First-Degree AV Block vs U Wave: Key ECG Distinctions
First-degree AV block and U waves are completely different ECG phenomena that occur at different times in the cardiac cycle and represent distinct electrophysiological processes—first-degree AV block reflects delayed AV conduction (PR interval >200 ms), while the U wave is a low-amplitude deflection that appears after the T wave during ventricular repolarization.
Fundamental Differences
First-Degree AV Block
- Timing: Occurs at the beginning of the cardiac cycle, measured from the start of the P wave to the start of the QRS complex 1
- Definition: PR interval prolongation >200 ms with 1:1 AV conduction (technically a "delay" rather than true block, as every P wave conducts) 1
- Location on ECG: Involves the P-QRS relationship
- Mechanism: Represents slowed conduction through the AV conduction system, typically within the AV node 1
U Wave
- Timing: Occurs at the end of the cardiac cycle, appearing after the T wave and before the next P wave 2
- Characteristics: Low-amplitude (approximately 0.33 mV or 11% of T wave amplitude), low-frequency deflection 2
- Location on ECG: Most evident in leads V2 and V3, frequently absent in limb leads 2
- Mechanism: A mechanoelectric phenomenon, with multiple proposed origins including delayed repolarization, Purkinje fiber repolarization, or mechanical stretch 3, 4
Clinical Recognition Features
First-Degree AV Block
- Measure the PR interval from the beginning of the P wave to the beginning of the QRS
- Present at all heart rates when pathologic
- May be associated with wider QRS complexes 5
- Clinical significance: Not always benign—associated with increased cardiovascular events and mortality in some populations 6, 5
U Wave
- Heart rate dependent: Rarely present at rates >95 bpm; present in 90% of cases at rates <65 bpm 2
- Normal when upright and small in amplitude
- Abnormal findings 2:
- Inverted U waves in V2-V5 (suggests ischemia or hypertension)
- U wave amplitude exceeding T wave amplitude (severe hypokalemia, K <2.7 mmol/L)
- Fusion with T wave (long QT syndromes, increased sympathetic tone)
Critical Pitfall to Avoid
Do not confuse a prominent U wave with a prolonged PR interval or mistake it for a P wave of the next cycle. The U wave occurs in the TP segment after ventricular repolarization is complete, while first-degree AV block affects the timing before ventricular depolarization 2, 7. In supraventricular tachycardias, U waves can simulate pseudo-P waves and mimic long RP tachycardias, leading to diagnostic confusion 7.
When to Report Each Finding
First-Degree AV Block
Report when PR interval exceeds 200 ms. Consider further evaluation when:
- PR interval >300 ms (extreme prolongation may cause pacemaker-like symptoms)
- Associated with symptoms suggesting hemodynamic compromise 1
- Present in high-risk populations (e.g., Brugada syndrome, where it predicts arrhythmic events) 5
U Wave 2
Report when:
- Inverted in precordial leads
- Merged/fused with T wave
- Amplitude greater than T wave amplitude
The U wave is often subtle and overlooked; its recognition requires careful ECG analysis, particularly in leads V2-V3 where it is most prominent 2.