position to help distinguish atrioventricular nodal reentrant tachycardia (AVNRT) from orthodromic atrioventricular tachycardia (AVRT). METHODS AND RESULTS: Para-Hisian entrainment/resetting was evaluated in 50 patients: 33 with AVNRT and 17 with AVRT. The maneuvers were performed using a standard quadripolar catheter placed at the His position: low output for right ventricular (RV) capture and high output for both RV and His capture. The retrograde atrial activation sequence, SA interval (interval from stimulus to earliest retrograde atrial activation), and "local" VA interval (interval between the ventricular and atrial electrograms at the site of earliest retrograde atrial activation) were compared between His and His/RV capture. The DeltaSA was > 40 ms in patients with AVNRT and was < 40 ms in all but one patient with AVRT. In concert with the DeltaSA interval, the DeltaVA interval was able to fully define the mechanism of the tachycardia in all patients studied. CONCLUSION: Para-Hisian entrainment/resetting can determine the course of retrograde conduction operative during narrow complex tachycardias. It is a useful diagnostic maneuver in differentiating AVNRT and orthodromic AVRT.     

QRS complex widening due to loss of left bundle branch capture: pitfall of para-Hisian pacing

During para-Hisian pacing, widening of the paced QRS complex usually indicates loss of His bundle capture. We describe a patient without any accessory pathways in whom widening of the paced QRS complex occurred due to loss of left bundle branch capture during para-Hisian pacing. After initial widening of the QRS complex, further widening was observed due to loss of His bundle capture. With the initial QRS widening, the stimulus-atrial interval and retrograde atrial activation sequence were almost unchanged, so the findings mimicked retrograde conduction over an accessory pathway. This may be a pitfall of the para-Hisian pacing technique.     

Diagnostic significance of the morphological change in the atrial electrogram during Para-Hisian pacing

Para-Hisian pacing (PHP), a pacing method to differentiate between conduction occurring over an accessory pathway (AP) from that over the atrioventricular node (AVN), is assessed essentially by comparing the timing in the atrial electrogams. Morphological change in the atrial electrograms is often observed during PHP, but its significance has not been investigated. Prior to the catheter ablation procedure, PHP was performed in 52 patients with an AP and in 36 patients with AV nodal reentrant tachycardia (AVNRT). The morphological change in the atrial electrograms, which was retrospectively assessed between the His bundle and proximal right bundle branch (HB-RB) captured and non-captured beats, was identified in 15 of 52 patients with an AP and in 26 of 36 patients with AVNRT. The atrial electrogram in the 6 of these 15 AP patients changed its morphology without overlapping the ventricular electrogram. All 6 AP patients exhibited a PHP pattern with the presence of 2 retrograde conduction routes, an AP and the AVN. In the patients demonstrating no morphological change in the atrial electrogram, 33 of 37 AP patients and all 10 AVNRT patients had only one retrograde conduction route. Morphological change in the atrial electrogram without overlapping the ventricular electrogram seems to have diagnostic significance indicating the presence of both AP and AVN conduction.     

Significance of pacing cycle lengths in manifest entrainment of orthodromic circus movement tachycardia by ventricular pacing

The physiology of entrainment of orthodromic circus movement tachycardia (CMT) was studied using ventricular pacing during 18 episodes of induced CMT in 7 patients with atrioventricular (AV) accessory pathways. The first paced impulse was delivered as late as possible in the tachycardia cycle (mean 88 +/- 5% of the spontaneous cycle length [CL]). Entrainment was demonstrated by the following criteria: 1:1 retrograde conduction via the accessory pathway; capture of atrial, ventricular and His bundle electrograms at the pacing rate; and resumption of tachycardia at its previous rate after cessation of pacing. The number of ventricular paced impulses ranged from 5 to 14 (mean 8 +/- 3), and entrainment occurred in 2 to 7 paced cycles (mean 4 +/- 2). Orthodromic activation of a major part of the reentry circuit (manifest entrainment) was demonstrated during 9 episodes by the occurrence of His bundle electrogram preceding the first CMT QRS at the time anticipated from the last paced beat. In the 9 other episodes, persistent retrograde His bundle activation and AV nodal penetration by each paced impulse caused a delay (mean 79 +/- 25 ms) in activation of the His bundle preceding the first CMT QRS after the last paced beat. The mean pacing CL achieving manifest entrainment was 92 +/- 3% of the tachycardia CL, compared with 84 +/- 3% for retrograde AV nodal penetration (p less than 0.01). In conclusion, manifest entrainment of orthodromic CMT can be demonstrated by ventricular pacing at very long CLs; shorter CLs may cause CMT termination due to retrograde AV nodal penetration.     

Sequential dual chamber extrastimulation: A novel pacing maneuver to identify the presence of a slowly conducting concealed accessory pathway

BACKGROUND: Transient VA block can be created in the AV node (AVN) when an atrial extrastimulus is delivered at the AVN effective refractory period (ERP) due to anterograde concealed conduction. OBJECTIVE: We hypothesized that ventricular stimulation during pacing-induced AVN refractoriness could identify concealed accessory pathways (APs) that remain hidden with standard maneuvers. METHODS: Patients undergoing electrophysiological study for supraventricular tachycardia were screened for presence of an AP using standard pacing maneuvers and/or V pacing during adenosine infusion. The dual-chamber sequential extrastimulation maneuver consisted of an 8-beat drive train of simultaneous AV pacing at 600 msec, followed by an A2 delivered at AVN ERP, followed by a V2 delivered at the drive train cycle length (600 msec). Repeat drives were then performed with decrements of 10 msec for V2 until VA block was seen. Retrograde AVN and AP ERP were recorded with standard (V1, V2) and dual-chamber extrastimulation (A1/V1, A2, V2). Patients with an AP identified with standard pacing, manifest pre-excitation, or A ERP < AVN ERP were excluded. RESULTS: Fourteen patients with and 19 patients without an AP were studied. In all patients with an AP, exclusive VA conduction over the AP, without fusion, was seen with the described pacing maneuver. In patients without an AP, retrograde AV nodal ERP was extended by a mean of 138 +/- 46 msec (range 50 to 210 msec) with the A2. Anterograde concealed conduction into the AP was also seen in some patients who showed AP conduction during standard V1V2 pacing (mean retrograde extension of ERP 12 +/- 8 msec, range 0 to 20 msec). CONCLUSION: Dual-chamber sequential extrastimulation is a useful maneuver for identifying slowly conducting APs not revealed with standard pacing maneuvers because of an ERP and conduction time similar to the AVN. The maneuver uses anterograde concealed conduction to prolong AVN refractoriness much more than that of a concealed AP, thereby allowing the AP to become manifest with the V2.     

Use of Only a Regular Diagnostic His-Bundle Catheter for Both Fast and Reproducible "Para-Hisian Pacing" and Stable Right Ventricular Pacing

Pacing from the Diagnostic His-Bundle Catheter. Introduction : Para-Hisian pacing, i.e., pacing the anteroseptal right ventricle (RV) with or without direct capture of the His bundle (HB), allows the differentiation of VA conduction over the AV node from conduction over an accessory pathway. Classically, it is performed by maneuvering a separate pacing catheter around the HB catheter, which may be difficult and time-consuming.
Methods and Results : This study prospectively evaluated the use of a single standard octapolar HB catheter with 2-mm interelectrode spacing for simultaneous (para-Hisian) pacing from the distal bipole and recording from the three proximal bipoles in 148 consecutive patients. Para-Hisian pacing was successful in 146 of 148 patients, performed within a median of only 10 seconds, and easily repeated several times during the course of an electrophysiologic study. Retrograde HB activation could he recorded in 132 of 146 patients; a clearly different surface ECG configuration confirmed the presence or absence of HB capture in all other patients. Interestingly, stable RV pacing could he performed from the HB catheter for the rest of the electrophysiologic study in 138 of 142 patients in whom this was tried. RV pacing from this site also led to better interpretation of retrograde conduction, due to clear visualization of retrograde HB activation.
Conclusion : Pacing from the distal dipole of a regular diagnostic HB catheter provides a fast and reliable way to perform para-Hisian pacing. Therefore, it may be advocated as a routine diagnostic protocol during electrophysiologic procedures. Moreover, pacing from this site obviates the need for a separate RV pacing catheter in most patients.     

Ectopic ventricular tachycardia in association with a concealed accessory pathway.

Ventricular tachycardia may result in retrograde atrial depolarization. In all previous examples studied by intracardiac recording techniques retrograde conduction to the atria has been thought to occur via the normal conduction system. In the patient reported here, 1 : 1 retrograde conduction with atrial preexcitation during ventricular tachycardia indicated the presence of a concealed accessory pathway. This was confirmed by the phenomenon of exact atrial capture. The achievement of atrioventricular dissociation during tachycardia by atrial overdrive pacing excluded a reentrant tachycardia involving such a pathway. The widely variable H--V interval in the presence of a constant V--A interval during tachycardia suggested anterograde His bundle depolarization following atrial activation over the accessory pathway. This hypothesis was supported by the long V--H interval during tachycardia and the relationship of the ventricular, atrial and His bundle electrograms of the first and least beat of the tachycardia. This report illustrates the value of careful electrophysiological investigation in evaluating the relationship of an accessory pathway to the tachycardia mechanism.     

希氏束旁旁路的射频消融体会

位于希氏束上、下0.5cm以内的右侧间隔旁路称为希氏束旁旁路,为避免射频消融时损伤希氏束,大头导管标测定位应在窦性心律,心室起搏及诱发心动过速三种方式以确定.消融时应密切观察体表QRS波形态及逸搏情况.需在心室起搏下消融时,确信旁路中断后应停止或间歇心室起搏,以观察逸搏情况确定是否希氏束受损.如果自然发作和诱发的室上性心动过速(室上速)均为旁路下传,心内标测为多旁路间折返,房室结构均为旁路者时测不能确认患者房室结传导是否正常,应首先消融其逆传旁路,对正传旁路消融应慎重,以避免消融后的房室传导障碍.     

Successful elimination of concealed accessory pathway-mediated tachycardia by ablation of AV nodal slow pathway

We report a case of atrioventricular reentrant tachycardia (AVRT) using a concealed para-Hisian accessory pathway for retrograde conduction, which also required anterograde conduction over the AV nodal slow pathway to maintain the tachycardia. The shortest VA interval during AVRT (70 ms) was noted at a site with His bundle electrogram amplitude of 0.25 mV. The AVRT was cured by radiofrequency ablation of the AV nodal slow pathway without affecting accessory pathway conduction. The patient has not reported any sustained palpitations at 2 years after ablation while receiving no medications. The case presented in this report illustrates a para-Hisian AVRT that was successfully eliminated by an unconventional approach of ablation of the atrial inputs to the AV nodal slow pathway.     

Reliability of retrograde atrial activation patterns during ventricular pacing for localizing accessory pathways

Definitive localization of accessory pathways is based on atrial activation patterns during orthodromic supraventricular tachycardia when retrograde conduction occurs exclusively through the accessory pathway. In some patients, supraventricular tachycardia cannot be induced or is deleterious. To determine whether accessory pathway sites can be identified accurately during ventricular pacing, retrograde atrial activation was assessed during orthodromic supraventricular tachycardia and ventricular pacing at multiple cycle lengths in 41 patients with a single accessory pathway. To obviate retrograde fusion due to concomitant conduction through the normal atrioventricular (AV) conduction system that may obscure the location of the accessory pathway, the difference in conduction time from the site of earliest atrial activation to the His bundle atrial electrogram (delta A-SVT) was measured during orthodromic supraventricular tachycardia and compared with values observed during ventricular pacing (delta A-VP). Characteristic values for the delta A-SVT interval were identified for left lateral (66 +/- 17 ms), left posterior (50 +/- 8 ms), posteroseptal (33 +/- 7 ms), right free wall (22 +/- 15 ms) and anteroseptal (0 +/- 0 ms) accessory pathway sites. During ventricular pacing, the site with the earliest atrial electrogram was used to define the accessory pathway location only if the maximal value of the delta A-VP interval over the range of cycle lengths assessed was comparable with the value of the delta A-SVT interval characteristic of that region. Values of the delta A-SVT interval correlated closely with the maximal values of the delta A-VP interval (r = 0.91). With this approach, 40 (98%) of 41 accessory pathway sites were identified correctly during ventricular pacing.(ABSTRACT TRUNCATED AT 250 WORDS)     

Concealed anomalous cardiac conduction pathways: A frequent cause of supraventricular tachycardia

Fifteen patients who had one or more documented episodes of supraventricular tachycardia but no periods of Wolff-Parkinson-White syndrome were studied with His bundle recording, atrial recordings from multiple left and right atrial sites and programmed atrial and ventricular stimulation. Ten of the 15 patients studied had a concealed anomalous conduction pathway as part of the reentry circuit responsible for the tachycardia. The criteria for concealed anomalous conduction pathways were (1) eccentric retrograde atrial activation during Supraventricular tachycardia, (2) increase in both the cycle length of Supraventricular tachycardia and the retrograde conduction time during ipsilateral bundle branch block, and (3) retrograde conduction through the concealed anomalous conduction pathway during the refractory period of the bundle of His. Six of the 10 anomalous pathways identified were in the left atrium, 3 in the right atrium and 1 in the septum.

Eight of the 10 patients with a concealed anomalous conduction pathway were successfully treated with drugs, 7 with propranolol and 1 with propranolol and quinidine. In two patients with recurrent Supraventricular tachycardia unresponsive to medical management, surgical division of the concealed anomalous conduction pathway stopped recurrences for 9 and 15 months, respectively.     

Simultaneous anterograde fast-slow atrioventricular nodal pathway conduction after procainamide

Three patients with paroxysmal supraventricular tachycardia underwent electrophysiologic studies that included His bundle recordings, incremental atrial and ventricular pacing and extrastimulation before and after intravenous infusion of 500 mg of procainamide. In all three patients the tachycardia was induced during atrial pacing or premature atrial stimulation, or both. Two of the three patients had discontinuous atrioventricular (A-V) nodal curves with induction of a slow-fast tachycardia during failure in anterograde fast pathway conduction and one patient had a smooth A-V nodal curve with induction of a slow-fast tachycardia at critical A-H interval delays. After procainamide: (1) in all three patients atrial pacing induced A-V nodal Wenckebach periodicity (cycle length 300 to 400 ms) resulting in simultaneous anterograde fast and slow pathway conduction (one atrial beat resulting in two QRS complexes) and retrograde fast pathway conduction initiating an echo response or a slow-fast tachycardia, or both; (2) in all three patients there was enhanced conduction and shortening of refractoriness of the anterograde fast pathway and depressed conduction and lengthening of refractoriness of the retrograde fast pathway; and (3) in two patients there was inability to sustain tachycardia because of selective block within the retrograde fast pathway. In conclusion: (1) procainamide altered conduction and refractoriness of the anterograde fast and slow pathways so that simultaneous conduction could occur during atrial pacing, resulting in a double ventricular response and a slow-fast echo or tachycardia, or both; and (2) the differential effects of procainamide on anterograde fast and retrograde fast pathways suggests two functional A-V nodal fast pathways, one for anterograde and the other for retrograde conduction.     

Sincronia Ventricular na Estimulação Cardíaca Parahissiana: Alternativa por Ativação Cardíaca Fisiológica (Estimulação Indireta do Feixe de His)?

Background Artificial cardiac pacing by direct or indirect His bundle capture results in synchronous ventricular contraction (physiological pacing).Objectives To compare cardiac synchronization, technical characteristics, and electronic parameters between two techniques of indirect His-bundle pacing: non-selective (NS-HBP) vs para-Hisian pacing (PHP).Methods The experimental intervention (between November 2019 and April 2020) consisted of implanting a DDD pacemaker in patients who had left ventricular ejection fraction (LVEF) > 35%. The resulting cardiac synchronization was compared using an electrocardiographic algorithm that analyzed QRS variation and the technical characteristics of non-selective Hisian pacing (DDD-His) and para-Hisian pacing (DDD-Var).Results Of 51 total patients (men: 28), 66.7% (34) were allocated to the DDD-Var group and 33.3% (17) to the DDD-His group. The mean ages in each group were 74 and 79 years, respectively. In the DDD-Var group, QRS variation (ventricular synchrony) improved after implantation (p < 0.001). In post-implantation ECG, 91.2% of the DDD-Var group presented a physiological pacing pattern, which was similar to the DDD-His group (88.2%; p = 0.999). The paced QRS axis was also similar (physiological) for both groups. Intraoperative fluoroscopy time (XRay) during implantation was lower for the para-Hisian technique (median 7 min in the DDD-Var group vs 21 min in the DDD-His group, p < 0.001). The mean QRS duration increased in the DDD-Var group (114.7 ms pre-implantation vs 128.2 ms post-implantation, p = 0.044). The mean post-implantation R-wave amplitude was 11.2 mV in the DDD-Var group vs 6.0 mV in the DDD-His group, p = 0.001.Conclusion Para-Hisian pacing appears to indirectly recruit the His bundle, which would make this an effective and comparable strategy for physiological pacing, resulting in synchronous ventricular contraction similar to that of non-selective Hisian pacing.     

Utilization of Retrograde Right Bundle Branch Block to Differentiate Atrioventricular Nodal from Accessory Pathway Conduction

Introduction: Defining whether retrograde ventriculoatrial (V-A) conduction is via the AV node (AVN) or an accessory pathway (AP) is important during ablation procedures for supraventricular tachycardia (SVT). With the introduction of ventricular extrastimuli (VEST), retrograde right bundle branch block (RBBB) may occur, prolonging the V-H interval, but only when AV node conduction is present. We hypothesized that when AP conduction was present, the V-A interval would increase less than the V-H interval, whereas with retrograde nodal conduction, the V-A interval would increase at least as much as the V-H interval.
Methods and Results: We retrospectively reviewed the electrophysiological studies of patients undergoing ablation for AVN reentrant tachycardia (AVNRT) (55) or AVRT (50), for induction of retrograde RBBB during the introduction of VEST, and the change in the measured V-H and V-A intervals. Results were found to be reproducible between independent observers. Out of 105 patients, 84 had evidence of induced retrograde RBBB. The average V-H interval increase with induction of RBBB was 53.7 ms for patients with AVRT and 54.4 ms for patients with AVNRT (P = NS). The average V-A interval increase with induction of RBBB was 13.6 ms with AVRT and 70.1 ms with AVNRT (P < 0.001). All patients with a greater V-H than V-A interval change had AVRT, and those with a smaller had AVNRT.
Conclusions: Induction of retrograde RBBB during VEST is common during an electrophysiological study for SVT. The relative change in the intervals during induction of RBBB accurately differentiates between retrograde AVN and AP conduction.     

Radiofrequency Ablation of a Concealed Nodoventricular Mahaim Fiber Guided by a Discrete Potential

INTRODUCTION: We present the case of a 17-year-old woman who underwent an electrophysiological study and radiofrequency (RF) ablation of supraventricular tachycardia refractory to medical treatment. Two right-sided, concealed, nondecremental atrioventricular accessory pathways (AV-APs) involved in orthodromic circus movement tachycardias were identified. After RF ablation of both AV-APs, evidence of bidirectional dual AV nodal conduction was demonstrated and regular narrow complex tachycardia was induced. METHODS AND RESULTS: During the tachycardia, retrograde slow and fast AV nodal pathway conduction with second-degree ventriculoatrial (VA) block and VA dissociation were observed. During the tachycardia with second-degree VA block, ventricular extrastimuli elicited during His-bundle refractoriness advanced the next His potential or terminated the tachycardia. Mapping the right atrial mid-septal region, a distinct high-frequency activation P potential was recorded in a discrete area, two thirds of the way from the His bundle toward the os of the coronary sinus. Detailed electrophysiologic testing with the recordable P potential demonstrated that the tachycardia utilized a concealed nodoventricular AP arising from the proximal slow AV nodal pathway. CONCLUSION: The tachycardia with slow 1:1 VA conduction could be reset by ventricular extrastimuli elicited during His-bundle refractoriness advancing the subsequent activation P potential and atrial activation. RF ablation guided by recording of the activation P potential resulted in elimination of both the slow AV nodal pathway and the nodoventricular connection with preservation of the normal AV conduction system.     

Reentry tachycardia with complete atrioventricular dissociation probably connected with the right Maha?m bundle]

A case of reentrant tachycardia with narrow and wide ventricular complexes without appearances of preexcitation is reported. Electrophysiological investigation showed complete retrograde atrioventricular block during tachycardia; left bundle branch block did not show the tachycardia rate. The reentry loop probably comprised: the His bundle, the right bundle branch, a right Maha?m bundle and possibly a myocardial bridge. Possible intra-hisian reentry is discussed. The initiation of the tachycardia is analysed together with the possible consequences of permanent cardiac pacing.     

Electrophysiologic characteristics and radiofrequency ablation of concealed nodofascicular and left anterograde atriofascicular pathways

INTRODUCTION: True nodoventricular or nodofascicular pathways and left-sided anterograde decremental accessory pathways (APs) are considered rare findings. METHODS AND RESULTS: Two unusual patients with paroxysmal supraventricular tachycardia were referred for radiofrequency (RF) ablation. Both patients had evidence of dual AV nodal conduction. In case 1, programmed atrial and ventricular stimulation induced regular tachycardia with a narrow QRS complex or episodes of right and left bundle branch block not altering the tachycardia cycle length and long concentric ventriculoatrial (VA) conduction. Ventricular extrastimuli elicited during His-bundle refractoriness resulted in tachycardia termination. During the tachycardia, both the ventricles and the distal right bundle were not part of the reentrant circuit. These findings were consistent with a concealed nodofascicular pathway. RF ablation in the right atrial mid-septal region with the earliest atrial activation preceded by a possible AP potential resulted in tachycardia termination and elimination of VA conduction. In case 2, antidromic reciprocating tachycardia of a right bundle branch block pattern was considered to involve an anterograde left posteroseptal atriofascicular pathway. For this pathway, decremental conduction properties as typically observed for right atriofascicular pathways could be demonstrated. During atrial stimulation and tachycardia, a discrete AP potential was recorded at the atrial and ventricular insertion sites and along the AP. Mechanical conduction block of the AP was reproducibly induced at the annular level and at the distal insertion site. Successful RF ablation was performed at the mitral annulus. CONCLUSION: This report describes two unusual cases consistent with concealed nodofascicular and left anterograde atriofascicular pathways, which were ablated successfully without impairing normal AV conduction system.     

Multiple reentry tachycardia in patients with ventricular preexcitation: report of three cases

The electrophysiologic studies of three patients with accessory pathways and multiple reentrant circuits are reported. The first patient had two atrioventricular accessory pathways: a left posterior capable of bidirectional conduction and a right paraseptal with retrograde conduction only. Four atrioventricular reentry circuits were documented: left and right orthodromic circuits and a left antidromic circuit with retrograde conduction over the right paraseptal accessory pathway. The second patient had a left lateral atrioventricular accessory pathway with type A preexcitation. Two reentrant tachycardias were noted: an atrial tachycardia where the accessory pathway remained concealed and an orthodromic atrioventricular tachycardia. The third patient had dual atrioventricular nodal pathways and a right nodofascicular accessory pathway. The accessory pathway became manifest only when a critical atrioventricular delay was reached, indicating its association with the slow atrioventricular nodal pathway. Wide QRS tachycardia with left bundle branch block contour was documented, by means of the slow atrioventricular nodal pathway and nodofascicular fiber antegradely, and the proximal right bundle branch, the His bundle, and the fast atrioventricular nodal pathway retrogradely.     

Entrainment of Typical AV Nodal Reentrant Tachycardia Using Para-Hisian Pacing:

INTRODUCTION: Despite the ability to cure atrioventricular nodal reentrant tachycardia (AVNRT) by radiofrequency catheter ablation with a high success rate, the exact localization of the tachycardia circuit is still not well established. The presence of AV nodal tissue between the typical AVNRT circuit and the His bundle, constituting a lower common pathway (LCP), remains controversial. METHODS AND RESULTS: Entrainment of AVNRT during para-Hisian stimulation allows accurate measurement of the His- to- atrial (HA) interval which is part of the same circuit as that of the tachycardia. With an LCP, during tachycardia, there is simultaneous conduction from the low turnaround of the circuit to the atrium (via the fast pathway) and to the His bundle (via the LCP). However, during entrainment by para-Hisian pacing, the impulse has to retrogradely depolarize sequentially the LCP and the fast pathway. Therefore, in the presence of an LCP, the HA interval duration during tachycardia (HAt) should be shorter than that of during entrainment by para-Hisian stimulation (HAe). We considered an LCP present when Hae - HAt was > or = 10 msec. Entrainment of typical AVNRT with para-Hisian stimulation was performed in 23 consecutive patients (21 females) with a mean age of 45+/-17 years. LCP was considered to be present in 18 of 23 patients (78%). In addition, transient His-bundle dissociation from the ongoing tachycardia occurred in seven patients (30%). CONCLUSION: These results support the presence of a LCP during typical AVNRT.     

Atrioventricular conduction in patients with clinical indications for transvenous cardiac pacing.

Eighty patients with clinical indications for cardiac pacing had atrioventricular conduction analysed by His bundle study. The indications for cardiac pacing included high grade atrioventricular block, sick sinus node syndrome without tachycardia, bradycardia-tachycardia syndrome, unstable bilateral bundle-branch block, and uncontrolled ventricular irritability. Complete heart block, Wenckeback block (Mobitz I), and 2:1 block were noted proximal and distal to the His bundle. Mobitz II block only occurred distal to the His bundle. Of special interest were the high incidence of distal conduction abnormalities by His bundle analysis (40/80, 50%), the re-establishment of normal atrioventricular conduction in acutely ill patients with recent evidence of heart block, and the high incidence of intraventricular conduction disturbances on standard electrocardiogram (48/80, 60%).