Name That Rhythm


I’m posting this here today because I really don’t know the answer to the question and I wanted to get some opinions from the couple of you who still read this blog.

A patient in his 70’s comes in by ambulance with a complaint of dizziness. As part of his workup, we get an EKG which is noted below. I wasn’t able to figure out the rhythm. A cardiologist came down to the ED to evaluate the patient and wasn’t able to tell me what the rhythm was, either. He only stated that it “isn’t malignant” and it “isn’t what’s causing the patient’s dizziness.”

There is a P wave before every other QRS. The PR interval appears to be constant on those beats.
The R to R intervals appear to alternate and are regularly irregular.
The QRS morphology is narrow and seems to be constant, so it doesn’t appear to be bigeminy.
I guessed that it was a Mobitz II. The cardiologist said “no way.”

Your opinions? If you want to get a better look at the EKG, you can click on the picture. Also, a link to a .pdf copy of the EKG is here.

Undetermined Rhythm EKG


  1. Interesting!
    I see grouped beats of 2 with pauses that are not clearly dropped beats. I see similar p’s every other beat, although I am unsure that there isn’t a p before EVERY beat- see the grouped beats in V2 and V3 for what appear to be p’s prior to both beats, and followed by a pause.

    Agree that the narrow complex and regularity argue against Mobitz II. In real life, I’d likely have run a rhythm strip and checked V2 as a lead where my p and T wave are very different (as opposed to the eerily simlar p and T in AVF).

    Best guess is that it’s a Wenkebach: again in V1-3 it’s unclear to me that the p’s are fixed.

  2. Looks like atrial bigeminy. If there were to be one more ectopic atrial site then you’d have WAP, but with the alternating cycle of sinus beat and PAC, the only thing I can think to call it is atrial bigeminy.

  3. Rate: ~70’s average
    Regularity: grouped beats, bigeminal pattern
    P-waves: sinus at an effective rate of 30ish (1800ms P-P), low voltage early P’s (appreciable in V2 and V3) in a bigeminal pattern
    PRi: normal, 1:1 association
    QRSd: narrow (rSr’ in V1/V2, lead positioning?)

    Axis: normal
    QTc: normal
    ST/T-waves: diffusely low amplitude T-waves

    Rhythm interpretation: sinus rhythm with bigeminal APB’s, complicated by wandering baseline in II (addition of alternative rhythm strip would be beneficial in diagnosis).

    A longer strip perhaps would reveal periods of more sustained sinus rhythm. Consider changing the patient’s position and recording a new ECG, or asking for a bit of light exercise to try and increase the sinus rate or to suppress the APB’s.

    In clinical context, I think the ECG to be somewhat benign and unlikely to be the cause of the dizziness.

  4. I’m in agreement with Christopher. The different morphology of the second P-wave in each pair confirms that those are PAC’s and we are seeing a fixed atrial bigeminy.

  5. Agree with Christopher and Vince – sinus rhythm with atrial bigeminy.

    The rhythm strip at the bottom being lead II complicates matters because the T wave is easily mistaken for a p-wave. V2 and V3 are your money leads for deciphering this. It may even be unnecessary to print a rhythm strip if you can set up a modified chest lead.

  6. I agree with christopher. Atrial Bigeminy with an incomplete RBBB.
    Hey, it ain’t a STEMI, and it ain’t 3rd degree heart block.

  7. Ditto C and V…. disagree with the cardiologist that this isn’t malignant or the source of this near-syncope. He needs a tilt-table and/or event monitor. I would have a very difficult time with my conscience if I discharged him.

  8. Stella Fitzgibbons MD on

    Wandering atrial pacemaker. You need 3 different shapes of P waves and this guy has it. He probably has COPD or some other lung disease.

    • The baseline wander appears to contribute to the irregular deformation of the P-waves, but WAP is excluded by the grouped beating with fixed coupling of the P-P’ and R-R’ intervals. Perhaps multifocal APB’s could be entertained, but the common P-P’ and PRi for the extrasystoles makes this unlikely.

  9. Definitely need a full rhythm strip. I think the confusing thing is that the p-waves and t waves look similar in morphology in lead II. As far as rhythm, best I can see is:
    –group beating
    — each p wave is conducted/ no dropped p waves
    —unchanged QRS complexes with no preceding p wave

    My guess is some form of SA block …. But definitely a guess

  10. I had never heard of atrial bigeminy, but I’m not seeing p waves before every QRS which is required for atrial bigeminy according to Dr. Google. Maybe it is just the leads. V2 seems to have p waves for each beat. If not, possibly junctional bigeminy?

    I didn’t send the patient home, either. Elderly patients and heart conduction issues just don’t make me feel all warm and fuzzy inside. Cardiologist was consulted for inpatient care. Now next time I see him, I can ask him how that patient with atrial bigeminy was doing 🙂

    • It’s rather hard to see the second p-wave of each pair in lead II but it’s there as a couple of tiny undulations in the baseline. It kinda looks like the first p-wave if you completely squashed it.

      For confirmation that it isn’t just artifact V2 and V3 show the second p-wave a bit more clearly.

  11. I disagree with the comments above. I think this is a mobitz pattern.

    Correct me if I’m wrong, and I am looking at this on a small phone screen, but the p waves appear to be regularly spaced, as do the qrs pairs. This is what you would see with mobitz type block. I would consider this a mobitz pattern regardless of what the cardiologist thinks.


    • Dr. N,

      Your small screen isn’t doing the P-waves or groupings justice. The initial P-waves are different from the coupled P-waves of the second QRS complexes in the group (compare II with V2), and the ectopic P-waves have a normalish PRi (best seen in V2). This excludes Mobitz Type I or Type II behavior.

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