Extensive variation in the signal amplitude of the atrial floating VDD pacing electrode.

The dependence of atrial signal amplitude on the site of the atrial sensing dipole of a single-pass lead was examined in 29 patients. The vertical location of the dipole was documented in supine fluoroscopy during quiescent and deep breathing and in upright chest roentgenogram, and was expressed as a proportion of the right atrial height. As the group average, the atrial signal amplitude was equal when tested in supine, sitting, standing, and right- or left-side positions in follow-up determinations. The signal amplitude varied markedly between postures, showing a coefficient of variation of 45% +/- 24% within the group. Coefficient of variation within the 6-month follow-up period in each tested position ranged from 31%-44%. Correlation between postures was weak (range of r = 0.53-0.81). Vertical location of the atrial dipole had no relationship to the signal amplitude. At least in one posture or test occasion the atrial signal amplitude was very low, < or = 0.35 mV in 20 patients, and below detection limit (0.25 mV) in 5 patients. Programmed to high sensitivity, atrial undersensing was rare in ambulatory electrocardiography, ranging from 0-9,000 missed atrial beats (0%-8%), with a median of 100 beats/24 hours. In conclusion, temporary variation in atrial signal amplitude is extensive. Despite occasionally measured large signal amplitudes atrial sensitivity in single lead VDD pacemakers should be programmed high, and if poor atrial tracking is suspected, other methods besides routine sensitivity testing should be adapted.