What are the signs and symptoms of normal sinus rhythm with 1st degree atrioventricular (AV) block?

Signs and Symptoms of Normal Sinus Rhythm with First-Degree AV Block

First-degree AV block is typically asymptomatic and represents a benign finding in most patients, characterized by a prolonged PR interval >200 ms on ECG with all atrial impulses still conducting to the ventricles. 1, 2

Clinical Presentation

Most Common Scenario: Asymptomatic

• The vast majority of patients with first-degree AV block have no symptoms whatsoever and the finding is discovered incidentally on routine ECG 2, 3
• Normal physical examination findings with regular heart rhythm 1
• No complaints of dizziness, syncope, or exercise intolerance in typical cases 2

When Symptoms Do Occur (PR Interval ≥300 ms)

Symptoms emerge when the PR interval becomes markedly prolonged, typically exceeding 300 ms, causing hemodynamic consequences: 1, 2, 3

• Fatigue and exercise intolerance - the most common symptomatic complaints 2, 3
• "Pseudo-pacemaker syndrome" symptoms including:
  • Exertional dyspnea 2
  • Decreased exercise capacity 3
  • Sensation of pulsations in neck or chest 4, 5
  • Presyncope or lightheadedness (rare) 5

Mechanism of Symptoms

The hemodynamic compromise occurs because: 1, 3, 5

• Left atrial systole occurs too close to or simultaneous with the previous left ventricular systole 1
• This causes inadequate left ventricular filling 1
• Increased pulmonary capillary wedge pressure 3
• Reduced cardiac output, particularly during exercise when the PR interval fails to shorten appropriately 5

Physical Examination Findings

Typical Findings

• Regular pulse with normal rate (sinus rhythm maintained) 1
• Normal blood pressure in most cases 3
• No signs of heart failure in isolated first-degree AV block 2

Subtle Findings in Marked First-Degree Block

• Cannon A waves may occasionally be visible in jugular venous pulsations when atrial contraction occurs against a closed tricuspid valve 5
• These findings are much less prominent than in complete heart block 5

Important Clinical Distinctions

Athletes and High Vagal Tone

• First-degree AV block is common and physiologic in trained athletes, present in up to 35% of athlete ECGs 1
• Resolution with exercise or hyperventilation confirms benign, vagally-mediated etiology 1, 6
• Athletes remain asymptomatic and can participate in all competitive sports 2

Red Flags Requiring Further Evaluation

Watch for these features that suggest the block may NOT be benign: 2, 3

• Any symptoms of fatigue, exercise intolerance, or presyncope 2
• PR interval ≥300 ms 2, 3
• Wide QRS complex suggesting infrahisian disease 1
• Failure of PR interval to shorten with exercise 1, 5
• Coexisting bifascicular block 2
• Presence of structural heart disease 2, 3

Key Clinical Pitfall

Do not assume all first-degree AV block is benign - while the PR interval between 200-300 ms is almost always asymptomatic 3, marked prolongation ≥300 ms can cause significant symptoms that improve with pacing 1, 2, 4. The challenge is that symptoms like fatigue are nonspecific, so exercise testing to demonstrate failure of PR interval adaptation is crucial for confirming the block as the cause 5.