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By Ken Grauer, MD
Clinical Scenario: An 87-year-old Hispanic woman had a postoperative ECG performed as part of her evaluation for dyspnea that developed following abdominal surgery. There was no chest pain. A permanent cardiac pacemaker ("marcapasos") had been implanted years earlier for some type of rhythm disorder. Her 12-lead ECG is shown above. Does it appear that the pacemaker is functioning appropriately? Can you identify two other findings on this ECG that may be relevant to her clinical situation?
Interpretation: With exception of two spontaneous beats (labeled X and Y), the lead II rhythm strip at the bottom of the tracing shows regular pacer spikes at a rate of 70 beats/minute. Each pacer spike reliably captures the ventricles (evidence by the fact that each pacer spike is followed by both a QRS complex and T wave). Sensing function of the pacer is also appropriate, as judged by the finding of a constant R-to-spike interval (interval from spontaneous beat until the next pacer spike) that is appropriately the same as the inherent pacer rate. The key to detecting the findings of concern on this tracing lies with focusing on the two spontaneous beats (X and Y)—and in viewing QRS morphology of these spontaneous beats in each of the three simultaneously recorded leads. Despite not being certain of the underlying spontaneous rhythm (impossible to determine if a P wave precedes beats X and Y in lead II)—what can be said is that spontaneous QRS morphology in leads II and aVF, as well as in leads V4, V5, and V6 show ST segment coving, slight ST segment depression, and moderately deep and symmetric T wave inversion. Serial Troponin I values were found to be positive for acute infarction.
The second ECG finding of concern on this tracing is more subtle, and relates to the appearance of the T wave of several paced complexes. Although ST segment/T wave morphology in paced complexes is rarely indicative of specific pathology, one is struck by the peaked appearance of many of the paced beats in this tracing. In further support of our suspicion, the T wave appearance in the spontaneous beat seen in lead aVL is peaked enough to merit checking serum electrolytes, which revealed moderate postoperative hyperkalemia.