[Electrocardiographic differences between standard 12-lead recording and modifications in exercise testing]

Modified limb electrode positions proposed by Mason and Likar are often used in exercise electrocardiograms. In some cases, such modifications induced considerable differences in electrocardiographic waveforms of the frontal plane leads. To assess the magnitude of these changes, 55 patients were studied with standard and modified limb electrodes. We observed a significant verticalization (p less than 0.01) of all electrical axis in the modified ones: there was a difference of 19.6 +/- 1.7 grades for the P wave, 36.4 +/- 8 grades for the QRS axis and 26.0 +/- 3.3 for the T wave in the frontal plane. When classified by electrocardiographic pathology, old transmural inferior myocardial infarction and S1, S2, S3 pattern did not change electrical axis. There was also a significant increase (5 +/- 1 mseg, p less than 0.01) in the intrinsic deflection onset of the unipolar lead a VF of the modified one. Such finding suggests the recording of a more lateral region when compared with the onset of intrinsic deflection of the precordial leads. We conclude that modified limb electrode position induces a significant verticalization of the electrical axis in the frontal plane. The recorded myocardial region in such electrocardiograms seems to be different from that recorded in the standard one. For some cases, we suggest to explore the inferior myocardial region by using the standard aVF location in the postexercise recordings.