Far-field sensing by the atrial channel

Tracing N° 1
Medtronic Devices: PM Field: AV delays, refractory periods management of tachycardia

This 74-year-old man received an Ensura DR dual chamber pacemaker for complete AV block after aortic valve surgery; the atrial lead was implanted in the right atrial appendage and the ventricular lead was placed in the high septum; he was asymptomatic and was seen in follow-up 3 months after implantation of the pacing system.

Graph and trace

Tracing 1a: tracing recorded upon arrival; programmed in DDDR mode; the first channel is lead I of the surface ECG upon which the markers are superimposed, the second channel is the atrial EGM, the third is lead II and the last channel is lead III;

  1. sequences AP-VP-AR with fixed VP-AR intervals; atrial pacing appears effective; the atrial EGM show atrial depolarization after each AP, confirming that the atrium was effectively captured; ventricular capture is also confirmed; behind each VP, the atrial EGM shows a signal of low amplitude, which probably represents far-field sensing of the end of the ventricular signal by the atrial channel;

Tracing 1b: change in programming with prolongation of the post-ventricular atrial blanking;

  1. same tracing and same programming as the preceding figure;
  2. change in programming with prolongation of blanking (successful programming);
  3. the atrial signal sensed behind each ventricular paced event is identical, though is now sensed in the post-ventricular atrial blanking (fixed VP-Ab intervals);

Tracing 1c: change in programming with lower atrial sensitivity;

  1. same tracing and same programming as the preceding figure;
  2. change in programming with lower atrial sensitivity;
  3. elimination of far-field sensing.

The post-ventricular atrial blanking is the first phase of the post-ventricular atrial refractory period. As illustrated on this tracing, in a recipient of an Ensura pacemaker, the programmer shows the sensing of atrial events that fall in that blanking period as “Ab” on the marker chain, and includes it in the count of arrhythmias. When an atrial event occurs in the post-ventricular atrial refractory period after blanking, it is identified as AR on the recording of event markers; however, it neither changes the synchronization of the pacing intervals nor initiates an AV delay. This prevents ventricular synchronization to a retrograde P wave that might trigger a PMT.
In this patient, the diagnosis of crosstalk is evident; compared with ventricular pacing, the additional sensed atrial signal is of low amplitude and very early.

To prevent crosstalk, one might consider 4 measures:
1) a) lower the strength of ventricular pacing, b) program ventricular pacing in bipolar configuration (as was already the case), or c) both; in the majority of cases, however, these changes have little impact on oversensing, since it is the depolarization of the ventricles instead of the ventricular pacing stimulus that is sensed in the atrium.
2) program atrial sensing in a bipolar configuration (as was already the case).
3) make the atrial channel less sensitive; a reprogramming to 0.5 mV (instead of 0.3 mV initially) eliminated oversensing. This reprogramming incurred a risk of decreasing the detection of atrial arrhythmias, AF in particular, due to the damping of the atrial signals.

Theoritical features / Basic concepts

The atrioventricular delay Refractory periods of single chamber pacemakers Refractory periods of dual chamber pacemakers BVPA The safety window VRP ARP PVAB PVARP Protection against pacemaker-mediated tachycardias Onset, duration and rate of pacemaker-mediated tachycardia Prevention of onset of pacemaker-mediated tachycardia Diagnosis of pacemaker-mediated tachycardia by the pacemaker Anti-pacemaker-mediated tachycardia therapy  THE ATRIOVENTRICULAR DELAY The AV delay determines the longest time interval between an atrial event and the programmed delivery of a ventricular pacing pulse. This interval is the electronic equivalent of the PR interval. The programming of the AV delay intends to ensure optimal mechanical coordination between atrial and ventricular contractions, whether the...