The sick sinus syndrome: natural history of dysfunction of the sinoatrial node

The Role of Electrocardiographic Exercise Testing for the Possibility of Permanent Pacemaker Implantation in Patients with Sinus Bradycardia

Objective

Study on the role of electrocardiographic (ECG) exercise testing for the possibility of permanent pacemaker implantation (PPI) in patients with sinus bradycardia (SB).

Methods

Cross-sectional descriptive, prospective study. The study on 60 patients with SB below 50 beats/minute on 12-lead electrocardiogram at rest, with or without symptoms of SB, conducted at the Vietnam National Heart Institute and Hanoi Heart Hospital from January 2020 to September 2021.

Results

Sixty patients with SB were studied, 36 male (60%) and 24 female patients (40%), p > 0.05. The average age was 55.12 ± 13.89 years old. Maximum exercise capacity (MEC) is low and only reaches 7.78 ± 3.59 metabolic equivalents (METs); Not reaching 85% of predicted MEC accounts for 53.5%; Maximum exercise time is 10.53 ± 0.46 minutes; Impaired heart rate (HR) variability in patients with SB is high: Chronotropic Index <0.8 accounts for 53.5%, not reaching 85% of predicted HR max accounts for 45%. The average HR max was 129.90 ± 29.22 beats per minute (BPM). The average maximum workload systolic blood pressure was 155.23 ± 20.59 mmHg. The average value of maximum exercise diastolic blood pressure was 88.10 ± 9.11 mmHg. The HR decreased by 27.87 ± 16.82 BPM in the first minute. Not achieving 85% of predicted MEC (p = 0.062), so it is not an independent factor predicting the ability for PPI. Only the HR variability index <0.8 is an independent predictor for PPI in bradycardic patients, which has OR = 21.521 (95% CI: 2.27-04.34, p < 0.05).

Conclusion

Results can be seen that Chronotropic Index <0.8 is an important marker for physicians to decide on PPI in ECG during exercise testing in SB patients and is a potential prognostic factor for the need for PPI.

Sinus node dysfunction and atrial fibrillation-Relationships, clinical phenotypes, new mechanisms, and treatment approaches

Although the anatomical basis of the pathogenesis of sinus node dysfunction (SND) and atrial fibrillation (AF) is located primarily in the left and right atria, increasing evidence suggests a strong correlation between SND and AF, in terms of both clinical presentation and formation mechanisms. However, the exact mechanisms underlying this association are unclear. The relationship between SND and AF may not be causal, but is likely to involve common factors and mechanisms, including ion channel remodeling, gap junction abnormalities, structural remodeling, genetic mutations, neuromodulation abnormalities, the effects of adenosine on cardiomyocytes, oxidative stress, and viral infections. Ion channel remodeling manifests primarily as alterations in the "funny" current (I_f) and Ca²⁺ clock associated with cardiomyocyte autoregulation, and gap junction abnormalities are manifested primarily as decreased expression of connexins (Cxs) mediating electrical impulse propagation in cardiomyocytes. Structural remodeling refers primarily to fibrosis and cardiac amyloidosis (CA). Some genetic mutations can also cause arrhythmias, such as SCN5A, HCN4, EMD, and PITX2. The intrinsic cardiac autonomic nervous system (ICANS), a regulator of the heart's physiological functions, triggers arrhythmias.In addition, we discuss arrhythmias caused by viral infections, notably Coronavirus Disease 2019 (COVID-19). Similarly to upstream treatments for atrial cardiomyopathy such as alleviating CA, ganglionated plexus (GP) ablation acts on the common mechanisms between SND and AF, thus achieving a dual therapeutic effect.

Genetic insight into sick sinus syndrome

Aims

The aim of this study was to use human genetics to investigate the pathogenesis of sick sinus syndrome (SSS) and the role of risk factors in its development.

Methods and results

We performed a genome-wide association study of 6469 SSS cases and 1 000 187 controls from deCODE genetics, the Copenhagen Hospital Biobank, UK Biobank, and the HUNT study. Variants at six loci associated with SSS, a reported missense variant in MYH6, known atrial fibrillation (AF)/electrocardiogram variants at PITX2, ZFHX3, TTN/CCDC141, and SCN10A and a low-frequency (MAF = 1.1–1.8%) missense variant, p.Gly62Cys in KRT8 encoding the intermediate filament protein keratin 8. A full genotypic model best described the p.Gly62Cys association (P = 1.6 × 10⁻²⁰), with an odds ratio (OR) of 1.44 for heterozygotes and a disproportionally large OR of 13.99 for homozygotes. All the SSS variants increased the risk of pacemaker implantation. Their association with AF varied and p.Gly62Cys was the only variant not associating with any other arrhythmia or cardiovascular disease. We tested 17 exposure phenotypes in polygenic score (PGS) and Mendelian randomization analyses. Only two associated with the risk of SSS in Mendelian randomization, AF, and lower heart rate, suggesting causality. Powerful PGS analyses provided convincing evidence against causal associations for body mass index, cholesterol, triglycerides, and type 2 diabetes (P > 0.05).

Conclusion

We report the associations of variants at six loci with SSS, including a missense variant in KRT8 that confers high risk in homozygotes and points to a mechanism specific to SSS development. Mendelian randomization supports a causal role for AF in the development of SSS.

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.

Sinus Node and Atrial Arrhythmias

Although sinus node dysfunction (SND) and atrial arrhythmias frequently coexist and interact, the putative mechanism linking the 2 remain unclear. Although SND is accompanied by atrial myocardial structural changes in the right atrium, atrial fibrillation (AF) is a disease of variable interactions between left atrial triggers and substrate most commonly of left atrial origin. Significant advances have been made in our understanding of the genetic and pathophysiologic mechanism underlying the development and progression of SND and AF. Although some patients manifest SND as a result of electric remodeling induced by periods of AF, others develop progressive atrial structural remodeling that gives rise to both conditions together. The treatment strategy will thus vary according to the predominant disease phenotype. Although catheter ablation will benefit patients with predominantly AF and secondary SND, cardiac pacing may be the mainstay of therapy for patients with predominant fibrotic atrial cardiomyopathy. This contemporary review summarizes current knowledge on sinus node pathophysiology with the broader goal of yielding insights into the complex relationship between sinus node disease and atrial arrhythmias.

Associated conduction disturbances in patients with symptomatic sinus node disease

Electrophysiological investigations were performed in 30 patients with symptomatic sinus node disease (SND) to assess the extent and distribution of associated functional disturbances in the conduction system. The tests were performed before and after inhibition of autonomous tone with propranolol, 0.1 mg/kg, and atropine, 0.02 mg/kg. Surface ECG had shown bundle branch blocks (BBB) in 5 patients and fascicular blocks in 2. AV block I had been recorded in 4 patients, while none had shown high-degree AV block. Malfunction was most often detected in the AV junction, 17 patients showing a prolonged conduction time or an abnormal effective AV node refractory period. Intraventricular conduction delay was present in 7 patients, with a prolonged HV interval in 3 and a complete permanent BBB in the others. Rate-dependent BBBs were demonstrated in a further 4 patients. Long cardiac arrests following interruption of atrial pacing, suggesting impaired automaticity also of subsidiary escape pacemakers, were seen in 11 patients. Only 6 patients, 20%, showed no signs of associated malfunction of the conduction system. Thus, detailed electrophysiological assessment demonstrated associated conduction abnormalities in the majority of these SND patients. The results agree with histopathological studies and show that sinus node malfunction is often the clinically apparent manifestation of a widespread degenerative process in the cardiac conduction system.

Sick sinus syndrome

Sinus-node dysfunction is common in the elderly and, in most cases, does not cause any symptoms. Despite the high number of laboratory investigations, most diagnoses of sinus-node dysfunction are made by 12-lead electrocardiography, which shows severe sinus bradycardia, sinus arrest, or sinoatrial block. Continuous electrocardiographic monitoring, exercise testing, and electrophysiologic investigations (including pharmacologic interventions to cause complete autonomic blockade) are sometimes useful in detecting transient or latent sinus-node abnormalities. The term sick sinus syndrome should be reserved for patients with symptomatic sinus-node dysfunction. Sick sinus syndrome has a protean presentation with variable degrees of clinical severity. Symptoms are often intermittent, changeable, and unpredictable. Because these symptoms can be observed in several other diseases, none are specific to sick sinus syndrome. Owing to the nonspecific nature of its symptoms, sick sinus syndrome can be diagnosed only when clear electrocardiographic signs corroborate symptoms. In the absence of a demonstrable link between signs and symptoms, a diagnosis can be presumed only when signs of severe sinus dysfunction are present and when every other possible cause of symptoms has been excluded carefully. Sinus-node dysfunction frequently is associated with diseases of the autonomic nervous system, and autonomic reflexes play a major role in the genesis of syncope. Survival does not seem to be affected by sick sinus syndrome. Atrioventricular block, chronic atrial fibrillation, and systemic embolism are major pathologic conditions that affect the outcome of the syndrome. Treatment should be aimed at controlling morbidity and relieving symptoms. Cardiac pacing is the most powerful therapy; physiologic pacing (atrial or dual-chamber) has been shown definitively to be superior to ventricular pacing.

Clinical dysrhythmias after surgical repair of congenital heart disease

Innovations in surgical and medical treatment continue to improve the outlook for children with complex congenital heart disease. Although mortality continues to decrease, disease-related morbidity is increasing as a large cohort of these patients is reaching young adulthood, pursuing careers, marrying, and in many cases having children of their own. Chronic recurrent dysrhythmias are a frequent cause of long-term morbidity in this population and result in frequent, unanticipated emergency room visits and hospitalizations. Although not usually life threatening, they can pose considerable challenges to the patients and the providers who care for them. This article provides an overview of the most common dysrhythmias encountered in this population, dysrhythmia substrates, and therapeutic options.

The natural course of untreated sick sinus syndrome and identification of the variables predictive of unfavorable outcome

We performed a prospective study in 35 untreated patients aged > or = 45 years, who had a mean sinus rate at rest of < or = 50 beats/min and/or intermittent sinoatrial block, and symptoms attributable to sinus node dysfunction. The patients were followed up for up to 4 years (mean 17 +/- 15 months). During follow-up, 20 patients (57%) had cardiovascular events that required treatment: 8 had syncope (23%); 6 had overt heart failure (17%); 4 patients had chronic atrial fibrillation (11%); and 2 patients had poorly tolerated episodes of paroxysmal tachyarrhythmias (6%). Actuarial rates of occurrence of all events were 35%, 49%, and 63%, respectively, after 1, 2, and 4 years. At univariate analysis, age > or = 65 years, end-systolic left ventricular diameter > or = 30 mm, end-diastolic left ventricular diameter > or = 52 mm, and ejection fraction < 55% were predictors of cardiovascular events. At multivariate analysis, age, end-diastolic diameter, and ejection fraction remained independent predictors of events. Actuarial rates of occurrence of syncope were 16%, 31%, and 31%, respectively, after 1, 2, and 4 years. Both univariate and multivariate predictors of syncope were history of syncope and corrected sinus node recovery > or = 800 ms. A favorable outcome was observed in the remaining 43% of patients. Thus, clinical cardiovascular events occur in most untreated sick sinus syndrome patients during long-term follow-up, even though a favorable course can be expected in 43% of patients. The outcome can be partly predicted on initial evaluation. In the patients with a favorable outcome, treatment can safely be delayed.

Incidence of atrioventricular block and chronic atrial flutter/fibrillation after implantation of atrial pacemakers; follow-up of more than ten years

The incidence of atrioventricular (AV) block and chronic atrial flutter/fibrillation was determined in 41 patients, mean age 62.2 years (22 to 88 years) who received atrial pacemakers before October 1980. The mean follow-up period was 12.3 years, range 11-14.4 years. Twenty-five patients suffered from bradycardia-tachycardia syndrome, 13 symptomatic bradycardia, and 3 bradycardia related ventricular tachycardia. At follow-up: 18 patients (44%) had permanent I-AV block; 11 patients had II-AV block, 9 patients (22%) transient Wenckebach block and 2 patients (5%) transient Mobitz block, all asymptomatic. The cumulative incidence of III-AV block was 1 (2.5%). Chronic atrial flutter/fibrillation occurred in 6 patients (15%), 5 were not pacemaker-dependent; 1 received a ventricular pacemaker. Seventeen patients died after a mean of 7 years (0.6-12.2 years). Cumulative survival rate of 58% for 14.4 years, did not differ from a matched cohort of the normal population. There were no pacemaker related deaths. We conclude that long-term atrial pacing was safe and effective. The incidence of III-AV block and chronic atrial flutter/fibrillation was low.

The natural history of sick sinus syndrome

A literature review has addressed the two major factors in disease progression in sick sinus syndrome: atrioventricular block and atrial fibrillation. An incidence of atrioventricular block of 8.4 percent in a follow-up period of 34.2 months is considered clinically significant and sufficient to justify use of a ventricular lead in pacemaker management. Atrial fibrillation occurs much more commonly (22.3%) in ventricular pacing than with atrial demand pacing (3.9%) in a 2 1/2 year observation period. Coincident with the reduction in arrhythmia achieved by atrial demand pacing is a significant reduction in systemic embolism (1.6% vs. 13%). This is considered largely to be due to the improved rhythm control with AAI pacing but also possibly to the avoidance of retrograde atrioventricular conduction. The benefits of AAI pacing in terms of mortality in sick sinus syndrome have not yet been fully assessed. In the future, DDI pacing is recommended with, in some patients, the addition of another sensor to provide rate responsiveness on exercise.

Reappearance of persistent normal sinus rhythm in a patient with the sick sinus syndrome following cardiac pacing for 10.5 years

A patient with symptomatic sick sinus syndrome which manifested as a brady-tachy syndrome, was admitted to our institute in 1969. Following cardiac pacing for 10.5 years, the brady-tachy syndrome disappeared and normal sinus rhythm was restored. Her stable normal sinus rate has persisted up to the present, 2.5 years after termination of pacing.

Sinus node function in first three weeks after cardiac transplantation

Donor sinus node function was studied in 10 patients from day 4 to day 24 after cardiac transplantation. Cycle length, atrial arrhythmias, corrected sinus node recovery time, and estimated sinoatrial conduction time were recorded daily. Five patients had at least two sets of results suggesting sinus node dysfunction (group A) while five patients had no such abnormalities (group B). The prognosis in group A was poor, with four of the five patients dying within four months of the operation; one unexpected death from arrhythmias was recorded by ambulatory electrocardiographic monitoring. All five patients in group B survived for at least eight months. In nine patients sinus node function varied from day to day, with corrected sinus node recovery time reaching a peak at 11 to 18 days after operation. The longest corrected sinus node recovery time was 11 160 ms. Neither the differences between the patients, nor the day to day variation, could be explained solely by the degree of rejection as assessed by biopsy or by the ischaemia time of the heart during procurement. Sinus node dysfunction soon after transplantation is associated with a poorer prognosis and might be the terminal event in some cases.

Natural history of severe sinus bradycardia discovered by 24 hour Holter monitoring

This study follows patients with severe sinus bradycardia (40 beats per minute for 6 seconds or greater) in order to evaluate mortality and the effectiveness of permanent pacemaker insertion. Severe sinus bradycardia was noted on a 24-hour Holter in 95 patients. There were 64 males and 31 females with a mean age of 69 +/- 10 years. All were available for follow-up at 26 +/- 13 months. Twenty-eight required a permanent pacemaker at an average of 2 +/- 3 months after the Holter. Of this group 12 had the Holter for arrhythmia, 11 for cerebral symptoms, 4 for palpitations and 1 for chest pain. Only 1 was taking digitalis and no patients were taking Inderal. Six (21%) died at a mean interval of 21 +/- 15 months following pacemaker insertion. Sixty-seven did not require pacemaker insertion. The indications for Holter monitoring were arrhythmia in 16, palpitations in 19, cerebral symptoms in 20 and chest pain in 12. Four of these patients were on digitalis, 8 on Inderal, and 4 on both. Eleven (16%) died at a mean interval of 12 +/- 7 months after the initial Holter recording. Dizziness and/or syncope reoccurred in 22. Five had these symptoms even after pacemaker insertion. We conclude that severe sinus bradycardia is associated with a significant mortality. Insertion of a permanent pacemaker may decrease recurrent symptoms and slightly increase time of survival, but does not appear to influence the overall survival rate.

Long-term follow-up after pacemaker implantation in sick sinus syndrome

In 74 symptomatic patients suffering from sick sinus syndrome, survival after pacemaker implantation and additional drug therapy was determined. The 5-year survival was poor (47.2%). Evidence of other cardiac disease (present in 44.6%) influenced the prognosis unfavorably, especially in combination with continuing symptoms. As yet the phase of SSS in which chronic cardiac pacing is required, should be considered as a critical stage.

Automaticity of subsidiary pacemakers of patients with dysfunction of the sinus node

Automaticity (postpacing impulse recovery time) of the sinus node or subsidiary pacemakers (or both) was determined by intracardiac recordings and atrial overdrive pacing in three patients with disease of the sinus node. Their electrocardiographic manifestations included sinus bradycardia, sinoatrial block, sinus arrest, atrioventricular junctional rhythm, and atrioventricular dissociation. One patient, who initially had syncope, demonstrated prolonged time for recovery of the sinus node (maximum corrected value, 3,485 msec) and impaired atrioventricular junctional automaticity (maximum corrected junctional recovery time, 1,460 msec). Of the remaining two patients without syncopal attacks, one had an adequate low atrial or junctional escape mechanism following cessation of pacing. The maximum low atrial escape interval was 1,660 msec, and the maximum junctional recovery time was 1,770 msec (corrected value, 140 msec). The third patient disclosed a prolonged maximum corrected recovery time for the sinus node (870 msec), while the maximum junctional recovery time was only 1,810 msec (corrected value, 160 msec). We conclude that automaticity of the subsidiary pacemakers in disease of the sinus node can be determined in the presence of atrioventricular dissociation or total sinus arrest, and that determination of the recovery time of the subsidiary pacemakers may be useful in identifying symptomatic patients with sinus nodal dysfunction.

Survival in sinoatrial disorder (sick-sinus syndrome)

A total of 381 patients with established (156) or potential (225) sinoatrial dysfunction were included in a 10-year prospective survey to determine the course of the disease and the benefits of pacing. With the exclusion of nine patients who were lost to follow-up, 61 were fitted with pacemakers. The overall survival of patients with established and potential dysfunction was similar and apparently indistinguishable from that of the normal population. Pacemaker implantation had little discernible effect on mortality though it reduced some incapacitating symptoms. These findings suggest that sinoatrial dysfunction is a relatively benign condition. Hence pacing should probably not be adopted as a routing measure but be reserved for patients with troublesome symptoms.

Termination of symptoms in the sick sinus syndrome by atrial fibrillation

A female patient with symptomatic sick sinus syndrome (SSS) had a permanent pacemaker implanted. She later developed chronic atrial fibrillation with an adequate ventricular rate. When the pulse generator failed she was found to be clinically asymptomatic. This patient had never adjusted emotionally to her pacemaker and, since she was asymptomatic and in a stable rhythm, the failed pacemaker was not replaced. Possibly, thought should be given to the discontinuance of pacing for the SSS when, in the presence of adequate A-V conduction, stable atrial fibrillation develops.