THE NORMAL ANATOMY OF THE CONDUCTION SYSTEM IN MAN AND ITS PATHOLOGY IN ATRIOVENTRICULAR BLOCK*

Disruption of the endothelial glycocalyx layer is associated with idiopathic complete atrioventricular block in the elderly population: An observational pilot study

Idiopathic atrioventricular block (iCAVB) is the most common reason for the need for a permanent pacemaker in the elderly population. The fibrotic process that occurs in the conduction system of the heart with aging is the main pathogenesis in the development of iCAVB. However, the processes that trigger the development of iCAVB in the elderly population have not been fully elucidated. In this study, we aimed to reveal the possible relationship between the endothelial glycocalyx (EG) layer and idiopathic complete atrioventricular block. A group of 68 consecutive patients who developed iCAVB and a group of 68 healthy subjects matched for age, sex, and cardiovascular risk factors were included in the study. The groups were compared for clinical, laboratory, and levels of Syndecan-1 (SDC1), an EG layer marker. In the study, SDC1 levels were found to be significantly higher in the iCAVB group compared to the control group (23.7 ± 7.5 vs 16.7 ± 5.2; p = 0.009). In multivariable regression analysis, SDC1 was determined as an independent potential predictor for iCAVB (OR: 1.200; 95% CI: 1.119-1.287; p < 0.001). In the receiver operating characteristic curve analysis, SDC1 predicted iCAVB with 74% sensitivity and 72% specificity at the best cut-off value of 18.5 ng/mL (area under the curve: 0.777; confidence interval: 0.698-0.856; p < 0.001). Disruption of the endothelial glycolic layer may be one of the main triggering factors for the process leading to iCAVB.

Fascicular Blocks: Update 2019

Many advances in the knowledge of medical science are due to the observation of an unknown phenomenon that remains an open question. A plausible hypothesis must be demonstrated and proved through a scientific method in order to be accepted by the scientific community and the same results must be reached by following either the same or different techniques. The original case described by Rosenbaum MB et al., in this review triggered a series of anatomic and physiologic investigations with clinical and experimental observations that supported the trifascicular nature of the intraventricular conduction system of the heart and the concept of hemiblocks. The recognition and description of the left fascicular blocks made by the Argentinian School of Electrocardiology bridged an important gap in electrocardiography and many electrocardiograms that could not be explained until that moment could finally be understood. This review intends to redefine reliable criteria for the electrocardiographic and vectorcardiographic diagnosis of left fascicular blocks [hemiblocks]. The anatomy of the left bundle branch is also discussed to better understand the incidence, prevalence, clinical significance and main causes of left anterior and left posterior hemiblock either isolated or associated with right bundle branch block. This review offers the reader a reappraisal of the trifascicular nature of the intraventricular conduction system regarding the anatomy of the left bundle branch system and its pathophysiological and clinical significance.

Toward detection of conduction tissue during cardiac surgery: Light at the end of the tunnel?

Postoperative conduction block requiring lifetime pacemaker placement continues to be a considerable source of morbidity for patients undergoing repair of congenital heart defects. Damage to the cardiac conduction system (CCS) during surgical procedures is thought to be a major cause of conduction block. Intraoperative identification and avoidance of the CCS is thus a key strategy to improve surgical outcomes. A number of approaches have been developed to avoid conduction tissue damage and mitigate morbidity. Here we review the historical and contemporary approaches for identification of conduction tissue during cardiac surgery. The established approach for intraoperative identification is based on anatomic landmarks established in extensive histologic studies of normal and diseased heart. We focus on landmarks to identify the sinus and atrioventricular nodes during cardiac surgery. We also review technologies explored for intraoperative tissue identification, including electrical impedance measurements and electrocardiography. We describe new optical approaches, in particular, and optical spectroscopy and fiberoptic confocal microscopy (FCM) for identification of CCS regions and working myocardium during surgery. As a template for translation of future technology developments, we describe research and regulatory pathways to translate FCM for cardiac surgery. We suggest that along with more robust approaches to surgeon training, including awareness of fundamental anatomic studies, optical approaches such as FCM show promise in aiding surgeons with repairs of heart defects. In particular, for complex defects, these approaches can complement landmark-based identification of conduction tissue and thus help to avoid injury to the CCS due to surgical procedures.

Left posterior fascicular block, state-of-the-art review: A 2018 update

We conducted a review of the literature regarding epidemiology, clinical, electrocardiographic and vectorcardiographic aspects, classification, and differential diagnosis of left posterior fascicular block.

Isolated left posterior fascicular block (LPFB) is an extremely rare finding both in the general population and in specific patient groups. In isolated LPFB 20% of the vectorcardiographic (VCG) QRS loop is located in the right inferior quadrant and when associated with right bundle branch block (RBBB) ≥40%.

The diagnosis of LPFB should always consider the clinical aspects, because a definite diagnosis cannot be made in the presence of right ventricular hypertrophy (RVH) (chronic obstructive pulmonary disease (COPD)/emphysema), extensive lateral myocardial infarction (MI) or extremely vertical heart.

Intermittent LPFBs are never complete blocks (transient or second degree LPFB) and even in the permanent ones, one cannot be sure that they are complete. When LPFB is associated with RBBB and acute inferior MI, PR interval prolongation is very frequent.

About the specialized myocardial conducting tissue

The chronological succession of discoveries on the location and structure of the atrio-ventricular conducting system are described. The starting point of this system is located in the sinus atrial node, identified by the English scientists A. Keith and M. W. Flack in 1907. The atrioventricular conducting system was pointed out by the Swiss physician Wilhelm His Jr. in 1893. The atrioventricular node (AV) was first identified by the Japanese pathologist Sumao Tawara and his German professor Ludwig Aschoff in 1906. Likewise the structure and routes of the three internodal bundles are described. These bundles include: Bachmann's bundle (1916) connecting the right with the left atrium and the AV node; the middle Wenckebach's bundle (1910) and the posterior or Thörel's bundle (1910), extending from the region of the sinus atrial node towards the posterior margin of the AV node. Lastly, the ventricular left and right conduction systems are detailed. These include the main trunk and their peripheral subdivisions with respective networks. Regarding the controversial existence of the left middle subdivision, it can exist in animal and human hearts. Nevertheless, an intermediate left septal network of specialized fibers seems to act as a functional equivalent of this subdivision.

Implications of left bundle branch block in patient treatment

Left bundle branch block (LBBB) causes an abnormal pattern of cardiac activation and affects regional myocardial function. Although recognition of LBBB on the surface electrocardiogram is straightforward, dissecting its effect on patient treatment and outcome can be more challenging. The altered pattern of cardiac activation in LBBB causes electrical and mechanical ventricular dyssynchrony, influences ischemia detection on the surface electrocardiogram, and affects stress testing and imaging modalities dependent on wall motion and thickening. Restoration of synchrony by biventricular pacing can improve symptoms and longevity in carefully selected patients. The diagnostic, prognostic, and therapeutic implications of LBBB across this spectrum are discussed in this review.

The Anatomical Basis of Complete Atrioventricular Block in Cats with Hypertrophic Cardiomyopathy

The cardiac conduction system was examined histologically in 13 feline cases of hypertrophic cardiomyopathy (HCM) with complete atrioventricular (AV) block. Marked degeneration and fibrous replacement of the AV conduction system were consistently observed in the combined regions of the branching portion of the AV bundle and the upper portion of the left bundle branch. These changes were associated with extensive fibrosis of the central fibrous body and endocardial and myocardial fibrosis in the upper border of the ventricular septum. Chondrometaplastic lesions or osseous lesions, or both, present in the base of the central fibrous body, compressed the underlying penetrating or branching (or both) portions of the AV bundle, causing apparent reduction of the conduction fibres. The pathological process and the nature and predilection sites of the lesions resembled those associated with ageing in human patients with complete AV block. It is possible that the pathological process in the cats was fundamentally related to the normal ageing phenomenon and may have been exacerbated by mechanical forces created by the cardiac hypertrophy associated with HCM.

A histological study of the cardiac conduction system in a heifer with complete atrioventricular block

A case of complete atrioventricular (AV) block of congenital origin in a 16-month-old Holstein heifer was studied histologically with serial sectioning of the cardiac conduction system. The heart was enlarged and showed moderate dilatation of the left and right ventricles. Histologically, the abnormally placed and poorly formed AV bundle was observed in association with abnormality in the tricuspid extension of the central fibrous body, suggesting that the pathological state of the AV bundle had been responsible for the complete AV block. This type of anatomical fault in the AV bundle is considered to be part of an embryological, developmental malformation of the central fibrous body.

Cardiac conduction system involvement in sudden death of obese young people

Involvement of the conduction system in the sudden death of obese young people has not been documented in the literature. We therefore studied the conduction system by serial section examination in 7 subjects, 5 obese and 2 mild to moderately obese, who died suddenly at ages 6, 11, 14, 16, 20, 30, and 32 years of age (5 males, 4 black and 1 white; two females, 1 black and 1 white). Three had a history of sleep apnea. The heart was hypertrophied and enlarged in 6; all 6 had a distinct ventricular septal bulge and epicardial coronary arteries were normal. All had focal mononuclear cells in and around the sinoatrial node and/or its approaches, with marked fat throughout the conduction system in 3, fibrosis of the atrioventricular (AV) bundle and/or the left bundle branch in 5, and the branching bundle sandwiched between the bulbar muscle and the summit of the ventricular septum in 5 (2 with left-sided bundle, 1 with loop formation, and 1 with a markedly fragmented bundle). The AV node was partly within the central fibrous body and/or the atrial septum in 6 patients; focal mononuclear cells were present to a varying degree, with focal fibrosis of the ventricular septum in 6 patients, arteriolosclerosis in 4, and myocardial disarray in 3. The mild to moderately obese patients demonstrated lesser amounts of fat with more fibrosis when compared with the markedly obese. In summary, there are significant pathologic findings in the conduction system in the sudden death of obese young people.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterizing His-Purkinje system signals observed at the body surface. A model study

A computer model of the His-Purkinje system (HPS), based on human geometry, has been developed and used in conjunction with an atrial model and an inhomogeneous torso model to calculate electrocardiograms (ECGs) and isofield potential and magnetic maps. The calculated electric potential maximum for penetrating His bundle (HB) activity is 0.6 microV, somewhat smaller than reported by others. A calculated ECG of combined atrial and HPS activity indicates that atrial repolarization contributes to the ramp pattern observed in the PR segment. Simulated isofield distributions indicate two features that should be present in serial-measured HPS maps: (1) maps preceding HB activity should be included to illustrate the nature of the onset of HB activity and (2) maps should show a change in the location and magnitude of the extrema commensurate with the change in direction and magnitude of the source current circumscribed by the anatomy of the HB and bundle branches.

Histopathologic studies of the conduction system in marked left-axis deviation

The histopathologic changes of the left ventricular (LV) conduction system were studied in 11 autopsy cases with marked left-axis deviation (LAD). In all cases the LV conduction system was fanlike and spread broadly over the left septal surface. However, the sizes and sites of the histopathologic lesions varied. In 2 cases the lesions were small and localized at the initial portion of the LV conduction system, whereas in 9 cases the lesions were located more peripherally and were more extensive, especially in 2 cases in which the lesions were mainly localized in the apical third of the LV conduction system. These differences in the sizes of lesions were believed to be due to the anatomic structure of the conduction system. At the initial portion of the LV conduction system, cells were oriented longitudinally with collagen partitions, which presumably resulted in functionally longitudinal dissociation. The variability in the lesions in these patients may explain why prognosis in terms of development of complete heart block is not always poor in patients with LAD and right bundle branch block.

Electrophysiologic cardiac function before and after surgery in children with atrioventricular canal

Thirty-two children with atrioventricular (AV) canal underwent electrophysiologic studies: 18 underwent preoperative studies at a median age of 3 years (range 6 months to 16 years); 14 underwent postoperative studies at a median age of 4 years (range 2 to 19); and 2 underwent both preoperative and postoperative matched studies. In the preoperative group the following abnormalities were observed: first-degree AV block in 5 patients (due to internodal conduction delay in 1, AV nodal conduction delay in 2 and normal intracardiac intervals in 2); internodal conduction delay but normal PR interval in 4; and disease of the sinus node in only 1. In the postoperative group the following abnormalities were observed: first-degree AV block in 9 (due to AV nodal conduction delay in 2, His-Purkinje system conduction delay in 1, upper normal intracardiac intervals in 3 and unidentified in 3); prolongation of the right ventricular apical activation time in 11 of 13 with right bundle branch block; abnormal sinus node function in 3; and abnormal AV nodal function in 4 (1 of whom had associated sinus node disease). Atrial and ventricular functions were normal in all preoperative and postoperative patients. Electrophysiologic dysfunction is rare in preoperative patients with AV canal; in postoperative patients electrophysiologic abnormalities occur in 38% and involve the sinus and AV nodes in 19 and 25%, respectively.

Correlates of prolonged HV conduction in aortic stenosis

Forty-eight patients with predominant aortic stenosis underwent His bundle electrography (HBE) at the time of diagnostic catheterization. Patients were divided into four groups based upon severity of calcification of the aortic valve fluoroscopically as judged independently by three angiographers. Of 48 patients, three had no calcification, 11 had mild, 18 had moderate, and 16 had severe aortic valve calcification. No correlation was found between HV interval and severity of aortic valve calcification. Significant correlation was found between HV interval prolongation and aortic valve area (p less than 0.02), history of congestive heart failure (p less than 0.02), and increasing left ventricular end-diastolic pressure (p less than 0.05). Left ventricular ejection fraction less than 45% had greater likelihood of HV interval prolongation (p less than 0.01). No correlation was established between HV interval and age, aortic valve gradient, left ventricular peak systolic pressure, syncope, and coronary artery disease. Aortic valve area was the most significant independent predictor of HV prolongation, with history of congestive heart failure second. We conclude that HV interval prolongation in aortic stenosis with calcified valves is best predicted by evidence of declining left ventricular function rather than severity of aortic valve calcification.

Left-axis deviation in otherwise healthy children

Eighteen children with left-axis deviation on the electrocardiogram but no other detected abnormalities are reported. The implications of this finding are discussed. No untoward cardiovascular events occurred during the period of follow-up and the electrocardiographic findings remained unchanged. This, therefore, appears to be a benign entity in the short term but longer follow-up is required before the ultimate prognosis can be determined.

Anatomic substrate for congenital atrioventricular block in middle-aged adults

Congenital atrioventricular block is usually a benign disorder not necessitating pacing. In some patients slowing of rate and/or mortality have been noted with aging. However an anatomic substrate has not been established for the progressive slowing of the escape rate. In this study we report an anatomic substrate in two such patients who were dying in congestive heart failure, ages 49 and 42, respectively. Multiple pre-mortem ECG's in both cases revealed wide QRS escape rhythms, and escape rates of approximately 35 and 28 beats/minute, respectively. Conduction system examination by serial section in both cases revealed lack of connection between the atrial septum with the peripheral conduction system with total replacement by fat of the AV nodal approaches and AV node, and advanced sclerosis of the summit of the ventricular septum which was more marked on the right side. In addition, the His bundle showed marked septation in case one and fragmentation in case two. Sclerosis of the summit of the ventricular septum involved the branching bundle and the bundle branches in both cases. In conclusion, both patients had the characteristic lesions of congenital atrioventricular block, namely replacement of the AV node and AV nodal approaches by fat, with lack of connection to the peripheral conduction system, and one also had a fragmented His bundle. In addition premature aging of the summit of the ventricular septum may have reflected the long-standing hemodynamic stresses of chronic bradycardia. This in turn resulted in trifascicular involvement of the conduction system leading to a shifting of the escape rhythm distally eventuating in a slower idioventricular escape rhythm.

Electrophysiologic and histologic correlations in chronic complete atrioventricular block

Electrophysiologic studies using the His bundle electrogram (HBE) and histologic studies of serial sections of the conduction system were correlated in two groups of deceased patients. Group 1 consisted of five patients with chronic complete atrioventricular block (CAVB) who had narrow QRS complexes and AH block (block proximal to the His bundle deflection). Group 2 consisted of four patients with chronic CAVB who had wide QRS complexes and HV block (block distal to the His bundle deflection). In group 1, the sites of the main lesion were not located in the approaches to the atrioventricular (AV) node or the AV node, but were found in the penetrating portion of the His bundle in one patient and in the branching portion of the His bundle in three patients. In the remaining patient, the main site of block could not be demonstrated histologically in the AV conduction system, but marked fibrosis of the approaches to the sinoatrial node and surrounding atrial muscle was found. In all patients of group 2, the site of the main lesion was located in the bilateral bundle branches, and thus was compatible with so-called trifascicular block. This correlation study between the His bundle electrogram and histologic findings of the AV conduction system showed that in some cases, CAVB presenting as AH block on the HBE can be associated with a lesion in the branching portion of the His bundle (distal His), and that CAVB presenting as HV block on the HBE is associated with a bilateral lesion of the bundle branches.

Correlation of the width of the QRS complex with the pathologic anatomy of the cardiac conduction system in patients with chronic complete atrioventricular block

A study correlating the electrocardiographic findings and the histology of the atrioventricular (AV) conduction system was carried out in 14 cases with chronic complete AV block and in 13 cases without chronic complete AV block. Patients with chronic complete AV block were divided into two groups, based on the width of the QRS complex. The QRS complexes were narrow (less than 0.12 second) in four cases (group 1) and wide (greater than or equal to 0.12 second) in 10 cases (group 2). In group 1, the main lesion was located in the penetrating portion of His bundle (Hisp) in one heart, in the branching portion of the His bundle (Hisb) in another and in the combined regions of Hisb and the left bundle branch in two. Three of the four cases in group 1 had idiopathic fibrosis of the conduction system and one had calcific nodules in the central fibrous body. In group 2, the main lesion was located in Hisb in two cases, in the combined regions of Hisb and the right bundle branch in one, in the Hisb and in the bilateral bundle branches in two, and in the bilateral bundle branches in five. All cases in group 2 were of the idiopathic type, except case 5, which had calcific aortic stenosis. In 13 cases without chronic complete AV block the AV conduction system was histologically normal, except for slight-to-moderate aging changes in the His bundle or the bundle branches. Lesions of the Hisb, which is believed to be the "distal His" electrophysiologically, may induce complete AV block with narrow or wide QRS complexes, depending upon the severity of the lesions in Hisb or adjacent bundle branches.

Initial ventricular activation in left-sided intraventricular conduction defects

Nine patients with ECG evidence of rate-related left bundle branch block (LBBB) were studied using His bundle electrograms, electrograms from the right ventricular (RV) apex and vectorcardiograms recorded as heart rate was increased to produce LBBB. In five patients, when LBBB occurred, initially normal septal activation reversed and the HV intervals increased 10-30 msec, while the H-RVA interval did not change (group 1). Four patients had initially normal QRS duration (90-100 msec) but reversed septal activation (group 2). When LBBB developed there was no shift in either the HV interval or H-RVA interval. Only the QRS complex itself widened to distinguish these patients from group 1. These studies defined ventricular septal activation in normal conduction and in complete LBBB (group 1) and incomplete LBBB (group 2). The conduction patterns of group 1 and 2 patients are similar but are both markedly different from normal.

Atrial fibrillation resulting from cardiac trauma

Reports in the literature concerning isolated traumatic lesions of the conductive system of the heart following blunt injury to the thorax are rare. A 69-year-old woman who was a front-seat passenger developed disturbances of conduction with atrial fibrillation after blunt injury to the thorax. She died 7 days later. Autopsy and subsequent histological investigation of the conductive system of the heart showed isolated contusion in the right atrium close to the atrio-ventricular node.

Anatomical configuration of the His bundle and bundle branches in the human heart

The relationships among the His bundle, the origin of both bundle branches, and the interventricular (IV) septum were examined histologically in 32 human hearts, and the entire bundle branch systems were delineated in 13 of these. The His bundle in five hearts traversed the right IV septal crest, and the LBB origin was a very narrow stem (maximum 1.5 mm in cross-section) crossing from right to left through the inferior margin of the membranous septum. Proximal LBB anatomy was extremely variable, demonstrating multiple fiber groups which fanned out over the entire left septal surface. The LBB did not divide into two discrete divisions without multiple interconnections. The RBB formed an obtuse angle with the His bundle in 27 of 32 hearts. In those five hearts with "right-sided His bundles," the right bundle branch was a direct continuation. The clinical, electrophysiologic, and electrocardiographic implications of these anatomical observations are discussed.

Electrophysiologic and pathologic correlations in two cases of chronic second degree atrioventricular block with left bundle branch block

This study concerns two cases of chronic 2 degrees atrioventricular (A-V) block with left bundle branch block (LBBB). Pathological studies included serial section of the conduction systems. Case 1 had type I 2 degrees block with LBBB. Electrophysiological studies revealed type I 2 degrees block proximal to the His bundle recording site and a prolonged H-V interval (60 msec). Pathologically there was a moderate to marked fibrosis of the approaches to the A-V node and of the A-V node, marked fibrosis of the left bundle branch, and moderate involvement of the right bundle branch. The changes proximal to the His bundle were more marked than the changes distal to this bundle. Case 2 had type II and 2:1 2 degrees A-V block with LBBB. Electrophysiologically the site of block was distal to the His bundle recording site, and there was a prolonged A-V node and severe involvement of both bundle branches. The changes distal to the His bundle were more severe than the changes proximal to the His bundle. This study reveals that the electrophysiologic data more closely approximated the pathologic findings than did surface electrocardiographic data alone. It also emphasizes that there may be multiple sites of disease in chronic 2 degrees block with bundle branch block.

The anatomic substrate of complete left bundle branch block

The conduction systems of eight cases diagnosed clinically as having complete left bundle branch block (LBBB) were serially sectioned. Left axis deviation was present in six and normal axis in two. In all cases there was evidence of ischemia of the myocardium and left bundle branch (LBB) in various parts of the coronary circulation. In all cases, the LBB showed pathologic change at its junction point with the bundle of His, with disruption, complete or incomplete, in six, and with recent and old lesions in two. There was no difference in lesions of the LBB between cases with and without left axis deviation. There is thus complete correlation between the electrocardiographic abnormality LBBB and lesions of the LBB in these cases. The pathogenesis of the LBB lesions in these cases is probably both ischemic and mechanical.

Cardiac arrhythmias. 5. Disturbances in cardiac conduction and their management

A simple classification of atrioventricular block, criteria for diagnosis, and suggested therapy are discussed. The differences between Mobitz I and II second-degree A-V block are reviewed.

Good management of patients who have bundle-branch or other forms of intraventricular block depends on informed judgment which includes consideration of (1) the effect of intraventricular conduction disturbances on the pumping action of the heart, and (2) the likelihood that a specific intraventricular conduction disturbance will be complicated by complete heart block.

Evidence suggests that common forms of intraventricular disturbance, unlike ventricular fibrillation, do not alter significantly the pumping action of the heart. The role of intraventricular conduction disturbances in the genesis of ventricular fibrillation is assessed.

We concluded that only when right bundle-branch block is combined with block of the anterior or posterior fascicle of the left bundle branch is complete heart block sufficiently imminent to warrant special concern. If bilateral block is associated with symptoms of episodic severe bradycardia, pacemaker therapy is indicated.

Atrioventricular conduction defects in patients presenting with syncope and normal PR interval

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Sites of congenital and surgical heart block as defined by His bundle electrocardiography

Catheter recordings of His bundle electrograms were obtained in seven patients with congenital heart block (CHB) and in two with surgical heart block (SHB). In the latter two patients block occurred following total correction of tetralogy of Fallot. In six patients with CHB block occurred proximal to H. In one CHB patient block occurred in the His bundle with "split" H potentials. Intraventricular conduction was normal in five of the patients with CHB who had narrow QRS and H-V intervals ranging from 35 to 45 msec. H-V intervals were short in two CHB patients (25 and 30 msec), one of whom had QRS widening with initial slowing. In the latter patient a functioning infranodal bypass (Mahaim tract) inserting into the right ventricular septum could explain the findings. In the two patients with SHB block was distal to H with P-H intervals of 125 msec and 160 msec, respectively.

The degree of bradycardia and the occurrence of symptoms partially correlated with a location of block in or distal to the His bundle. It is suggested that recording of H potentials is useful in the evaluation of children with complete atrioventricular block.

Association of right bundle-branch block with left superior or inferior intraventricular block. Its relation to complete heart block and Adams-Stokes syndrome

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