The Transvalvular Impedance (TVI) is derived between atrial and ventricular pacing electrodes. A sharp TVI increase in systole is an ejection marker, allowing the hemodynamic surveillance of ventricular stimulation effectiveness in pacemaker patients. At routine follow-up checks, the ventricular threshold test was managed by the stimulator with the supervision of a physician, who monitored the surface ECG. When the energy scan resulted in capture loss, the TVI system must detect the failure and increase the output voltage. A TVI signal suitable to this purpose was present in 85% of the tested patients. A total of 230 capture failures, induced in 115 patients in both supine and sitting upright positions, were all promptly recognized by real-time TVI analysis (100% sensitivity). The procedure was never interrupted by the physician, as the automatic energy regulation ensured full patient’s safety. The pulse energy was then set at 4 times the threshold to test the alarm specificity during daily activity (sitting, standing up, and walking). The median prevalence of false alarms was 0.336%. The study shows that TVI-based ejection assessment is a valuable approach to the verification of pacing reliability and the autoregulation of ventricular stimulation energy. 1. Introduction The automatic adaptation of ventricular pacing energy to the individual capture threshold has been available in the cardiac stimulation practice for the last two decades. Different systems have been proposed by the industry to prevent unnecessary high pacing output, based either on periodic threshold assessment performed by the implanted device [1, 2] or on capture check at every paced beat [3, 4]. The latter approach offers the additional advantage of continuous surveillance of pacing effectiveness, which increases the patients’ safety and allows the tracking of threshold changes keeping the pulse amplitude slightly above the minimum required for cardiac stimulation. In case of capture loss, a high-energy back up pulse is delivered with a short delay from the failing stimulus [4], so that the electromechanical activity of the heart is promptly restored. In beat-by-beat capture check as well as periodic threshold measurement, the confirmation of capture relies on the detection of pacing-induced active myocardial depolarization, that is, the action potential generated by excited myocardial fibers in the surrounding of the stimulating electrode, generally referred to as evoked potential. The evoked electrical response must be discriminated from the electrode polarization produced by the
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