The feasibility of ST-segment monitoring with a subcutaneous device.

We have developed a subcutaneous cardiac monitor/alarm for patients at high risk that alerts companions or bystanders if cardiac arrest occurs. The leadless device has 4 electrodes mounted on its corners. An added feature of the implantable monitor/alarm is the ability to detect ST-segment deviations present in acute ischemia. We obtained acute ischemia electrocardiogram (ECG) data of 30 patients from the Utah 192-site body surface maps and determined the left precordial region where our device would be implanted. (The device is implanted at the location where its electrodes yield maximum QRS amplitude.) Unipolar body surface potentials from the chosen area were further interpolated so that closely spaced bipolar electrocardiograms can be derived by performing algebraic subtraction of these "unipolar" ECGs. Results show that the summation of the absolute values of ST-deviations from a pair of diagonal bipolar leads from the optimal subarea (3 x 6 cm rectangle) provides a measure comparable to the maximum ST-deviation among the 4 corner unipolar leads of the same area. This suggests that ST-segment deviations seen at neighboring precordial sites are sufficiently different that significant cancellation of these deviations does not occur in forming close bipolar ECGs. Our conclusion is that during acute ischemia, bipolar ECG recordings between neighboring precordial sites can present ST-segment deviations that are large enough for automatic detection in many patients. Furthermore, the same electrodes used by our subcutaneous device in monitoring R-waves in arrhythmia detection can also be used in monitoring ST-segment deviations for acute ischemia detection.