Ventricular Pacing failure

Tracing N° 1
Medtronic Devices: PM Field: Pacing & Sensing

This 65-year-old man received an Ensura dual chamber pacemaker for treatment of syncope due to high-grade AV conduction defect; he suffered from end-stage renal failure and sustained a syncopal event a few hours before undergoing hemodialysis, 2 months after implantation of the pacemaker.

Graph and trace

The first channel is lead I of the surface ECG with the markers superimposed, the second shows the atrial EGM, the third and fourth show the ventricular EGM, and the last channel is lead III;

  1. accurate atrial sensing initiating an AV delay and ventricular pacing (AS-VP); the ventricular EGM and both surface leads show the absence of ventricular capture (no ventricular signal after the pacing stimulus);
  2. the spontaneous ventricular event after the preceding P wave is properly sensed (VS);
  3. a new atrial sensing-ventricular pacing sequence (AS-VP) is ineffective; the P wave is blocked; the rhythm is consistent with type I (Wenckebach) second degree AV block (progressive prolongation of the PR interval followed by blocked P wave);
  4. ineffective ventricular pacing; the following QRS was not sensed, not due, however, to faulty sensing; instead, the QRS fell in the post-ventricular pacing ventricular blanking period, a period of absolute ventricular refractoriness and, therefore, was not sensed;
  5. continuation of missing ventricular capture;

This pacemaker was programmed at a pacing amplitude of 2.5V / 0.4 ms. In hemodialyzed patients, the status of electrolytes, e.g. sodium, potassium, calcium, and glucose levels change constantly with possible concomitant changes in pacing threshold, requiring a regular monitoring of the pacing output and consequent modifications in order to reliably capture the myocardium. With this in mind, Capture Management is a noteworthy function. When Capture Management is active, the pacemaker automatically monitors the pacing threshold at regular intervals. After it has measured the threshold, the pacemaker sets a target output as a function of a programmable safety margin. This guarantees a reliable capture in patients whose pacing threshold varies widely, without having to permanently program high pacing amplitudes.
In this patient, variations in the ventricular threshold were observed, reaching 3.5 V / 0.4 ms. No lead dislodgement was found on chest X-ray.

Theoritical features / Basic concepts

Pacing Definition of pacing threshold Chronaxy and rheobase Factors that influence the safety matgin Influence of the electrode configuration on capture threshold and lead impedance Influence of polarity Sensing The frequency spectum Slope Signal amplitude Automatic sensing Automatic sensitivity PACING Definition of pacing threshold The pacing threshold is the lowest electrical pulse, delivered outside the natural refractory periods, that consistently elicits the propagation of a depolarizing wavefront. It is measured as voltage amplitude (V) or pulse width (ms). Chronaxie and rheobase Lapicques voltage - duration (or chronaxie - rheobase) relationship, expresses the non-linear relation between the threshold voltage and pulse duration. The pacing threshold voltage increases...