A NEW PACEMAKER ALGORITHM FOR CONTINUOUS CAPTURE VERIFICATION AND AUTOMATIC THRESHOLD DETERMINATION - ELIMINATION OF PACEMAKER AFTERPOTENTIAL UTILIZING A TRIPHASIC CHARGE BALANCING SYSTEM

A new pacemaker algorithm designed to automatically verify pacemaker capture and determine pacing threshold by detection of a stimulus evoked potential was studied in 20 patients undergoing permanent pacemaker implantation. To eliminate pacing stimulus afterpotential and detect an evoked response, a hardware feedback circuit and a software template matching algorithm were used to produce a triphasic charge-balanced pacing pulse. After charge balancing the pacing lead, a residual artifact is measured. A capture window is defined as the area integral of the first 24 msec of the evoked depolarization, and a capture threshold as one third the amplitude of the capture window. The maximum allowable residual artifact is one eighth the amplitude of the capture window. Once the stimulus afterpotential is eliminated and the evoked response detected, capture threshold is automatically and continuously determined and the algorithm adds a 0.8-V safety margin to the pacemaker output. This algorithm was run automatically and after simulated loss of capture, produced by manually decreasing pacer output below threshold, in the bipolar (13 patients) and unipolar (20 patients) pacing modes. in each patient loss of capture was immediately detected. The data were consistent (P = NS) between algorithm runs. During unipolar pacing the area integral of the first 24 msec of the evoked response was 412 +/- 137 versus 413 +/- 144 and the residual artifact 5.8 +/- 4.8 versus 8.1 +/- 7.5. The resulting ratio (signal/noise) of the two parameters was 150 +/- 141 versus 145 +/- 181. Automatically determined threshold was 0.69 +/- 0.43 V versus 0.69 +/- 0.42. During bipolar pacing the area integral of the first 24 msec of the evoked response was 266 +/- 102 versus 275 +/- 102 and the residual artifact 4.0 +/- 3.3 versus 3.9 +/- 3.0. The resulting ratio was 193 +/- 210 versus 157 +/- 170. The automatically determined threshold was 0.84 +/- 0.61 versus 0.87 +/- 0.72 V. This study has demonstrated that accurate detection of a stimulus evoked response is possible after charge balancing a permanent pacing lead, thus allowing for consistent and reliable verification of capture and automatic threshold determination. By continuously verifying capture and maintaining pacemaker output only 0.8 V above threshold, this automated system may increase patient safety and reduce pulse generator battery depletion.