| Sumario: | Fifty-one patients with Intermedics pacemakers and different chronic (> 12 months) ventricular lead models were investigated. Ventricular charge thresholds (uC) were measured telemetrically at 1.0, 2.0, 2.5, and 3.5 V, respectively. Then pulse duration was increased until charge per pulse (uC) was twice the threshold value in patients not being pacemaker dependent (n = 39) and three times the threshold in pacemaker dependent patients (n = 12), thus giving a 2;1 or 3:1 safety factor in terms of charge ('safety charge'). At safety charge settings, the battery current was measured telemetrically for all four pulse amplitudes (PA) in VVI mode at 70 beats/min. For safety purposes, only pulse amplitudes were considered that fulfilled two conditions: (1) pulse duration threshold (PDT) 0.30 ms at PA and (2) PDT 1.00 ms at a pulse amplitude of (PA-0.5 V). The combination of pulse amplitude and pulse duration that yielded the safety charge at the lowest battery current was defined as optimized ventricular output (Copt). It was found at 1.0 V in 27 patients and at 2.0 V in 24 patients. The safety pulse duration (SPD) that yielded a 2:1 safety charge in patients who were not pacemaker dependent was 0.32 ± 0.12 ms for both, 1.0 V (n = 23) and 2.0 V (n = 16), respectively. In pacemaker dependent patients, the SPD for the 3:1 safety charge was 0.61 ± 0.25 ms (at 1.0 V. n = 4) and 0.47 ±0.11 ms (at 2.0 V, n = 8), respectively. The safety factor for conversion of PDT into SPD was 3.15 (range 3.00-3.38) for pacemaker dependent patients and 2.04 (range 2.00-2.43) for patients who were not pacemaker dependent, respectively. Charge thresholds measured at study entrance, after 24 hours, and again after 6 months showed a median variation of 14% and a maximum individual variation of 55%. On day 0 and 180, 24-hour Holter recordings were obtained from ail patients and revealed constant ventricular capture at output settings Copt. When the output was changed from a fixed setting (2.5 V at 0.50 ms) to Copt, the battery current decreased by 17.5% (P < 0.0001). In conclusion, pacing thresholds in patients with chronic ventricular leads are stable enough to permit programming battery-saving low output settings, if pacemakers are followed on a regular basis. Titration of a 2:1 safety charge (a 3:1 safety charge in pacemaker dependent patients) by prolongation of pulse duration is safe, provided that pulse amplitude is chosen carefully. Using this approach, current consumption can significantly be reduced without jeopardizing patient's life.
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