Automatic left ventricular threshold measurement
A 64-year old man with a history of dilated cardiomyopathy and left bundle branch block was implanted with a triple-chamber Boston Scientific Autogen CRT-D. Interrogation revealed an automatic threshold measurement of the left ventricular lead.
Four channels are displayed: the atrial channel, the right ventricular channel, the left ventricular channel and the evoked response channel which enables validation of automatic threshold measurements.
- atrial sensing and left ventricular stimulation with capture (3 stimuli at 1.2 V then lowering of the amplitude by 0.1 V); right ventricular backup stimulation 80 ms after left ventricular stimulation.
- loss of left ventricular capture at 0.9 V, diagnosed correctly by the device (LOC: loss of capture) with distinctive change in the appearance on the evoked response channel; right ventricular backup stimulation with capture and left ventricular detection.
- second ineffective stimulus; the device diagnoses loss of capture; threshold raised to 1 V.
The automatic left ventricular threshold measurement is based on the analysis of the evoked response. The evoked response channel is a unipolar detection channel which is comprised of the distal or proximal electrode on the LV lead, according to the configuration of stimulation. Every 21 hours, a threshold measurement is performed which sets the pacing amplitude for the following 21 hours with a programmable safety margin (from 0.5 to 2.5 V) without beat-to-beat verification of capture. The amplitude is not programmed solely according to the last measurement but adds the programmable safety margin to the maximum threshold of the last 7 successful automatic measurements. The left ventricular thresholds are often elevated when compared to right ventricular thresholds because during epicardial stimulation, the lead is positioned but not screwed into a coronary vein. Determining the stimulation amplitude is sometimes difficult due to high thresholds or the existence of diaphragmatic stimulation which may result in a narrow margin of programming. Automatic threshold measurements with adaptive amplitudes and the possibility of using low safety margins can reduce the risk of overconsumption when the threshold is relatively high. In similar fashion, programming low safety margins (minimal value of 0.5 V) permits efficacious left ventricular stimulation while avoiding diaphragmatic stimulation in situations with narrow margin of programming.