A Rare ECG Event

Ventricular Standstill with Atrial Dissociation

Image Case

This strip is a rare ecg event. The initial strip shows sinus tachycardia with right bundle branch block (RBBB). A PVC initiates ventricular standstill and there is recovery of conduction several seconds after.  In the top strip, the P wave is marked with red arrow. As you go from left to right and the bottom, we can see a change in the morphology of the P wave.This is actually the separation of 2 p waves (black and red arrows at the bottom strip). The other P wave is smaller (black arrow) than the other (red arrow). This is from a post-cardiac transplant patient. So, what we have is ventricular standstill that unmasked the 2 p waves (atrial dissociation). One P wave is from the recipient and the other is from the donor.

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Paroxysmal AV block (Ventricular standstill)

Paroxysmal AV block (Ventricular standstill)

-characterized by an abrupt and persistent AV block in the presence of otherwise normal AV conduction.

-may be initiated by:

conducted or blocked PAC or PVC

acceleration or slowing of sinus rhythm

**** in this case, we can see the slowing of the sinus rhythm.... went into 2:1 AVB then complete AVB with ventricular standstill.

-once the block is initiated, the block will persist until terminated by an escape, usually ventricular, with a predictable 

relationship of the escape to the following P wave

-most likely infra-His or subnodal and due to bilateral BBB (exact mechanism is unclear).

Reference: 

Fisch C and Knoebel SB. 2000. Electrocardiography of Clinical Arrhythmia. New York. Futura Publishing Co.

#215

Paroxysmal AV block (ventricular standstill)

Looking at the case in a 6-second strip is soooo scary.

Strip # 1 - before the non-conducted beat is the gradual slowing of the rate (longer RR interval) and there is  prolongation of the PRI before the non-conducted beats.

Strip # 2 - there is a conducted beat followed by non-conducted P waves. This is called ventricular standstill or paroxysmal AV block.

Strip # 3 is the continuation of the ventricular standstill/paroxysmal AV block.

Strip # 4 - if seen in isolation can be mistaken at Mobitz II. It should be properly labeled as 2:1 AV block.

Strips # 5 and 6 also showed 2:1 AV conduction but here it is clear that this a type 1 mechanism due to the PRI prolongation before the dropped beat.

This case has a brain mass. It impaired the swallowing function, so the nurse is regularly suctioning the patient. The RR prolongation (slowing of the heart rate and PRI prolongation are characteristics of vagally-induced event.

So this is vagal-induced.

****

Paroxysmal AV block can be initiated by a conducted on non-conducted PAC, PVC, acceleration or deceleration of sinus beats. According to Fisch, the electrophysiology of paroxysmal AV block is unclear. One proposed mechanism is repetitive concealed conduction. Concealed conduction is the failure of a cardiac impulse to traverse the entire AV conducting system. Another mechanism is the prolonged PP interval after the PAC associated with deceleration-dependent (Phase 4) depolarization of the lower part of the junction and  resultant AV block.

Again according to Dr. Fisch, "resumption of normal AV conduction has been ascribed to conduction during the supernormal period of recovery following idioventricular impulse, summation of anterograde or retrograde  impulses, and Wedensky facilitation. In which a properly timed retrograde impulse makes it possible for an  otherwise non-threshold anterograde impulse to conduct and "peel" back the refractory period".

Reference: 

Fisch C and Knoebel SB. 2000. Electrocardiography of Clinical Arrhythmia. New York. Futura Publishing Co.

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