Cardiology · Arrhythmias
The facts most likely to be tested
Sinus bradycardia is defined as a sinus rhythm with a heart rate of less than 60 beats per minute.
Athletes and healthy individuals during sleep often exhibit asymptomatic sinus bradycardia due to high vagal tone.
Symptomatic sinus bradycardia manifests as syncope, dizziness, fatigue, or shortness of breath due to inadequate cardiac output.
Atropine is the first-line pharmacological treatment for hemodynamically unstable patients with symptomatic bradycardia.
Transcutaneous pacing is indicated for patients who are refractory to atropine or have high-degree AV blocks.
Inferior wall myocardial infarction is a classic cause of sinus bradycardia due to ischemia of the sinoatrial (SA) node.
Beta-blockers, calcium channel blockers, and digoxin are the most common medication-induced causes of sinus bradycardia.
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A 68-year-old male is brought to the emergency department after a syncopal episode. He reports feeling lightheaded and fatigued for the past two days. His medical history is significant for hypertension treated with metoprolol and diltiazem. Physical examination reveals a blood pressure of 92/60 mmHg and a heart rate of 42 beats per minute. An ECG shows a regular rhythm with P waves preceding every QRS complex.
What is the most appropriate initial management for this patient?
Atropine
The patient is hemodynamically unstable (hypotension, syncope) due to symptomatic sinus bradycardia, likely exacerbated by his current medications; therefore, the first-line treatment is atropine.
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Etiology / Epidemiology
Common in well-trained athletes and during sleep; pathological causes include inferior MI and hypothyroidism.
Clinical Manifestations
Often asymptomatic; symptomatic patients present with syncope, dizziness, or angina due to decreased cardiac output.
Diagnosis
ECG showing HR < 60 bpm with a normal, upright P-wave preceding every QRS complex.
Treatment
Asymptomatic: observe. Symptomatic/unstable: Atropine first-line; avoid in high-degree AV block.
Prognosis
Excellent if asymptomatic; symptomatic cases require pacemaker if reversible causes are excluded.
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Epidemiology & Etiology
Physiologic in athletes due to high vagal tone. Pathological causes include sick sinus syndrome, hypothyroidism, hypothermia, and increased intracranial pressure (Cushing's triad). Medications like beta-blockers and calcium channel blockers are frequent iatrogenic triggers.
Pertinent Anatomy
Originates in the SA node, the heart's primary pacemaker. Dysfunction here leads to failure of impulse initiation, often exacerbated by vagal nerve stimulation.
Pathophysiology
Results from either decreased automaticity of the SA node or SA exit block. Vagal stimulation releases acetylcholine, hyperpolarizing the nodal cells and slowing the phase 4 depolarization. This reduces the firing rate below the normal 60-100 bpm range.
Clinical Manifestations
Patients may report fatigue, lightheadedness, or near-syncope. Red flags include hypotension, altered mental status, or acute heart failure. Physical exam may reveal a bradycardic pulse and signs of poor perfusion.
Diagnosis
12-lead ECG is the gold standard. Diagnostic criteria include HR < 60 bpm with a constant P-wave morphology. If intermittent, a Holter monitor is indicated to correlate symptoms with rhythm.
Treatment
Asymptomatic patients require no intervention. For symptomatic/unstable patients, Atropine 1mg IV is the first-line agent. If refractory, use transcutaneous pacing or dopamine/epinephrine infusions. Do not delay pacing in patients with high-degree AV block.
Prognosis
Generally benign in healthy individuals. Persistent symptomatic bradycardia often necessitates a permanent pacemaker to prevent syncope and improve quality of life.
Differential Diagnosis
Sick Sinus Syndrome: alternating bradycardia and tachycardia
Complete Heart Block: P-waves and QRS complexes are dissociated
Hypothyroidism: associated with cold intolerance and delayed reflexes
Beta-blocker toxicity: associated with hypoglycemia and hypotension
Inferior MI: associated with ST-elevation in leads II, III, aVF