Basics of PR Logic

Tracing N° 33
Medtronic Devices: ICD Field: Discrimination

Patient implanted with a dual-chamber ICD for arrhythmogenic right ventricular dysplasia; this tracing shows the fundamentals of the functioning of the dual-chamber PR Logic discrimination algorithm.

Graph and trace

The graph shows a typical example of VT with atrioventricular dissociation; a burst allows terminating the episode.

  1. the EGM initially shows a sinus rhythm followed by the onset of a VT with a ventricular rate faster than the atrial rate;
  2. note that some intervals classified as VS recycle the VT counter;
  3. the VT counter is filled, analysis by PR Logic is indicative of VT; a burst is delivered with a return to sinus rhythm;
  4. the PR Logic algorithm and Wavelet were programmed to ON.

In a dual- or triple-chamber ICD, the PR Logic algorithm is used to discriminate the origin of the tachycardia and to differentiate between ventricular and supraventricular tachycardias based on the study of the relationship between atrial events and ventricular events and by comparing their respective rates. PR Logic discrimination applies only during the initial detection phase and is not functional during the redetection phase. PR Logic is only operational for cardiac rates below the programmed limit of supraventricular tachycardia and can therefore be active in the VF zone depending on the programming. PR Logic renders its diagnosis once the initial VT or VF counter is full.

PR Logic discrimination is based on the continuous and simultaneous calculation of 6 criteria, which can sometimes render the detailed explanation of its functioning complex.  In this example, the diagnosis of VT is obvious to the physician due to atrioventricular dissociation, which is the most common occurrence. Indeed, during a VT episode, the detection of a ventricular rate faster than the atrial rate is much more frequent than the observation of a 1:1 ratio (retrograde conduction) or a double tachycardia. The diagnosis is also evident for PR Logic. One of the essential steps of the diagnosis by PR Logic is the comparison between ventricular and atrial rates (measurement of the median value of the RR intervals and PP intervals over the last 12 cycles). When the ventricular rate is judged to be faster than the atrial rate, the device concludes to a VT without further analysis. In this instance (V> A), if as in this example, PR Logic and Wavelet are programmed simultaneously, the device does not consult the analysis by Wavelet which cannot therefore modify the diagnosis of VT carried out by PR Logic.

Indeed, when the 2 algorithms are programmed simultaneously, Wavelet can only divert therapies and correct the diagnosis rendered by PR Logic when PR Logic concludes to VT and the atrial rate is judged to be greater than or equal to the ventricular rate (1:1 VT or double tachycardia).