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Atrial bigeminy

Bigeminal Rhythm With Aberrant Conduction

Wed, 02/24/2016 - 20:09 -- Dawn

This ECG is a good example of sinus rhythm with aberrantly-conducted PACs.  The tracing was donated to the ECG Guru several years ago by Dr. Ahmed from Sanjiban Hospital in India.  We have no patient data for this tracing. 

The underlying rhythm here is normal sinus rhythm. Most of the parameters – rate, PR interval, and QRS duration – are normal.  The QTc interval, which is the QT interval corrected to a rate of 60 bpm, is prolonged at 568 ms.  We do not know the patient’s clinical condition or medications, so we cannot guess at the reason.  However, a prolonged QTc is associated with an increased risk of Torsades de pointes, a type of polymorphic ventricular tachycardia. 

The first three beats appear the same (Leads I, II, and III).  However, the first R-to-R interval is shorter than the second one.  This could be due to rate variation, a concealed sinus block, or a premature atrial contraction (PAC).   The P wave of the “early” beat, marked #1, looks slightly different from the other P waves in Lead II, but, because of the slow rate, there is no way to be sure without a longer rhythm strip.  After the possible PAC, the rhythm becomes coupled, probably atrial bigeminy, where every other beat is a PAC.  There are several mechanisms that cause grouped beating, but atrial ectopic bigeminy is the most common. Normally, PACs have different-looking P waves compared to the sinus beats.  In this ECG, the P waves are often buried in the preceding T waves, and are hard to evaluate. 

Interestingly, the PACs in this strip are conducted with varying amounts of aberrancyBecause they are premature, they find the ventricles in varying stages of repolarization.  When the ventricles are not completely repolarized, the electrical impulse may conduct down the more recovered conduction pathways, while going around the less recovered pathways.  A common form of aberrant conduction is right bundle branch block.  In RBBB aberrant conduction, the impulse finds the right bundle branch temporarily unable to conduct, so it follows the left bundle branch down into the ventricles.  The left ventricle is depolarlized slightly ahead of the right, and the common ECG pattern of a wide QRS with rSR’ pattern in V1 and Rs in Leads I and V6 will appear. 

The PAC marked “2” probably has a small degree of aberrancy.  The PACs marked “3” and “4” appear almost like the normal beats.  PACs “5” and “6” have clearly taken on the form of right bundle branch block, including the T wave changes commonly seen with RBBB.  

The clinical significance of the grouped beating, and of the prolonged QT interval, would have to be evaluated before treatment is decided.  But, this ECG provides students with a good opportunity to see variations on aberrant conduction.

Atrial Bigeminy Vs Possible Sino-atrial Exit Block

Mon, 10/05/2015 - 17:26 -- Dawn

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. 

Differential diagnosis includes many causes of “atrial bigeminy”.  When every other beat is a premature atrial contraction (APB or PAC), the pause will be caused by the PAC “re-setting” the sinus node, and will not be a multiple of the R-to-R interval on that strip.  We can differentiate this rhythm from the AV blocks because in sinus block, the P wave is missing.  In AV block, the P wave is present, but not always conducted. 

To better illustrate what may be happening in this ECG, we have included a laddergram.  If you are not familiar with using laddergrams, please refer to the following links. 

ECG Guru, Are You New To Laddergrams,   Dr. Ken Grauer, Making A Laddergram.  

Clinically, we would want to investigate the possible cause of the SA block or atrial bigeminy, and watch for worsening of any block, which could result in a dangerous bradycardia.

Sinus Rhythm With Atrial Bigeminy

Tue, 07/07/2015 - 15:56 -- Dawn

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.

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