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Dr A Röschl's picture


Why does this EKG indicate a sick sinus node? First, we observe a sinus rhythm with a rate just below 60 bpm. Then, there is a pause of approximately 3000 ms, followed not by a sinus beat, but by a junctional escape beat (retrograde/inverted P-wave immediately after the QRS complex). This ECG was recorded at the general practitioner's office, and it can be assumed that no vagal stimulus contributed to the arrhythmia.

Dawn's picture

An Irregular Bradycardia

Thank you to Alikuni Kllany from Toronto for donating these ECGs.  They are from a 59-year-old man who has a history of hypertension and depression.  Last year, he was on atenolol when he experienced a brief syncopal episode and bradycardia. He was taken off atenolol and started on amlodipine 5 mg.  He also takes ramipril 10 mg, atorvastatin 40 mg, and tamsulosin .4 mg.  He continues to have bradycardia and dizziness. 

The first ECG shows grouped beating, with repetitive groups of two and three complexes.  The P waves are very small, and hard to evaluate.  The best place to see them is in the Lead II rhythm strip at the bottom.  The rhythm strip is not run concurrently with the 12-Lead, making it even more difficult to evaluate P wave morphology. 

The beats that begin the groups also END a pause.  These are junctional escape beats.  After the junctional escape beats, the PR intervals vary.  This can be explained by RP / PR reciprocity, first described by Mobitz.  He demonstrated that the RP interval can affect the next PR interval. Longer RP intervals (slower rate) cause PR shortening.  Shorter RP intervals equal longer PR intervals.  The P waves are so small, it is difficult to determine whether there is a P wave in the last T wave of each group, which would indicate non-conducted PACs.  So, we are left with a sinus pause or sinus exit block (suggested by the timing of the first six beats.  We have used red arrows to suggest where the sinus node probably fired.  P waves indicate conduction, of course.  Lack of P waves, the impulse failed to exit the sinus node. 

There are many mechanisms by which grouped beating can occur.  Second-degree AVB, Type I (Wenckebach) comes to mind first.  That rhythm in it’s pure form would have a regular sinus rhythm underlying it.   Escape-capture bigeminy often occurs with slow rates and junctional escapes, but does not by itself cause “trigeminy”.  Sick sinus syndrome can cause all types of chaos in the rhythm. 

Dawn's picture

ECG Basics: Sinus Pause / Sinus Arrest

This example of sinus arrest, also called sinus pause, shows a spontaneous return to sinus rhythm.  There are many mechanisms by which pauses can occur on the ECG. One concept for beginner students to grasp is that, if the pause contains the equivalent of regular R-to-R intervals, and the first complex after the pause is "on time",  we can expect that the sinus node kept firing, but did not penetrate the atria (exit block). If the pause is irregular in length, with the first beat after the pause seeming to come in randomly, we can call this sinus arrest or pause, understanding that there are many different mechanisms that can be at work here. Because what little we can see of the underlying sinus rhythm is irregular, or speeding up, we cannot discern absolutely that this is sinus arrest. 

The bottom line for the patient, and for any level practitioner, is, "how is the patient tolerating this pause, and what does it mean to the patient's overall prognosis?  In the short term, the patient may require emergency pacing while the cause of the dysfunction is investigated. If pauses are long enough to lower cardiac output, they can cause fainting. More than a few automobile accidents have happened as a result of this type of dysrhythmia. 

For more advanced students, this short rhythm strip (Lead II) reveals sloping ST depression, and indicates the urgent need for a 12-lead ECG and other tests. 



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