Bradycardia is often found in athletic adults and is typically asymptomatic. Whether persons with bradycardia have higher levels of cardiovascular fitness or abnormalities in the conduction system is unclear. This article gives us an idea about the causes and management of such asymptomatic bradycardia.  

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The asymptomatic bradycardia

Multiple studies demonstrate that higher resting HR (RHR) predicts poor cardiovascular outcomes both independent of conventional risk factors and within subgroups of patients with cardiovascular disease. Resting heart rate has been demonstrated to be modifiable over time due to the interaction of genes and environmental factors such as exercise, medical conditions and medications. However, the impact of bradycardia is less clear.

Tresch and Fleg reported that sinus bradycardia (HR <50 bpm) in apparently healthy, nonathletic individuals older than 40 years showed no association with cardiovascular morbidity and mortality. Other studies suggested that asymptomatic bradycardia may be associated with modestly reduced cardiovascular disease (CVD) mortality.

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The symptomatic bradycardia

Common symptoms caused by bradycardia include:

• Syncope
• Pre-syncope dizziness
• Dizziness
• Chest tightness
• Chest pain
• Fatigue
• Exercise intolerance
• Inattention
• Forgetfulness
• Irritability, etc.

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Mechanism

1. The main mechanism is the decrease in cardiac output caused by bradycardia.
2. In addition, whether bradycardia causes symptoms are also related to factors such as individual differences in patients and whether they are combined with other diseases. For example, the first-degree atrioventricular block is very common in athletes and rarely causes symptoms; while for patients with underlying heart disease, such as left ventricular hypertrophy or preserved ejection fraction heart failure; first-degree atrioventricular block may cause deterioration of heart function.
3. In addition, the evaluation of symptoms should also pay attention to symptoms other than bradycardia, such as bradycardia secondary to diseases such as cardiac sarcoidosis or Lyme disease.
4. The symptoms of the primary disease are often more serious than bradycardia. Related symptoms other than bradycardia help to clarify the cause of bradycardia and guide further diagnosis and treatment.

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Causes of bradycardia:

Intrinsic

1. Cardiomyopathy (ischemic or nonischemic)

2. Congenital heart disease

3. Degenerative fibrosis

4. Infection/inflammation

• Chagas disease
• Diphtheria
• Infectious endocarditis
• Lyme disease
• Myocarditis
• Sarcoidosis
• Toxoplasmosis

5. Infiltrative disorders

• Amyloidosis
• Hemochromatosis
• Lymphoma

6. Ischemia/infarction

7. Rheumatological conditions

• Rheumatoid arthritis
• Scleroderma
• Systemic lupus erythematosus

8. Surgical or procedural trauma

• Cardiac procedures such as ablation or cardiac catheterization
• Congenital heart disease surgery
• Septal myomectomy for hypertrophic obstructive cardiomyopathy
• Valve surgery (including percutaneous valve replacement)

Extrinsic

1. Autonomic perturbation

2. Carotid sinus hypersensitivity

• Neurally-mediated syncope/presyncope
• Physical conditioning
• Sleep (with or without sleep apnea)
• Situational syncope

1. Cough
2. Defecation
3. Glottic stimulation
4. Medical procedures
5. Micturition
6. Vomiting

3. Metabolic

• Acidosis
• Hyperkalemia
• Hypokalemia
• Hypothermia
• Hypothyroidism
• Hypoxia

Sick sinus syndrome

The sinus node is the epicentre of the conduction system of the heart, and sinus node disease is also the most common cause of bradycardia. Sick sinus syndrome refers to a disease in which sinus node pacing function and/or sinus conduction dysfunction is caused by sinus node and adjacent tissue diseases, which leads to bradycardia.

The most common cause of sick sinus syndrome is degenerative changes caused by ageing. However, the following should also be considered:

• Sinus bradycardia - The sinus node produces an electrical charge at a slower rate than normal.
• Sinus arrest - Signals from the sinus node pause, causing skipped beats.
• Sinoatrial exit block - Signals to the upper heart chambers are slowed or blocked, causing a pause or skipped beats.
• Chronotropic incompetence -The heart rate is normal at rest but doesn't increase with physical activity.
• Bradycardia-tachycardia syndrome - The heart rate alternates between abnormally slow and fast rhythms, usually with a long pause (asystole) between heartbeats.

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Atrioventricular block

The atrioventricular node is the second hallmark of the cardiac conduction system. The atrioventricular block is also a common cause of bradycardia. The atrioventricular block includes first-degree, second-degree and third-degree atrioventricular blocks.

1. First degree AV blocks can originate from various locations within the conduction system. The levels of conduction delay include the atrium, AV node (most common in first-degree heart block), Bundle of His, bundle branches, fascicles, and Purkinje system.
2. Mobitz type I second degree AV block usually occurs within the AV block while Mobitz type II second degree AV block mainly originates from conduction system disease below the level of the AV node (in the bundle of His and in the bundle branches).
3. In third-degree AV block, no atrial impulses could reach the ventricle- it can occur in the AV node or in the intranodal specialised conduction system.

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Management of asymptomatic bradycardia

For asymptomatic bradycardia, reversible causes that may cause sick sinus syndrome and atrioventricular block should be ruled out first. If the cause is clear, treat the cause, and bradycardia can be relieved after treatment of the original disease. However, most clinical bradycardia especially that occurs in elderly patients has no clear cause.

A permanent cardiac pacemaker is the most effective treatment for bradycardia. The most commonly used are:

• Single-chamber pacemakers that stimulate one chamber of the heart, either an upper chamber (atrium) or a lower one (ventricle).
• Dual-chamber pacemakers send electrical impulses to both the atrium and the ventricle and pace both chambers. A dual-chamber pacemaker synchronizes the rhythm of the atrium and ventricles in a pattern that closely resembles the natural heartbeat.

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Patients with sick sinus syndrome may convert to atrial fibrillation, a rhythm that is amenable to medical therapy and may eliminate the need for pacemaker placement.

Disclaimer- The views and opinions expressed in this article are those of the authors and do not necessarily reflect the official policy or position of M3 India.