sinus arrhythmia
First Glance:
From across the room it looks like sinus rhythm with some irregularity.
Discussion:
When presented with an irregular pattern, first consider whether there is a repeating pattern. A repeating pattern (ie: regularly irregular) could be due to an interesting block, or correlated to a physiologic process.
The healthy autonomic system is constantly making finessed adjustments to the cardiac output to make up for changes in position, demand, respiration, etc. In the patient who is getting an EKG (ie: at rest) the only thing usually being adjusted for is respiration. Sinus arrhythmia is a wandering rate that is changing with respect to the respiratory cycle. There needs to be a 160ms (4 small box) different between R-R intervals.
Look here:
You might also see a similar strip with a second degree sinoatrial block. That is where there the sinus node is firing, and sometimes it doesn’t depolarize the atria around it resulting in a missing P wave (and subsequent missing QRS). A second degree SA block has two types like a second degree AV block. 2nd degree type I SA block is characterized by a progressingly shortening PP interval, followed by a missing P wave. However, that just doesn’t fit here because we would see lengthier pauses and probably more regularity in the cycles.
So, ok. Sinus arrhythmia. In sinus arrhythmia the R-R interval varies with respiratory cycle. The normal autonomic system should speed the rate up when preload is down, which is caused by expiration in the spontaneously breathing patient, or in inspiration in the patient on positive pressure ventilation. In this short strip the rate does not correlate perfectly with the respiratory cycle (lower trace) but a sinus arrhythmia is the most likely cause by far and a longer evaluation would likely show a more convincing trend.
And what about this?
The orange triangle shows a P’ wave with a shorter PR interval and a new morphology. The PR interval is within normal limits, but shorter than before. A shorter (but otherwise normal) PR interval could be a reflection of the same autonomic changes causing the sinus arrhythmia (momentarily decreased vagal tone), or be due to an ectopic atrial focus (which theoretically could also be unsuppressed by a momentarily decreased vagal tone). There is another one two beats later in this excerpt.
Final Impression?
Sinus tachy with mild arrhythmia @ ~100 bpm, probably with monomorphic PACs.
Management implications:
Nothing specific.
The Take-home Point:
Sinus arrhythmia is the same as sinus rhythm. It’s just a way to say “Yeah. I saw that the rate changes a bit”.
In some folks, the beat to beat interval change can be extremely marked such that it looks like 2nd degree block or afib from across the room- especially at slow rates.
Diagnosing sinus arrhythmia becomes more interesting when you can’t see the P waves well, but you still want to differentiate between afib and sinus arrhythmia and some other stuff. A ten second afib strip can have gently faster and slower periods too, but if you watch for a while longer you will see places where the rhythm gets choppy and that would push you towards afib (or sinus with premature supraventricular ectopy, depending), or if there is any regularity some kind of block. In those cases, hunting for P waves will be critical.
One star strip. Students should identify the rhythm correctly.