Chapter 1 · LV pacing and threshold · Case 6
LV impedance alert and an odd PaceSafe LV test
Patient and episode
Patient
- AUTOGEN X4 CRT-D, 6 years after implant; VVIR 60, the atrial lead out of use (impedance >3000 Ω); BiV pacing (92% of beats), LV offset 0 ms, LV vector LVTip1>>RV, LV output Auto (PaceSafe LV)
- A remote report with an LV impedance alert; three weeks later, a PaceSafe LV test stored by the device
The recording
Tap a number on the trace, or an entry in the list below
- Remote report: alert “Left ventricular pacing lead impedance out of range”, raised the evening before
- RV lead stable: pacing impedance 404 Ω at the last office visit and 431 Ω on the daily measurement, RV threshold 0.8 V @ 0.4 ms
- LV lead: pacing impedance 289 Ω and threshold 0.8 V at the last office visit; 612 Ω and 2.0 V @ 0.4 ms on the last daily measurements. On the trends both had been stable for 6 months (360-505 Ω, 0.6-0.8 V) and jump on the last day; PaceSafe LV sets the LV output to 3.0 V (threshold + safety margin)
- Presenting EGM of the report: BiV pacing at the lower rate, LV and RV paced together every 1000 ms (LVP 0, RVP 1000; LV offset 0 ms), with the same paced complex on the Shock channel at every beat
- Sharp deflections on the LV channel only (nothing on the RV and Shock channels) = noise, sensed as LVS 798: the LV pace due 200 ms later is inhibited (Inh-LVP, LV protection period) and only the RV is paced; the Shock complex changes and the LV is activated late (large LV deflection)
- Three weeks later: ambulatory PaceSafe LV test in VVIR. Each LV test pulse (LVP 775) is backed up by an RV pace 80 ms later (RVP 855); at 1.3 V the evoked response (ER, bottom channel) is the same after every LV pulse = capture
- 1.2 V: bursts of noise and sharp spikes appear on the LV channel only (the RV channel stays clean), some sensed in the refractory period ([LVS]); the evoked response is distorted and the device labels 2 beats “Fusion”
- An isolated sharp LV spike, with nothing on the RV channel, is sensed (LVS 448): the test pulse due 330 ms later is inhibited (Inh-LVP) and only the RV is paced; the same happens again 3 cycles later (LVS 445)
- 1.1 V: loss of capture (LOC) on 2 beats, the evoked response changes and the LV is activated late ([LVS]): end of the test (End). 1.2 V was the last amplitude that captured, a value measured among noise
Points to remember
- Read an LV impedance alert with the trends and the other leads: here the LV impedance and the LV threshold jump together after 6 stable months, while the RV lead is unchanged. With noise on the LV channel only, this points to the LV lead or its connection
- Noise on the LV channel alone is sensed as LVS, and the next LV pace is withheld (Inh-LVP): the beat is paced from the RV only, and CRT is lost on those beats
- Noise also spoils the automatic LV threshold test: a threshold measured among noise cannot be trusted. And LV capture is not checked beat by beat, so a lead that worsens between two tests can lose capture unnoticed
- Next step: see whether another vector of this quadripolar LV lead can replace LVTip1>>RV, testing each for threshold and phrenic nerve stimulation and keeping one with a low threshold and no phrenic capture. With a vector to the RV, also look for anodal capture (at high output the RV electrode, the anode, can capture the RV, and the device cannot see it); a 12-lead ECG confirms that the LV is captured
Summary
- An LV lead problem (the lead or its connection): the LV impedance and threshold jump together, and noise on the LV channel alone inhibits LV pacing on the presenting EGM, then spoils a PaceSafe LV test three weeks later
Clinical pearl
It is essential to understand the LV channel markers to understand how the LV lead works: an LVS on noise followed by Inh-LVP means that the LV was not paced on that beat
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.