Sinus Arrest

[nick46928]

This may be a stupid question but it is one that have asked a lot of people about and no one could answer. I am unable to find any info about it on the internet.

Why is there no ventricular escape beat or a junctional escape beat when someone goes into sinus arrest? It makes sense to me that if the SA node didn't fire that the rest of the hearts automaticity would take over and there would be some type of junctional or ventricular beat.

[Thunderchild145]

Well actually, there typically is a junctional or idioventricular escape rhythm to replace the sinus arrest. This is more common. What's uncommon is where the SA node fails AND the junctional pacemaker fails AND the ventricular pacemaker fails and they go into asystole. That's typically a very bad sign of some serious conduction/automaticity problems.

[Ridryder 911]

Your question is very valid and I am not making light of it. Sometimes, you have to just wonder about the ole heart. It does what it wants to do, you will find out as many cardiologist describe it can be very unpredictable and who knows why?

As discussed, sometimes when the intrinsic rate falls below, usually the next firing mechanism will pick up. However; not all the time and the heart will re-set itself. Like I describe as "re-booting" in computers. I believe it is all dependent upon the cardiac tissue of automaticity to detect if another pacemaker needs to fire or a clearing of the system and resetting itself is necessary. This of course can be dependent upon the fluctuation of depolarizing and as well as on how much is needed to regain firing on the absolute period.

R/r 911

[scope2776]

Okay, I understand what you're asking...

First, it might be helpful to look at sinus arrest as a result of another pathology or the result of another rhythm, not necessarily a rhythm of its own.

Second, there are a couple of different terms:

SA block is normally the result of a non-conducted impulse from the SA node. (No P waves) The P to P interval will remain unchanged.

Sinus Pause/Arrest (still no P waves) is a variable time period in which the SA node is not working. The time interval is not a multiple of the normal P to P interval. Therefore the P to P interval will not "march out". Normally a sinus pause is less than 3 seconds and a sinus arrest is greater than 3 seconds.

A patient in true SA block "rhythm" is in full arrest.

SA pauses/blocks are normally seen in the compensatory pauses after abberently conducted beats (PVCs, PJCs, etc). The myocytes need to "recharge" or continue to gather electrolytes for the next impulse that were depleted by the last depolarization. Therefore in this sense it's more of a result of another event, not a rhythm of its own... understand? The lesser pacemakers cannot fire at this point because they were just depolarized with the last premature beat and also need to "re-charge" or reach their action potential.

This is of course assuming the automatically of the SA myocytes has not been directly suppressed by pathological process such as hypoxia, channel blockers, trauma, ect. In which case the pt is also in "arrest".

Hope this helps...

[nick46928]

All that somewhat makes sense.

One more question kind of involving that same thing. If Adenosine slows down impulses through the AV node then why is it that it doesn't create something that looks like a 1st degree AV block (or 3rd degree if it completely stops the conduction as opposed to just slowing it down)? Shouldn't the atria continue to fire and there just be a delay at the AV node rather then the patient going to a sinus arrest?

(Remember, I am just a medic student so if I am just mistaken about the action of Adenosine please correct me.)

[chbare]

It can. You can see multiple rhythms transiently develop following adenosine. I have seen high grade AV blocks, ventricular escape beats, and even runs of ventricular tachycardia.

Take care,

chbare.

[FireMedic65]

thankfully the half life is only what... 6 seconds? and hopefully that rhythm goes away after that 6 seconds:)