Localization of maximal ST segment displacement in various ischemic settings by orthogonal ECG: Implications for lead selection and the mechanism of ST shift

BACKGROUND: ST segment deviation conveys crucial information concerning dia gnosis, therapy and prognosis juring acute coronary syndromes, but the unde rstanding of the genesis of different ST shift polarities and the rationale for optimal lead placement during ischemic monitoring are incomplete. PATIENTS AND METHODS: Ninety-nine continuous recordings were made with orth ogonal X, Y and Z leads in 35 patients during ST elevation myocardial infar ction (MI), in 30 patients during single vessel, elective coronary angiopla sty (PTCA), and in 34 patients with unstable angina or acute non-Q wave MI. Each lead was sampled at 500 Hz, and dominant QRS-T complexes were average d every 47 s. In PTCA, 10 s averages were analyzed. Trend plots of ST + 60 ms for each lead and ST vector angles phi and theta were constructed and ed ited. ST shift polarity (depression or elevation) and vector orientation (p hi and theta) were noted for the greatest ST shift 50 muV or greater on any lead fur each patient. Coronary angiographical data were consulted when av ailable. RESULTS: By constructing polar plots of phi and theta, it was evident that ST depression vectors were confined to a small, lateral cardiac region desp ite a variety of coronary lesions, while ST elevation vectors were oriented according to the territory of the occluded artery (difference of direction means, P<0.002). CONCLUSION: ST depression in acute coronary syndromes is maximal over the l eft thorax regardless of coronary lesion location, indicating that the mech anism of ST depression is not fully understood. In ambulatory monitoring wh ere ST depression is expected, a lateral lead may suffice.