PR Logic and conducted atrial fibrillation

Tracing N° 36
Medtronic Devices: ICD Field: Discrimination

Patient with ischemic cardiomyopathy implanted with a dual-chamber ICD (Evera XT DR); the following tracings will allow detailing the operation of the PR Logic algorithm in the setting of an episode of fast conducted AF.

Graph and trace

The graph shows a very fast atrial rhythm and an irregular ventricular rhythm with intervals initially oscillating between sinus zone and VT zone in favor of a conducted AF; after a stabilization in the VT zone, the VT counter is filled although no therapy is delivered given a diagnosis of SVT.

  1. the EGM is indicative of a conducted AF;
  2. all intervals classified as VS reset the VT counter to 0;
  3. after stabilization of the ventricular rhythm in the VT zone, the VT counter is filled; no therapy is delivered, the device having arrived at a diagnosis of conducted AF;
  4. the PR Logic and Wavelet algorithms are both programmed.

This patient presented episodes of fast conducted AF detected in the VT zone. This tracing demonstrates the interest of the VT counter (requirement of consecutive rapid intervals) in allowing a first "discrimination" during an episode of conducted AF. Indeed, any interval ranked VS resets the VT counter.
This tracing also allows detailing the different steps leading to the diagnosis of conducted AF by the PR Logic algorithm, which intervenes only when the VT or VF counter is filled:

  1. the atrial rate is clearly faster than the ventricular rate; for a diagnosis of conducted AF, the median atrial interval should be less than or equal to 94% of the median ventricular interval;
  2. there is no oversensing of the R wave at the atrial channel with large variability of the atrial intervals and absence of characteristic repetitive sequences; for a diagnosis of conducted AF, less than 10 out of the last 12 ventricular intervals are required with diagnosis of oversensing;
  3. the AF counter is > 6; the possible diagnoses at this stage are thus conducted AF, conducted atrial flutter, double tachycardia;
  4. the ventricular rhythm is considered to be irregular with a large variability of the RR intervals (between 260 and 330 ms) and therefore indicative of a conducted AF (the 2 most frequent intervals represent less than 50% of the total number of intervals) .

If the PR Logic concludes to a conducted AF, Wavelet (if set to ON) is not integrated in the discrimination and therefore cannot change the diagnosis. Conversely, if PR Logic concludes to double tachycardia, Wavelet (if programmed ON) is integrated in the discrimination.