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Dawn's picture

Atrial Bigeminy Vs Possible Sino-atrial Exit Block

This three-lead rhythm strip is from an 85 year-old-woman for whom we have no other information, unfortunately.  It shows an obvious, regular bigeminal pattern.  There are many rhythms that can cause bigeminy, with regular, paired beats.  So, how do we analyze this rhythm? 

First, we can see that all the QRS complexes are fairly narrow and look alike in each lead.  So, we know this is a supraventricular rhythm.  The rhythm “marches out”, with the same spacing between all the paired beats, and exactly one P-QRS missing after each pair. 

Sino-atrial exit block occurs when the sinus node fires, but the impulse is unable to exit the sinus node. No P wave will occur, but the sinus node continues to fire in a regular rhythm.  There are four types of SA block, just as there are four types of AV block.  This ECG meets the criteria for a Second-degree, Type II SA block. 

First-degree SA block would not be decipherable on the ECG, as we do not see the SA node fire, and therefore cannot measure an interval between the SA node firing and the production of a P wave. 

Second-degree SA block, Type I would have progressively prolonging exit times for the impulse leaving the SA node.  This would produce progressively shortening R-to-R intervals, until the pause. 

Second-degree SA block, Type II has regular R-to-R intervals until the pause, and the pause will equal two or more (usually two) R-to-R intervals. 

Third-degree SA block produces NO P WAVES, and would have to be diagnosed on electrophysiology studies, as it would usually result in a junctional escape rhythm. 

Dawn's picture

Sinus Rhythm With Atrial Bigeminy

This ECG is from an 88-year-old man with congestive heart failure.  No other clinical information is known.  It shows an underlying sinus rhythm with atrial bigeminy - every other beat is a premature atrial contraction.  There is very little, if any, difference in the morphology of the sinus P waves and the ectopic P waves, indicating that the ectopic focus is in the vicinity of the sinus node.  There is no "compensatory" pause, because PACs penetrate the sinus node, resetting it.  So, the underlying sinus rate here is about 72 beats per minute.  There are several mechanisms for bigeminy to occur, but ectopic bigeminy is the most common.

Dawn's picture

ECG Basics: Sinus Rhythm With Ventricular Bigeminy

This rhythm strip offers two leads taken at the same time, Lead II and Lead V1.  The Lead II strip may not look "typical" to a beginning student, because the sinus beats are very small and biphasic.  This is due to an axis shift, which cannot be evaluated without more leads.

One of the best teaching opportunities in this strip is the concept of "underlying rhythm" with ectopy.  The underlying rhythm here is sinus.  But there are sinus P waves which are hidden, making the sinus rate twice what it appears to be.  The P waves are invisible in the Lead II strip, with baseline artifact making them even harder to see.  But in V1, we are able to find them at the end of the PVCs' T waves.  The sinus rhythm is a bit irregular toward the end of the strip.  There are probably many things a more advanced practitioner could say about this strip, but it usually requires more than one or two leads to do a complete evaluation.  For your basic student, it is a good example of sinus rhythm with ventricular bigeminy.

Dawn's picture

Bigeminal Rhythm

To continue a recent theme, we offer this ECG, which appears on quick glance to be an atrial bigeminy.  On closer inspection, we do not see ectopic P' waves in the second conducted beats of each pair.  In fact, the P waves look the same in both the first and second beats. However, if you look very closely (we enlarged the screen), you may notice that the second T wave of each pair is slightly larger with a slightly different morphology than the first T wave.  If this distortion of the second T waves represents a PAC, the PAC is non-conducted and concealed in the T wave. If that concealed P wave is able to penetrate and depolarize the sinus node, we might expect the sinus node to be reset, and the next P wave and QRS combination is right on time.

So, what looks like an "atrial bigeminy" might actually be an "atrial trigeminy" - albeit with non-conducted PACs.

Also notable is the subtle ST elevation in V1, V2, and V3, which should be evaluated with clinical data. Unfortunately, we do not have information about the patient.  Also, V2 and V3 do not represent normal or expected R wave progression.  The chest leads should have rS patterns, and they should progress from small r waves and deep S waves in V1 with gradually taller R waves and shorter S waves across the chest wall.  V6 should be, finally, all upright.  We suspect lead reversal between V2 and V3, or misplacement of the electrodes in general.  Also, the q waves in those leads seem misplaced, without the usual configuration of pathological Q waves in those leads.

We would love to hear your opinions about this interesting rhythm.


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