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The 7 Fr "split-tip electrode" (2.5-mm tip electrode divided longitudinally into four electrodes with an adjacent 2-mm ring electrode) improves mapping resolution due to its small recording electrodes and narrow interelectrode distances (0.1 mm). The purpose of this study was to examine the temperature-controlled ablation properties of this electrode. In seven anesthetized dogs, the thigh muscles were exposed and superfused with canine blood. A split-tip catheter electrode (with a thermocouple in each of the five electrodes) and a conventional 4-mm catheter electrode were positioned at constant pressure perpendicular or parallel to the surface of the thigh muscle. Impedance measured between each split electrode and a skin patch correlated with the degree of contact with blood and tissue. In the parallel catheter to tissue orientation, split electrodes not in contact with tissue had a low impedance (mean 210-224 ohms), and the split electrode almost entirely in contact with tissue had the highest impedance (380 +/- 56 ohms). In the perpendicular catheter to tissue orientation all split electrodes had a similar impedance (mean 279-286 ohms). A total of 75 radiofrequency (RF) lesions were produced in the temperature-controlled mode with the 4-mm electrode (target 60 degrees C) or the split-tip electrode (power limited by the hottest electrode reaching 70 degrees C) with current delivered to all five electrodes simultaneously, or only to electrodes in contact with tissue. Lesion depth was not significantly different between electrodes in the parallel orientation (5.2 +/- 0.9 vs 5.1 +/- 1.4 vs 5.3 +/- 1.1 mm), but significantly deeper with the conventional 4-mm tip electrode in the perpendicular orientation (6.7 +/- 1.2 vs 5.3 +/- 1.3 vs 5.6 +/- 0.9 mm, P < 0.05). This was due to higher power delivered to the conventional 4-mm electrode (27 +/- 9 vs 17 +/- 7 vs 15 +/- 7 W, P < 0.05) because convective cooling by the blood flow was less effective for the split-tip electrode due to a reduced heat conduction across the interelectrode space from the hottest electrode to cooler areas of the group of five electrodes (mean temperature difference between the hottest split electrodes and the ring electrode: 24 degrees C). Electrode cooling or heat conduction was not effected by the elimination of current delivery to non-contact electrodes. Steam pops occurred in 36% of applications with the conventional 4-mm electrode in the perpendicular orientation but never with the split-tip electrode in spite of the higher target temperature. Measurement of impedance from the split electrodes allow the determination of electrode tissue contact and RF lesions produced with the split-tip electrode in the temperature-controlled mode using a target of 70 degrees C were of reasonable size and not associated with steam pops.

Ablation catheters with multiple long coiled electrodes have been recently developed to induce continuous linear lesions for the treatment of atrial fibrillation. The efficacy and safety of ablation catheters with multiple long coiled electrodes has not been evaluated. The aim of the present in vivo study was to investigate the influence of saline irrigation on lesion dimensions and coagulum formation during RF current ablation using those ablation catheters. In 14 anesthetized sheep, the thigh muscle was prepared as a cradle and filled with heparinized blood (37 degrees C). The quadripolar coiled ablation catheter (electrode length 7 mm, electrode distance 2 mm) was placed parallel to the muscle with standardized 10-g contact pressure. RF current energy was delivered sequentially temperature-(70 degrees C) or power-controlled (10, 20, 30, or 40 W) with additional irrigation of the electrode (10 mL/min normal saline) for 90 seconds. Forty-two of 129 RF current lesions were induced by temperature-controlled and 87 by irrigated ablation. Except for three lesions following low energy irrigated application (10 W), all lesions were continuous. Significantly larger lesions following irrigated RF current applications were produced with a power output of 30 W (depth 0.74 +/- 0.13 cm, width 0.78 +/- 0.13 cm) and 40 W (depth 0.75 +/- 0.16 cm, width 0.92 +/- 0.28 cm) as compared to 20 W (depth 0.47 +/- 0.13 cm, width 0.82 +/- 0.22 cm). Coagulum formation adherent to the electrode was exclusively observed following 18 of 42 nonirrigated RF current ablations. In conclusion, irrigated coiled ablation electrodes induce continuous linear lesions with a power output of 20-40 W. The risk of coagulum formation at the coiled electrode can be avoided by irrigation.

The surgical atrial maze procedure has provided proof that atrial fibrillation can be cured by performing atrial incisions based on anatomical and electrophysiological principles. Preliminary reports of attempts at radiofrequency catheter ablation of atrial fibrillation utilizing an anatomy-based "linear incision" method have shown the feasibility of the method. However, postprocedural atrial fibrillation recurrence has been common and in addition new, uniform tachycardias have developed in some patients. Both of these outcomes may be in part due to incomplete or inconsistent lesion deployment. This article details the use of the CARTO system for deploying anatomy guided linear atrial lesions for the purpose of curing atrial fibrillation. The procedure is comprised of three phases, which are discussed in detail: (1) baseline map; (2) lesion deployment and; (3) lesion assessment. Using a single standard ablation electrode, lesions can be deployed safely, and complete lesions can be confirmed. Paradigms for right and left atrial incisions are proposed.

Supraventricular and ventricular arrhythmias, particularly nonsustained ventricular tachycardia, and ventricular premature beats are a common finding in patients with hypertrophic cardiomyopathy. Several investigations have demonstrated that nonsustained ventricular tachycardia on Holter monitoring is associated with an increased risk of sudden cardiac death. It has been a long lasting controversial discussion whether suppression of these arrhythmias with drugs, such as amiodarone is capable to reduce the incidence of sudden cardiac death. While recent studies have indicated that nonsustained ventricular tachycardia in asymptomatic patients without additional risk factors, such as a positive family history of sudden cardiac death or syncope should not be treated prophylactically with amiodarone. Symptomatic patients with sustained ventricular tachycardias and/or syncope related to ventricular arrhythmias should undergo ICD implantation. The implantation of an ICD in asymptomatic patients should be limited to those who have several risk factors for sudden cardiac death. It is questionable whether other risk stratifiers, such as programmed electrical stimulation may be helpful to identify asymptomatic patients who are at risk to die suddenly. Moreover, whether the demonstration of electrocardiogram fractionation during electrophysiological study is superior to the induction of sustained ventricular arrhythmias for risk stratification, needs further investigation.

Atrial fibrillation is one of the most commonly encountered clinical arrhythmias. Different treatment options for this rhythm disorder exist with the electrical and/or pharmacological cardioversion to sinus rhythm with subsequent antiarrhythmic drug therapy to prevent recurrences being one of the primary therapeutic goals. Another alternative, however, is represented by the control of the ventricular rate in patients with persistent atrial fibrillation. The question of which these two strategies should be preferred in the majority of patients with atrial fibrillation has not been studied in a prospective way. Given the background of conflicting data with respect to the prognostic impact of atrial fibrillation and of the increasing evidence concerning the risks of antiarrhythmic drug treatment in atrial fibrillation, a prospective multicenter trial has been initiated to compare these two therapeutic alternatives prospectively. Patients will be randomly assigned to cardioversion with subsequent antiarrhythmic drug therapy to prevent recurrent atrial fibrillation or to a therapy aiming exclusively at control of the ventricular rate during persistent atrial fibrillation. All patients will receive anticoagulation by means of warfarin (target INR 2.5-3.5) to prevent thromboembolic complications. The rationale and the design of the PIAF trial (Pharmacological intervention in Atrial Fibrillation) are discussed below. The pilot phase of this study has begun patient enrollment in the spring of 1995.

Presently available bipolar pacemakers connected to bipolar leads allow the programming of polarity to either bipolar or unipolar configuration. The bipolar configuration improves sensing because of its lack of oversensing. In respect to the pacing impulse, it may be beneficial to program the bipolar to unipolar configuration, because the unipolar pacing configuration seems to lead to lower pacing thresholds. The aim of the study was to investigate the effects of the two configurations on pacing and sensing in 11 patients with a new ventricular steroid-eluting lead (Encor Dec 033-301, Telectronics Inc.) connected to the same types of VVI(R) pacemakers. Follow-up was 12 months after implantation. Pacing was assessed by pulse duration thresholds at 0.8- 1.6-, and 2.5-V pulse amplitude and by pacing impedance. Parameters for sensing included sensing thresholds, R wave amplitudes, and amplitude of intrinsic deflection. Pulse duration thresholds were similar for the two configurations. At implantation, pacing impedance was significantly higher with bipolar (672 +/- 60 omega) than with unipolar pacing configuration (626 +/- 82 omega, P < 0.005). The difference lasted until 1 month after implantation (bipolar: 670 +/- 49 omega; unipolar: 63053 omega, P < 0.01). No significant differences were detected after 3 (bipolar: 683 +/- 63 omega; unipolar: 662 +/- 70 omega) and 12 months (bipolar: 658 +/- 64 omega; unipolar: 660 +/- 63 omega) between the 2 configurations. Although intrinsic deflections of ventricular electrograms were different, mean R wave amplitudes were similar in unipolar and bipolar configuration. Pacing and sensing characteristics of unipolar and bipolar configuration were similar for the studied bipolar pacing lead. Pacing impedance was initially higher for bipolar than for unipolar configuration and equilibrated within 3 months after implantation.

A case is presented of a 38-year-old male with dextrocardia in whom radiofrequency current ablation of an incessant atrial tachycardia originating within the infero-lateral pulmonary vein was achieved. Activation mapping with detection of the earliest atrial activation was used for identification of the arrhythmogenic focus. In addition to fluoroscopy, transesophageal echocardiography was used for catheter guidance during the transseptal puncture. The present experience suggests that location of an arrhythmogenic focus within the pulmonary venous system should be considered whenever early atrial activation during ectopic atrial tachycardia is recorded at the junction between the left atrium and the pulmonary veins.

Unlabelled: With the aim to detect dysfunction of pacing leads, most present-day pacemakers measure pacing impedance by means of real-time telemetry. The recommended setting for impedance measurement is 5.0 V for pulse amplitude and 0.5 ms for pulse duration. Availability of reliable settings would facilitate impedance measurements during follow-up. The purpose of the present study was twofold: (1) to assess whether telemetrically measured impedance of the studied pacemaker is similar to impedance measured at implantation; and (2) to evaluate whether the pacemaker setting influences telemetrically measured impedance. Sixty-five consecutive patients receiving VVI(R) pacemakers were studied; in all patients, impedance measured at implantation by a pacing system analyzer was compared to measurements obtained telemetrically within 1 day after implantation. In 44 other patients, impedance was determined 3 months after implantation at 60 and 120 ppm (n = 44), twice at 60 ppm (n = 42), and 12 months after implantation at 60 ppm (n = 34). For each measurement, pulse amplitude was programmed to 0.8, 1.6, 2.5, 5.0, and 8.0 V, and pulse duration to 0.05, 0.25, 0.50, and 1.00 ms. Impedance at implantation (606 +/- 113 ohms) did not differ from the data obtained with telemetry (629 +/- 113 ohms; P = NS). Different pacing rates, repeat measurements, and follow-up time failed to show any influence on impedance. Measurement tolerance was < 10% for 15 of 19 studied settings other than 5.0 V and 0.5 ms., Conclusion: The studied pacemakers provide reliable data for telemetrically measured impedance. Telemetrical impedance does not necessarily have to be measured at 5.0 V and 0.5 ms. These findings should be considered for measurement and interpretation of real-time telemetry impedance during follow-up.

The purpose of the study was to compare the feasibility of low amplitude output programming (2.5 V/0.5 msec) 3 or more months after pacemaker implantation in patients receiving steroid and nonsteroid lead systems. Chronic pacing voltage, current, and energy thresholds were determined from 0.05- to 1.0-msec pulse duration in 44 patients with steroid lead systems, and in 36 patients with nonsteroid lead systems; all patients received pacemakers from the same manufacturer, which utilized the same programming and telemetry features. Chronaxie, pulse duration at the lowest pacing current, and energy were assessed from individual threshold curves. Steroid-eluting leads had significantly lower pacing voltage, current, and energy thresholds than nonsteroid leads. A 100% safety threshold margin could be achieved in 43 (98%) patients with steroid lead systems and in 27 (75%; P < 0.05) patients with nonsteroid lead systems with output programming of 2.5 V/0.5 msec. Chronaxie (0.22 +/- 0.17 msec vs 0.44 +/- 0.31 msec; P < 0.05), pulse duration at lowest pacing current (0.28 +/- 0.12 msec vs 0.49 +/- 0.22 msec; P < 0.05), and pulse duration at lowest pacing energy (0.31 +/- 0.17 msec vs 0.53 +/- 0.22 msec; P < 0.05) were significantly shorter for steroid than for nonsteroid lead systems. In 42 patients of the former group, a 100% safety margin could be maintained either with a 2.5 V/0.3 msec or with a 1.6 V/0.5 msec output. Conclusions Low amplitude output programming can be obtained in almost all pacemakers connected to steroid-eluting lead systems, and in a significantly higher number of patients than when connected with nonsteroid leads systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Unlabelled: The effects of lower pulse amplitude on battery current and pacemaker longevity were studied comparing the new, small-sized VVI pacemaker, Minix 8341, with the former model, Pasys 8329. Battery current was telemetrically measured at 0.8, 1.6, 2.5, and 5.0 V pulse amplitude and 0.05, 0.25, 0.5, and 1.0 msec pulse duration. Internal current was assumed to be equal to the battery current at 0.8 V and 0.05 msec. Pacing current was calculated subtracting internal current from battery current. The Minix pacemaker had a significantly lower battery current because of a lower internal current (Minix: 4.1 +/- 0.1 microA; Pasys: 16.1 +/- 0.1 microA); pacing current of both units was similar. At 0.5 msec pulse duration, the programming from 5.0-2.5 V pulse amplitude resulted in a greater relative reduction of battery current in the newer pacemaker (51% vs 25%). Projected longevity of each pacemaker was 7.9 years at 5.0 V and 0.5 msec. The programming from 5.0-2.5 V extended the projected longevity by 2.3 years (Pasys) and by 7.1 years (Minix). The longevity was negligibly longer after programming to 1.6 V., Conclusion: extension of pacemaker longevity can be achieved with the programming to 2.5 V or less if the connected pacemakers need a low internal current for their circuitry.

Unlabelled: Fourteen patients (12 men, 2 women; 61 +/- 9 years) with ventricular tachycardia and underlying heart disease underwent an attempt at radiofrequency energy catheter ablation. Twelve patients had coronary disease and two patients had dilated cardiomyopathy. Two patients had two clinical tachycardias, the ejection fraction was 38% +/- 11%. All tachycardias were inducible and hemodynamically well tolerated (cycle length = 357 +/- 56 msec). Ablation was initially successful in nine patients (no tachycardia inducible after ablation and before discharge). Two patients had recurrences (in-hospital and 4 months) and one patient had a tachycardia of a different morphology, which was also successfully ablated. Ablation was overall successful in seven patients and unsuccessful in seven patients (including all patients with cardiomyopathy). Mid-diastolic potentials were observed in all the patients in whom ablation was successful but not observed in four of seven unsuccessful patients. The successful patients remain free of recurrences at 9 +/- 8 months follow-up., Conclusions: (1) in ventricular tachycardia following an old infarction radiofrequency energy ablation is possible with a high success rate if a critical component of the tachycardia circuit can be localized. Localizing isolated mid-diastolic potentials and ensuring these potentials are part of the reentrant circuit with concealed entrainment can help to enhance the results. (2) A negative predischarge electrophysiological study may be predictive of success.

Electromagnetic signals coming from the pacemaker programmer can permanently damage multiprogrammable pacemakers. At a follow-up of a sixfold programmable VVI pacemaker (Phoenix 251-6, Pacesetter), the emergency VVI button was activated on the programmer after the programming of the pacing impulse had induced a loss of capture. The next telemetrical interrogation revealed an alteration to a threefold programmable Programmalith III. The inadvertent programming of the model number occurred, because the programming head was removed at one very specific time in the programming sequence of the emergency VVI command.

One hundred five patients with an accessory atrioventricular pathway underwent catheter ablation of the pathway using radiofrequency current. There were 79 accessory pathways located on the left and 32 on the right side of the heart. In patients with right-sided pathways ablation was attempted via a catheter positioned at the atrial aspect of the tricuspid annulus. In patients with a left-sided free-wall accessory pathway a novel approach was utilized in which the ablation catheter was positioned in the left ventricle directly below the mitral annulus. Accessory pathway conduction was permanently abolished in 93 patients (89%). Failures were mainly due to inadequate catheters used initially. It is concluded that catheter ablation of accessory atrioventricular pathways using radiofrequency current is an effective and safe therapeutic modality for patients with symptomatic tachyarrhythmias mediated by these pathways.

The purpose was to test whether a reduction of pacemaker electrode surface area below 8 mm2 improves leads that elute steroid from the electrode tip to the surrounding myocardium. A standard-sized 8 mm2 lead with 1 mg dexamethasone was implanted in 12 patients and a lead with 4 mm2 electrode surface area and 0.5 mg dexamethasone in ten patients. Pacing threshold, impedance, and sensing threshold were measured at implantation and after 1, 4, and 12 weeks. Pacing thresholds were similar for both groups and were always less than or equal to 0.8 V at 0.5 msec pulse duration in all patients. Impedance was significantly higher (P less than 0.05) for the 4 mm2 lead (implantation: 726 +/- 119 ohms; 1 week: 596 +/- 71 ohms; 4 weeks: 624 +/- 68 ohms; 12 weeks: 643 +/- 56 ohms) than for the 8 mm2 lead (implantation: 422 +/- 43 ohms; 1 week: 402 +/- 48 ohms; 4 weeks: 439 +/- 57 ohms; 12 weeks: 449 +/- 61 ohms). R wave amplitudes did not differ between both groups; no sensing failure occurred at 5 mV sensitivity. Compared to the 8 mm2 lead the reduction of surface area to 4 mm2 did not influence pacing threshold, but resulted in a higher pacing impedance. The amount of pacing energy was lower in the smaller-sized electrode. For clinical impact, low pacing threshold and high impedance leads are the condition to implant pulse generators with smaller battery capacity.

Unlabelled: The pacing rate of activity-modulated pacemakers is triggered by vibrations running through the body. Whether the body constitution predicts maximum pacing rate and may facilitate rate response programming was studied in 16 patients with Activitrax pacemakers. Rate response parameters were programmed to a fixed setting in VVIR/VOOR mode (lower pacing rate 60 ppm, upper pacing rate 125-130 ppm, activity threshold medium/7). Body vibrations were induced by a treadmill exercise test with increasing speed. Maximum pacing rates were measured at the stage of symptom-limited tolerance. Exercise tests with a duration of 7.3 +/- 2.9 minutes resulted in a maximum pacing rate of 98 +/- 22 ppm ranging from 60-122 ppm. Maximum pacing rates did not differ between male (n = 10; 102 +/- 21 ppm) and female (n = 6; 92 +/- 24 ppm). Correlations between maximum pacing rates and body constitutional factors were not significant with r = -0.15 (weight), r = 0.39 (height), r = 0.07 (body surface area), and r = -0.27 (skin-fold thickness). The correlations with body mass index (r = -0.53) and age (r = -0.53) were initially significant, but not after Bonferroni-Simes-Hommel correction. The age-dependent relationship may be caused by the shorter exercise duration of older patients indicated by the correlation between exercise duration and maximum pacing rate (r = 0.77), as well as with age (r = -0.73)., Conclusions: body constitution did not modify body vibrations and did not allow prediction of maximum pacing rates; therefore, it is no aid for the programming of rate response parameters.

A case is presented of a 68-year-old male patient with a history of myocardial infarction and recurrent ventricular tachycardia who was successfully treated with a single 20-second transcatheter application of radiofrequency current. Prior to current application a complete endocardial map had been obtained of an area of slow conduction that extended caudo-cranially for approximately 2 cm along the lower left ventricular septum. Stimulation techniques yielded evidence that this area was critically related to tachycardia initiation and maintenance. Its central part was subsequently chosen as the site for current delivery.

The aim of steroid-eluting leads is to reduce chronic pacing thresholds. Whether steroid-eluting leads also modify acute and chronic R wave amplitudes as well as R wave sensing of the pulse generator was investigated in 31 patients with a unipolar ventricular pacemaker. Four different leads were implanted: Two steroid-eluting leads with different electrode surface areas (8 mm2 and 5.8 mm2) and two conventional leads (Target Tip, Elgiloy lead). At implantation filtered R wave amplitudes, peak-to-peak values, and slew rates were measured by a pacing system analyzer. One year after implantation R wave amplitudes were directly determined from intracardiac electrograms and compared to the peak-to-peak data at implantation. Additionally, R wave inhibition was evaluated at a sensitivity setting of 5 mV. There were no differences among the four leads in respect to any of the parameters studied at implantation. At follow-up, no differences in R wave amplitudes were found leading to an appropriate sensing in all leads. Steroid-eluting leads did not differ from conventional leads and a smaller electrode surface area of 5.8 mm2 had no influence on ventricular electrogram. Together with a pacemaker with an input impedance of 37 kohms R wave sensing was a correct setting of 5 mV.

The pacing threshold of the human heart may be altered by physiological factors such as physical exercise. These changes may influence the individual programming of a pacemaker, since pacemakers can be programmed at pulse amplitudes of 2.5 volts and less. We investigated 22 patients with a multiprogrammable ventricular demand pacemaker 3 months after implantation; 16 patients had received a steroid-eluting lead and six patients had an Elgiloy lead. Parameters measured at rest and immediately after exercise were: Voltage threshold at pulse durations between 0.05 and 0.6 ms, impedance, R wave amplitude and energy consumption for the pacing threshold at 0.5 ms pulse duration. All patients performed a symptom limited supine bicycle exercise test. None of the investigated parameters showed a significant difference between rest and exercise, neither for the steroid eluting lead nor for the Elgiloy lead. The data suggest that the individual programming of a pacemaker adapted to the measurements at rest is also reliable and safe during exercise.

For potential application in ablating left free-wall accessory AV pathways with direct current shocks, a new epicardial-endocardial electrode configuration, designed to focus the current field across the mitral annulus, was tested in dogs. A catheter electrode in the coronary sinus (epicardial electrode) was used as the cathode, and a catheter electrode in the left ventricle (endocardial electrode) placed beneath the mitral valve, high against the mitral annulus and directly across from the epicardial electrode formed the anode. Two shocks, each of 30, 40, or 50 joules (J) were delivered in nine, three, and four dogs, respectively. The first shock was applied to the anterior or lateral wall and the second shock to the posterior wall, except in one dog which received one anterior and one lateral shock. Two dogs receiving 50 J shocks died acutely, one due to rupture of the coronary sinus and cardiac tamponade and the other had unexplained electromechanical dissociation. The remaining 14 dogs tolerated the two shocks well and were sacrificed 3-5 days later for pathological examination of the heart. Shocks in the anterior and lateral regions produced atrial necrosis (height 1.5-11 mm, width 1.5-12 mm and depth 1-3 mm) in 10 of 14 dogs and ventricular necrosis (height 4-27 mm, width 4-33 mm, and depth 5-14 mm) in all 14 dogs. Ideal lesions with atrial necrosis extending down to the annulus and ventricular necrosis extending to the epicardial aspect of the ventricular crest occurred in five dogs in which the endocardial electrode was positioned high against the annulus. In the other nine dogs, the endocardial electrode was located 6-18 mm below the annulus, as estimated by the center of ventricular necrosis. In these dogs, the ventricular lesions did not extend to the epicardial aspect of the crest and, in four dogs, no atrial necrosis was found. Shocks delivered to the posterior wall produced no atrial or ventricular necrosis except in one dog receiving a 50 J shock. It is concluded that, using the epicardial-endocardial electrode configuration with the LV catheter positioned high against the annulus, shocks of less than 50 J in dogs safely produce atrial and ventricular necrosis adjacent to the mitral annulus in the anterior and lateral regions but not in the posterior regions. Similar lesions in man may be capable of interrupting left anterior and lateral accessory AV pathways.

A case is presented of a 20-year-old woman with a history of three episodes of syncope within the last 4 years, which was caused by a rapid ventricular response to atrial fibrillation via a left-sided posterior accessory pathway. A variety of antiarrhythmic agents had failed to control the arrhythmia. Using a novel dual catheter approach, with one catheter in the coronary sinus and an adjacent catheter in the left ventricle close to the mitral annulus, accessory pathway conduction was successfully interrupted by two radio-frequency current applications between the tip electrodes of the two catheters. During follow-up, 12-lead electrocardiograms have been normal and the patient has been asymptomatic.

Programmed electrical cardiac stimulation was performed in a 40-year-old man with documented recurrent, sustained ventricular tachycardia which was refractory to standard medical therapy. Both the presence of several discontinuities in the AV nodal conduction curves during atrial and ventricular stimulation and the time intervals of the AV nodal echo phenomena suggested the presence of multiple AV nodal pathways. The results of this study are of interest in further increasing our understanding of the electrophysiologic behavior of the human AV node.

A case is presented of a patient with incessant ventricular tachycardia of left bundle branch block morphology. Endocardial mapping revealed the site of earliest activation during tachycardia to be the proximal right ventricular septum. Pacing at this site elicited the clinical tachycardia, whereas pacing at the proximal left ventricular septum induced a right bundle branch block morphology identical to that of a previously recorded spontaneous ventricular tachycardia. Electrophysiological evidence is given that both types of tachycardia originate from a single reentry circuit located in the proximal ventricular septum in which the reentrant wavefront may travel either orthodromically (during spontaneous tachycardia and right ventricular pacing) or antidromically (during left ventricular pacing).