By Ken Grauer, MD
Clinical Scenario: The ECG in the Figure was obtained from a 90-year-old woman admitted to the hospital for shortness of breath. In view of this history, how would you interpret her 12-lead ECG? Can you also account for the finding of early transition (R wave becoming taller than the S wave between leads V1 and V2)?
Interpretation: The rhythm is sinus at a rate of about 65 beats/minute. PR, QRS, and QT intervals are normal. Mean QRS axis is +25°. There is low voltage in the limb leads, and no ECG evidence of chamber enlargement. There appears to be an isolated Q wave in lead III, although low QRS amplitude makes this difficult to determine. As already noted, transition occurs early (between leads V1 to V2), with a dominant R wave (Rs complex) in lead V2. However, the most striking finding on this 12-lead ECG is the flat (if not slightly downsloping) ST depression in precordial leads V2 through V5.
The clinical history provides an important clue for arriving at the correct ECG diagnosis in this case. This clue is based on the premise that silent myocardial infarction (MI) occurs in at least one-third of all MIs, with this entity being most common in diabetic patients and the elderly. Half of all patients with "silent" MI have no symptoms at all. In such patients, the diagnosis of infarction sometimes only will be made retrospectively when a follow-up ECG shows changes that suggest infarction has taken place since the last ECG was obtained. The other half of patients with "silent" MI do not have chest pain, but instead manifest other symptoms (i.e., shortness of breath, change in mental status, malaise, gastrointestinal symptoms, etc.). Of these nonchest pain symptoms associated with "silent" MI, shortness of breath is the most common, especially in older patients. This diagnosis, therefore, needs to be ruled out for the 90-year-old woman who presents with acute shortness of breath in this scenario.
Acute posterior infarction most often results from sudden occlusion of the right coronary artery. As a result, acute posterior MI most often also is associated with acute inferior MI—a finding suggested by the admittedly subtle but definitely present ST segment covering and elevation in leads III of the ECG in the Figure (possibly also with a Q wave in this lead III). None of the standard leads on a 12-lead ECG directly assess the posterior wall of the left ventricle. As a result, we invoke "mirror-image" leads, namely V1, V2, and/or V3, when looking for signs of acute posterior MI. A positive "mirror test" is present in leads V2 and V3 of this tracing—in that turning this ECG over and holding it up to the light will reveal Q waves and ST segment elevation (the mirror image of the tall R wave and ST depression seen in leads V2 and V3 of the Figure).
Dr. Grauer, Professor and Assistant Director, Family Practice Residency Program, University of Florida, Gainesville, is on the editorial board of Emergency Medicine Alert.