Chapter 1 · LV pacing and threshold · Case 3

PaceSafe LV test with an unexpected beat

Patient and episode

Patient

  • RESONATE CRT-D in sinus rhythm at 96 bpm with atrial tracking; LV output programmed Auto (PaceSafe LV)
  • An ambulatory LV threshold test stored by the device and sent through LATITUDE, with an unexpected beat in the middle: is the measured threshold valid?

The recording

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

Boston Scientific CRT electrogram, segment 1/1 — PaceSafe LV test with an unexpected beat
  1. Ambulatory PaceSafe LV test in sinus rhythm (AS ≈ 625 ms): an LV test pulse just after each atrial sensed event and a backup RV pace 80 ms later; 1.0 V for 3 beats
  2. 0.9 V for 3 beats, with capture
  3. Premature ventricular beat: RV sensed (RVS 538) about 20 ms after the atrial event and LV sensed 53 ms later, with a different morphology on every channel = PVC; no test pulse on this cycle and no loss of capture counted
  4. The test resumes on the next tracked beat, at the next step: 0.8 V for 3 beats (LVP 630: the LV interval restarts from the PVC)
  5. 0.7 V for 3 beats, with capture
  6. 0.6 V: the first beat still captures
  7. Loss of capture: the evoked response changes and the LV is activated late ([LVS])
  8. Second loss of capture at 0.6 V: end of the test; the LV threshold is 0.7 V

Points to remember

  • A PVC, or any sensed ventricular beat, during PaceSafe LV only cancels that test cycle: no test pulse, no loss of capture counted, and the step-down resumes on the next tracked beat
  • The test stops only after 2 losses of capture at one step, so a single odd beat cannot fake a high threshold: the threshold measured here, 0.7 V, can be trusted
  • Tell a PVC from a loss of capture on the markers and the EGM: a PVC is sensed (RVS, then LVS), with a new shape on every channel; a loss of capture is an LV pace (LVP) followed by a changed evoked response and a late LV activation ([LVS])

Summary

  • A PVC during an ambulatory PaceSafe LV test: the device skips that cycle, counts no loss of capture and goes on; the LV threshold, 0.7 V, is valid

Clinical pearl

An odd beat in an LV threshold test is not always a loss of capture: if its markers show it was sensed (RVS, LVS), it is a PVC that the test simply skips, and the threshold stays valid

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.