Myocardial ischemia induces changes in the electrophysiological properties of the myocardium, resulting in a decrease in resting membrane potential and conduction velocity, with a dispersion of the activation wavefront. Although in terms of field distribution MCG and ECG are complementary, the signal morphology is comparable for both methods. By performing the first single-channel MCG in a CAD patient after exercise testing, Saarinen et al. (1974) were able to show a depression of the ST segment in the MCG signal. They also found that the ratio of the ischemic ST segment depression to R wave amplitude after stress was even greater in the MCG than in the ECG. Cohen et al. (1983) described TQ baseline elevation and ST depression after a two-step exercise test of a patient with CAD, demonstrating the noninvasive measurement of injury currents.
When analyzing ST segment and T-wave amplitude after exercise in both MCG and BSPM in 24 CAD patients with specified ischemic regions and no previous myocardial scar, Hanninen et al. (2001) identified optimal MCG locations sensitive to ST depression and elevation as well as to the increase or decrease of the T-wave amplitude. These authors noted that the locations were dependent on the stenosed vessel region, that the T-wave changes could separate stenosed vessel regions as well as ST segment changes, and that the most informative sites were outside the 12-lead standard ECG. In a later study in 44 patients with ischemia documented by coronary angiography and exercise thallium scintigraphy, the same group was able to show a decrease in MCG ST amplitude, ST slope and T-wave amplitude under exercise testing (Hanninen et al., 2002). The optimal sites for the measurement of these parameters were over the abdomen, but a reciprocal increase was found over the left parasternal area.
Using a nine-channel MCG system in an unshielded setting, Chen et al. (2004) analyzed MCG time traces and waveform morphology parameters in healthy subjects and 11 patients with documented exercise-induced ischemia. On the basis of ST- and T-wave signal amplitudes determined under resting and stress conditions, differences were found between the healthy subjects and patients, in particular post exercise. With regard to these differences, the T-wave amplitude over the upper left thorax was most effective, and changes persisted longer post exercise than changes in the ST segment.
Myocardial viability in CAD was evaluated by Morguet et al. (2004), who examined several time and area MCG parameters using a 49-channel system. In 15 patients with single-vessel disease whose myocardial viability was determined on the basis of single photon emission computed tomography (SPECT) as well as positron emission tomography (PET), the amplitudes of the R and T waves were identified as parameters with the highest selectivity in terms of myocardial viability.
Taken together, these results show that ischemia affects the MCG signal amplitudes, in particular during the ST segment and at T-wave apex. The location of these changes may also be associated with the affected vessels.
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