Chapter 1 · LV pacing and threshold · Case 4

Non-responder despite 100% BiV pacing

Patient and episode

Patient

  • COGNIS CRT-D in a 67-year-old man with dilated cardiomyopathy and left bundle branch block
  • No response 6 months after the implant (still breathless on daily activities), although interrogation shows 100% BiV pacing

The recording

Tap a number on the trace, or an entry in the list below

Boston Scientific CRT electrogram, segment 1/3 — Non-responder despite 100% BiV pacing
Boston Scientific CRT electrogram, segment 2/3 — Non-responder despite 100% BiV pacing
Boston Scientific CRT electrogram, segment 3/3 — Non-responder despite 100% BiV pacing
  1. Recorded at the visit: sinus rhythm (AS ≈ 680 ms), each atrial sensed event tracked by an RV and an LV pace delivered together (RVP, LVP 0: no LV offset), so the counters show 100% BiV pacing; the paced QRS on Lead I is the same on every beat, positive
  2. Same programming, LV output 2.5 V/0.4 ms: this QRS is the same as on the first tracing (positive on Lead I) = RV capture only, the LV is not captured
  3. The next QRS is different (negative on Lead I) = BiV capture; the two alternate from beat to beat, and the LV EGM (VG) alternates with them. The LV threshold measured at this visit is 2.5 V/0.4 ms: the output sits exactly at threshold
  4. LV output reprogrammed to 3.5 V/0.4 ms, same pacing configuration: the BiV capture QRS on every beat (negative on Lead I; LV EGM now at 2 mm/mV instead of 0.5). So the first tracing, 100% BiV pacing on the counters, was RV capture only

Points to remember

  • The counters count paced beats, not captured ones: an LV output set at the LV threshold gives lost or intermittent LV capture that they do not show
  • In a non-responder, look at the paced QRS (on an ECG, or Lead I on the programmer) and measure the LV threshold; a paced QRS that changes from beat to beat points to intermittent LV capture
  • Here the chest X-ray showed no visible dislodgement; the LV threshold may have risen from a micro-dislodgement or from local changes in the myocardium
  • What to do: measure the LV threshold in each pacing configuration of the lead, keep the one with the lowest threshold without phrenic nerve stimulation, and leave a safety margin above it; here 3.5 V restored BiV capture on every beat

Summary

  • The LV threshold has risen to the programmed LV output (2.5 V): LV capture is lost or intermittent while the counters show 100% BiV pacing; at 3.5 V every beat is BiV captured

Clinical pearl

100% BiV pacing on the counters is not 100% BiV capture: in a non-responder, look at the paced QRS and measure the LV threshold

Practice out loud

Just describe the tracing. What do you think is going on? Describing the numbers can help.

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From Boston Scientific CRT — clinical cases by M. Strik. Published by Cardiocases. Every numbered marker on a recording is explained in the list beneath it; tap a marker on the trace or an entry in the list.