Rate-Responsive Pacing Controlled by the TVI Sensor in the Treatment of Sick Sinus Syndrome

Cardiac rate adaptation to changes in metabolic demand for blood supply is essential for the optimisation of exercise capacity and general well-being, especially for people inclined to an active life-style. Since patients affected by sick sinus syndrome often present with different forms of chronotropic incompetence [1], dual-chamber rate-responsive pacing is usually indicated in the electrical treatment of this disease. Sensors of various kind are currently applied to regulate the pacing rate, but the ideal goal to precisely meet the physiological needs by an artificial control system has not yet been fully achieved [2]. Indeed, activity sensors, which are generally based on an accelerometer, are highly sensitive and quickly reactive during dynamic exercise, but they cannot detect conditions of isometric exercise, post-exercise recovery, or mental stress, which would normally entail cardiovascular compensation. In addition, the accelerometer indications are not specific, since the sensor can induce a rate increase even in response to passive movements. Sensors designed to record changes in physiological parameters indicating exercise or fatigue, such as minute ventilation, are more specific, but usually slow and less sensitive. Sensors of different manifestations of adrenergic tone, e.g. Q–T interval, pre-ejection interval, unipolar ventricular impedance, and peak endocardial acceleration (PEA), generally provide a good approximation of the expected rate regulation [3–6], but may require complex hardware [7], can be affected by positive feedback from the pacing rate itself [8], or may be unreliable under particular conditions [9].