What is the best summary of ECG findings showing a rate of 88 beats per minute (bpm), regular rhythm, axis -20 degrees, prolonged P-R interval of 0.26 seconds, and normal QRS complex duration of 0.08 seconds?

ECG Interpretation: First-Degree Heart Block

The best summary of this ECG is first-degree heart block (Option A), characterized by the prolonged P-R interval of 0.26 seconds with otherwise normal conduction parameters.

Key Diagnostic Features

The ECG demonstrates a P-R interval of 0.26 seconds (260 ms), which exceeds the upper limit of normal (200 ms) and defines first-degree atrioventricular block 1. According to the American College of Cardiology guidelines, first-degree AV block is defined as a PR interval between 200-400 ms, which this patient clearly meets 1.

Why This is First-Degree Heart Block

• Prolonged P-R interval (0.26 seconds) is the defining feature, with every P wave followed by a QRS complex in a 1:1 relationship 1
• Regular rhythm with constant P-R interval confirms this is not a higher-degree block 1
• The QRS duration of 0.08 seconds (80 ms) is normal, ruling out bundle branch blocks 1

Why the Other Options Are Incorrect

Left Axis Deviation (Option B)

• While the axis is -20°, this falls within the normal range (normal axis is typically -30° to +90°) 1
• Left axis deviation is defined as an axis between -30° to -90° according to American College of Cardiology criteria 1
• An axis of -20° is considered borderline in athletes but normal in the general population 1

Left Bundle Branch Block (Option C)

• Complete LBBB requires QRS duration ≥120 ms in adults 1
• This patient has a QRS of only 0.08 seconds (80 ms), which is completely normal 1
• LBBB also requires broad notched R waves in leads I, aVL, V5, and V6, with absent q waves in these leads 1

Clinical Significance

• 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 presenting to the emergency department, first-degree AV block may warrant evaluation for underlying causes (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

Common Pitfalls

• Do not confuse first-degree AV block with higher-degree blocks—the 1:1 AV relationship with constant PR interval is key 1
• A slightly leftward axis (-20°) should not distract from the primary abnormality of prolonged AV conduction 1
• Normal QRS duration definitively excludes bundle branch blocks, which require QRS ≥120 ms 1