Yes, it does usually cause a distortion of some kind in the preceding T-wave, but if they're all identical (1:1 relationship to the QRS/T) then it's hard to know if the distortion is native to the T-wave itself. This is the kind of ECG-reading expertise that only comes through lots of practice and experience, or what you might call a "trained eye".
As for the refractory period, the atria are separate from the ventricles and have their own refractory period. The P wave might occur during the ventricle's absolute refractory period, but if there's a long enough delay through the AV node (PR interval) then the ventricles will be recovered in time to be depolarized by the impluse when it finally hits the His/Purkinje system.
Tom
On Tue, Jul 14, 2009 at 1:38 PM, Robert Vroman <r.vroman@...> wrote:
“Once you accept that sinus tachycardia is a form of SVT, this view is no longer tenable.”
This of course may be semantics and the opinions of various people, but my understanding has been that the umbrella term of SVT does not include sinus initiated rhythms. However, if it is looked at as any tachycardia originating above the ventricles then sinus would be included.
“What would you call junctional tachycardia at 140 beats/min? Something other than SVT?”
I would call it junctional tach. But I see your point; if one is not able to identify it as junctional then you have to fall back to SVT. In general though I try to avoid the SVT nomenclature unless it is a situation where all I can truly say is “fast and narrow”. Hopefully this is the exception rather than the rule.
Your post has raised another question though. You mention that in the setting of a 1AVB the P wave could be buried in the preceding QRS complex or T wave. If in the QRS complex wouldn’t that be in the absolute refractory period and as such not depolarize the ventricles? In other words I would think it would cause dissociation between the atria and ventricles. If in the T wave, or relative refractory period, I would propose that more often than not there is some distortion of the T wave to indicate the presence of a P wave in there. Perhaps I am missing something you were saying?
"As far as the subsequent discussion of rates goes, my understanding is that the category of SVT is not really considered until the rate is over 150. That is because at rates less than that if P waves are present they will be seen. However the key word is considered, and this classification should be a last ditch usage. It is very possible to be able to identify the actual rhythm at rates over 150, and it can indeed be sinus as well as others. I have often heard 180 used as the theoretical upper limit of the sinus node. However is there any evidence to support this? In school I remember learning the formula Paul mentioned (220 – age). Also I have not seen it this year in the coverage of the Tour de France, but I remember a few years ago they would show the riders heart rates which were many times above 180. I would assume those were sinus rhythms."
I see the disconnect now.
Once you accept that sinus tachycardia is a form of SVT, this view is no longer tenable. As the most frequently encountered SVT, sinus tachycardia is the first heart rhythm I consider when I'm dealing with a narrow complex tachycardia, and as Paul mentioned, you always have to consider whether or not it's a compensatory tachycardia. I consider the Hs and Ts for all tachycardias. The other issue with regard to atrial complexes is that when you have sinus tachycardia with 1AVB, the P waves can be obscured by the preceding T wave at rates below 150. What would you call junctional tachycardia at 140 beats/min? Something other than SVT? On occasion I've seen 2:1 atrial tachycardia that was mistaken for sinus tachycardia because every other atrial complex was buried in the QRS/T complex. While atrial flutter with 2:1 conduction is often right at 150, sometimes it's faster and sometimes it's slower. It's still SVT. Same with AF w/RVR. That's the problem with imprecise definitions and ambiguous umbrella terms like SVT that are used as if they are actual heart rhythms as opposed to a specific category of heart rhythms. By definition, a tachycardia is a heart rate > 100. By definition, if it's supraventricular then it's anything that originates above the ventricles. Why complicate matters by throwing in arbitrary criteria that add no value in terms of figuring out the diagnosis and can in fact be misleading?
Tom
-- Tom Bouthillet Lieutenant / Paramedic Town of Hilton Head Island Fire & Rescue Division 843-247-3453 (cell) ems12lead.blogspot.com
-- Tom Bouthillet Lieutenant / Paramedic Town of Hilton Head Island Fire & Rescue Division 843-247-3453 (cell) ems12lead.blogspot.com
This is a woman come in ewith plapitation and normal BP. Canon A wave was seen. No P wave can be seen.Rate is less than 150. R wave in Avr lead. No retrograde...
Just to clarify, SVT is an umbrella term for all non-ventricular tachycardias. So the right question is, "What kind of SVT is this?" Tom Sent from my iPhone...
Only because I know someone has the answer...? Where did the rate of 150 come to be the determinant between "SVT" and non?? Why don't all monitors allow the...
That is a horrible EMS myth, Jon. I've reviewed cases where Adenocard was given to sinus tachycardia, simply because it was 160 bpm and "couldn't have been...
Tom: My point is clarified here and in the other posts.? SVT is a largely non-specific term (vs VT) for rhythms with a rate at or above 150, when, really,...
a few things, using heart rate alone is not a good way to make an ecg dx, think about a "nornal" stress test, a 20y old would be expected to hit 200bpm (sinus)...
What kind of SVT is it, Haval Lutfalla? Sinus tachycardia? AF w/RVR? AVNRT? AVRT? Junctional tach? Calling it SVT only points toward a group of abnormal heart...
Chill out all we are on the same side :-). It is important to label things correctly because when you for example say VT everyone knows what you mean, can draw...
 Sorry for my quick typing before.  I know SVT is a large pool of all kind include the junctional tachycardia.  No P wave: not sinus tachycardiaof course...
I will come out of "lurker mode" and give my two cents on this one. I would go with junctional tach for a couple of reasons. First, the rate is such that if P...
Robert just to chime in the heart rate thing. 220 - age * 0.85 is the normal "expected" heart rate for someone during exercise. I've seen it be much lower (if...
Robert Vroman wrote: <snip> "As far as the subsequent discussion of rates goes, my understanding is that the category of SVT is not really considered until the...
"Once you accept that sinus tachycardia is a form of SVT, this view is no longer tenable." This of course may be semantics and the opinions of various people,...
Yes, it does usually cause a distortion of some kind in the preceding T-wave, but if they're all identical (1:1 relationship to the QRS/T) then it's hard to...
Tom- It is hard to get people to understand that, it is far easier to accept the easy term than to read the fine print. ===================================== ...
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