Spontaneous daily variability of atrial sensitivity. A study with an implanted DDD pacemaker

Spontaneous variations in the chronic stimulation threshold were widely investigated in the last few years and formed the basis of the safety margin concept of stimulation energy. The dynamic variability of sensitivity, particularly in the atrium, is less studied and, in the literature, contrasting data are present about the actual entity of these variations. A 2:1 safety margin is still recommended by many authors, although some papers reported that this value was inadequate to guarantee a correct sensing function.

We evaluated daily variability of atrial sensitivity (AS) in patients implanted with the Cosmos 3 DDD cardiac pacemaker, with a special algorithm that continuously measures the minimum-maximum sensitivity values in atrium or in the ventricle during a 12-h sampling period.

34 patients, 20 males and 14 females, with a mean age of 72+/-5 years, were implanted with a Cosmos 3 DDD pacemaker (PM). The indication to implant a PM was sick sinus syndrome in 21 patients and a high degree of AV block in 13 patients. Atrial bipolar leads were always employed. At 1, 2, 3 and 6 months after the implant, we analyzed the minimum-maximum values of atrial sensitivity for the last 12-h. This analysis was performed in the late afternoon.

The mean AS varied more than threefold, from a mean minimum of 0.68 to a mean maximum value of 2.19 mV at 1 month, from 0.68 to 2.21 mV at 2 months, from 0.69 to 2.19 mV at 3 months and from 0.67 to 2.17 mV at 6 months, without significant differences among the four follow-up controls. In seven patients, there was a fourfold variation, while the variation was fivefold in five patients.

In conclusion, from our data, we can confirm that atrial sensitivity may vary widely during the day and, consequently, we recommend that a 3:1 safety margin should be programmed and the recommended 2:1 ratio should be abandoned. Moreover, at implant, particular attention should be paid to obtaining the best atrial signal amplitude, at least threefold higher than the highest sensitivity value available in the implanted PM. To achieve this goal and to obviate to the oversensing phenomena, bipolar leads are preferable to unipolar leads.