ECG Interpretation of First-Degree Heart Block

Diagnostic Criteria

• The American College of Cardiology defines first-degree AV block as a PR interval between 200-400 ms, which is met in this case with a PR interval of 0.26 seconds 1
• A prolonged P-R interval of 0.26 seconds is the defining feature of first-degree heart block, with every P wave followed by a QRS complex in a 1:1 relationship 1
• A regular rhythm with a constant P-R interval confirms that this is not a higher-degree block 1

ECG Characteristics

• The QRS duration of 0.08 seconds (80 ms) is normal, ruling out bundle branch blocks 3, 4, 5
• Left axis deviation is defined as an axis between -30° to -90°, and an axis of -20° is considered borderline in athletes but normal in the general population 1, 2, 4

Clinical Implications

• First-degree AV block is often physiologic in athletes due to increased vagal tone and is considered a normal variant when asymptomatic 1
• In non-athletes, first-degree AV block may warrant evaluation for underlying causes, such as medications, ischemia, electrolyte abnormalities, or intrinsic conduction disease 1
• No immediate treatment is required for isolated first-degree AV block in hemodynamically stable patients 1

Diagnostic Differentiation

• Complete LBBB requires a QRS duration of ≥120 ms in adults, which is not met in this case with a QRS of 0.08 seconds (80 ms) 3, 4, 5
• LBBB also requires broad notched R waves in leads I, aVL, V5, and V6, with absent q waves in these leads, which is not present in this case 4, 5