Narrow QRS Tachycardia with Ventriculoatrial Dissociation Mediated by a Left Fasciculoventricular Fiber

A 30-year-old man presented with narrow QRS tachycardia. The intracardiac electrocardiogram showed an atrial-HIS (AH) interval of 75 msec and a HIS-ventricular (HV) interval of 44 msec during baseline. Atrial incremental pacing revealed HV shortening, with apparent incomplete right bundle branch block (RBBB) morphology without QRS complex axis deviation. The induced tachycardia exhibited several QRS morphologies: a narrow QRS, complete RBBB and complete left bundle branch block (LBBB) morphology. Spontaneous conversion of the QRS pattern from wide to narrow was observed. The cycle length of the tachycardia was significantly shortened (from 316 to 272 ms) from LBBB morphology to narrow QRS complex. The atrial activation was dissociated from the ventricular activation during all tachycardias. Each QRS complex during tachycardia was preceded by a HIS deflection and HV interval was 35 ms, which was shorter than that of sinus rhythm. HIS deflection was earlier than right bundle potential during all kinds of tachycardia. This tachycardia is most likely mediated by a left fasciculoventricular fiber which connects the HIS bundle below the atrioventricular node to the myocardial tissue of the left ventricle. The HIS-Purkinje system is used as an antegrade conduction limb and the fasciculoventricular fiber as a retrograde limb in the tachycardia circuit.     

Influence of ventricular conduction defects on ventricular repolarisation and coupling intervals of ventricular extrasystoles.

In all conduction disorders of the left ventricle the Q-T interval is prolonged in relation to the prolongation of the QRS duration, but the S-T interval (measured from the end of the S wave) is within normal limits. Right ventricular conduction defects are associated with prolongation of the Q-T interval, but the S-T interval in incomplete and complete right bundle branch block is subnormal. In the absence of a broad QRS, the Q-T and S-T intervals are not prolonged in left anterior hemiblock. However, an associated non-specific left intraventricular conduction defect or right bundle branch block does prolong the QRS and thus the Q-T interval. The S-T interval is normal or subnormal in the presence of right bundle branch block and left anterior hemiblock confirming that left anterior hemiblock does not effect the Q-T interval and repolarisation. In the presence of any conduction defect which prolongs the QRS duration, extrasystoles have longer coupling intervals. The coupling intervals are longer in the presence of left bundle branch block than right bundle branch block. It is postulated that this is due to the differences in the time of depolarisation and thus repolarisation of the left ventricle.     

His bundle recordings in right bundle-branch block coexisting with iatrogenic right ventricular pre-excitation

Iatrogenic right ventricular pre-excitation failed to abolish right bundle-branch block in two patients. When `exclusive' His bundle pacing was performed, the QRS complexes, St-V, and St-LVE intervals were similar to the ventricular deflections, H-V, and V-LVE (intervals) recorded during sinus rhythm. `Exclusive' pacing of the ordinary muscle at the right ventricular inflow tract produced a complete left bundle-branch block pattern without abnormal left axis deviation. Pacing of both His bundle and ordinary muscle yielded combination complexes in which the right bundle-branch block pattern persisted.     

Pathologic correlations in three cases of bilateral bundle branch disease with unusual electrophysiologic manifestations in two cases.

Examination of the conduction system in three patients with bifascicular block who had electrophysiologic studies forms the basis for this report. Patients 1 and 2 had left bundle branch block and Patient 3 right bundle branch block and left axis deviation. The H-V interval was prolonged in each case (70, 65 and 60 msec, respectively). Serial section examination of the conduction system revealed sclerodegenerative involvement of both bundle branches in all cases. In Case 1, atrial extrastimulus testing converted left to right bundle branch block; in Case 2, it delineated a sinus echo zone with repetitive sinus nodal reentrance. In the latter case serial section revealed extensive amyloid infiltration of the approaches to the sinoatrial (S-A) node and the atrial preferential pathways. In Case 3, with right bundle branch block and left axis deviation, serial section revealed greater involvement of the anterior part of the main left bundle branch than of the posterior portion as well as involvement of the second part of the right bundle branch. The study revealed excellent correlation between electrophysiologic and pathologic findings in three cases of intraventricular conduction disease and demonstrated an anatomic basis for the electrophysiologic findings resembling alternating bilateral bundle branch block. Sinus nodal reentrance may be related to disease in the approaches to the S-A node thereby causing delay in perinodal tissue allowing sinus reentrance. Finally in Case 3, the anatomic substrate for left axis deviation may lie in a greater involvement of the anterior portion than of the posterior portion of the main left bundle rather than in the corresponding portions of the periphery.     

Re-entry via Mahaim fibres as a possible basis for tachycardia.

Paroxysmal tachycardia with widened QRS complexes was recorded in a 21-year-old man. In sinus rhythm there was no evidence of pre-excitation. His bundle studies revealed an abnormally short HV interval of 30 ms. Premature atrial stimuli produced an increased PR interval. At short coupling intervals the His bundle activity became incorporated within the QRS complex. Concurrently, a left bundle-branch block pattern appeared identical to that seen during tachycardia. Closely coupled ventricular extrastimuli initiated a tachycardia identical to the initial episode. A re-entry mechanism via anterograde Mahaim fibres and retrograde His bundle -AV node pathway is postulated.     

Spontaneous cardiac resynchronization

A 73-year-old woman with dilated cardiomyopathy presented with heart failure. The ECG showed sinus rhythm with left bundle branch block, left-axis deviation and prolonged QRS duration and frequent ventricular premature complexes from the left ventricular septal wall were present. Ventricular premature beats had narrower QRS duration than sinus node beats conducted through the His-purkinje fibers consistent with resynchronizing beats. The mechanisms of narrowing of the QRS complex produced by premature beats in cases of impaired intra and interventricular conduction are discussed.     

The relative risk of spontaneous complete atrioventricular block in elderly patients with impaired intra-ventricular conduction.

We reviewed 144 consecutive patients with symptomatic high grade atrioventricular block. Cases due to congenital heart disease, acute myocardial infarction, cardiac surgery or digitalis toxicity were excluded. Of the remaining, we chose 71 patients in whom atrioventricular conduction was observed either intermittently during complete heart block (CHB) or in electrocardiograms taken within two years prior to documentation of CHB. The mean age was 69 years, with the peak incidence in the seventh decade in 43 men and eight decade in 28 women. Bundle branch block (BBB) was present in 76% of patients as follows: 47% had right BBB (20% with normal QRS axis, 20% with left axis deviation and 7% with right axis deviation), 17% had left BBB (11% with normal QRS axis and 6% with left axis deviation) and 12% had either alternating BBB, right BBB with alternating axis deviation or atypical BBB. "Trifascicular block" patterns accounted for 21% of the total group of CHB. We also studied the prevalence of various patterns of BBB in a group of 2000 random hospital patients of comparable age and sex exclusive of those with acute myocardial infarction and heart surgery. The risk of CHB for the various patterns of BBB was calculated relative to normal intraventricular conduction. All patterns of BBB carried a considerably increased relative risk of CHB, (P smaller than .01). The relative risk was highest for RBBB with left axis deviation and lowest for LBBB with normal or left axis deviation. In the men, 74% had QRS patterns of "bifascicular" or "trifascicular" block during atrioventricular conduction. By contrast, 71% women had atrioventricular beats showing either no BBB or right BBB with normal QRS axis. QRS pattern during CHB was unchanged from that during atrioventricular conduction in 52% if cases (rabge 38%-76% with different QRS patterns) suggesting idiojunctional pacemaker. CHB in these cases was thought to be due probably to coexistent disease in the AV node or His bundle. Although the concept of uni-, bi- and trifascicular block patterns has been useful in identifying patients at greater risk of CHB, the predictability of the electrocardiogram has obvious limitations, particularly in women.     

Mahaim conduction producing left axis deviation and normal QRS.

An unusual patient is described in whom electrophysiological studies strongly suggest the occurrence of Mahaim conduction. The patient whose electrocardiogram previously showed a left anterior hemiblock pattern then developed advanced atrioventricular (AV) block (AH block). Beats conducted through the atrioventricular node always had a short HV interval (20 ms) and QRS complexes of left anterior hemiblock pattern. Junctional escape beats always had a normal HV interval (50 ms) with normal intraventricular conduction. His bundle pacing showed the StV interval and QRS contour of escape beats. These findings suggest the existence of an accessory pathway (Mahaim fibres) passing from the area of block, presumably the uppermost portion of the His bundle, to the posteroinferior division of the left bundle-branch. The surface electrocardiogram did not show the characteristic delta wave of the Wolff-Parkinson-White syndrome. Our observations suggest that patients in whom there is conduction along Mahaim fibres may show only the pattern of intraventricular conduction defect without a delta wave.     

Mahaim conduction producing left axis deviation and normal QRS.

An unusual patient is described in whom electrophysiological studies strongly suggest the occurrence of Mahaim conduction. The patient whose electrocardiogram previously showed a left anterior hemiblock pattern then developed advanced atrioventricular (AV) block (AH block). Beats conducted through the atrioventricular node always had a short HV interval (20 ms) and QRS complexes of left anterior hemiblock pattern. Junctional escape beats always had a normal HV interval (50 ms) with normal intraventricular conduction. His bundle pacing showed the StV interval and QRS contour of escape beats. These findings suggest the existence of an accessory pathway (Mahaim fibres) passing from the area of block, presumably the uppermost portion of the His bundle, to the posteroinferior division of the left bundle-branch. The surface electrocardiogram did not show the characteristic delta wave of the Wolff-Parkinson-White syndrome. Our observations suggest that patients in whom there is conduction along Mahaim fibres may show only the pattern of intraventricular conduction defect without a delta wave.     

Significance of first degree heart block (prolonged P--R interval) in bifascicular block

To test the hypothesis that first degree heart block in the presence of a QRS pattern of bifascicular block is related to conduction delay in the remaining fasclcle, we reviewed the His bundle records of 63 patients with a pattern of bifascicular block and compared the H-V intervals of the 41 patients without first degree heart block with those of the 22 patients with P-R prolongation. The following conclusions were drawn from analysis of our cases and those studied and reported by others: (1) In the presence of first degree heart block a significant number of these patients will have a normal H-V interval. (2) More than 50 percent of patients with a pattern of left or right bundle branch block and right axis deviation have a prolonged H-V interval regardless of the P-R interval, and the correlation of P-R and H-V prolongation is not statistically significant (P > 0.05 and < 0.1). (3) In patients with a pattern of right bundle branch block and left axis deviation, the presence of P-R prolongation suggests abnormality of the H-V interval (P < 0.005), although 30 percent of the patients with this finding will have a normal H-V interval and the H-V interval cannot be predicted in individual patients. (4) His bundle electrography is essential to determine accurately the presence of trifascicular block in individual patients whether the P-R interval is normal or prolonged.     

Bundle branch block. Demonstration of the incomplete nature of some "complete" bundle branch and fascicular blocks by the extrastimulus technique

The incomplete nature of some electrocardiographic “complete” bundle branch and fasclcular blocks is demonstrated using the atrial extrastimulus technique. Patient 1, with a QRS pattern of “complete” left bundle branch block, manifested a QRS pattern of right bundle branch block at a shorter coupling interval, indicating that the left bundle could conduct. Patient 2, with a QRS pattern of right bundle branch block and “complete” left anterior hemiblock, manifested a pattern of left posterior hemiblock at a shorter coupling interval, indicating that the left anterior fascicle could conduct. Patient 3, with a normal QRS complex, showed left bundle branch block at shorter coupling intervals and then a pattern of right bundle branch block as the coupling interval was further decreased, indicating that functional left bundle branch block was incomplete. This demonstration of partial bundle branch block depends on a discordance of conduction time and refractory period, the bundle or fascicle with depressed conduction (incomplete block) having a shorter refractory period than the more normally conducting bundle or fascicle. This discordance may be related to the development of trifascicular block in patients with bifascicular block and a normal H-V interval. It is a predisposing factor in the complex patterns of aberrant conduction seen during supraventricular tachyarrhythmias with varying cycle lengths.     

On axis deviation in human bundle branch block

Fifty-four cases with records showing both normal intraventricular conduction and bundle branch block have been collected; 192 records were available, ninety showing bundle branch block, ninety-two showing normal intraventricular conduction, and ten showing both bundle branch block and normal conduction.The axis deviation has been measured in each record and the average position of the electrical axis of QRS during normal conduction compared with that during bundle branch block.When right or left bundle branch block appeared or disappeared there was a significant change in the direction of the electrical axis of QRS (15° or more) in less than one-half of the cases, and the average change was 12° to the left in left bundle branch block and 12° to the right in right bundle branch block.When a significant change of axis was associated with the appearance of bundle branch block, the direction of the change was always to the left when the block was of the left bundle branch and, except in one case, to the right when the right bundle branch was blocked.Even in cases in which a significant change of axis was found, the general pattern of the bundle branch block electrocardiogram conformed closely to the pattern with normal intraventricular conduction.Factors, other than the bundle branch block, that might have caused a shift of the electrical axis have been discussed; it is believed that they did not materially influence the averaged results.It is concluded that the axis deviation and the general pattern of the electrocardiogram are not greatly modified by the appearance of bundle branch block, and that the axis deviation associated with right and left bundle branch block is due principally to the position of the electrical axis of QRS before the bundle branch block appeared.Certain implications arising from this conclusion have been briefly discussed.     

Electrocardiographic criteria of left ventricular hypertrophy in left bundle-branch block.

In order to determine whether the electrocardiographic criteria of left ventricular hypertrophy apply in the presence of left bundle-branch block we studied 79 cases of intermittent left bundle-branch block and compared the QRS voltage and axis before and after its onset. Cases of incomplete left bundle-branch block were excluded. There was a statistically significant correlation between pre- and post-left bundle-branch block values of R or S wave voltage in leads I, V1, V2, V5, and V6, the Sokolow index (R V5 or V6 + S V1), and the QRS axis. There was a statistically significant reduction in R wave voltage in leads I, V5, and V6, an increase in S wave voltage in V1 and V2, and leftward shift of QRS axis, but the Sokolow index remained unchanged, after the onset of left bundle-branch block. The Sokolow criteria for left ventricular hypertrophy apply satisfactorily even in the presence of left bundle-branch block, though specificity is low, but QRS axis is unhelpful.     

Electrocardiographic criteria of left ventricular hypertrophy in left bundle-branch block.

In order to determine whether the electrocardiographic criteria of left ventricular hypertrophy apply in the presence of left bundle-branch block we studied 79 cases of intermittent left bundle-branch block and compared the QRS voltage and axis before and after its onset. Cases of incomplete left bundle-branch block were excluded. There was a statistically significant correlation between pre- and post-left bundle-branch block values of R or S wave voltage in leads I, V1, V2, V5, and V6, the Sokolow index (R V5 or V6 + S V1), and the QRS axis. There was a statistically significant reduction in R wave voltage in leads I, V5, and V6, an increase in S wave voltage in V1 and V2, and leftward shift of QRS axis, but the Sokolow index remained unchanged, after the onset of left bundle-branch block. The Sokolow criteria for left ventricular hypertrophy apply satisfactorily even in the presence of left bundle-branch block, though specificity is low, but QRS axis is unhelpful.     

Atrioventricular and intraventricular conduction in hyperkalemia

Twelve patients exhibited electrocardiographic evidence of fascicular block during hyperkalemia. Isolated left posterior hemiblock occurred in four, isolated left anterior hemiblock in two, right bundle branch block with left anterior hemiblock in two, right bundle branch block with left posterior hemiblock in one, left bundle branch block with abnormal left axis deviation in two and advanced atrioventricular block in one. In all seven patients with sinus rhythm the P-R interval shortened after correction of hyperkalemia. Electrophysiologic studies using His bundle recording and atrial pacing in one patient revealed intraatrial conduction delay and marked prolongation of conduction time in the His-Purkinje system. It is concluded that conduction defects in the specialized intraventricular conduction system are common in hyperkalemia and result in electrocardiographic patterns of fascicular block.     

Lidocaine and His bundle extrasystoles. His bundle discharge conducted with functional right of left bundle-branch block, or blocked entirely (concealed).

In this patient, discharge from the bundle of His either conducted normally, conducted with functional right or functional left bundle-branch block, or blocked entirely (concealed), depending on the preceding cycle length and the coupling interval of the premature His bundle depolarization. The presence of both functional right and left bundle-branch block may have been attributable to differences in effective and functional refractory periods between the two bundle branches. Concealed His bundle extrasystoles mimicked first-degree, and types I and II second-degree AV block, according to the interval between His bundle discharge and the subsequent P wave. Lidocaine eliminated His bundle extrasystoles that blocked entirely (concealed) or conducted with functional left bundle-branch block by improving His-Purkinje conduction and by lengthening the coupling interval of the premature His bundle extrasystole. Lidocaine had no effect on AV nodal conduction time. This patient has been known to have concealed His bundle discharge for at least three years and has not required permanent pacemaker insertion.     

A computer heart model incorporating anisotropic propagation: II. Simulations of conduction block

This study describes the simulation of the more common types of conduction blocks with a computer model of the heart incorporating anisotropic propagation. The rationale was to test the model as to its ability to simulate these blocks by physiologically justifiable adjustments of the conduction system alone. The complete blocks were generated by simply blocking conduction totally at selected sites in the proximal conduction system, and the incomplete blocks by slowing down the conduction velocity in the proximal system. Also simulated were the left fascicular blocks and the bilateral blocks. All simulated electrocardiograms, vectorcardiograms, body surface potential maps, and epicardial isochrones for these blocks were similar to clinically observed data, with the exception of the left posterior hemiblock, which was slightly atypical. This could be because such blocks are usually accompanied by other cardiac pathologies not included in our simulations. The model also supports van Dam's observation that during left bundle branch block the passage of activation from right to left occurs via slow myocardial activation with no evidence of a local delay due to a septal barrier. Finally, the model suggests that a left bundle branch block with a normal frontal plane QRS axis may simply represent a case of an incomplete left bundle block, whereas one that exhibits a left axis QRS deviation in the frontal plane represents a more severe complete left bundle branch block.     

Wolff-Parkinson-White syndrome type B with tachycardia-dependent (phase 3) block in the accessory pathway and in left bundle-branch coexisting with rate-unrelated right bundle-branch block.

A patient with Wolff-Parkinson-White syndrome type B developed 2:1 atrioventricular block resulting from the association of persistent right bundle-branch block with tachycardia-dependent (phase 3) left bundle-branch block. Electrophysiological studies disclosed the coexistence of a tachycardia-dependent (phase 3) block in the accessory pathway. This conduction disturbance was exposed, not by carotid sinus massage as in previous studies, but by pacing-induced prolongation of the interval between two consecutively conducted atrial impulses. Furthermore, the surface electrocardiogram showed, at different times, ventricular complexes resulting from: (1) exclusive atrioventricular conduction through the normal pathway without bundle-branch block; (2) predominant, or exclusive, atrioventricular conduction through a right-sided accessory pathway; (3) exclusive atrioventricular conduction through the normal pathway with right bundle-branch block; (4) exclusive conduction through the normal pathway, with left bundle-branch block; (5) fusion between (1) and (2); and finally, (6) fusion between (2) and (3) However, QRS complexes resulting from simultaneously occurring Wolff-Parkinson-White syndrome type B and left bundle-branch block could not be identified. Future electrophysiological investigations should re-evaluate the criteria used to diffrentiate between true and false patterns of Wolff-Parkinson-White syndrome type B coexisting with left bundle-branch block.     

Coexistence of left branch block and ectopic rhythm simulating alternating block

During the acute phase of diaphragmatic myocardial infarction with septal extension, the ECG of a patient with a chronic left bundle branch block changed in a period of seconds from complete left bundle branch block to incomplete right bundle branch block then to narrow QRS complexes followed by incomplete and then complete left bundle branch block: the same QRS changes then occurred in reverse order; the atrial rhythm was absolutely stable during the recording. These appearances are explained by fusion of sinus and of an ectopic rhythm arising distal to the zone of block, the rate of which (sometimes faster and sometimes slower than the sinus rhythm) could have been influenced by an electrotonic effect after retrograde activation of the right bundle and concealed conduction in the left bundle. Appearances of bundle branch block may be recorded when the ventricle is partially activated from the point of breakthrough of the blocked branch.     

The delay in the onset of ejection of the left ventricle in bundle-branch block

The Q-E interval, from the beginning of the QRS complex to the onset of ejection of the left ventricle as given by the rise of the subclavian arterial pulse, was determined in 9 normal subjects, in a control group of 30 patients with heart disease without intraventricular block (QRS duration 0.10 second or less), and in 29 patients with intraventricular block of various sorts as shown by a QRS duration of 0.12 second or longer. In the control groups the Q-E interval varied from 0.08 to 0.16 second (to 0.19 second when cases with auricular fibrillation are included).This wide range in Q-E duration is due to the fact that the Q-E interval measures a number of variable factors besides the relative time of stimulation of the left ventricle.In evaluating the meaning of the Q-E interval in individual instances of intraventricular block, it is inferred from the above that the range between 0.14 and 0.18 second is of no value in determining the presence or absence of “functional” left bundle-branch block. In intraventricular block a Q-E interval less than 0.14 second indicates the probable absence of “functional” left bundle-branch block and therefore suggests “functional” right bundle-branch block. In intraventricular block a Q-E interval longer than 0.18 second suggests the probable presence of “functional” left bundle-branch block but does not exclude the simultaneous presence of “functional” right bundle-branch block.On this basis it was shown that the common bundle-branch type of intraventricular block included instances of “functional” right as well as “functional” left bundle-branch block.It is therefore concluded that these results confirm our previous view that the direction of the QRS complex is determined by other factors besides the block, and it cannot be used to locate the bundle branch involved in intraventricular block. Some other indication such as the Q-E duration must be employed if the location of block is desired. It is preferable, we believe, to describe intraventricular block, for the present at least, as (1) common bundle-branch type, (2) uncommon bundle-branch type, or (3) indeterminate types.