Figure 4. Reversal of LA and LL Electrodes

The left-hand tracing appears to show ST segment/T wave changes inferiorly (leads II, III, and aVF); it does not appear obviously artifactual. The second tracing, recorded when the patient became free of chest pain, shows the initial ST segment/T wave changes are gone, and now similar changes are seen in leads I and aVL. Closer scrutiny reveals that lead III has become inverted, and that leads I and II are actually transposed�as well as leads aVL and aVF. These changes suggest reversal of the left-sided limb leads. Examining P wave height in leads I and II, as well as P wave morphology in lead III, does not help determine which tracing is incorrectly recorded in this case (see text). An old ECG showed that the right-hand tracing was indeed the �real� ECG.