[Frequency-dependent rise in absolute threshold during DDD-R stimulation--clinical consequences]

A 65-year-old female patient had been supplied with a rate-adaptive DDD-R pacemaker system because of symptomatic sick sinus syndrome with insufficient rate increase one year ago (first implantation of a DDD-system had taken place 14 years ago). In addition she had coronary atherosclerosis without relevant haemodynamic changes. Now she was taken into hospital because of dizziness and dyspnoea on exertion, without loss of consciousness. The physical examination showed normal results concerning heart, lungs and vessels.

During pacemaker control a significant rate-dependent stimulation threshold increase was found (stimulation rate 70-160/min). Similar results were seen in threshold measurements during operation as well as for the ventricular lead implanted in 1982, and for the new electrode just placed in the right ventricle. Several pacemaker controls within 6 months after implanting the new lead showed a significant increase in pacing threshold.

Because of a new rate-dependent increase of pacing threshold the implantation of a further ventricular lead became necessary. Again a rate dependent threshold increase during and immediately after surgery was detected. This time the programming of the pacemaker allowed a good safety interval because of a generally low stimulation threshold.

This case report demonstrates a rate-dependent rise in threshold on DDD-R stimulation. It clearly shows that this rise occurred more often at high than at low rates of stimulation, but there were marked individual differences. There was no clear correlation between cycle length and the interval from implantation to the investigation. Further investigations including stimulation with various cycle lengths should be carried out in patients at high risk. Furthermore, the development of modern pacemaker systems with autocapture might help to solve this problem.