THE AETIOLOGY OF COMPLETE HEART-BLOCK

Perioperative management of patients with cardiac implantable electronic devices

The use of cardiac implantable electronic devices (CIEDs) has increased significantly in recent years. Consequently, more patients with CIEDs will undergo surgery during their lifetime, and thus the involvement of anesthesiologists in the perioperative management of CIEDs is increasing. With ongoing advancements in technology, many types of CIEDs have been developed, including permanent pacemakers, leadless pacemakers, implantable cardioverter defibrillators, cardiac resynchronization therapy-pacemakers/defibrillators, and implantable loop recorders. The functioning of CIEDs exposed to an electromagnetic field can be affected by electromagnetic interference, potential sources of which can be found in the operating room. Thus, to prevent potential adverse events caused by electromagnetic interference in the operating room, anesthesiologists must have knowledge of CIEDs and be able to identify each type. This review focuses on the perioperative management of patients with CIEDs, including indications for CIED implantation to determine the baseline cardiovascular status of patients; concerns associated with CIEDs before and during surgery; perioperative management of CIEDs, including magnet application and device reprogramming; and additional perioperative provisions for patients with CIEDs. As issues such as variations in programming capabilities and responses to magnet application according to device can be challenging, this review provides essential information for the safe perioperative management of patients with CIEDs.

High prevalence and distinctive clinical features of LMNA-associated atrioventricular block in young patients

BACKGROUND AND AIMS

Atrioventricular block (AVB) is a degenerative disease and more commonly encountered in elderly patients, but unusual and often of unknown etiology in young patients. This study aimed to investigate the potential contributions of genetic variations to AVB of unknown reasons in young patients.

METHODS

We enrolled 41 patients aged <55 years with high-degree AVB of unknown etiology whose clinical and genetic data were collected.

RESULTS

Genetic variants were identified in 20 (20/41, 48.8%) patients, 11 (11/20, 55%) of whom had LMNA variants including 3 pathogenic (c.961C > T, c.936+1G > T and c.646C > T), 4 likely pathogenic (c.1489-1G > C, c.265C > A, c.1609-2A > G and c.1129C > T) and 3 of uncertain significance (c.1158-3C > G, c.776A > G and c.674G > T). Compared to those without LMNA variants, patients with LMNA variants demonstrated a later age at onset of AVB (41.45 ± 9.89 years vs 32.93 ± 12.07 years, P = .043), had more prevalent family history of cardiac events (81.8% vs 16.7%, P < .000), suffered more frequently atrial (81.8% vs 10.0%, P < .000) and ventricular (72.7% vs 10.0%, P < .000) arrhythmias, and were more significantly associated with enlargement of left atrium (39.91 ± 7.83 mm vs 34.30 ± 7.54 mm, P = .043) and left ventricle (53.27 ± 8.53 mm vs 47.77 ± 6.66 mm, P = .036).

CONCLUSIONS

Our findings provide insights into the genetic etiology of AVB in young patients. LMNA variants are predominant in genotype positive patients and relevant to distinctive phenotypic properties.

Effects of body mass index and blood pressure on atrioventricular block: Two-sample mendelian randomization

BACKGROUND

Observational studies have suggested associations between some atherogenic risk factors and atrioventricular (AV) block.

OBJECTIVE

The purpose of this study was to investigate the causal effects of several cardiometabolic exposures on AV block and evaluate the role of coronary artery disease (CAD) as a mediator on the causal pathway by mendelian randomization analysis.

METHODS

Two-sample bidirectional mendelian randomization was performed to assess the causal effects of cardiometabolic traits on AV block and examine causality inversely. The exposures of interest included body mass index (BMI), systolic blood pressure (SBP), diastolic blood pressure (DBP), fasting glucose, fasting insulin, low-density lipoprotein, high-density lipoprotein, and triglyceride. Multivariable mendelian randomization was then conducted to disentangle the effect of each significant exposure. Mediation effect of CAD on the causal pathways were estimated by two-step, two-sample mendelian randomization.

RESULTS

Genetically predicted elevation of BMI (odds ratio [OR] 1.40; 95% confidence interval [CI] 1.10-1.78; P = .006), SBP (OR 1.02; 95% CI 1.00-1.03; P = .015), and DBP (OR 1.04; 95% CI 1.01-1.07; P = .005) were significantly associated with increased AV block risk. Effects of the other exposures were insignificant. There were no reverse causal effects. Multivariable mendelian randomization showed causal effects of increased BMI, SBP, and DBP on AV block after mutual adjustment. CAD mediated 14.20% (8.82%, 16.46%), 26.32% (25.00%, 26.47,%) and 12.20% (7.69%, 15.94%) of AV block risk from BMI, SBP and DBP, respectively.

CONCLUSION

Elevated BMI, SBP, and DBP exhibited causal effects on AV block. The impacts were partly mediated by CAD.

Muscarinic acetylcholine receptors M2 are upregulated in the atrioventricular nodal tract in horses with a high burden of second-degree atrioventricular block

Background

Second-degree atrioventricular (AV) block at rest is very common in horses. The underlying molecular mechanisms are unexplored, but commonly attributed to high vagal tone.

Aim

To assess whether AV block in horses is due to altered expression of the effectors of vagal signalling in the AV node, with specific emphasis on the muscarinic acetylcholine receptor (M2) and the G protein-gated inwardly rectifying K+ (GIRK4) channel that mediates the cardiac IK,ACh current.

Method

Eighteen horses with a low burden of second-degree AV block (median 8 block per 20 h, IQR: 32 per 20 h) were assigned to the control group, while 17 horses with a high burden of second-degree AV block (median: 408 block per 20 h, IQR: 1,436 per 20 h) were assigned to the AV block group. Radiotelemetry ECG recordings were performed to assess PR interval and incidence of second-degree AV block episodes at baseline and on pharmacological blockade of the autonomic nervous system (ANS). Wenckebach cycle length was measured by intracardiac pacing (n = 16). Furthermore, the expression levels of the M2 receptor and the GIRK4 subunit of the IKACh channel were quantified in biopsies from the right atrium, the AV node and right ventricle using immunohistochemistry and machine learning-based automated segmentation analysis (n = 9 + 9).

Results

The AV block group had a significantly longer PR interval (mean ± SD, 0.40 ± 0.05 s; p < 0.001) and a longer Wenckebach cycle length (mean ± SD, 995 ± 86 ms; p = 0.007) at baseline. After blocking the ANS, all second-degree AV block episodes were abolished, and the difference in PR interval disappered (p = 0.80). The AV block group had significantly higher expression of the M2 receptor (p = 0.02), but not the GIRK4 (p = 0.25) in the AV node compared to the control group. Both M2 and GIRK4 were highly expressed in the AV node and less expressed in the atria and the ventricles.

Conclusion

Here, we demonstrate the involvement of the m2R-IK,ACh pathway in underlying second-degree AV block in horses. The high expression level of the M2 receptor may be responsible for the high burden of second-degree AV blocks seen in some horses.

A Dexamethasone-Loaded Polymeric Electrospun Construct as a Tubular Cardiovascular Implant

Cardiovascular tissue engineering is providing many solutions to cardiovascular diseases. The complex disease demands necessitating tissue-engineered constructs with enhanced functionality. In this study, we are presenting the production of a dexamethasone (DEX)-loaded electrospun tubular polymeric poly(l-lactide) (PLA) or poly(d,l-lactide-co-glycolide) (PLGA) construct which contains iPSC-CMs (induced pluripotent stem cell cardiomyocytes), HUVSMCs (human umbilical vein smooth muscle cells), and HUVECs (human umbilical vein endothelial cells) embedded in fibrin gel. The electrospun tube diameter was calculated, as well as the DEX release for 50 days for 2 different DEX concentrations. Furthermore, we investigated the influence of the polymer composition and concentration on the function of the fibrin gels by imaging and quantification of CD31, alpha-smooth muscle actin (αSMA), collagen I (col I), sarcomeric alpha actinin (SAA), and Connexin 43 (Cx43). We evaluated the cytotoxicity and cell proliferation of HUVECs and HUVSMCs cultivated in PLA and PLGA polymeric sheets. The immunohistochemistry results showed efficient iPSC-CM marker expression, while the HUVEC toxicity was higher than the respective HUVSMC value. In total, our study emphasizes the combination of fibrin gel and electrospinning in a functionalized construct, which includes three cell types and provides useful insights of the DEX release and cytotoxicity in a tissue engineering perspective.

Strong independent association between third-degree AV-block and diabetes mellitus using a large database

BACKGROUND

Recent data suggests an association between DM and third-degree AV- Block. The goal of this study was to evaluate the independent association between diabetes and third-degree AV-Block using a very large database.

METHOD

We used ICD-9 Codes for DM and third-degree AV-block from the Nationwide Inpatient Sample (NIS) database. We randomly selected the 1992 and 2002 databases which are 10 years apart as two independent samples. We used uni- and multi-variate analysis to evaluate this association.

RESULTS

1992 database contained a total of 6,195,744 patients. Diabetes occurred in (0.5 %) of patients with third-degree AV-block vs. (0.2 %) of the control (OR: 2.15, CI 2.06-2.25, p < 0.0001). 2002 database contained a total of 7,853,982 patients. Diabetes occurred in (0.4 %) of patients with third-degree AV-block vs. (0.2 %) of the control (OR: 1.86, CI: 1.80-1.93, p < 0.0001). Using Multivariate analysis adjusting for age, congestive heart failure, and coronary artery disease, DM remained independently associated with third-degree AV block in both databases. (for 1999: OR: 2.54, CI 2.51-2.57, p < 0.0001 and for 2002 OR: 1.56, CI 1.55-1.57, p < 0.0001).

CONCLUSION

DM is independently associated with third-degree AV-block with persistent association over a period of 10 years. The cause of this association warrants further investigation.

Anatomy and Pathology of the Cardiac Conduction System

The cardiac conduction system is formed of histologically and electrophysiologically distinct specialized tissues uniquely located in the human heart. Understanding the anatomy and pathology of the cardiac conduction system is imperative to an interventional electrophysiologist to perform safe ablation and device therapy for the management of cardiac arrhythmias and heart failure. The current review summarizes the normal and developmental anatomy of the cardiac conduction system, its variation in the normal heart and congenital anomalies, and its pathology and discusses important clinical pearls for the proceduralist.

Reversible Causes of Atrioventricular Block

Atrioventricular blocks may be caused by a variety of potentially reversible conditions, such as ischemic heart disease, electrolyte imbalances, medications, and infectious diseases. Such causes must be always ruled out to avoid unnecessary pacemaker implantation. Patient management and reversibility rates depend on the underlying cause. Careful patient history taking, monitoring of vital signs, electrocardiogram, and arterial blood gas analysis are crucial elements of the diagnostic workflow during the acute phase. Atrioventricular block recurrence after the reversal of the underlying cause may pose an indication for pacemaker implantation, because reversible conditions may actually unmask a preexistent conduction disorder.

Association of anti-Ro seropositivity with cardiac rhythm and conduction disturbances

AIMS

Anti-Ro/La autoantibodies are especially prevalent in autoimmune diseases but are also relatively frequent in healthy adults. Their arrhythmogenic effect on the immature cardiac conductive system is well established, with substantial evidence demonstrating an increased risk for congenital atrioventricular block in neonates of seropositive mothers. Despite their wide distribution and their arrhythmogenic potential effect, there are no large population studies conducted in seropositive adults. Thus, this is the first large population-based study to examine the association of anti-Ro/La seropositivity with cardiac rhythm and conduction disturbances.

METHODS AND RESULTS

This cross-sectional designed study involved the electronic health records of the largest health maintenance organization in Israel. All subjects that were tested positive for anti-Ro/anti-La antibodies between the years 2002 and 2019 were included and were matched by age, gender, and place of residence, with controls. Rates of different cardiac rhythm and conduction disturbances were compared between groups. Sensitivity analyses were performed using propensity score matching. The study population included 17 231 anti-Ro/La seropositive subjects and 84 368 controls. Anti-Ro seropositive patients had higher rates of conduction disturbances (3.0 vs. 1.7%, P < 0.001) and rhythm disturbances (10.5 vs. 7.0%, P < 0.001). Patients who tested positive for anti-La alone did not demonstrate a significant association with arrhythmias. Multivariate logistic regression analysis, controlling for possible confounders, showed an increased risk for cardiac conduction disturbances [odds ratio (OR) 1.44, 95% confidence interval (CI) 1.25-1.66, P < 0.001], as well as for cardiac rhythm disturbances (OR 1.21, 95% CI 1.11-1.31, P < 0.001) among anti-Ro seropositive patients. However, the association with rhythm disturbances was more robust in certain subgroup analyses.

CONCLUSIONS

Anti-Ro seropositivity is positively associated with adult cardiac conduction disturbances and, to a lesser extent, cardiac rhythm disturbances, regardless of the presence of concurrent autoimmune disease.

Survivors of Sudden Cardiac Arrest Presenting With Pulseless Electrical Activity: Clinical Substrate, Triggers, Long-Term Prognosis

BACKGROUND

The proportion of sudden cardiac arrest (SCA) presenting as pulseless electrical activity (PEA) is rising, and survival remains low. The pathophysiology of PEA-SCA is poorly understood, and current clinical practice lacks specific options for the management of survivors.

OBJECTIVES

In this study, the authors sought to investigate clinical profile, triggers, and long-term prognosis in survivors of SCA presenting with PEA.

METHODS

The community-based Oregon SUDS (Sudden Unexpected Death Study) (since 2002) and Ventura PRESTO (Prediction of Sudden Death in Multi-ethnic Communities) (since 2015) studies prospectively ascertain all out-of-hospital SCAs of likely cardiac etiology. Lifetime clinical history and detailed evaluation of SCA events is available. We evaluated all SCA survivors with PEA as the presenting rhythm.

RESULTS

The study population included 201 PEA-SCA survivors. Of these, 97 could be contacted for access to their clinical records. Among the latter, the mean age was 67 ± 17 years and 58 (60%) were male. After in-hospital examinations, 29 events (30%) were associated with acute myocardial infarction, and 5 (5%) had bradyarrhythmias. Among the remaining 63 patients (65%), specific triggers remained undetermined, although 31 (49%) had a previous history of heart failure. Of the 201 overall survivors, 91 (45%) were deceased after a mean follow-up of 4.2 ± 4.0 years. Survivors under the age of 40 years had an excellent long-term prognosis.

CONCLUSIONS

Survivors of PEA-SCA are a heterogeneous group with high prevalence of multiple comorbidities, especially heart failure. Surprisingly good long-term survival was observed in young individuals. Acute myocardial infarction as the precipitating event was common, but triggers remained undetermined in the majority. Provision of individualized care to PEA survivors requires a renewed investigative focus on PEA-SCA.

3rd-Degree Atrioventricular Block

Audience

This oral boards case is appropriate for emergency medicine residents and medical students on emergency medicine rotations.

Introduction/Background

Third-degree heart block (also known as complete heart block) is a cardiovascular emergency that requires prompt recognition. Complete heart block is a type of atrioventricular (AV) block whereby no atrial impulses reach the ventricular conduction system. The most common etiology of AV block is ischemic heart disease, with up to 1 in 5 patients developing some type of conduction disturbance after an MI.1 Complete heart block is seen in 8% of patients post-MI.2 Other causes include myocarditis, infectious endocarditis, infiltrative cardiac disease, congenital AV blocks, non-ischemic cardiomyopathy, electrolyte disturbances, and drug side effects.3 In complete heart block, the heart rate is dependent on the location of the block and a functioning secondary pacemaker within the conduction system. Analysis of the EKG will determine the location of this escape rhythm. For escape rhythms originating at the AV node or high in the His-Purkinje system, the QRS complex will typically be narrow, and the ventricular rate typically in the 40-60 bpm range. For blocks with ventricular escape rhythms, the QRS will appear wide, with rates of 20-40 bpm. Patients presenting with 3rd-degree AVB with ventricular escape rhythms can destabilize. If no escape rhythm generates, patients develop asystole and cardiac arrest. Since 1 in 600 adults over the age of 65 will develop a form of supraventricular conduction abnormality each year, this disease process is important to identify and treat.4 Effective management includes accurate interpretation of a 12-lead EKG, assessment of hemodynamic stability and systemic perfusion, and time-sensitive pharmacologic or procedural intervention.

Educational Objectives

At the end of this oral board session, examinees will: 1) demonstrate ability to obtain a complete medical history including detailed cardiac history, 2) demonstrate the ability to perform a detailed physical examination in a patient with cardiac complaints, 3) investigate the broad differential diagnoses which include acute coronary syndrome (ACS), electrolyte imbalances, pulmonary embolism, cerebrovascular accident, aortic dissection and arrhythmias, 4) obtain and interpret the cardiac monitor rhythm strip to identify complete heart block, 5) list the appropriate laboratory and imaging studies to differentiate arrhythmia from other diagnoses (complete blood count, comprehensive metabolic panel, magnesium level, EKG, troponin level, chest radiograph), 6) identify a patient with complete heart block and manage appropriately (administer IV atropine, attempt transcutaneous pacing, place a transvenous pacemaker, emergent consultation with interventional cardiology), 7) provide appropriate disposition to intensive care after consultation with interventional cardiologist.

Educational Methods

This is a straight-forward case which was written to assess learners' ability to rapidly recognize an unstable cardiac rhythm and to subsequently treat and stabilize the patient. Oral board testing is used as a proxy for the emergency department (ED) and can assist with periodic assessment of resident performance while in the ED.We have found that oral board testing is a useful tool to assess residents' critical thinking while still applying pressure that is needed to pass the examination itself. Large groups of residents can be assessed in a short time period without needing to "wait" for a particular clinical condition to present to the ED.In this case, learners were assessed using a free online evaluation tool, ie, Google forms. Multiple questions were written for each critical action, and the Google form served as the online evaluation and repository. The critical actions of the case were then tied to Emergency Medicine Milestones, and the results were compiled for use during residency clinical competency evaluations. Residents were provided with immediate verbal feedback of their performance and were also given their electronic evaluations when requested.

Research Methods

Learners and instructors were given the opportunity to provide electronic feedback after the case was completed to assess strengths and weaknesses, and subsequent modifications were made. Additionally, learners answered written multiple-choice questions after the case to assess for retention of the material.

Results

Senior learners found this to be a more enjoyable way to refresh their skills than direct lecture. Junior residents and students who encountered this clinical entity first in the oral board rather than in the ED, stated that they enjoyed the ability to "trial run" the case before being faced with an emergent and uncontrolled setting of the ED. Overall, the learners rated the case as 4.7 (1-5 Likert scale, 5 being excellent) after the mock oral board examination was completed.

Discussion

Students and residents who were assessed with a mock oral board session found this to be an improvement over traditional "lecture" and were pleased to have participated. The content is highly relevant to emergency medicine and the format forces learners to be actively engaged in review of the material. The case is a good model for the high stakes testing of written and oral board examinations, and is an effective way to assess a resident's ability to rapidly assess and manage a life-threatening condition in the ED.

Topics

Third-degree AV block, complete heart block, 3^rd-degree block, hypotension, syncope, bradycardia, cardiovascular emergency.

Late-onset atrioventricular block after transcatheter aortic valve replacement

Background

Conduction disturbances leading to permanent pacemaker implantation (PPI) rarely occur late after transcatheter aortic valve replacement (TAVR). The clinical features of this phenomenon and its association with periprocedural conduction disturbances remain uncertain.

Objectives

We aimed to determine the incidence and characteristics of late-onset atrioventricular block (AVB) after TAVR.

Methods

This single-center study included 246 patients undergoing TAVR. Late-onset AVB was defined as AVB ≥1 month after the TAVR.

Results

Periprocedural AVB (periAVB) occurred in 43 patients (17%). Patients with periAVB had a higher rate of right bundle branch block (47% vs 7%, P < .0001). Of the 43 patients with periAVB, 15 underwent PPI (35%) at a median duration of 6 days, whereas 1 of the remaining 203 patients without periAVB underwent PPI within 1 month (0.5%). During a median follow-up duration of 365 days, late-onset AVB occurred in 10 of 230 patients without PPI within 1 month (4%) at a median duration of 76 days. All 10 patients presented transient periprocedural atrioventricular conduction disturbances, including 8 patients with periAVB (80%), all of whom recovered within 1 month, and 9 patients underwent self-expanding valve implantation (90%). The mortality rate in patients with PPI within 1 month was higher than in those without, although the difference was not statistically significant (hazard ratio 2.68, 95% confidence interval 0.97–9.05, log-rank P = .09).

Conclusion

Late-onset AVB occurred in a minority of patients undergoing TAVR. Greater vigilance is warranted, particularly in patients with transient conduction disturbances during the periprocedural period following self-expanding valve implantation.

Reversible Causes of Atrioventricular Block

Atrioventricular blocks may be caused by a variety of potentially reversible conditions, such as ischemic heart disease, electrolyte imbalances, medications, and infectious diseases. Such causes must be always ruled out to avoid unnecessary pacemaker implantation. Patient management and reversibility rates depend on the underlying cause. Careful patient history taking, monitoring of vital signs, electrocardiogram, and arterial blood gas analysis are crucial elements of the diagnostic workflow during the acute phase. Atrioventricular block recurrence after the reversal of the underlying cause may pose an indication for pacemaker implantation, because reversible conditions may actually unmask a preexistent conduction disorder.

Anatomy and Pathology of the Cardiac Conduction System

The cardiac conduction system is formed of histologically and electrophysiologically distinct specialized tissues uniquely located in the human heart. Understanding the anatomy and pathology of the cardiac conduction system is imperative to an interventional electrophysiologist to perform safe ablation and device therapy for the management of cardiac arrhythmias and heart failure. The current review summarizes the normal and developmental anatomy of the cardiac conduction system, its variation in the normal heart and congenital anomalies, and its pathology and discusses important clinical pearls for the proceduralist.

Fascicular heart blocks and risk of adverse cardiovascular outcomes: Results from a large primary care population

BACKGROUND

Fascicular heart blocks can progress to complete heart blocks, but this risk has not been evaluated in a large general population.

OBJECTIVE

The purpose of this study was to investigate the association between various types of fascicular blocks diagnosed by electrocardiographic (ECG) readings and the risk of incident higher degree atrioventricular block (AVB), syncope, pacemaker implantation, and death.

METHODS

We studied primary care patients referred for ECG recording between 2001 and 2015. Cox regression models were used to estimate hazard ratios (HRs) as well as absolute risks of cardiovascular outcomes.

RESULTS

Of 358,958 primary care patients (median age 54 years; 55% women), 13,636 (3.8%) had any type of fascicular block. Patients were followed up to 15.9 years. We found increasing HRs of incident syncope, pacemaker implantation, and third-degree AVB with increasing complexity of fascicular block. Compared with no block, isolated left anterior fascicular block (LAFB) was associated with 0%-2% increased 10-year risk of developing third-degree AVB (HR 1.6; 95% confidence interval [CI] 1.25-2.05), whereas right bundle branch block combined with LAFB and first-degree AVB was associated with up to 23% increased 10-year risk (HR 11.0; 95% CI 7.7-15.7), depending on age and sex group. Except for left posterior fascicular block (HR 2.09; 95% CI 1.87-2.32), we did not find any relevant associations between fascicular block and death.

CONCLUSION

We found that higher degrees of fascicular blocks were associated with increasing risk of syncope, pacemaker implantation, and complete heart block, but the association with death was negligible.

Complete heart block in patients infected with SARS-CoV-2: A case series from a developing country

Coronavirus Disease 19 (COVID-19) has led to a global pandemic and has been the center of attention across the entire medical community. This novel virus was initially thought to affect primarily the respiratory system, but now it is evident that it has a multitude of effects on the human body. Our point of interest is to establish the effect of COVID-19 infection on the conducting system of the heart. We present a case series of four patients who developed complete heart block (CHB) shortly after being infected with COVID-19 without any previous known risk factors of complete heart block. There have only been a few previous case reports on the occurrence of CHB in COVID-19 patients highlighting the importance and the need of our case series to the literature of cardiovascular outcomes in COVID-19 patients. Our case series highlight that COVID-19 can indeed affect the conduction system of the heart and cause CHB in patients who then recovered spontaneously further elucidating the transient nature of cardiovascular effects caused by the novel virus.

A Patient Infected with SARS-CoV-2 Presenting with Complete Heart Block

Complete heart block is a rare presentation in a patient with COVID-19 infection that may result when the virus enters the myocardial cell by the angiotensin-converting enzyme-2 receptor. Here, we report a case of forty-nine-year male with COVID-19 with complete heart block (CHB).

Complete Heart Block as a Clinical Feature in Critically Ill Coronavirus Disease 2019 (COVID-19) Patients: A Case Series of Three Cases

Background. Novel coronavirus-19 disease (COVID-19) is associated with significant cardiovascular morbidity and mortality. However, there have been very few reports on complete heart block (CHB) associated with COVID-19. This case series describes clinical characteristics, potential mechanisms, and short-term outcomes of critically ill COVID-19 patients complicated by CHB. Case Summary. We present three cases of new-onset CHB in critically ill COVID-19 patients. Patient 1 is a 41-year-old male with well-documented history of Familial Mediterranean Fever (FMF) who required mechanical ventilator support for acute hypoxic respiratory failure from severe COVID-19 pneumonia. He developed new-onset CHB without a hemodynamic derangement but subsequently had acute coronary syndrome complicated by cardiogenic shock. Patient 2 is a 77-year-old male with no past medical history who required intubation for severe COVID-19 pneumonia acute hypoxic respiratory failure. He developed CHB with sinus pause requiring temporary pacing but subsequently developed multiorgan failure. Patient 3 is 36-year-old lady 38 + 2 weeks pregnant, gravida 2 para 1 with no other medical history, who had an emergency Lower Section Caesarean Section (LSCS) as she required intubation for acute hypoxic respiratory failure. She exhibited new-onset CHB without hemodynamic compromise. The CHB resolved spontaneously after 24 hours. Discussion. COVID-19-associated CHB is a very rare clinical manifestation. The potential mechanisms for CHB in patients with COVID-19 include myocardial inflammation or direct viral infiltration as well as other causes such as metabolic derangements or use of sedatives. Patients diagnosed with COVID-19 should be monitored closely for the development of bradyarrhythmia and hemodynamic instability.

Prevalence of bradyarrhythmias needing pacing in COVID-19

Background

The Sars‐Cov‐2 infection is a multisystem illness that can affect the cardiovascular system. Tachyarrhythmias have been reported but the prevalence of bradyarrhythmia is unclear. Cases have been described of transient high‐degree atrioventricular (AV) block in COVID‐19 that were managed conservatively.

Method

A database of all patients requiring temporary or permanent pacing in two linked cardiac centers was used to compare the number of procedures required during the first year of the pandemic compared to the corresponding period a year earlier. The database was cross‐referenced with a database of all patients testing positive for Sars‐Cov‐2 infection in both institutions to identify patients who required temporary or permanent pacing during COVID‐19.

Results

The number of novel pacemaker implants was lower during the COVID‐19 pandemic than the same period the previous year (540 vs. 629, respectively), with a similar proportion of high‐degree AV block (38.3% vs. 33.2%, respectively, p = .069). Four patients with the Sars‐Cov‐2 infection had a pacemaker implanted for high‐degree AV block, two for sinus node dysfunction. Of this cohort of six patients, two succumbed to the COVID‐19 illness and one from non‐COVID sepsis. Device interrogation demonstrated a sustained pacing requirement in all cases.

Conclusion

High‐degree AV block remained unaltered in prevalence during the COVID‐19 pandemic. There was no evidence of transient high‐degree AV block in patients with the Sars‐Cov‐2 infection. Our experience suggests that all clinically significant bradyarrhythmia should be treated by pacing according to usual protocols regardless of the COVID status.

Long-term outcomes in young patients with atrioventricular block of unknown aetiology

Aims

Atrioventricular block (AVB) of unknown aetiology is rare in the young, and outcome in these patients is unknown. We aimed to assess long-term morbidity and mortality in young patients with AVB of unknown aetiology.

Methods and results

We identified all Danish patients younger than 50 years receiving a first pacemaker due to AVB between January 1996 and December 2015. By reviewing medical records, we included patients with AVB of unknown aetiology. A matched control cohort was established. Follow-up was performed using national registries. The primary outcome was a composite endpoint consisting of death, heart failure hospitalization, ventricular tachyarrhythmia, and cardiac arrest with successful resuscitation. We included 517 patients, and 5170 controls. Median age at first pacemaker implantation was 41.3 years [interquartile range (IQR) 32.7–46.2 years]. After a median follow-up of 9.8 years (IQR 5.7–14.5 years), the primary endpoint had occurred in 14.9% of patients and 3.2% of controls [hazard ratio (HR) 3.8; 95% confidence interval (CI) 2.9–5.1; P < 0.001]. Patients with persistent AVB at time of diagnosis had a higher risk of the primary endpoint (HR 10.6; 95% CI 5.7–20.0; P < 0.001), and risk was highest early in the follow-up period (HR 6.8; 95% CI 4.6–10.0; P < 0.001, during 0–5 years of follow-up).

Conclusion

Atrioventricular block of unknown aetiology presenting before the age of 50 years and treated with pacemaker implantation was associated with a three- to four-fold higher rate of the composite endpoint of death or hospitalization for heart failure, ventricular tachyarrhythmia, or cardiac arrest with successful resuscitation. Patients with persistent AVB were at higher risk. These findings warrant improved follow-up strategies for young patients with AVB of unknown aetiology.

Neurologic complications of brady-arrhythmias

Brady-arrhythmias are responsible for both overt as well as subtle neurologic signs and symptoms, from the seemingly benign and nonspecific symptoms associated with presyncope, to sudden focal neurologic deficits. A brief background on nodal and infra-nodal brady-arrhythmias is provided, followed by extensive discussion regarding neurologic complications of brady-arrhythmias. The multiple mechanisms of and associations between Brady-arrhythmias and transient ischemic attacks and ischemic stroke are discussed. Controversial associations between brady-arrhythmias and neurologic disease are discussed as well, such as potential roles of brady-arrhythmias in cognitive impairment and sequelae of chronotropic incompetence; and the contribution of brady-arrhythmias to syncope and associated injuries to the nervous system. The chapter is written to stand on its own, with guidance toward other pertinent sections of this text where appropriate for further reading.

First-degree atrioventricular block is significantly associated with incident atrial fibrillation in the population predominantly including participants aged ≥ 60 years

Some previous studies demonstrated that first-degree atrioventricular block (f-AVB) was associated with incident atrial fibrillation (AF), while evidence is scarce regarding the association between f-AVB and incident AF in older populations. Therefore, we sought to investigate the association of f-AVB with incident AF in the population predominantly including participants aged ≥ 60 years. Eligible participants were residents in Kanazawa City, Japan aged ≥ 40 years who underwent 12-lead ECG at the National Japanese Health Check-up in 2013. Participants with AF detected at the baseline exam and those without adequate follow-up were excluded. f-AVB was defined as PR interval ≥ 220 ms based on the Minnesota code (6-3). The cumulative incidence of AF was estimated by the Kaplan-Meier curve analysis, and statistical significance was evaluated by the Log-rank test. Unadjusted and adjusted hazard ratios (HRs) were computed by Cox proportional hazard models. HRs were adjusted for conventional risk factors for AF. 37,730 participants (mean age, 72.3 ± 9.6 years; male, 37%) were included. Baseline f-AVB was observed in 667 (1.8%) participants. During the median follow-up period of 5 years (interquartile range, 4.0-5.0 years), 691 cases of incident AF were observed. A 5-year cumulative incidence of AF was significantly higher in f-AVB (+) group compared with f-AVB (-) group (6.8% vs 2.1%, p < 0.01). In the fully adjusted model, f-AVB was significantly associated with incident AF (HR, 1.75; 95% confidence interval 1.25-2.45; p value < 0.01). f-AVB was independently associated with incident AF in the population predominantly including participants aged ≥ 60 years.

Routine Measurement of Thyroid Stimulating Hormone in Patients Presenting With Third-Degree Atrioventricular Block: Do We Really Need It?

Background Hypothyroidism can be a cause of sinus bradycardia. However, thyroid laboratory evaluation is often performed routinely in patients with complete heart block (CHB) though there is little data to support this practice. This study aimed to assess the frequency of thyroid dysfunction in patients presenting with CHB without clinical features of hypothyroidism. Methodology All patients referred for permanent pacemaker implantation for CHB were included in this cross-sectional study. Patients with known thyroid disorder or clinical features of thyroid disorder were excluded. Demographic, electrocardiography (EKG), and routine thyroid stimulating hormone (TSH) screening results were recorded. Results A total of 102 patients with complete atrioventricular (AV) block were enrolled in the study of which 50.0% (51) were male. The mean age was 61.09 ± 11.74. Co-morbidities included diabetes mellitus 44.1% (45), smoking 36.3% (37), and hypertension 55.9% (57). Mean EKG atrial rate was 82.97 ± 31.31 mmHg with a mean ventricular escape rate of 36.17 ± 5.93. Permanent pacemakers were implanted in all of the patients. Only one patient had an abnormal TSH. Conclusions We found a very low prevalence of thyroid dysfunction among patients without clinical features of thyroid dysfunction presenting with third-degree AV block. This calls for cautious prescription of thyroid testing in clinically euthyroid patients.

Characteristics and outcomes of cats with and without pacemaker placement for high-grade atrioventricular block

INTRODUCTION

Available information on characteristics and outcome in cats with high-grade atrioventricular block (AVB) that receive a pacemaker is limited.

ANIMALS

Twenty-two privately owned cats presenting with high-grade AVB.

MATERIALS AND METHODS

Medical records were retrospectively examined. Cats were grouped as having received a pacemaker (PACE group) or not having received a pacemaker (non-PACE group). Clinical characteristics and outcomes of groups were evaluated.

RESULTS

There were 10 cats in the PACE group and 12 cats in the non-PACE group. At presentation, syncope or seizure-like behavior (p = 0.004) and bradycardia (p = 0.043) were more common in the PACE than the non-PACE group. Historical lethargy (p = 0.015) and dull mentation (p = 0.045) were more common in the non-PACE group, as was clinically relevant systemic disease. Pacemaker placement improved syncope or seizure-like behavior in 100% of cats. The degree of AVB at presentation was not associated with pacemaker placement nor the future degree of AVB. The major complication rate of pacemaker placement was 40%. Intergroup survival was not significantly different (PACE group 1278 days, 95% confidence interval: 0-2145 days; non-PACE group 213 days, confidence interval: 1-not available, p = 0.77).

CONCLUSIONS

Pacemaker placement improved clinical signs in cats with high-grade AVB. A difference in survival time could not be demonstrated between cats that received and did not receive a pacemaker in this retrospective study. High-grade AVB can be transient and accompanied by systemic disease in some cats.

Aetiologies and temporal trends of atrioventricular block in young patients: a 20-year nationwide study

Aims

To describe aetiologies and temporal trends in young patients with atrioventricular block (AVB).

Methods and results

We identified all patients in Denmark, receiving their first pacemaker because of AVB before the age of 50 years between 1996 and 2015. Medical records were reviewed and clinical information and diagnostic work-up results were obtained to evaluate the aetiology. We used Poisson regression testing for temporal trends. One thousand and twenty-seven patients were identified, median age at time of implantation was 38 (interquartile range 25–45) years, 584 (56.9%) were male. The aetiologies were complications to cardiac surgery [n = 157 (15.3%)], congenital AVB [n = 93 (9.0%)], cardioinhibitory reflex [n = 52 (5.0%)], congenital heart disease [n = 43 (4.2%)], complication to radiofrequency ablation [n = 35 (3.4%)], cardiomyopathy [n = 31 (3.0%)], endocarditis [n = 18 (1.7%)], muscular dystrophy [n = 14 (1.4%)], ischaemic heart disease [n = 14 (1.4%)], sarcoidosis [n = 11 (1.1%)], borreliosis [n = 9 (0.9%)], hereditary [n = 6 (0.6%)], side-effect to antiarrhythmics [n = 6 (0.6%)], planned His-ablation [n = 5 (0.5%)], complication to alcohol septal ablation [n = 5 (0.5%)], and other known aetiologies [n = 11 (1.1%)]. The aetiology remained unknown in 517 (50.3%) cases. While the number of patients with unknown aetiology increased during the study period (P < 0.001), we observed no significant change in the number of patients with identified aetiology (P = 0.35).

Conclusion

In a nationwide cohort, the aetiology of AVB was identified in only half the patients younger than 50 years referred for first-time pacemaker implantation. The number of patients with unknown aetiology increased during the study period. These findings indicate need for better insight into aetiologies of AVB and improved diagnostic work-up guidelines.

Incidence and predictors of complete atrioventricular conduction recovery among patients with heart block

Aim: In this study, our aim was to determine clinical factors related to the recovery of the conduction system in patients presenting with atrioventricular (AV) block. Materials & methods: A total of 178 patients who were hospitalized at a tertiary center due to second- or third-degree AV block were retrospectively analyzed. Results: During hospital follow-up, 19.1% of patients had fully recovered from AV block. According to a logistic regression analysis; younger age (odds ratio [OR]: 0.950; 95% CI: 0.932-0.967; p < 0.001), presenting with acute coronary syndrome (OR: 18.863; 95% CI: 3.776-94.222; p < 0.001), β-blocker usage (OR: 12.081; 95% CI: 3.498-41.726; p < 0.001), high serum creatinine levels (OR: 4.338; 95% CI: 2.110-8.918; p < 0.001) and no calcification at the aortic valve (OR: 0.189; 95% CI: 0.050-0.679; p = 0.011) were found to be related to resolution of AV block. Conclusion: It is crucial to know the reversible causes of AV block to prevent unnecessary permanent pacemaker implantation.

Risk Factors Associated With Atrioventricular Block

IMPORTANCE

Pacemaker implantations as a treatment for atrioventricular (AV) block are increasing worldwide. Prevention strategies for AV block are lacking because modifiable risk factors have not yet been identified.

OBJECTIVE

To identify risk factors for AV block in community-dwelling individuals.

DESIGN, SETTING, AND PARTICIPANTS

In this population-based cohort study, data from the Mini-Finland Health Survey, conducted from January 1, 1978, to December 31, 1980, were used to examine demographics, comorbidities, habits, and laboratory and electrocardiographic (ECG) measurements as potential risk factors for incident AV block. Data were ascertained during follow-up from January 1, 1987, through December 31, 2011, using a nationwide registry. A total of 6146 community-dwelling individuals were included in the analysis performed from January 15 through April 3, 2018.

MAIN OUTCOMES AND MEASURES

Incidence of AV block (hospitalization for second- or third-degree AV block).

RESULTS

Among the 6146 participants (3449 [56.1%] women; mean [SD] age, 49.2 [12.9] years), 529 (8.6%) had ECG evidence of conduction disease and 58 (0.9%) experienced a hospitalization with AV block. Older age (hazard ratio [HR] per 5-year increment, 1.34; 95% CI, 1.16-1.54; P < .001), male sex (HR, 2.04; 95% CI, 1.19-3.45; P = .01), a history of myocardial infarction (HR, 3.54; 95% CI, 1.33-9.42; P = .01), and a history of congestive heart failure (HR, 3.33; 95% CI, 1.10-10.09; P = .03) were each independently associated with AV block. Two modifiable risk factors were also independently associated with AV block. Every 10-mm Hg increase in systolic blood pressure was associated with a 22% higher risk (HR, 1.22; 95% CI, 1.10-1.34; P = .005), and every 20-mg/dL increase in fasting glucose level was associated with a 22% higher risk (HR, 1.22; 95% CI, 1.08-1.35; P = .001). Both risk factors remained statistically significant (HR for systolic blood pressure, 1.26 [95% CI, 1.06-1.49; P = .007]; HR for glucose level, 1.22 [95% CI, 1.04-1.43; P = .01]) after adjustment for major adverse coronary events during the follow-up period. In population-attributable risk assessment, an estimated 47% (95% CI, 8%-67%) of AV blocks may have been avoided if all participants exhibited ideal blood pressure and 11% (95% CI, 2%-21%) may have been avoided if all had a normal fasting glucose level.

CONCLUSIONS AND RELEVANCE

In this analysis of data from a population-based cohort study, suboptimal blood pressure and fasting glucose level were associated with AV block. These results suggest that a large proportion of AV blocks are assocated with these risk factors, even after adjusting for other major adverse coronary events.

Macrophages Facilitate Electrical Conduction in the Heart

Organ-specific functions of tissue-resident macrophages in the steady-state heart are unknown. Here, we show that cardiac macrophages facilitate electrical conduction through the distal atrioventricular node, where conducting cells densely intersperse with elongated macrophages expressing connexin 43. When coupled to spontaneously beating cardiomyocytes via connexin-43-containing gap junctions, cardiac macrophages have a negative resting membrane potential and depolarize in synchrony with cardiomyocytes. Conversely, macrophages render the resting membrane potential of cardiomyocytes more positive and, according to computational modeling, accelerate their repolarization. Photostimulation of channelrhodopsin-2-expressing macrophages improves atrioventricular conduction, whereas conditional deletion of connexin 43 in macrophages and congenital lack of macrophages delay atrioventricular conduction. In the Cd11b^DTR mouse, macrophage ablation induces progressive atrioventricular block. These observations implicate macrophages in normal and aberrant cardiac conduction.

Clinical Characteristics and Outcomes of Patients with Ischemic and Non-Ischemic Complete Heart Block

INTRODUCTION

Third degree or complete heart block (CHB) is a medical emergency and usually requires permanent pacemaker placement. CHB can be caused by cardiac ischemia or non-ischemic conditions such as infiltrative diseases or fibrosis. The purpose of this study is to identify the baseline clinical characteristics associated with ischemic versus non-ischemic causes of complete heart block and compare their outcomes.

MATERIALS AND METHODS

An institutional review broad approval was granted by the Cleveland Clinic Health System. In our study, 216 patients with CHB presenting to Fairview Hospital between January 2014 and June 2016 were initially identified using the International Classification of Disease (ICD) codes at discharge. Only the patients with a new diagnosis of complete heart block (CHB) were included in the study (total N=62), which led to the exclusion of 154 patients. The patients were characterized into non-ischemic and ischemic groups based on cardiac marker elevation, electrocardiogram changes and/or cardiac catheterization findings. In all the patients, data including the following was collected: demographics such as age, gender, ethnicity and body mass index; pre-existing comorbidities such as hypertension, chronic kidney disease, diabetes mellitus, thyroid disease, previous coronary artery disease, history of cancer; use of nodal blocking agents, electrolyte abnormalities on admission, echocardiographic parameters such as ejection fraction (EF), right ventricular systolic pressure (RVSP), left ventricular end diastolic and systolic volumes (LVEDV and LVESV, respectively). The primary outcome was all-cause mortality and the secondary outcome was pacemaker placement. Categorical variables were analyzed using chi-square and continuous variables using ANOVA.

RESULTS

Out of 62 patients (N=62), 48 had non-ischemic and 14 had ischemic CHB. The mean age was 67 years (95% CI, 60.55-74.73) in the ischemic group and 75 years (95% CI, 71.52-78.80) in the non-ischemic group, p=0.04. Patients with ischemic CHB had a lower mean EF of 49.6% (95% CI, 42.04%-57.23%) compared to 57.42% in non-ischemic CHB patients (95% CI, 53.97%-60.87%), p=0.05. History of coronary artery disease was present in 71.4% (10/14) patients with ischemic CHB compared to 37.5% (18/48) patients with non-ischemic CHB, p=0.02. There was no statistically significant difference between the two groups in terms of gender, diabetes, hypertension, thyroid dysfunction, chronic kidney disease, nodal blocking agents, electrolyte abnormalities or smoking status. For outcomes, 6/48 (12.5%) of patients with non-ischemic CHB had died compared to 3/14 (21.4%) ischemic CHB (p=0.327). Permanent pacemaker was implanted in 45/48 patients (93.75%) of the non-ischemic CHB compared to 6/14 (42.83%) in the ischemic group (p<0.001).

CONCLUSIONS

Patients with ischemic CHB are younger, and they have a lower ejection fraction but they are less likely to get a pacemaker compared to non-ischemic CHB. Further studies with a bigger sample size are required to understand the long term mortality outcomes of patients with CHB.

Management of Bradyarrhythmias in Heart Failure: A Tailored Approach

Patients with heart failure (HF) may develop a range of bradyarrhythmias including sinus node dysfunction, various degrees of atrioventricular block, and ventricular conduction delay. Device implantation has been recommended in these patients, but the specific etiology should be sought as it may influence the choice of the type of device required (pacemaker vs. implantable cardiac defibrillator). Also, pacing mode must be carefully set in patients with heart failure (HF) and left ventricular systolic dysfunction.In this chapter, we summarize the knowledge required for a tailored approach to bradyarrhythmias in patients with heart failure.

Complete heart block in a patient with POEMS syndrome: A case report

BACKGROUND

Polyneuropathy, organomegaly, endocrinopathy, monoclonal syndrome (POEMS) is a rare paraneoplastic syndrome associated with plasma cell dyscrasia.

CASE REPORT

A 48-year-old man presented with a 1-year history of paresthesia and progressive weakness of extremities. Diagnosis of POEMS syndrome was made for him on the basis of clinical presentation, additional physical findings, typical sclerotic bone lesion, and bone marrow findings. In last admission, he explained episodes of dyspnea and chest pain that associated with frequent premature ventricular contraction in his electrocardiograph. Patient heart monitoring showed some episodes of complete heart block. Infra-His atrioventricular block in electro-physiologic study was detected. He had no history of ischemic heart disease. His cardiopulmonary findings on examination were normal. All results of cardiac biomarkers and serum electrolytes and repeated echocardiography were within normal range. Cong red staining of rectal fat pad biopsy was negative. After pacemaker insertion radiation of sclerotic bone, lesion started for him, but radiotherapy was ineffective, and he expired with respiratory failure. Complete heart block in POEMS syndrome has not been reported previously, and it is the first POEMS case with complete heart block.

CONCLUSION

Complete heart block is a cardiac manifestation of POEMS syndrome.

A case of an acquired high-degree AV block in a pregnant woman

Acquired heart block during pregnancy is rare. We describe the case of a 29-year-old pregnant female with fatigue due to an unexplained high-degree AV conduction disorder. She was treated with a dual chamber pacing system. Further pregnancy and delivery were without complications. The sparse literature on this topic is reviewed. (Neth Heart J 2008;16:419-21.).

The prevalence and prognosis of third-degree atrioventricular conduction block: the Reykjavik study

OBJECTIVES

The objectives of this study were to find the prevalence of third-degree atrioventricular block in representative population sample and to estimate its prognostic significance. Most earlier studies have been performed on hospital patients and some professional groups.

SETTING AND SUBJECTS

In the Reykjavik Study, a prospective cardiovascular population study, 9139 men and 9773 women aged 33-79 years were examined in 1967-91. Electrocardiograms were taken and coded according to the Minnesota code. Third-degree atrioventricular block was found in 11 persons, seven male and four female, an overall prevalence of 0.04%. All of these individuals had signs of dysrhythmia on electrocardiograms taken later, and in addition some other heart disease. The heart block was temporary in seven individuals (64%); six (55%) needed a pacemaker.

CONCLUSIONS

The prevalence of third-degree atrioventricular block in this general population was low. The block was temporary in the majority of subjects. All had some underlying heart disease, which may affect the prognosis more than the heart block. Fewer subjects than expected were found to need a pacemaker.

Maternal antibodies against fetal cardiac antigens in congenital complete heart block

An immunologic basis for congenital heart block has been proposed previously. To investigate the association between congenital heart block and maternal antibodies capable of crossing the placenta, we used immunofluorescence to examine serum samples from 41 mothers and 8 affected children, together with serum from controls, for antibodies to fetal cardiac tissue. Twenty-one mothers (51 percent) had IgG antibody reactive with fetal heart tissue, as compared with only 9 of 94 controls (10 percent; P less than 0.001). Three of 8 affected babies, but none of 50 healthy babies, had similar antibodies. The antibodies reacted with all myocardial tissue and were not directed specifically to the conduction system. They also reacted with other fetal tissues and could be distinguished from nuclear and smooth-muscle autoantibodies. We also observed a higher occurrence of antibodies to cytomegalovirus, but not to Epstein-Barr virus, in these mothers. Autopsy specimens from babies with congenital heart block examined by immunoperoxidase staining showed deposition of immunoglobulin and complement components in all cardiac tissues. These findings strengthen the case implicating immune reactivity related to maternal antibody in the development of some but not all cases of congenital heart block.

Significance of complete right bundle-branch block when an isolated finding. An echocardiographic study

Twenty-seven patients with complete right bundle-branch block as the only abnormal finding were studied using high speed M-mode echocardiography to determine the effect of the electrical delay on the mechanical events of right ventricular systole. Pulmonary valve opening (PVOm) was delayed in all cases. In some the delay was mainly between mitral valve closure (MVC) and tricuspid valve closure (TVC), and this was designated proximal block. In the others the main delay was between tricuspid valve closure and pulmonary valve opening and this was designated distal block. The patients were divided into those with proximal and those with distal block by calculating the ratio TVC-PVOm/MVC-TVC. Twelve out of 13 of those with distal delay but only one out of 14 of those with proximal delay had episodes of syncope or near syncope. These results are consistent with previous theories about the pathophysiology of right bundle-branch block. Echocardiography may offer a non-invasive method to estimate the prognosis in isolated right bundle-branch block.

Pathological studies on atrioventricular conduction disturbances

A histopathological study was made on 150 autopsied hearts according to the method of Lev in order to investigate the correlation of conduction disturbances with the branching pattern of the bundle branches. Subsequently, the study of bundle branch block and A-V block cases revealed that no explicit correlation existed between histopathological changes and functional disturbances nor between disturbances in conduction (i.e. breakdown of the physiological mechanism) and histopathological findings.