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    ECG Challenge: It's the Level of the Block that Counts

    An 83-year-old man with hypertension but who is otherwise healthy and very active for his age presents to your office for routine follow-up. He just returned from a trip out of the county where he was walking several miles per day and states he “feels like Tarzan.” His vitals are normal except his pulse is noted to be 45/beats min. He takes carvedilol 25 mg twice daily and losartan-HCTZ 40/25 mg once daily for hypertension. ECG is obtained and shown below (Figure 1a).

    Figure 1a.

    What is your ECG read?

    The ECG shows sinus rhythm at ~80 beats/min with 2:1 AV block for an effective ventricular rate of ~40 bpm. A cursory look at the rhythm strip lead II could lead one to identify this as marked sinus bradycardia. But note in V1 and V2 (Figure 1b) that there are two clear p-waves (red lines) in between the QRS complexes (blue lines), with the sinus rate occurring at twice the ventricular rate.

    Figure 1b.

    What is the best next step in patient management?

    It is not possible to discern based on this single surface ECG if we are dealing with a Mobitz I (proximal AV nodal) versus Mobitz II (distal AV nodal) block. Determining the level of block is critical because asymptomatic Mobitz I or Wenkebach AV block is benign whereas Mobitz II AV block can progress to complete heart block and requires pacemaker implantation.

    Characteristics that differentiate this as proximal AV block include the narrow QRS, the absence of symptoms, and the patient’s high dose carvedilol as a possible causative agent. A simple non-invasive method to further demonstrate proximal vs distal AV block is to increase the sinus rate with exercise while performing continuous ECG monitoring and note whether AV conduction improves. A stress treadmill ECG is one option and having the patient perform leg lifts while supine and hooked up to telemetry or a rhythm strip is also sufficient.

    Next: Stress ECG results

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