Bradycardia
Pathophysiology: The Cardiac Conduction System
To understand the mechanisms of bradycardia, it is helpful to know the normal functioning of the heart's electrical system:
- The sinus node, located in the right atrium, is the natural pacemaker of the heart: it spontaneously generates electrical impulses at a rate of 60 to 100 bpm at rest.
- The impulse propagates to both atria, causing them to contract, then reaches the atrioventricular node (AV node), which introduces a physiological delay allowing for ventricular filling
- The impulse then travels down the bundle of His, then its right and left branches, to the Purkinje fibers that stimulate the simultaneous contraction of both ventricles
- An abnormality at one of these steps can cause bradycardia: sinus dysfunction (slow pacemaker), atrioventricular block (slowed or blocked AV conduction), or infrahisian conduction disorder.
Causes and Classification
The causes of bradycardia are numerous and can be divided into physiological causes, those intrinsic to the heart, and extrinsic causes (medications, systemic diseases):
| Category | Common causes | Mechanism |
|---|---|---|
| Physiological | Intensive sports training, sleep, vagal reflex (emotion, pain, defecation, cough) | High vagal tone slowing sinus node automaticity; normal adaptation without underlying pathology |
| Medicines | Beta-blockers, bradycardic calcium channel blockers (diltiazem, verapamil), digoxin, amiodarone, ivabradine, antidepressants, lithium, opioids | Direct pharmacological action on the sinoatrial node or AV conduction; the most frequent cause of symptomatic bradycardia in clinical practice |
| Sinus node dysfunction (sick sinus syndrome) | Age-related fibrous degeneration of the sinus node, coronary artery disease, infiltrative cardiomyopathy (amyloidosis, sarcoidosis) | Insufficient sinus automatism; can manifest as sinus pauses, persistent bradycardia, or sick sinus syndrome |
| Atrioventricular block (AV block) | Idiopathic degeneration of the conduction system (Lev-Lenègre disease), inferior or anterior myocardial infarction, endocarditis, Lyme disease, sarcoidosis | Slowdown or interruption of conduction between the atria and ventricles; classified as AV block of the 1st, 2nd, or 3rd degree depending on severity |
| Metabolic and endocrine causes | Hypothyroidism, hyperkalemia, hypermagnesemia, hypothermia, severe hypoxia | Alteration of myocardial cell metabolism and conduction system |
| Infectious and inflammatory causes | Lyme disease (2nd or 3rd degree AV block), viral myocarditis, infective endocarditis with conduction system extension | Inflammation or direct infiltration of the conduction system by the pathogen or the immune response |
| Reflex neurological causes | Vasovagal syncope, carotid sinus syndrome, intracranial hypertension | Excessive activation of the parasympathetic nervous system (vagus nerve) slowing the sinoatrial node |
| Post-surgical | Cardiac surgery (valve replacement, bypass), catheter ablation, heart transplant | Injury or edema of the conduction system during surgery |
Types of bradycardia by location of the problem
The location of the conduction disturbance determines the type of bradycardia, its prognosis, and its management:
| Type | Seat | ECG Features | Clinical risk |
|---|---|---|---|
| Sinus bradycardia | Sinus node | Regular rhythm, normal P waves, narrow QRS complexes, heart rate less than 60 bpm | Often benign if asymptomatic; may require a pacemaker if severe sinus dysfunction |
| 1st-degree rape | AV node | Prolongation of the PR interval beyond 200 ms, all P waves conducted | Generally benign, simple monitoring; may progress to higher degrees |
| Mobitz I second-degree AV block (Wenckebach) | AV node | Progressive lengthening of the PR interval until a non-conducted P wave, then the cycle resumes | Often benign, sometimes vagal or medication-induced; rarely symptomatic |
| Second-degree Mobitz II AV block | Infrahissian (branches) | Unconducted P waves unpredictably, constant PR before block; QRS often wide | Potentially serious; high risk of progression to complete heart block; pacemaker often indicated |
| 3rd-degree AV block (complete AV block) | AV node or infranodal | Complete dissociation between P waves and QRS complexes; junctional escape rhythm (40-60 bpm) or ventricular escape rhythm (20-40 bpm) | Cardiology emergency; risk of syncope, cardiac arrest, or sudden death; urgent pacemaker |
| Sinus pauses | Sinus node | Absence of electrical activity for more than 3 seconds on the ECG or Holter | Symptomatic if greater than 3 seconds during wakefulness; pacemaker according to clinical context |
Symptoms
Bradycardia is asymptomatic in many cases. When it becomes symptomatic, the manifestations result from cardiac output insufficient to meet the body's needs:
- Unusual and persistent fatigue, often the first reported symptom, disproportionate to activity
- Dizziness, vertigo, or lightheadedness, especially with changes in position or exertion
- Shortness of breath on exertion, progressive intolerance to physical activity
- Paradoxical palpitations: sensation of perceived irregular heartbeats or skipped beats
- Chest pain or tightness, particularly with exertion, in case of associated myocardial ischemia
- Presyncope: imminent sensation of fainting with vision darkening, cold sweats, nausea
- Syncope: Brief, spontaneously resolving loss of consciousness, often related to a prolonged cardiac pause or complete AV block
- Confusion or difficulty concentrating in chronic forms due to prolonged cerebral hypoperfusion
- Decompensated heart failure in very severe and prolonged forms
Diagnosis
The investigation of bradycardia aims to confirm the diagnosis, identify the cause, assess the severity, and quantify the hemodynamic impact:
- Complete medical history: symptoms, their relation to exertion or rest, current medications, cardiac history, infectious exposures (ticks, travel to Lyme-endemic areas)
- Clinical examination: pulse taking, blood pressure measurement in both arms, cardiac auscultation, search for signs of heart failure
- 12-lead electrocardiogram (ECG): first-line examination, identifies the type of bradycardia and the location of the conduction disorder
- Holter ECG for 24 to 72 hours: Continuous ambulatory recording to detect intermittent bradycardias, sinus pauses, and paroxysmal AV blocks not visible on a resting ECG
- Implantable Cardiac Monitor (Reveal): Multi-month recording for unexplained syncope with normal ECG
- Biological assessment: TSH (hypothyroidism), ionogram (serum potassium, serum magnesium), creatinine, Lyme serology if suggestive context, drug level monitoring (digoxin level)
- Transthoracic echocardiogram: evaluation of ventricular function, chamber size, and search for underlying structural heart disease
- Stress Test: Evaluation of Chronotropic Response to Exercise; an inability to increase heart rate with exercise (chronotropic incompetence) suggests sinus dysfunction
- Carotid sinus massage: performed in a medical setting to test carotid sinus sensitivity in unexplained syncope in the elderly
- Intracavitary electrophysiological study: invasive exploration of the conduction system reserved for complex cases or before pacemaker implantation
Treatments
The management of bradycardia is closely dependent on the presence or absence of symptoms, the identified cause, and the severity of the conduction disorder:
| Situation | Treatment | Remarks |
|---|---|---|
| Physiological asymptomatic bradycardia | Simple surveillance, no treatment necessary | Athletes, young healthy adults; re-evaluate if symptoms appear |
| Drug-induced bradycardia | Review of the drug in question: dose reduction, substitution, or discontinuation of the bradycardizing medication | Always in consultation with the prescribing doctor; never stop a beta-blocker or antiarrhythmic abruptly without medical advice. |
| Reversible cause identified | Etiologic treatment: levothyroxine (hypothyroidism), IV antibiotics (Lyme disease), correction of dyskalemia, warming (hypothermia) | Bradycardia is generally resolved with treatment of the cause; a temporary pacemaker if the bradycardia is severe during the treatment phase. |
| Symptomatic acute bradycardia (emergency) | Atropine IV 0.5 to 1 mg (first line); IV isoproterenol infusion or temporary external cardiac pacing if atropine resistant | Emergency hospital care; continuous monitoring; preparation for transcutaneous or transvenous temporary pacemaker insertion |
| Chronic symptomatic sinus dysfunction | Permanent pacemaker implantation | Primary indication for symptomatic bradycardia without a reversible cause; significantly improves symptoms and quality of life |
| Mobitz II second-degree AV block or complete heart block | Definitive pacemaker, urgently if hemodynamically unstable | Formal indication even in the absence of symptoms for Mobitz II due to the risk of unpredictable progression to complete heart block. |
| Recurrent vasovagal syncope with cardioinhibitory component | Pacemaker with heart rate drop detection algorithm, in selected patients over 40 years old with frequent disabling syncope | Proven efficacy in cases with documented asystole during syncope; complementary hygienic and dietary measures |
Possible complications
An undiagnosed or untreated bradycardia can lead to complications that vary depending on its severity and duration:
| Complication | Description | Contributing factors |
|---|---|---|
| Recurrent syncope | Repeated knowledge loss exposing to trauma (falls, accidents) and altering quality of life | Paroxysmal atrial fibrillation, prolonged sinus pauses, bradycardia-tachycardia syndrome |
| Heart failure | Cardiac decompensation due to chronic insufficient output maintained by persistent severe bradycardia | Severe and prolonged bradycardia, pre-existing underlying heart disease |
| Sudden cardiac arrest | Cardiac arrest due to asystole or ventricular fibrillation secondary to prolonged cardiac pause | Complete untreated BAV, severe sinus node dysfunction with prolonged pauses |
| Secondary tachyarrhythmias | Appearance of ventricular tachycardias or torsades de pointes favored by slowed rhythm and QT interval prolongation | Severe bradycardia, QT-prolonging medications, associated hypokalemia |
| Strokes | Thromboembolisms favored by blood stasis in the atria in cases of bradycardia associated with paroxysmal atrial fibrillation | Bradycardia-tachycardia syndrome with intermittent atrial fibrillation |
Certain clinical presentations associated with bradycardia constitute absolute cardiac emergencies: syncope or loss of consciousness with associated trauma, intense chest pain with a slow pulse suggesting myocardial infarction with atrioventricular block, extreme bradycardia below 40 bpm with hypotension and altered mental status, acute respiratory distress due to decompensated heart failure, or cardiac arrest with absent pulse.
In the presence of these signs, call 911 immediately or go to the nearest emergency room without delay. For any non-urgent bradycardia symptoms such as unexplained fatigue, recent dizziness, or a perceived slow pulse, a consultation at Clinique Omicron allows for a rapid assessment including an ECG and appropriate guidance.
Consult at Clinique Omicron
Clinique Omicron provides cardiac rhythm disorder evaluations, including bradycardia, at multiple service points across Quebec. A physician or nurse practitioner (NP) can perform an on-site ECG, order a cardiac Holter monitor or a complete biological assessment, and refer to a cardiologist or electrophysiologist based on the results. Whether your symptoms are recent or have persisted for some time, early medical evaluation can help differentiate between benign bradycardia and a conduction disorder requiring intervention. In-person and telemedicine consultations are available. To book an appointment at one of our service points in Montreal, the South Shore, or elsewhere in Quebec, visit cliniqueomicron.ca.
The content of this page is provided for informational purposes only and is not intended to replace the advice of a qualified healthcare professional. Consult a physician for any symptoms, questions or decisions you may have regarding your health.
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