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PQ Interval

PR (PQ) interval

ECG Curve of the PQ Interval

1. Impulse originates in the SA node
• When it propagates to the atrial myocardium, the P wave begins to form
• Simultaneously, it spreads through the conduction system toward the AV node
• The impulse in the conduction system does not form a curve
2. The impulse enters the AV node
• The impulse spreads from the SA node
• through the myocardium (P wave)
• and through the conduction system (internodal pathways) to the AV node
• At the time of atrial activation (peak of the P wave)
• It arrives via the conduction system to the AV node
3. Delayed (decremental) conduction in the AV node
• The impulse “lingers” in the AV node for about 0.1s (no curve is formed)
• Then it passes to His bundle (no curve is formed)
4. Activation of the ventricular septum
• From His bundle the impulse through Purkinje fibers
• Begins to activate the myocardium of the ventricular septum
• Formation of the Q wave begins

Shortened PQ Interval (<0.12s)

• Sinus tachycardia
• Junctional rhythm
• Pre-excitation (WPW syndrome, LGL syndrome)

Junctional Rhythm

• Heart rate: 50/min
• PQ Interval: 0.08s (2 squares)
• In junctional rhythm, the impulse originates in the AV junction
• Atria and ventricles are activated almost simultaneously
• Therefore, there is a shortened PQ interval
• The atria are activated retrogradely (from bottom to top), resulting in:
• retrograde (negative) P wave

LGL Syndrome

• PQ Interval 0.08s (2 squares)
• LGL Syndrome features a connection between the atria and the AV junction
• The connection (James bundle) bypasses the AV node
• The impulse does not slow down in the AV node
• Therefore, the PQ interval will be shortened

First-Degree AV Block

• Laddergram illustrates the propagation of impulses through the conduction system
• A - Atria (1), AV - AV Junction (2), V - Ventricles (4)
• PQ Interval: 0.28s (280ms)
• In First-Degree AV Block, conduction through the AV node is prolonged
• The PQ interval is prolonged (>0.2s)

Second-Degree AV Block - Mobitz I (Wenckebach)

• Laddergram illustrates the propagation of the impulse through the conduction system
• A - Atria (1), AV - AV Junction (2), V - Ventricles (4)
• Sinus Rhythm
• PP Interval is constant (810ms)
• PQ interval progressively lengthens until the P wave is blocked at the AV node
• 1. PQ (190ms)
• 2. PQ (290ms)
• 3. P wave is blocked
• The cycle then repeats
• This is Second-Degree AV Block - Mobitz I (Wenckebach) with ventricular conduction (3:2)
• From 3 P waves, 2 P waves are conducted to the ventricles (the cycle then repeats)