PR Logic Feature

PR Logic uses pattern and rate analysis to discriminate between supraventricular tachycardia (SVT) and true ventricular tachyarrhythmias. It can withhold inappropriate VT/VF detection and therapy during episodes of rapidly conducted SVT in dual-chamber devices.

This feature can be found in some Medtronic ICD and CRT-D devices. Please go to or consult with your local Medtronic representative regarding device models available in your geography.

There are three distinct PR Logic features that can be programmed On: AF/Afl (Atrial Fibrillation/Atrial Flutter), Sinus Tach (Sinus Tachycardia), and Other 1:1 SVTs (Supraventricular Tachycardias). AF/Afl and Sinus Tach are automatically enabled when VF Detection is programmed On and the SVT V. Limit is also automatically programmed to 260 ms in Protectadevices and 320 ms in all other devices. High Rate Timeout can be programmed On or Off for this feature.

To program this feature, go to Params -> Detection(V.)…

Detection intervals and the SVT V. Limit – The SVT V. Limit value must be less than the VT detection interval. If VT Detection is programmed to Off, the SVT V. Limit must be shorter than or equal to the VF detection interval for detection enhancements to be in effect in the VF zone.

Warning: Do not program the Other 1:1 SVTs feature to On until the atrial lead has matured (approximately 1 month after implant). If the atrial lead dislodges and migrates to the ventricle, the Other 1:1 SVTs feature could inappropriately withhold detection and therapy.

Caution: Use caution when programming the Other 1:1 SVTs feature in patients who exhibit slow 1:1 retrograde conduction during VF or VT. This feature could inappropriately withhold VT/VF therapy in such patients.

PR Logic analyzes the activation patterns and timing in the atrium and ventricle to withhold inappropriate VT/VF detection during episodes of rapidly conducted supraventricular tachycardias while maintaining sensitivity for true VT and VF. PR Logic uses the rhythm’s AV pattern and rate to discriminate between SVTs and true ventricular tachyarrhythmias. PR Logic discrimination is applied to rhythms that fall between the SVT limit (nominally programmed to 260 ms or 320 ms) and the slowest programmed detection interval (FDI or TDI).

PR Logic is only applied at initial detection. It does not apply during redetection.

PR Logic will classify a rhythm as an AF/Afl, Sinus Tach, Other 1:1 SVT, or a Double Tachycardia. If PR Logic does not classify the rhythm and the Wavelet feature is turned On (available starting with Protecta devices) , Wavelet will analyze the rhythm before a final VF or VT classification is determined.

Following are descriptions of what PR Logic looks for to make a rhythm classification determination.

PR Logic Rhythm
AF/Afl (Atrial Fibrillation/Atrial Flutter) The AF/Afl feature is designed to withhold ventricular tachyarrhythmia detection when the ratio of sensed atrial to ventricular events is greater than 1:1 and the ventricular cycle length is not regular, indicating irregularly conducted atrial fibrillation. The AF/Afl feature is also designed to withhold ventricular tachyarrhythmia detection when AV pattern information shows regular 2:1 or 3:2 conduction, indicating atrial flutter.
Sinus Tach (Sinus Tachycardia) The Sinus Tach feature is designed to withhold ventricular tachyarrhythmia detection when there is sensed 1:1 atrial to ventricular conduction of rhythms that exhibit a gradual increase in rate into the detection zone and a similar PR interval to normal antegrade conduction.
Other 1:1 SVTs (Other Supraventricular Tachycardias) The Other 1:1 SVTs feature is designed to discriminate between ventricular tachyarrhythmias and rhythms that are the result of closely coupled atrial and ventricular depolarizations (for example, junctional rhythms such as AV nodal reentrant tachycardia). The Other 1:1 SVTs feature withholds ventricular tachyarrhythmia detection when AV pattern information indicates a 1:1 SVT in which there are consistent, near-simultaneous depolarizations of the chambers.
Double Tachycardia(Dissociated supraventricular and ventricular tachyarrhythmia) If both supraventricular and ventricular tachyarrhythmias occur at the same time, and the PR Logic analysis shows that the rhythms are dissociated and that the ventricular rhythm is not a conducted SVT, the device detects ventricular tachyarrhythmia and delivers the zone-appropriate ventricular therapy. In the episode text, the detected VT, VF, or FVT is recorded as VT (+SVT), VF (+SVT), or FVT (+SVT).

PR Logic elements – PR Logic analyzes the following rhythm characteristics to determine the origin of the arrhythmias just discussed: rate, pattern, regularity, AV dissociation, AF evidence, and presence of far-field R-waves.

PR Logic Rhythm
Rate Rate is calculated by the median of the last 12 atrial events and last 12 ventricular events. When the median of the 12 most recent sensed ventricular intervals is longer than the programmed SVT Limit, PR Logic will analyze the rhythm.
Pattern Pattern examines the position and number of P-waves relative to R-waves and is used to examine AV pattern information. For example, an AV pattern showing regular 2:1 or 3:2 conduction is one criterion indicating atrial flutter. An AV pattern showing 1:1 conduction with near simultaneous depolarization of both chambers indicates a 1:1 SVT classifying as Other 1:1 Supraventricular Tachycardias (junctional rhythms such as AV nodal reentrant tachycardia).
Regularity Regularity count examines how often the two most commonly occurring intervals (of at least 240 ms) occurred among the last 18 ventricular intervals up to detection. The number of intervals falling in these two most frequent bins is divided by the 18 intervals reviewed to calculate a regularity percentage. For a double tachycardia to be detected, the regularity count must be at least 75%. To withhold detection under the atrial fibrillation rule, the regularity count must be 50% or less. To withhold detection for Other 1:1 SVTs, the regularity count must be at least 25%.
AV Dissociation A rhythm is considered dissociated if four out of the most recent eight V-V intervals exhibit either:
  • No atrial events within a ventricular interval, or
  • An AV interval that differs from the average of the previous AV intervals by more than 40 ms
This criterion is used to help determine if a double tachycardia is present.
AF Evidence The AF evidence counter is used to identify atrial fibrillation, or to help detect a double tachycardia. The AF evidence counter is incremented when two or more atrial events occur within one V to V interval. The AF Evidence criterion is satisfied when the AF Evidence count is greater than or equal to 6. Once the criterion is met, it remains satisfied for as long as the AF Evidence count is greater than or equal to 5.
Far-Field R Waves The ICD identifies far-field R-wave oversensing in the atrium in order to exclude far-field R-waves from the process of SVT classification. If two atrial events occur in one V-V interval, the device further analyzes P-R pattern information to determine if one of the atrial events is actually a far-field R-wave. A far-field R-wave is identified on a beat-by-beat basis if there are exactly two atrial events for each R-R interval, there are consistent R-P or P-R intervals, and both the following are detected:
  • An A-A interval that exhibits a short-long pattern, AND
  • Either an A-V (PR) interval, which is < 60 ms,
  • A short V-A (RP) interval, which is < 160 ms
To identify sinus tachycardia with far-field R-wave sensing, the device requires a far-field R-wave on at least 4 of the 12 most recent ventricular intervals.

High Rate Timeout – The High Rate Timeout parameter allows you to program a maximum duration for which PR Logic and the other SVT discrimination features can withhold detection and therapy.

In the example below, the patient’s rate increases into the VT zone. The device uses PR Logic upon detection and therapy is successfully withheld. The Sinus Tach rule within PR Logic recognizes sinus tachycardia by the 1:1 conduction pattern and consistent R-R and P-R intervals. The annotation ST on the EGM decision channel indicates that therapy is withheld. Note: there are 3 Decision Channel annotations specific to PR Logic that display when that feature withholds ventricular tachyarrhythmia detection: AF (AF/Afl), ST (Sinus Tach), SV (Other 1:1 SVTs).

PR Logic programming

Interval plot


Using the Protecta Performance Model, PR Logic, combined with ATP, TWave Discrimination, and other SVT discriminators applied in the VF zone, significantly reduced shock frequency when tested on stored episodes from the SCD-HeFT Trial. This model, which combined existing and new shock reduction features, predicted 98% of patients would be free of inappropriate shocks 1 year after implant, and 92% at 5 years post-implant.1


  1. Volosin K, Exner DV, Wathen MS et al., Virtual ICD: A Model to Evaluate Shock Reduction Strategies, HRS Conference, May 2010.

Sources: Medtronic Protecta XT DR Clinician Manual; Medtronic Protecta XT CRT-D Clinician Manual.

Last updated: 
21 Mar 2013