PR interval on ECG

Pre-excitation syndromes and interval shortening

A decrease in the electrical impulse conduction time (when the interval is less than 0.12 s) indicates the presence of specific ventricular pre-excitation syndromes.

A striking example is Lown-Ganong-Levine (LGL) syndrome. This anomaly is characterized by clear ECG criteria: against the background of a shortened P-Q interval, an absolutely unchanged, normal ventricular QRS complex is recorded.

The prognosis in pre-excitation syndromes is often regarded as unfavorable. The main danger lies in the high probability of paroxysmal arrhythmias. The development of atrial fibrillation against the background of anomalous conduction poses a particular threat to the patient's life.

Conduction slowing: first-degree AV block

An increase in interval duration of more than 0.21 s indicates slowed impulse conduction through the atrioventricular node. It is important to note that all atrial impulses reach the ventricles without exception.

This condition is classified as first-degree AV block. The pathology is diagnosed solely by instrumental methods, based on ECG data. The normal rhythm is preserved on the ECG, and QRS complex dropouts are completely absent.

Historical fact: in the Kischel-Jones diagnostic criteria, the prolongation threshold of more than 0.2 s is traditionally used as a reliable sign of impaired AV conduction in rheumatic fever.

Since such a block may be transient, patients must undergo dynamic monitoring and repeat ECG recordings to assess conduction stability.

Dynamic changes: second-degree AV blocks

As conduction disorders progress, the P-Q interval begins to behave variably, which serves as the key to differential diagnosis of block types:

• Type I (Mobitz I): characterized by Wenckebach periodicity. The ECG shows progressive prolongation of the P-Q interval from one cardiac cycle to the next. This process culminates in the dropout of a single QRS complex, after which the cycle repeats. The level of impulse blocking is usually proximal (located within the AV node itself), so the prognosis for the patient is generally more favorable.
• Type II (Mobitz II): differs in mechanism. The P-Q interval remains absolutely stable and does not change before the complex dropout. Impulse blocking occurs suddenly. QRS dropout may be regular (e.g., every third or fourth impulse — 3:1 or 4:1 block) or irregular. The site of lesion here is distal (located below the AV node), which is prognostically less favorable due to the high risk of progression to complete heart block.

Complete rhythm dissociation (third-degree AV block)

When complete heart block develops, atrial impulses cease to be conducted to the ventricles. In this situation, the P-Q interval cannot be evaluated standardly, as complete rhythm dissociation occurs.

Sinus P waves are recorded regularly and at a normal rate, but they have no relation to the QRS complexes. Ventricular activity is maintained by an ectopic (escape) pacemaker. The nature of the QRS complexes depends on the level of the block:

• Proximal level (from the AV junction): QRS complexes remain narrow or slightly modified, with a relatively high rate (about 40 per minute).
• Distal level (idioventricular rhythm): QRS complexes become wide and deformed, and the rate drops to critical values (less than 30 per minute).

Clinically, complete heart block is extremely dangerous: it may be accompanied by Stokes-Adams attacks (episodes of fainting due to a sharp drop in cardiac output), which is a direct indication for cardiac pacing.

Class and exam questions

What else to study on this topic

• ECG criteria and topical diagnosis of bundle branch blocks
• Brugada syndrome: genetics, pathogenesis, and risk of sudden death
• Differential diagnosis of bundle branch blocks and ventricular extrasystoles
• Signs of pulmonary hypertension and right ventricular overload on ECG
• Complete AV block and indications for cardiac pacing

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