ECG Review-An Overly "Busy" 12-Lead
Clinical Scenario: The ECG shown in the Figure was obtained from a 74-year-old woman with chest discomfort. Given this history, how would you interpret this tracing? What is the rhythm? Can you explain why QRS morphology changes slightly in simultaneously recorded leads V4-V6 (beat X)?
Interpretation: As suggested by the title of this ECG review—a lot is happening on this tracing. Appreciation of several key points may greatly facilitate interpretation. First, remember that despite the change in leads after every three or four beats—time is continuously recorded along each longitudinal channel (i.e., a total of 14 consecutive beats are displayed on the middle channel involving successively leads II, aVL, V2, and V5). Second, look to see if an underlying sinus rhythm is present. That this is the case in the Figure is confirmed by inspection of the 1st and 3rd beats in lead II, both of which show narrow, normal-appearing QRS complexes preceded by an upright P wave with a constant and normal PR interval. Third, identify the abnormal (non-sinus conducted) beats—and ignore these abnormal beats when interpreting the ECG for axis, hypertrophy, and changes of infarction.
Applying these suggestions we note (by assessing the middle recording channel as a multilead rhythm strip) that the rhythm is ventricular bigeminy (every other beat is a PVC). Use of calipers reveals that P waves continue at a regular rate of 85/minute throughout the tracing (the arrow in lead II points to the second P wave in this lead, which is partially hidden by the initial part of the PVC). We suspect the reason for the different but still narrow QRS complex near the end of the tracing (beat X in lead V4) is that this is a fusion beat.
Interpretation of the rest of the 12-lead ECG is based on morphology of sinus-conducted beats in each lead. Q waves in leads III and aVF, in association with subtle ST elevation in these leads and T wave inversion in lead III, suggest inferior infarction that could be acute may account for this patient’s chest pain and frequent ventricular ectopy.
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