June 2019

A remote alert for AT/AF daily burden exceeding threshold

Jason Collinson – Chief Cardiac Physiologist
Essex Cardiothoracic Centre, Basildon and Thurrock University Hospital


A remote alert was received from an 80 year old male with a Medtronic CRT-pacemaker for AT/AF daily burden exceeding threshold a few weeks after handing out home monitoring. There were 73 episodes of AT/AF recorded since his last follow up (approx. 2 weeks) with the most recent episode having lasted more than 6 hours. No previous atrial arrhythmias had been documented. The presenting EGM (figure 1) and device settings are displayed below.

Mode: DDD
LR/UR: 60/130 bpm.
Paced/Sensed AV delay: 130/100 ms
PVAB: 150 ms
PVAB method: Partial +
Mode switch: On
Sensitivity thresholds: Atrial 0.3 mV, Ventricular 2.5 mV

Figure 1
Presenting EGM – Channel 1 shows the atrial tip to ring EGM. Channel 2 the RV tip to RV ring EGM. Below these are the A-A interval, marker and V-V interval channels.


A) What is the cause of the AT/AF episodes?

  • Atrial bigeminy
  • Far field R wave oversensing
  • Atrial undersensing
  • Atrial tachycardia

Answer a)
The answer is Far field R wave (FFRW) oversensing


This trace is an example of FFRW oversensing in a Medtronic CRT-pacemaker causing inappropriate mode switch (MS) recorded as an AT/AF episode. The sharp signals on the atrial channel are atrial paced (AP) events and atrial sensed intrinsic P waves (AS markers). FFRWs are the small signals seen on the atrial channel occurring after every biventricular paced (BV) complex. In this case all FFRWs fall in the post ventricular atrial blanking period and are marked (Ab).

Historically, in most pacemakers/ICDs events falling in the PVAB would be ignored and would not result in inappropriate mode switching. However, modern device platforms as in this Medtronic pacemaker mark sensed atrial events falling in the PVAB. These do not affect pacing timing cycles but are used in tachyarrhythmia detection. Consequently, the persistent FFRWs (Ab markers) combined with the AS (intrinsic P waves) and occasional AR events (intrinsic P waves in the post ventricular atrial refractory period, PVARP) contribute to the calculated atrial rate which meets the rate criteria for mode switching to occur.

B) Which two programming options should be considered to correct the problem?

  • Increase the PVAB
  • Shorten the PVAB
  • Decrease atrial sensitivity
  • Increase atrial sensitivity
  • Change the PVAB method

Answer b)
1. Decrease atrial sensitivity, 2. Change the PVAB method


The problem in this case is that FFRWs are present and being marked as an Ab event all the time which is being used in the rate count to trigger MS. To correct this problem the solution must result in eliminating the FFRWs entirely from being sensed and marked as Ab events. From the answers available there are only two options which may achieve this.

1. Decrease atrial sensitivity

A first option to consider if the intrinsic P wave amplitude is large and is notably larger than the FFRW amplitude, is to decrease atrial sensitivity (make the atrial channel less sensitive, increase the threshold value e.g. 0.3 mV  0.6 mV). This may help eliminate FFRWs from being seen and hence not trigger an Ab marker although does risk delaying detection of a true AT/AF episode. If amplitudes are similar this may not be an option as could result in atrial undersensing.

2. Change the PVAB method

In Medtronic devices it is worth understanding the three possible PVAB methods which can be programmed. These are PVAB partial, partial + (already programmed here) and absolute.

  • PVAB Partial works by marking atrial events which fall in the programmed PVAB interval (in this case 150ms) as an Ab. Marked events do not affect pacemaker timing cycles but do count towards tachyarrhythmia detection features.
  • PVAB Partial + operates in a similar way to PVAB Partial but differs by decreasing the atrial sensitivity (increases the threshold value) after a ventricular event for the duration of the programmed PVAB interval. The sensitivity then gradually returns to the programmed value after the PVAB interval times out. Events are still marked Ab do not affect pacemaker timing cycles but do count towards tachyarrhythmia detection features (as in this case)
  • PVAB Absolute is a true blanking period where no atrial events are sensed within the PVAB interval. The atrial channel is completely blind to atrial events within the programmed window and therefore no Ab markers can be allocated.

With the above in mind in this case programming the PVAB method to Absolute could help eliminate the FFRWs being marked. This solution however again is not perfect as caution is advised when selecting the Absolute method as the absolute blanking may reduce the devices ability to sense AT/AF episodes and in ICDs may reduce the ability to discriminate between VT and SVT.