The significance of second degree atrioventricular block and bundle branch block. Observations regarding site and type of block

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.

2022 HRS expert consensus statement on evaluation and management of arrhythmic risk in neuromuscular disorders

This international multidisciplinary document is intended to guide electrophysiologists, cardiologists, other clinicians, and health care professionals in caring for patients with arrhythmic complications of neuromuscular disorders (NMDs). The document presents an overview of arrhythmias in NMDs followed by detailed sections on specific disorders: Duchenne muscular dystrophy, Becker muscular dystrophy, and limb-girdle muscular dystrophy type 2; myotonic dystrophy type 1 and type 2; Emery-Dreifuss muscular dystrophy and limb-girdle muscular dystrophy type 1B; facioscapulohumeral muscular dystrophy; and mitochondrial myopathies, including Friedreich ataxia and Kearns-Sayre syndrome, with an emphasis on managing arrhythmic cardiac manifestations. End-of-life management of arrhythmias in patients with NMDs is also covered. The document sections were drafted by the writing committee members according to their area of expertise. The recommendations represent the consensus opinion of the expert writing group, graded by class of recommendation and level of evidence utilizing defined criteria. The recommendations were made available for public comment; the document underwent review by the Heart Rhythm Society Scientific and Clinical Documents Committee and external review and endorsement by the partner and collaborating societies. Changes were incorporated based on these reviews. By using a breadth of accumulated available evidence, the document is designed to provide practical and actionable clinical information and recommendations for the diagnosis and management of arrhythmias and thus improve the care of patients with NMDs.

Trifascicular block in a cat infected with Cytauxzoon felis

A 3-year-old female spayed indoor/outdoor Siamese cat was diagnosed with cytauxzoonosis and irregular heart rhythm was appreciated during a physical examination. On electrocardiogram, the cat had an average ventricular rate of 144 beats/min with Wenckebach type (Mobitz type I) atrioventricular block. The QRS complexes of sinus conducted beats had left anterior fascicular block morphology, they were followed by two subsequent beats presented with a prolonged P-R interval and QRS conducted with a left bundle branch block. The finding of alternating left anterior fascicular block and left bundle branch block along with atrioventricular block is consistent with trifascicular block. No structural heart changes were noted on echocardiography. Approximately 48 h after presentation to the emergency service, the cat acutely decompensated and spontaneously died.

High-grade atrioventricular block

Atrioventricular (AV) block is an AV conduction disorder that can manifest in various settings, with varying symptomaticity and severity. The electrocardiogram is a key diagnostic tool for management, and careful interpretation is necessary to institute the correct management. We described two cases of patients with bradycardia due to AV blocks and discussed the electrocardiogram interpretation and management.

Vagomimetic effects of fingolimod: physiology and clinical implications

Fingolimod is a sphingosine 1-phosphate (S1P) receptor modulator approved to treat relapsing-remitting multiple sclerosis (MS). Initiation of treatment with fingolimod has been found to produce transient bradycardia and/or slowing of atrioventricular impulse conduction in a small proportion of patients. This effect is thought to be due to the interaction of fingolimod with S1P receptors on the surface membrane of atrial myocytes causing a vagomimetic effect, similar to the action of acetylcholine on muscarinic receptors. As a precaution, patients are under electrocardiogram (ECG) monitoring for 6 h after receiving their first dose. Fingolimod is contraindicated in patients with overt or concealed cardiac diseases. However, the Fingolimod Initiation and caRdiac Safety Trial (FIRST), which was designed specifically to investigate the cardiac profile of fingolimod, did not show an increased risk of clinically relevant cardiac events with fingolimod. This review examines the electrophysiology and pathophysiology of cardiac impulse formation in the context of fingolimod. It concludes that these vagomimetic effects should be considered benign and should not prevent the effective use of fingolimod in the treatment of patients with MS.

The evaluation and management of drug effects on cardiac conduction (PR and QRS intervals) in clinical development

Recent advances in electrocardiographic monitoring and waveform analysis have significantly improved the ability to detect drug-induced changes in cardiac repolarization manifested as changes in the QT/corrected QT interval. These advances have also improved the ability to detect drug-induced changes in cardiac conduction. This White Paper summarizes current opinion, reached by consensus among experts at the Cardiac Safety Research Consortium, on the assessment of electrocardiogram-based safety measurements of the PR and QRS intervals, representing atrioventricular and ventricular conduction, respectively, during drug development.

Signalment, clinical signs, and prognostic indicators associated with high-grade second- or third-degree atrioventricular block in dogs: 124 cases (January 1, 1997-December 31, 1997)

OBJECTIVE

To evaluate signalment, clinical signs, and prognosis associated with high-grade second- or third-degree atrioventricular block (AVB) in dogs.

DESIGN

Retrospective case series.

ANIMALS

124 dogs.

PROCEDURES

Data were gathered from ECGs, veterinarian questionnaires, echocardiograms, and radiographs submitted for review; compared with data from a large control group; and examined for association between variables and duration of survival. A new classification system for AVB was evaluated.

RESULTS

Afghan, Catahoula Leopard Dog, Chow Chow, Cocker Spaniel, German Wirehaired Pointer, and Labrador Retriever breeds were predisposed to high-grade second- or third-degree AVB. Heavier, older, and sexually intact female dogs were overrepresented in the study group. Weakness, lethargy, exercise intolerance, and syncope were the most common clinical signs. The presence of clinical signs was not associated with duration of survival. Dogs with high-grade second-degree AVB had a duration of survival similar to that of dogs with third-degree AVB. Dogs with high-grade second- or third-degree AVB were at high risk for sudden death in the first 6 months after diagnosis. High ventricular escape rhythm rate and narrow escape-complex QRS width were negatively associated with duration of survival. Pacemaker implantation had a significant positive association with survival.

CONCLUSIONS AND CLINICAL RELEVANCE

Pacemaker implantation should be strongly considered in all dogs with high-grade second- or third-degree AVB regardless of whether clinical signs are evident. If medical treatment is warranted, vagolytic medications may be the best choice. A new classification system for AVB may merit further investigation.

Permanent pacemakers in older persons

OBJECTIVE

To review (1) the physiologic changes of aging that may lead to the need for a permanent pacemaker; (2) the current standard indications for pacemaker implantation as reported in expert guidelines; (3) newer investigational uses of pacemakers; (4) advances in pacemaker technology; and (5) cost-effectiveness of permanent pacing.

DATA SOURCES

Computer-assisted search of the English language literature (MEDLINE database), manual search of articles bibliographies, and review of data provided by a major pacemaker manufacturer.

DESIGN

Pertinent articles were reviewed and data extracted. Studies and data involving older persons were emphasized, and these data were extracted and presented.

RESULTS

Abnormalities in impulse generation and conduction are common in older people and form the substrate for the need of pacemaker implantation. Pacemaker use is high in older people, with an estimated 70 to 80% of all permanent pacemakers implanted in individuals aged 65 years and older. The hemodynamic changes of aging include a reduction of ventricular compliance and increased contribution of atrial contraction to ventricular filling. Pacemakers that maintain synchrony between atria and ventricles may, therefore, be particularly advantageous in older adults. Recent studies have validated this theoretical reasoning. Chronotropic incompetence is common in older people, and rate responsive ventricular pacing has been shown to improve quality of life compared with fixed rate devices in older patients. Sequential, dual chamber pacemakers reduce the symptoms of pacemaker syndrome and recurrences of atrial fibrillation in certain groups of patients. Potential utility of permanent pacing is being investigated in patients with severe left ventricular dysfunction, markedly prolonged atrioventricular conduction time, hypertrophic and dilated cardiomyopathy, and after cardiac transplantation. Biventricular pacing as therapy for severe heart failure is in the very early phases of investigation. Newer implantable pacemakers provide a host of technological options but are somewhat more expensive and require more frequent follow-up. Controversies still exist regarding the need for pacemakers in certain clinical conditions but are decreasing as new high quality studies are completed.

CONCLUSIONS

Permanent pacing is highly cost-effective, safe, and simple to perform. Pacemakers are implanted in patients with sinus node dysfunction, acquired (both native and postsurgical) atrioventricular block, some forms of neurally mediated syndromes, fascicular blocks, and, occasionally, for the prevention of supraventricular or ventricular tachyarrhythmias. Although pacemakers are implanted in individuals of all ages, they are most often utilized in older adults; it is estimated that 70 to 80% of all pacemakers are implanted in patients 65 years of age or older. This is attributable to an increase in abnormalities of impulse generation and conduction with advancing age. Dual chamber pacemakers that maintain synchrony between atria and ventricles are preferable in older patients because of the increased contribution of atrial contraction to ventricular filling in this age group. This theoretical advantage has been confirmed by prospective studies in limited patient subgroups.

Clinical intracardiac electrophysiologic testing: technique, diagnostic indications, and therapeutic uses

Clinical cardiac electrophysiologic testing has evolved rapidly since 1968, when the technique was first described. In an electrophysiologic study, electrode catheters are positioned within the heart to record electrical activity from the atrium, atrioventricular conduction tissue, and ventricle. Programmed stimulation is then performed, which involves pacing of the atrium or ventricle and introducing critically timed premature stimuli during sinus rhythm or paced rhythm. The use of programmed stimulation in conjunction with intracardiac recordings in electrophysiologic studies has facilitated the diagnosis of mechanisms of arrhythmias and the assessment of therapy. Electrophysiologic testing is useful in selected patients with sinus node dysfunction, conduction system disorders, supraventricular tachycardia, ventricular tachycardia, or ventricular fibrillation and in survivors of out-of-hospital cardiac arrest and patients with symptomatic but unsubstantiated rhythm disturbances. Therapeutic approaches that can be assessed by electrophysiologic testing include serial drug testing to determine the effectiveness of antiarrhythmic agents, antitachycardia pacing, the implantable defibrillator, transcatheter ablation, and electrophysiologically guided surgical procedures. In this review, we discuss the methods of electrophysiologic testing, its clinical applications in diagnosing the various cardiac rhythm disturbances, and its use in assessing various therapeutic modalities.

Survival in second degree atrioventricular block

Two hundred and 14 patients with chronic second degree heart block were seen and followed up in the Devon Heart Block and Bradycardia Survey between 1968 and 1982. The patients were divided into three groups according to the type of block. In group 1 there were 77 patients with Mobitz type I block (mean age 69 years), in group 2, 86 patients with Mobitz type II block (mean age 74 years), and in group 3, 51 with 2:1 or 3:1 block (mean age 75 years). The five year survival was similar in all groups, being 57%, 61%, and 53% in groups 1, 2, and 3 respectively. The presence or absence of bundle branch block did not appear to influence prognosis. In particular, patients in group 1 without bundle branch block did not fare any better than those in group 2 both with and without bundle branch block. One hundred and three of the patients were fitted with pacemakers, the proportion being greatest in group 2. In each group a significantly larger number of paced patients survived than unpaced. The five year survival for all the paced patients in the study was 78% compared with 41% for the unpaced. Since the paced patients were slightly younger than the unpaced two age matched groups of 74 patients each were selected from the paced and unpaced patients, but the five year survival of those paced was still significantly better. It is concluded that in the patients in the present study chronic Mobitz type I block has a similar prognosis to that of Mobitz type II block. Unpaced patients with both types did very badly, whereas those fitted with pacemakers had a five year survival similar to that expected for the normal population. These results refute the benign reputation of chronic Mobitz type I block and imply that patients with this condition should be considered for pacemaker implantation on similar criteria to those adopted for patients with higher degrees of block.

Indications for and management of cardiac pacemakers

Cardiac pacemakers are now at the forefront of medical technology. The ability to provide pacing therapy adapted to the individual patient has resulted in significant patient benefit. Although the indications for cardiac pacing have changed little over the past decade, the types of pacemakers available for specific bradyarrhythmias have proliferated in leaps and bounds. This technology is complex and sophisticated and requires in-depth knowledge for appropriate utilization. Artificial replacement of the human conduction system is now a reality. Still other advances remain to be achieved in this area, mainly in sensor technology and pacing therapy for tachyarrhythmias. At the current rate of development, the outlook is indeed promising.