AF Detection Feature

The AF Detection feature can detect atrial fibrillation (AF) by evaluating variations in the ventricular rhythm.  This feature is found in the Visia AF family of single chamber ICDs. It is intended for diagnostic purposes only, and does not provide atrial therapy. AF Detection uses pattern recognition of R-wave intervals that are highly irregular and uncorrelated when AF is present. 
This feature is appropriate for:
  • Patients at risk of developing AF  
  • Patients without a high percentage of ventricular pacing
  • Patients experiencing palpitations and rapid heartbeats potentially associated with atrial arrhythmias
  • Monitoring AF burden and the occurrence of asymptomatic episodes of AF, to assess whether medical treatment is necessary or should be adjusted.

This feature can be found in some Medtronic ICD devices. Please go to manuals.medtronic.com or consult with your local Medtronic representative regarding device models available in your geography.

The AF Detection feature is nominally programmed to Monitor.  It may also be programmed to Off.  When AF detection is programmed to Off, no AF diagnostics will be collected.    There are three distinct AF Detection parameters that can be programmed: 
  • AF Sensitivity
  • Ectopy Rejection 
  • Record EGM for 

To program this feature go to Params -> AF…  

Review the feature operation for more information on programming each of the AF Detection parameters.

AF Detection is based on R-R variability. The R-R interval differences between consecutive intrinsic R-wave intervals are plotted in a Lorenz plot.  R-wave intervals during an AF episode are highly irregular and uncorrelated. Pattern recognition is used to identify the presence of AF within 2-minute periods. The differences between consecutive R-wave intervals are registered to a Lorenz plot internally within the device.  An AF evidence score representing the degree of correlation to an AF rhythm is determined for each 2-minute period. The AF evidence score is assessed against the programmed AF Sensitivity threshold.  When the AF evidence score is greater than the AF Sensitivity threshold, AF is met for that 2-minute period.1 The Lorenz plots seen below are representations of how the AF Detection feature detects atrial fibrillation.
 
Lorenz Plot of an AF Episode
 
Lorenz Plot of Normal Sinus Rhythm
 
AF Sensitivity defines the threshold of AF evidence required to detect AF; it is not related to the sensitivity of detecting R-waves.  It can be programmed to Least Sensitive, Less Sensitive, Balanced Sensitivity, More Sensitive, and Most Sensitive. Balanced Sensitivity is the nominal setting.  It is important to note that the AF Sensitivity threshold has an inverse relationship to the evidence of AF.  For example: 
  • If the device is programmed to be Less Sensitive it raises the threshold and requires more AF evidence (or RR interval variability) to detect AF.  
  • If the device is programmed to be More Sensitive it decreases the threshold and requires less AF evidence (or RR interval variability) to detect AF.

Ectopy Rejection is designed to reduce the AF evidence score in the presence of ectopy.  It is nominally programmed to On, but can be programmed to Off. This parameter can be used to control AF detection performance when a patient has runs of ectopy (PACs or PVCs).When Ectopy Rejection is enabled, the device looks for patterns of ectopy by the density of points in the Lorenz Plot. The device will not detect AF when the algorithm detects evidence of ectopy during any 2-minute period.
 
Record EGM for… controls the duration of AF that is required to store an AF episode. An AF episode will not be stored until the programmed “Record EGM for” duration is met.  The duration can be programmed to 6, 10, 20, 30, or 60 minutes and is nominally set to 6 minutes.
Considerations:
  • AF detection occurs when the device detects atrial fibrillation for 6 minutes, this is nonprogrammable.  AF episode storage is controlled by the “Record EGM for” parameter
  • Ventricular pacing can regularize the rate and impact the ability to detect AF.
  • If the patient has complete heart block and requires continuous ventricular pacing, AF will not be detected.
Once an AF episode is detected, the device identifies the onset of an atrial fibrillation episode as the beginning of the first 2-minute period classified as AF leading to detection.  
 
AF Episode Termination
Termination occurs:
  • If the R-wave interval variability is low, the 2-minute period is not classified as AF and the episode terminates. 
  • AF Detection is suspended, if any parameter(s) are programmed, or MRI SureScan is programmed to ON, during  the 2-minute AF detection period. 
  • If there are 5 consecutive unclassified 2-minute periods. If any of the following events occur within a 2-minute period, the 2-minute period is considered unclassified:
    • High level of ventricular pacing
    • EGM Signal Noise
    • Higher priority episode (e.g. VT/VF, SVT)
    • T Wave Discrimination criteria is met
    • RV Lead Noise Discrimination criteria is met
    • Not enough R-wave intervals because events were excluded due to device operation, such as automatic battery measurements or capture threshold tests.
An asterisk (*) will be listed next to an AF episode to indicate that the episode was terminated prematurely.
 
Data Collection Set-up
There some notable changes to the data collection in Visia AF devices to enhance the evaluation of the AF episodes.
The LECG source includes two vectors, RVcoil to SVC and Can to RVcoil for enhance P-wave visibility.  Can to RVcoil is the nominal source.  The device will automatically switch the source to RVcoil to SVC prior to an AF episode if a valid SVC lead impedance is measured, ensuring the presence of a viable SVC coil electrode in the implanted system.  It is not necessary or recommended that you reprogram the LECG source or nominal millivolt range of +/- 2 mV.
 
The Stored (Ventricular) and Stored (Atrial) sources control which EGM vectors are stored for the respective episode types.  Three EGMs are collected; one dedicated to atrial episodes, one dedicated to ventricular episodes and one shared vector, in order to ensure dual episode collection. The device will be nominally programmed with EGM 2 (Wavelet) as the shared vector.  Therefore both AF and VT/VF episodes will have EGM 2 in the stored episode logs.  The second vector for an AF episode will be LECG as the high gain P-wave visible vector for AF episodes.  The second vector for a VT/VF episode will be EGM 1.
 
Evaluation of AF Detection
Diagnostic data on AF detection and AF burden are available on: Quick Look™ II, Arrhythmia Episode screen, Flashback Memory, Cardiac Compass™ Trends, Rate Histograms, and Episode counters.
 
Quick Look II screen – Shows the total percentage of time that the patient has spent in AF and the number of monitored AF episodes since the last session. 
 
Arrhythmia Episode screen – Shows list of all recorded episodes. AF episodes can be found by selecting the AF checkbox.  The longest AF episode should be reviewed first.  This can be done by filtering to view the longest AF episode or by finding the AF episode labeled as the longest within the AF episode list.  Each AF episode record includes the following:  
  • Interval Plot
  • EGM data
  • Episode Text
  • Episode Counters
The Plot option displays a diagram of the R-R intervals and shows the time stamp (Minimum EGM Storage Duration Met) when the EGM was collected for the episode. 
 

The EGM option displays two stored EGMs. The first is the high-gain LECG vector for P-wave visibility. The second is the standard EGM2 (Wavelet), far-field vector that provides a zoomed out view to see morphology.  To optimize P-wave visibility, each stored atrial episode will have two stored EGMs:  LECG and EGM2.   If the LECG EGM and the EGM 2 (Wavelet) sources are the same, then you will see the same EGM on both channels in your EGM display.

The Text provides additional information about the episode such as total duration and median ventricular rate during the episode. Also, if the episodes terminated for any reason other than a low AF evidence score, the termination reason will be provided in the episode text.

AF Episode Counters (Data -> Counters) provide a summary of AF activity. The following summary data (Counters) are available for AF episodes:
Prior Session and Last Session Data
  • % of Time AF – The percentage of total time in AF.
  • Average AF time/day – The average time in AF per day.
  • Monitored AF Episodes – The average number of monitored AF episodes per day.
Since Last Session Data
  • AF Durations – The number of detected AF episodes for each of a series of time durations.
  • AF Start 

AF diagnostic information can also be found in the Cardiac Compass Trends.  The AF Detection feature allows for the monitoring and recording of AF Burden and V-rate during AF trended information to support the clinical management of the patient. 

 

References

  1. Boriani G,Glotzer TV, Santini M, et al. Device-detected atrial fibrillation and risk for stroke: an analysis of > 10,000 patients from the SOS AF project (Stroke preventiOn Strategies based on Atrial Fibrillation Information from implanted devices). Eur Heart J. 2014;35(8):508-516.
Sources: Medtronic Visia AF™ Reference Manual, Medtronic Visia AF MRI™ SureScan™ Defibrillation System MRI Technical Manual 
Last updated: 
13 Jul 2016