Pacing systems capture threshold

On occasion, the pacing system will appear to be entirely normal as judged by the measured data telemetry, capture threshold, and sensing threshold evaluations yet the patient has reported intermittent symptoms or the pulse generator or lead is the subject of a manufacturer s advisory. At these times, it is appropriate to subject the patient to a variety of provocative tests in an effort to both unmask a problem and reproduce the patient s symptoms. At other times, it is tpropriate to screen the patient for specific clinical events that are commonly associated with pacing in an effort to prevent an adverse patient-pacemaker interaction. 

Fig. 19.19 Pacemaker output settings for lowest current drain and maximum longevity induction of an endless loop tachycardia during an atrial capture threshold test in a patient with a Pacesetter Affinity DR pacing system. The automatic PMT detection and termination algorithm recognized the tachycardia and promptly terminated it. Meanwhile, the recording provided details as to the tachycardia interval and the retrograde conduction interval providing guidance as to programming the PVARP.

Fig. 19.20 Lateral chest x-ray from a patient with a dual-unipolar DDD pacing system. The course of the ventricular lead is bizarre and attributed to a very pronounced pectus excavatum chest wall deformity. Capture thresholds were excellent, the paced QRS had a LBBB pattern, and a transesophageal echo confirmed that the lead was in the right ventricle.

As an aide to the evaluation so that nothing is overlooked, we have developed a worksheet that is then used to generate the final summary note. This is shown in Fig. 19.24. The front side of the sheet has clinical data based on history and the examination while the reverse side has the detailed measurements from the pacing system including capture and sensing thresholds. As noted at the bottom (Fig. 19.24), special information that may impact the follow-up schedule such as unstable threshold, pacemaker dependency, on advisory, and for devices that have been implanted for many years, a indication of their age can be identified. 

Levine PA, Confirmation of atrial capture and determination of atrial capture thresholds in DDD pacing systems, Chn Prog Pacing Electrophysiol 1984 2 465 73. 

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