Feasibility of an Implantable Arrhythmia Monitor

Conventional Holter monitoring is of limited benefit in patients with infrequent symptoms suspected to be related to arrhythmia. A small recorder implanted subcutaneously might obviate many limitations of conventional monitoring. To determine the feasibility of obtaining adequate electrocardiographic signals from such a device, a prototype was temporarily implanted in 17 patients undergoing pacemaker implantation. The prototype contained four disc‐shaped titanium electrodes, 0.21 inches in diameter embedded in epoxy. The four electrodes were in a square configuration spaced 0.72 inches center to center and were placed face down in a subcutaneous pocket in the left pectoral region. Bipolar recordings were made from a horizontal pair, a vertical pair, and both diagonal pairs of electrodes (interelectrode distance 1.02 inches) and recorded on electromagnetic tape after filtering at 0.5‐250 Hz. The mean peak‐to‐peak amplitude in each configuration was determined over a five‐beat interval. Clear recordings were obtained from all 17 patients with recognizable P, QRS, and T waves. The amplitude of the signals obtained from the diagonal pairs of electrodes (175 ± 51 and 170 ± 54 μV) were greater than obtained from either the vertical pair (142 ± 62 μV, P = 0.08 compared to diagonal electrodes] or the horizontal pair of electrodes (105 ± 54 μV, P < 0.01). The maximum amplitude recorded from any configuration was 189 ± 54 μV. In six patients the device was also tested with the electrodes face up in the subcutaneous pocket. In this position, there was a slight decrease in signal amplitude (maximum amplitude electrodes face up 136 ± 49 μV compared to 144 ± 49 μV face down, P = NS), but baseline stability was greatly improved during pectoral muscle contraction. These results demonstrate that a device with an interelectrode distance of 1 inch is able to record adequate electrocardiographic signals when placed subcutaneousiy in the left pectoral region. A fully implantable device of similar size, with continuous loop memory and telemetry, might be useful in patients with recurrent unexplained syncope.