The same is likely to be true for beats #1 and 7 - each of which is preceded by a PR interval slightly shorter than the PR interval of known sinus-conducted beats #8 and 9. Returning to Figure-4 - I believe the fact that the QRS complex of beat #10 is identical to the QRS of known sinus-conducted beats #8 and 9 - tells us that beat #10 is most probably sinus-conducted with a slightly shorter PR interval.Then WHY does the QRS complex of beat #10 look exactly like the QRS of sinus-conducted beats #8 and 9, despite being preceded by a shorter PR interval? I believe the answer is that this patient manifests a component of Vagotonic AV Block ( albeit not yet with clear demonstration of AV block.In addition, R wave amplitude of junctional beats #4 and 5 is clearly taller than the R wave of sinus-conducted beats #8 and 9! This PEARL is sometimes invaluable for telling us which beats in a tracing with AV dissociation are sinus-conducted vs of junctional origin vs fusion beats. We see a small-but-definitely-present initial q wave in these beats that is not seen in junctional beats #3, 4, 5 and 6. We know beats #8 and 9 are sinus-conducted. The reason is that the path of the electrical impulse for a sinus beat - may differ slightly from the path of a junctional beat that begins lower down or more to one side or the other of the AV node. P EARL # 4 : The QRS complex of junctional escape beats sometimes looks a little bit different than the QRS of sinus-conducted beats.This is not what we see in Figure-3 - as there is an obvious marked difference in P-P and R-R intervals for much of the long lead II rhythm strip. The effect is like a horse race - in which one rhythm temporarily “takes the lead” (ie, takes over the rhythm ) - until it either slows slightly, or until the other rhythm accelerates just enough to take over the rhythm - and then this back-and-forth process begins anew.
Most often, there is an underlying sinus ( atrial ) rhythm competing with an AV nodal rhythm beating at an almost identical rate. True “ isorhythmic” AV dissociation is an uncommon phenomenon, in which there are independent atrial and ventricular pacemakers that are beating at nearly identical (ie, “equal” ) rates. The word “iso” - is from the Greek word “isos” - which means “equal”. This is not “isorhythmic” AV dissociation.
This is because we never see any P waves that fail to conduct despite adequate opportunity to conduct! Instead, the reason why certain P waves do not conduct - is simply that the PR interval is too short. There is absolutely no evidence of any form of AV block.It is because of marked slowing in the rate that AV dissociation arises. Instead, as we state above - the rhythm is sinus bradycardia with marked sinus arrhythmia. AV dissociation is never the etiology of a rhythm. The rhythm in Figure-3 is not “AV dissociation”.
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