Tachycardia-Dependent Versus Bradycardia-Dependent Intermittent Bundle-Branch Block

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British Heart Journal, I972, 34, I67-I76. Br Heart J: first published as 10.1136/hrt.34.2.167 on 1 February 1972. Downloaded from

Tachycardia-dependent versus bradycardia-dependent intermittent bundle-branch block

Nabil El-Sherif From the Cardiology Department, Faculty of Medicine, Cairo University, Egypt, U.A.R.

Analysis of I6 cases with intermittent bundle-branch block showing rate-dependence revealed the presence of 12 instances with tachycardia-dependent bundle-branch block, 3 with bradycardia- dependent bundle-branch block, and one instance showing both types at two different occasions. Tachycardia-dependent bundle-branch block was explained on the basis of cycle length-recovery time relation and revealed a critical rate for normal intraventricular conduction. However, some overlap in the RR cycles of normal and aberrant beats in the same record was frequently seen, especially in the presence of atrialfibrillation. This was attributed to the sensitivity of the action potential duration of the diseased bundle to the possible cumulative effects of cycle length changes. On the other hand, the demonstration of bundle-branch block at widely varying heart rates in records taken on different occasions probably reflects the influence of slowly acting parameters. Bradycardia-dependent bundle-branch block was diagnosed whenever bundle-branch block appeared on slowing of the heart rate or disappeared on its acceleration, and was best explained on the basis of enhanced phase-4 depolarization of the bundle-branch system. The occasional demonstration of a paradoxical relation to cycle length changes under the influence of certain physiological manoeuvres and/or pharmacological agents was explained by the role of these agents in enhancing or depressing phase-4 depolarization. http://heart.bmj.com/

Transient and intermittent conduction dis- comitant disturbances at the AV conduction turbances are more familiar at the atrioven- system. tricular (AV) junctional tissue as compared Transient bundle-branch block is usually z with the intraventricular conduction system. defined as an intraventricular conduction Though study of the electrophysiological defect that subsequently returns, if only characteristics of the conduction system at temporarily, to normal conduction, while on October 1, 2021 by guest. Protected copyright. the two sites may reveal some variations, yet intermittent bundle-branch block on the these are not at all sufficient to explain the other hand is characterized in the same cardio- observed difference in the relative incidence graphic record of complexes showing bundle- of the two conditions. The consideration of branch block and normally conducted beats at least two factors may be helpful in this con- (Bauer, I964). However, since some cases of cern. Firstly, autonomic control may be bundle-branch block may prove to be rate operating in a good percentage of transient dependent (not considering for the moment and intermittent AV blocks (though some of other intricate factors) and since in many of the organic affections of the region may be these cases no attempt is usually taken to un- totally reversible). On the other hand, the cover probable instances of normal intra- presence or otherwise of autonomic control ventricular conduction (e.g. by inducing a on impulse conduction in the bundle-branch sufficient degree of bradycardia), it will be system and its possible role is still contro- hard sometimes to make a clear-cut distinc- versial. Secondly, some of the disturbances of tion not only between transient and inter- impulse conduction at the bundle-branch mittent bundle-branch block but even be- system may be either difficult to recognize tween permanent and transient blocks. In from conventional cardiographic records or fact, many cases of intermittent block are may be partially or totally obscured by con- usually diagnosed from fortuitously obtained Rezeived 17 March 1971. cardiographic records and are rarely actively I68 Nabil El-Sherif Br Heart J: first published as 10.1136/hrt.34.2.167 on 1 February 1972. Downloaded from

TABLE Pertinent data of (I6) cases of intermittent bundle-branch block

Case Age (yr) Diagnosis Conduction Rhythm Electrocardiogram showing normal No. and sex defect* conduction I 55 F Rheumatic mitral stenosis LBBB Atrial fibrillation; ventricular Left ventricular ischaemic pattern and insufficiency ectopic systoles 2 55 F Diabetes Left anterior Sinus; supraventricular ectopic Normal hemiblock systoles and tachycardia 3 49 F Recent infarction; hyper- LBBB Sinus Extensive anterior injury pattern tension; diabetes 4 I9 F Ostium secondum defect RBBB Sinus Right ventricular hypertrophy ofrsR' pattern 5 55 F Hypertensive coronary LBBB Sinus Left ventricular ischaemic pattern; heart disease ? old anteroseptal infarction 6 26 M Ventricular septal defect; LBBB Sinus; supraventricular ectopic Left ventricular ischaemic pattern subacute bacterial endo- systoles carditis 7 48 F Rheumatic mitral insuffici- LBBB Atrial fibrillation Left ventricular hypertrophy and ency ischaemic pattern 8 58 M Hypertensive heart disease LBBB Atrial fibrillation Left ventricular ischaemic pattern 9 6i M Hypertensive coronary LBBB Sinus; ventricular ectopic systoles Left ventricular ischaemic pattern; heart disease old inferior infarction IO 65 F Recent infarction, diabetes LBBB Sinus; ventricular ectopic systoles Recent extensive anterior infarction II 53 M Hypoxic cor pulmonale, RBBB Sinus; ventricular ectopic systoles Biventricular hypertrophy hypertension I2 57 F Diabetes RBBB Sinus; ventricular ectopic systoles Normal 13 45 F Massive pulmonary RBBB Sinus arrhythmia; supraventricu- Normal embolism lar ectopic systoles I4 52 M Coronary heart disease LBBB Sinus Incomplete LBBB; ? septal infarction I5 49 M Recent infarction LBBB Sinus Acute anterior injury pattern I6 55 M Recent infarction RBBB (i) Sinus+ ventricular, ectopic Anterolateral injury pattern systoles (2) Sinus

* Left or right bundle-branch block. t Obtained two weeks after the first record. http://heart.bmj.com/ searched for. The situation is further compli- cases with bradycardia-dependent block, and I cated by the fact that records showing normal case showing both at different occasions (Table). intraventricular conduction at a 'reasonable' In IO cases multiple records were usually analysed heart rate may be masking a latent degree of and the cases were followed for variable durations block which up to 5 years. In these cases, the role of certain bundle-branch would only be physiological manoeuvres and/or pharmacological uncovered by slight acceleration of the rate agents were repeatedly studied. The remaining 6 or other appropriate measures. cases (Cases 7 to I2) were found during a critical on October 1, 2021 by guest. Protected copyright. Though many reports on both transient and review of 50 consecutive cases of bundle-branch intermittent bundle-branch block have been block. Inthese cases,there were oneormorerecords published and various studies have been con- available for analysis but an effective follow-up ducted to elucidate the various factors con- was usually lacking. In these 50, ectopic systoles trolling impulse conduction in the bundle- were present in I4 and atrial fibrillation in another branch system, yet much controversy and a 6. In 4 records showing ectopic systoles and in 2 deal of still others with atrial fibrillation, evidence of normal good misconception prevail. The intraventricular conduction was seen in the beats following review represents a critical dis- following the compensatory pauses of the ectopic cussion of the subject through the study of i6 systoles or the long RR intervals in atrial fibrilla- cases of intermittent bundle-branch block tion. On the other hand, no single evidence of observed in the past 5 years (from January normal intraventricular conduction was seen in I965 to December I969) at Kasr El-Aini the 30 records showing regular sinus rhythm. Faculty of Medicine of the Cairo University. This limited statistical survey stresses not only the high incidence of intermittent bundle-branch block but more essentially the value of long RR and methods cycles in revealing probable instances of tachy- Subjects cardia-dependent block. In the presence ofectopic Sixteen cases showing rate-dependent intermittent systoles or atrial fibrillation, fortuitously long bundle-branch block were analysed. These include RR intervals are usually seen. In their absence, I2 cases with tachycardia-dependent block, 3 however, other bradycardia measures, like caro- Br Heart J: first published as 10.1136/hrt.34.2.167 on 1 February 1972. Downloaded from Tachycardia-dependent versus bradycardia-dependent intermittent bundle-branch block 169

Physiological manoeuvres and/or pharmacological agent applied Mechanism of bundle-branch block

Carotid sinus compression, amyl nitrite; exercise; atropine, adenosine Tachycardia-dependent triphosphate Tachycardia-dependent Amyl nitrite Tachycardia-dependent Exercise; atropine Tachycardia-dependent Carotid sinus compression Tachycardia-dependent Carotid sinus compression Tachycardia-dependent

Tachycardia-dependent Tachycardia-dependent Tachycardia-dependent Tachycardia-dependent Tachycardia-dependent Tachycardia-dependent Bradycardia-dependent Atropine Bradycardia-dependent Carotid sinus compression, atropine Bradycardia-dependent (I) - (i) Tachycardia-dependent (2) Carotid sinus compression (2) Bradycardia-dependentt http://heart.bmj.com/

tid sinus compression, should be consistently rates induced by amyl nitrite inhalation (record used. E), exercise, or atropine injection, left intraven- Pertinent data of all the cases are summarized in tricular aberrant conduction, was always observed. the Table, while illustrative examples of each The effect of intramuscular injection of ioo mg group will be critically analysed. adenosine triphosphate on the intraventricular conduction was repeatedly studied and was found to lead to normal intraventricular conduction, but Tachycardia-dependent bundle-branch this was always associated with slowing of the on October 1, 2021 by guest. Protected copyright. block heart rate below the critical level. The patient was Case i A 55-year-old woman with rheumatic followed for the next two years and intermittent mitral stenosis and incompetence was first ad- left bundle-branch block could always be seen mitted to hospital in January I965. Her cardio- but the critical rate varied from 52 to 82 beats a graphic record revealed atrial fibrillation and minute in several occasions. There was, however, 'intermittent left bundle-branch block (Fig. i). no specific pattern for the change in the critical The beats with normal intraventricular conduc- rate (in particular, no progressive decrease of the tion showed left ventricular ischaemic pattern. critical rate was observed). Records A to D in the figure are consecutive but not continuous. Analysis of these records reveals Comment This is a simple case of tachycardia- that long RR cycles favour normal intraventricu- dependent left bundle-branch block of unusually lar conduction while short cycles end with aber- long duration. It is interesting to observe both rant beats. There is a significant overlap of the the significant overlap of the critical RR cycles 'critical RR cycles for both types of conduction for both types of conduction in the same record between 77 and 921 (heart rate of 65-78 beats a and the wide variation of the critical rate for nor- minute). However, slowing of the heart rate below mal intraventricular conduction in records taken the critical level induced by carotid sinus com- on different occasions. pression (record D) consistently resulted in normal intraventricular conduction, while at higher Case 2 A 55-year-old diabetic woman was ad- 1 All intervals are expressed in hundredths of a second. mitted to hospital in September I966 with a Br Heart J: first published as 10.1136/hrt.34.2.167 on 1 February 1972. Downloaded from 170 Nabil El-Sherif

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C t 97 j 120 108 j 128 128 134 135 127 112 1781 4 E) V6 AAA FIG. I Case I, tachycardia-dependent bundle-branch block, record (D) shows the effect of carotid sinus compression (CSC) while record (E) shows the influence of amyl nitrite inhalation. X represents ventricular ectopic systoles. recent history of recurrent attacks of palpitation. rate of IOO beats a minute with normal intraven- The record in Fig. 2 was obtained at one sitting tricular conduction (a QRS pattern with a dura- and was originally divided into two separate tion of o-o8 sec). There are multiple supraventri- sheets that were remounted together with no loss cular ectopic systoles showing aberrant intra- of substance. Lead II shows sinus rhythm at a ventricular conduction (an rS pattern with a duration of o-Ii sec). This pattern represents a http://heart.bmj.com/ left anterior hemiblock (Rosenbaum et al., I969). FIG. 2 Case 2, In lead III, the first and third complexes represent tachycardia-dependent left normally conducted sinus beats, while the second anterior hemiblock. complex is an aberrantly conducted supraventricular beat. The fourth to eighth complexes are conducted sinus beats at a rate of 97 to I07 beats a minute showing aberrant intraventricular con- 60 60 74 duction of the same type seen in the ectopic beats 1401 68 1401 7.0 1381 (a left anterior hemiblock). The last two beats in on October 1, 2021 by guest. Protected copyright. lead III start a paroxysm of supraventricular tachycardia and are showing aberrant intraven- 70 60 57 56 62 tricular conduction. In lead Vi, the first, third, 401 1601 1 140136 and fifth complexes are normally conducted sinus Vi beats, the second and fourth complexes represent aberrant conduction, as the last three complexes 4, do. Lead V2 shows sinus rhythm at a rate of 103 | 68 1I40 72 56 57 1581 beats a minute with aberrant intraventricular conduction. Lead V3 shows a paroxysm of supra- V2 ventricular tachycardia at a rate of I36 beats a minute, with aberrant intraventricular conduction. The paroxysm ends before the last beat which reveals normal intraventricular conduction. 585 8 8 58 1 1 581 58 5 Comment This is a simple case of tachycardia- dependent left anterior hemiblock. The case V3 shows the close similarity between tachycardia- dependent bundle-branch block andintraventricu- lar aberrant conduction of both supraventricular ectopic systoles and tachyarrhythmias. This may help to illustrate the fact that aberrant intra- Br Heart J: first published as 10.1136/hrt.34.2.167 on 1 February 1972. Downloaded from Tachycardia-dependent versus bradycardia-dependent intermittent bundle-branch block 171 ventricular conduction of either an ectopic supra- rhythm in the middle of the record are all for- ventricular beat showing a relatively long coup- tuitously preceded by sinus P waves at practically ling interval or a supraventricular tachyarrhyth- constant PR intervals. In records A and B, it is mia with only moderate acceleration of the rate seen that normal intraventricular conduction may betray an underlying latent bundle-branch occurs at a critical RR cycle of 57 (heart rate I05 block. beats a minute). This is best seen during the re- turn of aberrant intraventricular conduction. Case 6 A 26-year-old man known to have a Records C, D, and E were obtained one week ventricular septal defect was admitted in Febru- later. The sinus tachycardia had disappeared but ary I969 with the diagnosis of subacute bacterial the left bundle-branch block was still present at endocarditis. His cardiogram showed sinus tachy- a heart rate of 88 to go beats a minute. Normal cardia (rate 120 beats a minute), grade I AV block, intraventricular conduction could still be easily and a left ventricular ischaemic pattern. The re- induced either by holding the breath after deep cord obtained 3 days later still revealed sinus inspiration (record C), or carotid sinus com- tachycardia (rate II0 beats a minute) and grade I pression (record D), or in the pause after an AV block but showed left bundle-branch block ectopic supraventricular beat (record E). In pattern. An attempt was made to uncover normal records C and D, normal intraventricular con- intraventricular conduction by inducing brady- duction is seen to occur at a critical RR cycle of cardia through carotid sinus compression (Fig. 3, 68 to 70 (heart rate of 85-88 beats a minute). In records A and B). Record A shows left bundle- record D, the escape rhythm, which occurs at a branch block for the first three beats followed by relatively slower rate of 68 beats a minute, ends two beats with normal intraventricular conduction, by an early capture of the ventricle after an RR then an escape rhythm occurs at a rate of 75 beats cycle of 6I that results in aberrant intraventricular a minute (this represents either an idioventricular conduction. The following sinus beats occurring tachycardia or an idiojunctional tachycardia with at longer RR cycles reveal normal intraventricular aberrant intraventricular conduction of classical conduction. The patient was successfully treated right bundle-branch block pattern). The escape for the subacute bacterial endocarditis and his rhythm ends by sinus capture of the ventricle records obtained one month later failed to show with normal intraventricular conduction that any evidence of left bundle-branch block at a continues for the next four beats before the re- heart rate of 130 beats a minute induced by amyl sumption of aberrant intraventricular conduction nitrite inhalation. in the last two beats. Record B reveals essentially similar findings; however it is interesting to ob- Comment This is a classical case of tachycardia- serve that the five beats that represent the escape dependent left bundle-branch block. It is interest- http://heart.bmj.com/ FIG. 3 Case 6, tachycardia-dependent left bundle-branch block. Records A, B, and D show the effect of carotid sinus compression (CSC), while record C reveals the influence of deep inspiration, and record E the effect of a supraventricular ectopic systole (marked X). Note the occurrence of ectopic escape rhythm with right bundle-branch block pattern after carotid sinus compression. (A) I Ss601 62 160 61 80 I 80 80 80 1 79 1 79 1 79 63 159 5J5J56 56 on October 1, 2021 by guest. Protected copyright.

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FI G . 4 Case 13, bradycardia-dependent right bundle-branch block. ing to observe that left bundle-branch block and with evidence of left ventricular failure. His occurred at widely varying heart rates in records cardiogram showed left bundle-branch block with taken on different occasions. small Q waves in the left surface leads suggestive of septal infarction. The patient was repeatedly Bradyeardia-dependent bundle-branch seen later on and his cardiograms always showed block bundle-branch block. During a last admission in May I969, intermittent bundle-branch block was Case 13 A 45-year-old woman with cancer of observed for the first time. There were frequent the body of the uterus was admitted to hospital in changes from QRS complexes showing complete September I966 for a hysterectomy. Her cardio- left bundle-branch block with a duration of O I7 vascular system was clinically free and her pre- sec to complexes showing incomplete left bundle- operative cardiogram was within normal limits. branch block and a duration of o II sec. Fig. 5 Nine days after operation the patient developed shows instances of transition between the two severe dyspnoea with chest oppression and lower- types of intraventricular conduction. Lead I ing of the blood pressure, and was diagnosed as record (A) shows sinus arrhythmia with two beats having a massive pulmonary embolism. The cardiogram taken on the same day showed sinus tachycardia and complete right bundle-branch block. Fig. 4 was recorded three days later and FIG. 5 Case I4, lead I in record (A) shows showed intermittent bundle-branch block. The two beats with left bundle-branch block during two strips of lead Vi show four types of QRS the slow phase of sinus arrhythmia. Record

pattern: (a) rS pattern with a duration of o-o8 sec (B) shows the effect of atropine injection. The http://heart.bmj.com/ denoting normal intraventricular conduction; observation of both types of intraventricular (b) rs pattern with notched s wave and a duration conduction at widely varying heart rate repre- of o o85 sec; (c) rsR' pattern with a duration of sents a bradycardia-dependent block. See text o og sec (both the second and third patterns repre- for details. sent incomplete right bundle-branch block; (d) rsR' pattern with a duration of O I3 sec repre- senting complete right bundle-branch block. With (A) the exception of the first, third, and sixth beats in strip Vi (a), which are ectopic supraventricular on October 1, 2021 by guest. Protected copyright. systoles, all complexes are preceded by identical P waves and constant PR intervals and represent 76 174 172 76 j80 78 68 75 76 conducted sinus beats. Obvious variation in the RR intervals is seen in strip Vi (a), while sinus arrhythmia is seen in strip Vi (b). It is noted that beats with complete bundle-branch block follow V1 m1 the longest RR intervals, and beats with normal intraventricular conduction follow the shortest intervals, while beats with incomplete bundle- branch block occur at intermediate cycle lengths. 76 1 76 1 74 1 76 1 76 1 76 1 75 1 74 A cardiogram taken on the next day revealed normal intraventricular conduction. (B) Comment This is a classical case of bradycardia- I dependent right bundle-branch block. The role of the acute but regressive right ventricular overload 5958 56 58 in the genesis of the intermittent conduction dis- 158 1 1 158 158 1 turbance is remarkable. AVL Case 14 A 52-year-old man was first seen in March I965 with a history ofmyocardial infarction 1 60 1 61 1 61 61 [ 61 56 1 60 | 58 | 59 | 59 | Br Heart J: first published as 10.1136/hrt.34.2.167 on 1 February 1972. Downloaded from T"hycardia-dependentTversus,bradycardia-dependent intermittent bundle-branch block 173

showing complete bundle-branch block occurring which follow the compensatory pause of the during the slow phase of the arrhythmia in the second ectopic systole reveal normal intraven- middle of the strip. Lead Vi on the other hand tricular conduction. The records under (B) were shows regular sinus rhythm with the shift from obtained two weeks later. Strip Vi shows frequent complete to incomplete bundle-branch block spontaneous transition from a right bundle- unaccompanied by change in the heart rate. branch block pattern to beats showing a slight Record (B) shows the effect of intravenous injec- degree of incomplete right bundle-branch block tion of i mg atropine sulphate administered in the (rSr'). The second to fourth strips show the effect same sitting. Atropine has resulted in sinus tachy- of carotid sinus compression. In strip Vi, carotid cardia; still however beats with incomplete sinus compression gives rise to a pause followed bundle-branch block could be observed at RR by a junctional escape beat which still reveals intervals of 56-6I (heart rate of 98-IO7 beats a complete bundle-branch block. This is followed minute). by two junctional escape beats with normal intraventricular conduction before the resumption of Comment There are two interesting observations conducted sinus beats showing slight degree of in the case. Firstly, instances of incomplete left incomplete right bundle-branch block. Lead aVR bundle-branch block were only seen for the first shows nearly similar findings. In lead I on the time afterfour years from the first observation of other hand, immediately after carotid sinus com- complete block. Secondly, the demonstration of pression there is a sinus beat with slight aberrant bundle-branch block during the slow phase of conduction followed by three junctional escape sinus arrhythmia and the observation of both beats all showing complete bundle-branch block types of intraventricular conduction at a widely before the resumption of conducted sinus beats varying heart rate represent bradycardia-depen- with slight aberration at the end of the strip. dent left bundle-branch block. Comment The observation under record (A) Case I6 A 55-year-old man was first seen in where the compensatory pause after ventricular February I967 with a history of recent myo- ectopic systoles allows for functional recovery of cardial infarction. His cardiogram showed com- the diseased bundle, represents a tachycardia- plete right bundle-branch block with ST-T dependent bundle-branch block. On the other changes suggestive of recent anterolateral injury hand, the findings in record (B) obtained two pattern. There were frequent ventricular ectopic weeks later, are more interesting: the persistence systoles that usually occurred in pairs. Record (A) of bundle-branch block in the beat immediately in Fig. 6 was obtained shortly after admission. In after carotid sinus compression though preceded the record, both the conducted sinus beat (strip by an RR interval which is longer than the inter- (Via) and the junctional escape beat (strip Vib) vals during the following sinus beats with normal http://heart.bmj.com/ FIG. 6 Case I6, record (A) shows multiple ventricular ectopic systoles revealing a tachycardia-dependent right bundle-branch block during an acute myocardial infarction. Record (B) obtained two weeks later shows the effect of carotid sinus compression (CSC) which reveals a bradycardia-dependent bundle-branch block. See text for details. (A)

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intraventricular conduction, and the demonstra- constantly decreasing critical heart rate until tion of a slow junctional rhythm with bundle- such a time as the block was no longer revers- branch block, represent a bradycardia-dependent ible (Shearn and Rytand, I953). Much evi- bundle-branch block. It is interesting to show dence however argues against incrimination the operation of two different mechanisms of re- intermittent bundle-branch block in the same of the heart rate as the sole determinant case on two different occasions. sponsible for intermittent bundle-branch block. Thus, though in some records of intermittent bundle-branch block a critical RR Discussion cycle could be seen that varied for no more The occurrence of bundle-branch block when than a few hundredths of a second, yet in a critical heart rate is exceeded is in accord- other records more overlap could usually be ance with known electrophysiological charac- seen. On the other hand, bundle-branch block teristics of the conduction system. Thus an at widely varying heart rates may be observed impulse arriving to the bundle-branches at an in records taken on different occasions or in RR interval shorter than its effective refractory the same record under the influence of a period will be either delayed or blocked. This variety of physiological manoeuvres and is readily explained in terms ofimpulse spread pharmacological agents. These and similar through incompletely repolarized fibres. How- observations entailed the search of other fac- ever, bundle-branch block developing at very tors besides a critical rate that may influence short RR intervals (whether these are ob- impulse conduction in the bundle-branch served to occur once as in the case of an early system. Out of these factors, the effect of vagal coupled supraventricular ectopic systole or impulses and momentary changes in coronary repetitively as in the case of a supraventricular perfusion received much attention. Vagal im- tachyarrhythmia) will be explained by the time pulses were thought to act on the conduction necessary for the physiochemical processes of at the bundle-branch system in a similar recovery (Burch, I962) and will not be con- fashion to its established role on conduction sidered pathological. The refractory periods of at the AV junctional tissue. Vesell and Lowen the specialized conducting system are known (I963) had critically discussed the subject; to shorten with abbreviation of the preceding however, controversy prevails not only in cycle length (Hoffman and Cranefield, I960; relation to clinical observations (Wilson, Linhart, Braunwald, and Ross, I965; Moe, I915; Holzmann, I943; Katz and Pick, I956; Mendez, and Han, i965). Usually during Scherf, Blumenfeld, and Yildiz, I96I) but

tachycardia, the functional refractory period even in experimental and anatomical studies http://heart.bmj.com/ of the bundle-branch system shortens rela- (Mitchell, Brown, and Cookson, I953; Crane- tively more than that of the AV junctional field, Hoffman, and Paes de Carvalho, I959; tissue and bundle of His, permitting after a Eliakim et al., I96I; Truex, I96I). The effect few beats the passage through that bundle- of momentary changes in coronary perfusion branch system again (Moe et al., I965). In is even more controversial. Reduced blood certain instances, however, the impulse con- supply to the bundle-branch system was duction may be interfered with (delayed or thought to act through local metabolic and

blocked) at the AV junctional-bundle of His physico-chemical changes altering the trans- on October 1, 2021 by guest. Protected copyright. level before reaching the bundle-branch sys- membrane ionic gradient, with secondary tem, which simply means a longer effective prolongation of the refractory period (Vesell refractory period at the former regions. This and Lowen, I963). will help us to remember that disturbances of The presence of slight overlap of the RR conduction at the AV junctional-bundle of cycles of normal and aberrant beats in records His level may completely mask significant showing tachycardia-dependent bundle- disease at the bundle-branch system. On the branch block does not argue against the pre- other hand, bundle-branch block observed to sence of a cycle length-recovery time relation. occur at relatively long RR intervals is patho- This observation, which is more commonly logical and denotes abnormal prolongation of seen in the presence of atrial fibrillation (Fig. the effective refractory period of the bundle- I), may reflect, in fact, the sensitivity of the branch system. action potential duration of the diseased The demonstration of a cycle length- bundle to the possible cumulative effects of recovery time relation stimulated various cycle length changes. Alteration in cycle authors to formulate the concept of critical length results in altered cell action potential. heart rate in intermittent bundle-branch It has been found that when the heart rate block. Some even suggested that the change suddenly changes, it may take several beats from intermittent to permanent bundle- before a constant action potential is achieved branch block might be effected through a for that rate (Hoffman and Cranefield, I960). Br Heart J: first published as 10.1136/hrt.34.2.167 on 1 February 1972. Downloaded from Tachycardia-dependent versus bradycardia-dependent intermittent bundle-branch block 1175

In contrast to the slight overlap of the critical the exposure of occult diastolic depolariza- RR cycle in the same record, the demonstration, the latter leading to impaired conduction tion of bundle-branch block at widely varying in the corresponding bundle-branch. The heart rates in records taken on different occa- occurrence of enhanced phase-4 depolariza- sions probably reflects the influence of slowly tion and bradycardia-dependent bundle- acting parameters, such as progressive or re- branch block in the case two weeks after gressive ischaemic changes, that may alter the records showing tachycardia-dependent block action potential duration of the diseased may be related to variations in the blood sup- bundle-branch system. ply of the diseased bundle. On the other hand, On the other hand, it is interesting to ob- atropine used in Case I4 had probably 'over- serve that most of the reports that invoked driven' the slow diastolic depolarization vagal influence or momentary changes in present and in consequence bundle-branch coronary perfusion especially when certain block appearing at a slow rate was replaced by physiological manoeuvres or pharmacological a narrow QRS complex when the rate was agents were applied, had shown either bundle- speeded by this drug. branch block occurring at relative slowing of In spite of the previous considerations, the heart rate or alternatively normal intraven- bradycardia-dependent bundle-branch block tricular conduction at relative acceleration of is still the subject of a good deal of miscon- the rate. This situation, termed bradycardia- ception. This has been recently outlined by dependent bundle-branch block (signifying Massumi (I968) who has only accepted 4 'slower' rather than 'slow' heart rate), finds cases from the earlier published reports a more ready explanation through the concept (Dressler, I959; Wallace and Lazlo, I96I; of enhanced phase-4 depolarization. It is Bauer, Julian, and Valentine, I956) to which obvious that in the interpretation of records he added another 4 cases of his own. However, showing bradycardia-dependent bundle- 3 of his cases (Cases 2 tO 4) represented equi- branch block, the abnormality cannot be ex- vocal examples. It is interesting to observe plained on the basis of impulse spread that in his second case the diagnosis of brady- through incompletely repolarized fibres as in cardia-dependent block was made on the the case of tachycardia-dependent bundle- assumption that the occurrence of six con- branch block. It has been shown however secutive escape beats, fortuitously preceded that a 'slower' ventricular rate may facilitate by P waves, was highly improbable; in addi- phase-4 depolarization of automatic cells. tion, the QRS configuration was that of typi- Sufficient reduction of the diastolic membrane cal right bundle-branch block. Both situations http://heart.bmj.com/ potential will cause reduction in amplitude are however illustrated in Fig. 3 record (B) and dv/dt of action potentials initiated in in the presence of an unequivocal escape automatic cells and alteration in conduction, rhythm. This stresses the value of the analysis ranging from simple slowing to decrimental of long strips for correct interpretation of unidirectional and bidirectional block (Singer, cardiac arrhythmias. As a matter of fact, if the Lazzara, and Hoffman, I967). The abnor- escape rhythm in our case and in the second malities are comparable to those that occur if case of Massumi were of junctional origin, the the impulse is conducted during repolariza- aberrant intraventricular conduction will on October 1, 2021 by guest. Protected copyright. tion at corresponding level of membrane po- represent a form of bradycardia-dependent tential (Kao and Hoffman, 1958; Van Dam, bundle-branch block involving the right Moore, and Hoffmnan, I963). Agents and bundle-branch system. The aberration of means that enhance or suppress phase-4 de- junctional escape beats can be generally ex- polarization in the human heart can conse- plained on the same lines as bradycardia- quently induce or improve conduction abnor- dependent block (Sarachek, I970). In our malities caused by this mechanism (Singer et case, the left bundle will also reveal a tachy- al., I967). Thus, some of the physiological cardia-dependent block. manoeuvres and pharmacological agents that were suggested to act through their influence on a presumed vagal control on the bundle- References branch system or through momentary changes in coronary perfusion can be more reasonably Bauer, G. E. (I964). Transient bundle-branch block. Circulation, 29, 730. explained through their effect on phase-4 Bauer, G. E., Julian, D. G., and Valentine, P. A. depolarization. On these grounds, the bundle- (I965). Bundle-branch block in acute myocardial branch block appearing after carotid sinus infarction. British Heart3journal, 27, 724. compression in Case i6 (Fig. 6) can be Burch, G. E. (I962). Relationship of heart rate to cardiac output, work, power, and tension in man. explained by vagal-induced AV slowing with journal of the American Medical Association, I82, junctional escape rhythm which permitted 339. 176 Nabil El-Sherif Br Heart J: first published as 10.1136/hrt.34.2.167 on 1 February 1972. Downloaded from

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(I969). Five cases of intermittent left anterior hemi- Cardiology Department, Faculty of Medicine, http://heart.bmj.com/ block. American Journal of Cardiology, 24, I. Cairo University, Cairo, Egypt, U.A.R. on October 1, 2021 by guest. Protected copyright.