Magnetic measurement of S-T and T-Q segment shifts in humans. Part II: Exercise-induced S-T segment depression.

The direct-current magnetocardiogram, which shows the T-Q (baseline) shift, is used to clarify the cause of S-T depression induced by stress testing in the human heart. Measurements are made of the amount of baseline shift associated with the S-T depression. Results are presented of a well-documented patient, with typical coronary artery disease, undergoing a two-step exercise test. Before exercise, there was no S-T or baseline shift. During exercise, the S-T segment became depressed and the baseline segment was simultaneously elevated, at about 70% of the S-T amplitude. After termination of exercise, the baseline elevation disappeared somewhat more rapidly than the S-T depression. These results were consistent in repeated tests of this patient. Because the baseline shift is a reflection of an injury current, these results confirm the belief that exercise-induced S-T depression is mostly due to an injury current which is interrupted during the S-T interval. The baseline shift seen here is the first non-invasive measurement of an injury current in the human heart, and its presence and time-course generally agree with measurements in the animal heart. This work also confirms that the direct-current magnetocardiogram, although not practical for clinical purposes, is useful as a research tool.