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ECG Basics: Sinus Bradycardia

Dawn's picture
Sat, 08/13/2016 - 23:56 -- Dawn

Sinus bradycardia.  This strip meets the criteria of:  regular rhythm, rate less than 60 bpm (40 bpm in this case), regular P waves before every QRS.  Sinus bradycardia can have many causes from a completely normal variation to a malfunction of the sinus node.  In some cases, enhanced parasympathetic tone causes sinus bradycardia.  Well-conditioned athletes typically have sinus bradycardia. Treatment depends upon the cause and the patient's response to the rate.  If the rate does not cause hemodynamic impairment, treatment may not be necessary.

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Jerry W. Jones MD FACEP's picture

If I were teaching one of my classes, I would have the students pay very, very close attention to the T waves. Under normal circumstances, they aren't usually very large at all - especially in the limb leads. These are quite normal. Surprisingly, we rarely anguish over whether a finding is abnormal or not because that is usually apparent (even if you aren't sure what is causing it). What we usually anguish over is whether something is normal or not and should we wake up the cardiologist at 3 am!

The next thing I would have my students do is find the U waves, because there is an upright U wave after each of the T waves. Get used to finding them. They are usually a bit more apparent during a bradycardia. These U waves appear quite normal - given the amplitude of the T waves. They really add nothing to the interpretation of this particular strip. But try to imagine the same, very subtle U wave that you see here as though it were inverted! That is even more difficult for the untrained eye to see, but the implications are enormous! I call these "Eye Exercises."

There is always a bit of danger in reading too much into a finding. The onset of each R wave is just slightly slurred. Does this represent a bit of pre-excitation? No! Just look at the PR intervals! They are normal.

Another thing to notice here is that you don't see any Ta waves (atrial T waves). They tend to be more obvious with faster rhythms.

And a bit of advice to all the more advanced ECG "nerds": sinus bradycardia is always an opportunity for a junctional escape pacemaker to manifest. Take a close look to make sure all the PR intervals are the same. If there is AV dissociation by default or usurpation, the PR intervals tend to get shorter and shorter.

Dawn, thanks again for posting this short strip.

Jerry W. Jones MD FACEP FAAEM
https://www.medicusofhouston.com
Twitter: @jwjmd

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