Far Field R Wave sensing is a fairly common pacemaker malfunction occurring when the atrial lead sees and interprets a paced or spontaneous ventricular signal as coming from the atrium. This can result in inappropriate mode switch, or suppression of atrial pacing.
Factors predisposing to FFRW oversensing are:

  • Short Post Ventricular Atrial Blanking (PVAB) period
  • The amplitude of the ventricular EGM signal (FFRW over-sensing correlated with LVH)
  • Proximity of atrial and ventricular leads
  • Unipolar Sensing
  • High sensitivity setting of atrial lead
  • Features of the atrial lead:
    • distance between anode and cathode of atrial lead
    • proximity to ventricular muscle
    • orientation of the sensing dipole in relation to the orientation of the wave of ventricular depolarization

In 25% of cases, the atrial detection occurs prior to detection of the R wave in the Right ventricle. This is particular likely to be true if there are intra-ventricular conduction blocks (esp. RBBB). This is clinical pertinent as the usual fix (increase the PVAB) will not have any effect. Decreasing atrial sensitivity may be helpful, but at the cost of atrial under-sensing. The best way to avoid this risk is to assure a low FFRW amplitude at implant (i.e. less than 20% of the Atrial EGM amplitude) – if necessary by experimenting with atrial lead placement.