• Conduction System and Concealed Conduction
• Mechanism of Concealed Conduction
• Ventricular Extrasystole and Concealed Conduction
• Atrial Fibrillation and Concealed Conduction
• Pseudo AV Block and Concealed Conduction
• Reciprocal Echo Beat and Concealed Conduction

Conduction System and Concealed Conduction

• Impulse (action potential) propagates through
• the conduction system and working myocardium
• During propagation, it generates an electrical vector
• Electrical vector in different parts of the heart
• Is seen on ECG paper as an ECG curve
  
  
• Concealed conduction
• Is impulse conduction through a certain part of the conduction system
• that is not visible on the ECG curve (is concealed)
• We diagnose it based on the subsequent ECG curve
• Concealed conduction most commonly manifests in the AV junction

Ventricular Extrasystole and Concealed Conduction

Concealed Conduction and Ventricular Extrasystole

• A laddergram illustrates impulse propagation through the conduction system
• A - Atria, AV - AV junction, V - Ventricles
• (1) An interpolated ventricular extrasystole (VES) - occurs between 2 sinus beats
• The impulse from the VES retrogradely penetrates the AV junction and alters its electrophysiological properties
• The following sinus impulse reaches the AV junction during the partial refractory period
• The impulse conducts more slowly through the AV junction
• Which is seen on the ECG as a prolonged PQ interval
• Concealed conduction is the retrograde conduction of the VES into the AV junction, which is not visible on the ECG
• We only see the subsequent prolonged PQ interval
• (2) Ventricular Extrasystole
• Retrogradely penetrates the AV junction
• The following sinus impulse reaches the AV junction during the refractory period and gets blocked

Ventricular Extrasystole (VES)

• The retrograde impulse from the VES penetrated the AV junction
• The P wave after the VES is blocked
• In the AV junction, the sinus and retrograde impulses from the VES met in the refractory phase, blocking each other
• After the VES, a complete compensatory pause follows
• 2x the RR interval without VES = RR interval with VES
• Concealed conduction is not present

Pseudo AV Block and Concealed Conduction

• Concealed conduction can sometimes mimic an AV block on the ECG
• However, this is a pseudo AV block with a different electrophysiological mechanism

Concealed Conduction and Pseudo AV Block II Degree (Mobitz II)

• The ECG shows AV block II degree (Mobitz II)
• Every 2nd P wave is blocked
• This is concealed conduction that mimics an AV block on the ECG
• Junctional extrasystole and a sinus impulse meet in the AV junction during sinus rhythm
• However, the impulses meet in the refractory phase and block each other
• The sinus and junctional impulses are not conducted to the ventricles
• In a long-term ECG recording, the ECG pattern of AV block II degree will not continue
• Because the junctional extrasystole would have to occur exactly at the time
• when the AV junction is in the refractory phase due to the impulse from the SA node
• This is impossible in a long-term ECG recording

Reciprocal Echo Beat and Concealed Conduction

• The AV junction has 2 pathways (slow and fast)
• If an impulse starts to revolve around the pathways (Re-entry), it leads to AV nodal re-entry tachycardia
  
  
• Reciprocal Echo Beat
• Occurs with specific timing of an extrasystole, when the impulse "bounces off" the AV junction
• The impulse revolves around the AV junction (through the 2 pathways) so that it does not encounter its own refractory period
• One impulse thus activates both the atria and ventricles from the AV junction

Concealed Conduction and Reciprocal Echo Beats

• This is concealed conduction, where one impulse creates multiple ECG waveforms (P waves, QRS complexes)
• Atrial Echo
• The impulse from an atrial extrasystole
• Revolves around the AV junction
• One impulse activates the atria twice and the ventricles once
• Junctional Echo
• The impulse from a junctional extrasystole
• Revolves around the AV junction
• One impulse activates the atria once and the ventricles twice
• Ventricular Echo
• The impulse from a ventricular extrasystole
• Revolves around the AV junction
• One impulse activates the atria once and the ventricles twice

Atrial Echo and Second-Degree AV Block (Wenckebach)

• On the ECG, there is second-degree AV block (Mobitz I - Wenckebach) with variable conduction to the ventricles
• PQ interval progressively lengthens until the P wave is blocked
• In the middle of the ECG recording, the ventricular conduction ratio is 6:5 (6th P wave is blocked)
• Notice on the laddergram, the delay of impulse in the AV node is 27ms, 28ms, 30ms
• For the impulse to turn around in the AV junction, it must not encounter the refractory period (must be precisely timed)
• Atrial Echo occurs only when
• The conduction in the AV node is extended to 28ms
• Then the impulse turns around in the AV junction and does not encounter the refractory period