ECG Review

Need for a Pacer?

By Ken Grauer, MD, Professor, Assistant Director, Family Practice Residency Program, University of Florida.

Figure. Non-continuous rhythm strips obtained from a 70-year-old man with a complaint of fatigue.

Clinical Scenario: The three non-continuous rhythm strips shown above (labeled A, B, and C) were obtained from a 70-year-old man complaining of "a little fatigue." Based on initial assessment of these tracings, he was diagnosed as having sick sinus syndrome (SSS) with marked sinus bradycardia. Permanent pacing was recommended. A second opinion questioned this diagnosis, and the need for pacing. Who is correct?

Interpretation/Answer: The main indication for permanent pacing with SSS is symptomatic bradycardia. Diagnostic work-up of the patient should exclude other potentially treatable causes of bradycardia such as rate-slowing drugs, cardiac ischemia or recent infarction, hypothyroidism, hypoxemia and/or electrolyte abnormality. None of these factors were contributing to the bradycardia in this case.

Initial interpretation of the first part of rhythm strip A, and the entire rhythm strip B was marked sinus bradycardia at a rate of about 40/minute. If this was the sustained rhythm in a symptomatic older adult without evidence of other potential cause of bradycardia, permanent pacing would be indicated.

However, reinspection of the tracings shown here suggests a different interpretation. Note that the last three beats in Tracing A (beats #5, 6, 7) are sinus at a normal rate of 75/minute. Tracing C shows a bigeminal rhythm. Although every other beat in Tracing C is widened, telltale premature P waves precede each widened complex, and define this rhythm as atrial bigeminy. QRS widening is the result of right bundle branch block aberrant conduction of each PAC (premature atrial contraction). Herein lies the key to this case. This elderly man was in near persistent atrial bigeminy. Scrutiny of the terminal descending portion of the T wave in Tracings A and B (especially in lead II of these tracings) reveals a notched ledge not present in the three normally conducted sinus beats at the end of Tracing A (beats 5, 6, 7). Note especially the notched point of the T wave of beat #4. These notchings represent PACs occurring so early in the cycle that they are blocked (nonconducted). Thus, rather than marked sinus bradycardia from SSS, this patient had near persistent atrial bigeminy with frequent blocked PACs producing a pseudobradycardia. Treatment with low-dose disopyramide suppressed much of his ectopic atrial activity, resulting in a near normal heart rate and no need for pacing.