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First Degree AV block

First degree AV (heart) block

PQ Interval

• Physiological PQ interval ranges from (0.12 - 0.2s)
• PQ interval = P wave (about 0.08s) + PQ segment (about 0.1s)

1. Impulse originates in SA node
• When it moves to the atrial myocardium, it starts generating the P wave
• Simultaneously, it spreads through the conduction system towards the AV node
• The impulse in the conduction system does not create 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 through 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 (does not create a curve)
• Then it moves to His bundle (does not create a curve)
4. Activation of the ventricular septum
• From the His bundle the impulse through Purkinje fibers
• Begins to activate the myocardial septum
• Starts generating the Q wave

ECG and First-Degree AV Block

• Prolonged PQ interval > 0.20s (>5 small squares)
• Narrow QRS complex (<0.12s)
• Because the ventricles are activated through intact Tawara branches
• QRS may be widened (>0.12s) in case of aberrant conduction

First-Degree AV Block

• Prolonged PQ interval: 0.28s (> 0.2s)
• (7 small squares)
• Sinus rhythm

First-Degree AV Block

• Prolonged PQ interval: 0.28s (> 0.2s)
• Due to prolonged conduction through the AV junction (AV node)
• Sinus rhythm

First-Degree AV Block

• Prolonged PQ interval 0.28s (> 0.2s)
• P wave is embedded within the T wave
• Sinus Rhythm

First-Degree AV Block and Sinus Bradycardia

• Prolonged PQ interval 0.32s (> 0.2s)
• Sinus Bradycardia
• Heart Rate: 47/min. (< 50/min.)
• In the case of First-Degree AV Block and Sinus Bradycardia, consider:
• Overdose of beta-blockers or calcium channel blockers
• Inferior Wall Myocardial Infarction