Makindo Medical Notes

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Sinoatrial block occurs when the sinoatrial (SA) node fails to initiate an electrical impulse, or the generated impulse is blocked before it can exit the SA node and depolarize the atria. This results in intermittent pauses in atrial and ventricular activity. It can be caused by increased vagal tone, certain medications that impair cardiac conduction, or intrinsic SA node disease. Continuous monitoring with a 24-hour Holter monitor may be necessary for diagnosis, especially if symptoms are infrequent. Pauses greater than 3 seconds, particularly if accompanied by symptoms like dizziness, syncope, or fatigue, may warrant consideration for a pacemaker.

About Sinoatrial Block

• Definition: Sinoatrial block is an interruption in the normal electrical activity where impulses from the SA node are delayed or fail to reach the atria.
• Mechanism: This may occur due to:
  • The SA node's failure to initiate an impulse (intrinsic dysfunction).
  • A block in the impulse before it exits the SA node, preventing it from depolarizing the atria.

Aetiology

• Increased Vagal Tone: High vagal tone, often seen in athletes or during sleep, can slow down or block SA node impulses temporarily.
• Medications: Drugs that impair cardiac conduction and may cause SA block include:
  • Beta-blockers
  • Calcium channel blockers
  • Digoxin
  • Antiarrhythmic agents (e.g., amiodarone)
• Intrinsic SA Node Disease: Age-related degenerative changes or fibrosis in the SA node impair its ability to conduct impulses effectively.
• Other Causes: Conditions such as myocarditis, ischemic heart disease, or electrolyte imbalances (e.g., hyperkalemia) may also contribute to SA block.

Types of Sinoatrial Block

• First-Degree SA Block: A delay in impulse conduction from the SA node to the atria, usually not visible on an ECG and typically asymptomatic.
• Second-Degree SA Block: Some impulses from the SA node are blocked, leading to occasional dropped beats.
  • Type I (Wenckebach): Progressive lengthening of conduction time until a beat is dropped.
  • Type II: Intermittent failure without progressive lengthening, resulting in random dropped beats.
• Third-Degree (Complete) SA Block: Complete failure of SA node impulses to reach the atria, leading to a complete pause and potentially causing escape rhythms from lower pacemaker sites.

Clinical Features

• Symptoms: Patients may experience dizziness, lightheadedness, palpitations, fatigue, and in more severe cases, syncope due to prolonged pauses or bradycardia.
• Signs: Physical examination may reveal bradycardia or irregular pauses.

Investigations

• 24-Hour Holter Monitor: Continuous ECG monitoring to detect intermittent SA block episodes.
• Electrolyte Panel: Assess for any contributing electrolyte imbalances, especially hyperkalemia.
• Thyroid Function Tests: Hyperthyroidism or hypothyroidism may exacerbate arrhythmias or conduction blocks.

Management

• Identify and discontinue any offending medications, such as beta-blockers or calcium channel blockers.
• Evaluate for underlying causes, such as electrolyte imbalances or thyroid dysfunction, and treat accordingly.
• Pacemaker Consideration: If pauses exceed 3 seconds or if the patient experiences symptoms (e.g., syncope), pacemaker insertion may be indicated to maintain regular rhythm.