What is the approach to assessing sinus bradycardia?

Approach to Assessment of Sinus Bradycardia

Definition

Sinus bradycardia is defined as a sinus heart rate <50-60 bpm, though the 2018 ACC/AHA/HRS guidelines specifically use <50 bpm as the threshold. 1, 2

• Sinus node dysfunction (SND) encompasses multiple manifestations including sinus bradycardia, ectopic atrial bradycardia (<50 bpm), sinoatrial exit block, sinus pause (>3 seconds), sinus node arrest, tachy-brady syndrome, chronotropic incompetence (failure to attain 80% of expected heart rate reserve during exercise), and isorhythmic dissociation 1

Classification

• Intrinsic SND: Abnormal intrinsic sinus automaticity with abnormal intrinsic heart rate (calculated as 118.1 - [0.57 x age]) and abnormal corrected sinus node recovery time 1
• Extrinsic/Autonomic SND: Normal intrinsic sinus automaticity with bradycardia due to excessive vagal tone or reversible causes 1, 3
• Tachy-brady syndrome: Alternating periods of sinus bradycardia with atrial tachycardia, atrial flutter, or atrial fibrillation, often with prolonged sinus pause after tachycardia termination 1, 4

Differential Diagnosis

Reversible/Treatable Causes (Must Exclude First)

• Medications: Beta blockers, non-dihydropyridine calcium channel blockers (diltiazem, verapamil), digoxin, sodium-channel and potassium-channel blocking antiarrhythmic drugs 1, 5
• Metabolic/Endocrine: Hypothyroidism, severe systemic acidosis, hypokalemia 1, 5
• Acute conditions: Acute myocardial infarction, elevated intracranial pressure, severe hypothermia 1, 5
• Other: Obstructive sleep apnea, hypoxemia, hypercarbia 5

Physiologic/Benign Causes

• Young athletic individuals, normal aging, sleep 2

Pathologic Causes

• Intrinsic sinus node disease (fibrosis, ischemia, infiltrative disease), conduction system disease 1, 2

History

Symptom Characterization

• Key symptoms to assess: Syncope, presyncope, lightheadedness, dizziness, palpitations, fatigue, shortness of breath, exercise intolerance 6, 4
• Timing: Frequency of symptoms, relationship to activity/rest, duration of episodes 5
• Correlation: Critical to establish temporal correlation between symptoms and documented bradycardia 1

Red Flags (Require Urgent Evaluation)

• Syncope with trauma or injury 1
• Morgagni-Adams-Stokes seizures (sudden loss of consciousness with bradycardia) 6
• Symptoms of heart failure or hemodynamic compromise 7
• Syncope in patient with structural heart disease or bundle branch block 1

Risk Factors

• Advanced age (>65 years), history of cardiac surgery, prior myocardial infarction, family history of sudden cardiac death 1
• Medications with negative chronotropic effects 1, 5
• Hypothyroidism, sleep apnea, athletic training 1, 5

Physical Examination (Focused)

• Vital signs: Heart rate, blood pressure (including orthostatic measurements), respiratory rate, oxygen saturation 7
• Cardiovascular: Jugular venous pressure, heart sounds (bradycardia rate, regularity, murmurs), peripheral pulses, signs of heart failure 5
• Neurologic: Mental status, focal deficits (to assess for elevated intracranial pressure or stroke) 1
• Thyroid: Palpation for goiter, signs of hypothyroidism (dry skin, delayed reflexes) 1
• General: Temperature (hypothermia), signs of sleep apnea (obesity, large neck circumference) 1, 5

Investigations

Initial/Essential

• 12-lead ECG: Document rhythm, rate, PR interval, QRS duration, screen for structural heart disease, conduction abnormalities, and acute ischemia 5, 7

  • Expected findings: Sinus bradycardia with rate <50 bpm, normal P wave morphology, 1:1 AV conduction 1

• Laboratory tests (based on clinical suspicion): 5

  • Thyroid function tests (TSH, free T4)
  • Electrolytes (potassium, magnesium, calcium)
  • pH/blood gas (if acidosis suspected)
  • Lyme titer (if endemic area and clinical suspicion)

• Medication review: Complete list of all medications including over-the-counter and herbal supplements 1, 5

Cardiac Monitoring (Essential for Symptom-Rhythm Correlation)

The type of monitor should be selected based on symptom frequency: 5

• Holter monitor (24-72 hours): For daily or near-daily symptoms 5
• Event recorder or mobile cardiac telemetry: For symptoms occurring weekly to monthly 5
• Implantable cardiac monitor: For very infrequent symptoms (>30 days between episodes) or when initial monitoring is non-diagnostic 5

Expected findings: Documentation of bradycardia during symptomatic episodes, assessment of minimum heart rate, pauses, chronotropic response 1

Advanced/Selective Testing

• Electrophysiology study (EPS): 1

  • May be considered (Class IIb) in symptomatic patients when diagnosis remains uncertain after noninvasive evaluation
  • May be considered when performed for another indication (e.g., syncope evaluation with bundle branch block)
  • Should NOT be performed in asymptomatic patients (Class III)
  • Measures: Sinus node recovery time (SNRT; abnormal if corrected SNRT >500-550 ms), sinoatrial conduction time (SACT)
  • Limitations: Variable and modest sensitivity/specificity; abnormal results alone do not justify pacemaker implantation 1

• Exercise stress test: To assess chronotropic incompetence (failure to achieve 80% of age-predicted maximum heart rate) 1

Empiric Treatment

Acute Symptomatic Bradycardia (Hemodynamically Unstable)

1. Atropine 0.5-1 mg IV bolus, repeat every 3-5 minutes as needed, maximum 3 mg 7, 8

   • Most effective for sinus bradycardia, AV nodal block, or sinus arrest
   • Less effective for Mobitz type II or third-degree AV block with wide QRS 7
   • Use cautiously in acute coronary ischemia/MI 7

2. Transcutaneous pacing: Initiate immediately if unresponsive to atropine 7

3. IV catecholamines (dopamine or epinephrine): Alternative or bridge to pacing 7

4. Temporary transvenous pacing: For persistent hemodynamically unstable bradycardia refractory to medical therapy 7

Chronic Management

First priority: Address reversible causes 1, 5

• Discontinue or reduce dose of offending medications (switch beta blocker to alternative antihypertensive without negative chronotropic effect) 1
• Treat hypothyroidism with thyroxine replacement 1
• Manage obstructive sleep apnea, correct metabolic abnormalities 1

Permanent pacing indications (Class I): 1

• Symptoms directly attributable to SND with documented correlation
• Symptomatic bradycardia as consequence of guideline-directed therapy that cannot be discontinued and is clinically necessary

Permanent pacing reasonable (Class IIa): 1

• Tachy-brady syndrome with symptoms attributable to bradycardia
• Symptomatic chronotropic incompetence (use rate-responsive programming)

Trial of oral theophylline may be considered (Class IIb): 1

• In patients with symptoms likely attributable to SND to assess potential benefit of permanent pacing

Indications to Refer

Cardiology/Electrophysiology Referral

• Urgent referral: Syncope with trauma, recurrent syncope, hemodynamically unstable bradycardia 1, 7
• Routine referral: 1, 5
  • Documented symptomatic bradycardia requiring pacemaker evaluation
  • Tachy-brady syndrome
  • Symptomatic chronotropic incompetence
  • Uncertain diagnosis after initial noninvasive evaluation
  • Bradycardia due to essential medications that cannot be discontinued

Other Specialist Referral

• Endocrinology: Suspected hypothyroidism requiring management 1
• Sleep medicine: Suspected obstructive sleep apnea 1
• Neurology/Neurosurgery: Elevated intracranial pressure 1

Critical Pitfalls

1. Treating asymptomatic bradycardia: Asymptomatic sinus bradycardia should NOT be treated; patients with minimal or infrequent symptoms without hemodynamic compromise do not require pacing 1, 7, 2

2. Failing to identify and treat reversible causes first: Always exclude medications, hypothyroidism, metabolic abnormalities, and elevated intracranial pressure before considering permanent pacing 1, 5, 7

3. Proceeding to pacemaker without symptom-rhythm correlation: Permanent pacing should only be considered when unequivocal correlation between symptoms and bradycardia is established 1

4. Relying on EPS alone for pacemaker indication: Abnormal SNRT or SACT in isolation should NOT be used to justify pacemaker implantation; EPS findings must be used in conjunction with clinical findings 1

5. Performing EPS in asymptomatic patients: Risk of invasive testing (8% complication rate including hematoma and atrial fibrillation) outweighs benefit; incidental abnormal findings have no clinical importance 1

6. Delayed escalation in unstable patients: Do not delay transcutaneous pacing in unstable patients failing atropine; consider underlying rhythm when choosing treatment 7

7. Using atropine inappropriately: Atropine may be ineffective for infranodal block (Mobitz type II, third-degree AV block with wide QRS) and may worsen ischemia in acute MI 7

8. Inadequate monitoring duration: Selecting monitoring strategy that does not match symptom frequency leads to missed diagnoses; use implantable monitor for very infrequent symptoms 5