Which ECG finding best distinguishes a prolonged PR interval due to first-degree AV block from a prolonged QT interval?

Question

Accepted Answer

C. PR interval involves AV nodal delay; QT interval reflects ventricular repolarization duration. ## Distinguishing PR Interval Prolongation from QT Prolongation

### Anatomical and Physiological Basis

**Key Point:** The PR interval and QT interval represent conduction and repolarization in different cardiac compartments, and their response to physiological stress differs fundamentally.

| Feature | PR Interval (First-Degree AV Block) | QT Interval (Prolongation) |
|---------|--------------------------------------|---------------------------|
| **Anatomical site** | AV node, bundle of His, bundle branches | Ventricular myocardium (repolarization phase) |
| **Reflects** | Atrial depolarization + AV conduction delay | Ventricular depolarization + repolarization |
| **Normal duration** | 120–200 ms | Varies with HR (corrected QTc: < 440 ms males, < 460 ms females) |
| **Response to exercise** | May shorten (increased sympathetic tone, faster AV conduction) | Shortens (increased HR → shorter QT) |
| **Pathophysiology** | Slowed conduction through AV node | Delayed repolarization (prolonged action potential) |

### Clinical Pearl

**High-Yield:** The PR interval is a *conduction* interval (depolarization), while the QT interval is a *repolarization* interval. This distinction is critical:
- **First-degree AV block** = slowed conduction → prolonged PR, normal QT
- **Long QT syndrome** = delayed repolarization → normal PR, prolonged QT (corrected for rate)

### Mechanism of Differentiation

The most reliable discriminator is the **physiological compartment involved**:
1. PR interval prolongation → AV nodal conduction delay (electrical pathway)
2. QT interval prolongation → ventricular repolarization delay (ion channel dysfunction or electrolyte abnormality)

Both may coexist in certain conditions (e.g., amiodarone, hypocalcemia), but the *primary* pathophysiology differs.

### Practical Bedside Approach

**Tip:** When analyzing an ECG:
- Measure PR interval from start of P wave to start of QRS
- Measure QT interval from start of QRS to end of T wave
- Correct QT for heart rate using Bazett formula: $QTc = \frac{QT}{\sqrt{RR}}$ (RR in seconds)
- If PR is prolonged but QTc is normal → first-degree AV block
- If QTc is prolonged but PR is normal → repolarization abnormality

[cite:Harrison 21e Ch 297]

![ECG — Waves and Intervals diagram](https://mmcphlazjonnzmdysowq.supabase.co/storage/v1/object/public/blog-images/explanation/3734.webp)

Answer

A. PR interval normalizes with exercise; QT interval remains prolonged

Answer

B. PR interval > 200 ms; QT interval varies with heart rate

Answer

D. PR interval is fixed; QT interval shortens with increased heart rate

Which ECG finding best distinguishes a prolonged PR interval due to first-degree AV block from a prolonged QT interval?

Explanation

Distinguishing PR Interval Prolongation from QT Prolongation

Anatomical and Physiological Basis

Clinical Pearl

Mechanism of Differentiation

Practical Bedside Approach

Practice similar questions

Join our NEET PG community

Feature	PR Interval (First-Degree AV Block)	QT Interval (Prolongation)
Anatomical site	AV node, bundle of His, bundle branches	Ventricular myocardium (repolarization phase)
Reflects	Atrial depolarization + AV conduction delay	Ventricular depolarization + repolarization
Normal duration	120–200 ms	Varies with HR (corrected QTc: < 440 ms males, < 460 ms females)
Response to exercise	May shorten (increased sympathetic tone, faster AV conduction)	Shortens (increased HR → shorter QT)
Pathophysiology	Slowed conduction through AV node	Delayed repolarization (prolonged action potential)