Left ventricular pacing
Content
General information
Stimulation configuration
Medtronic’s range of left ventricular leads includes monopolar, bipolar, and quadripolar leads. Stimulation polarity can be programmed in the left ventricle using different selections depending on whether the device supports a quadripolar (4-electrode) or non-quadripolar LV stimulation lead (1 or 2 electrodes). The quadripolar lead offers 16 pacing polarities, while non-quadripolar leads with 1 (unipolar) or 2 electrodes (bipolar) provide 1 to 4 (5 for the Consulta CRTP) pacing polarities, respectively.
These different options allow you to select a pacing polarity that ensures threshold control while limiting the energy used in order to increase the longevity of the device and select a pacing polarity that avoids pacing of the phrenic nerve.
Unipolar Lead
The single electrode of a unipolar lead is called the LV distal. If a unipolar left ventricular lead is implanted, only one programming option is available on new-generation defibrillators: LV Distal/RV Tip. The lead cannot be integrated into the pacing circuit. In a triple-chamber pacemaker, stimulation between the electrode and the device is possible, with the distal electrode of the lead being the active electrode and the device being the neutral electrode.
Bipolar lead
The two electrodes are called distal LV and anode LV. If a bipolar left ventricular lead is implanted, four programming options are available on new-generation defibrillators: Distal LV/Spire RV, Anode LV/Spire RV, Distal LV/Anode LV, and Anode LV/Distal LV. Thresholds are often higher for the LV Anode/RV Lead configuration, and the distal electrode (cathode) is generally used as the active electrode, with the RV lead or LV ring (anode) used as the neutral electrode.
Quadripolar probes
The four electrodes are named VG1, VG2, VG3, and VG4, with VG1 being the most distal electrode. Sixteen different configurations are available.
- The VectorExpress VG automatic test: general information
This test, available on devices compatible with a quadripolar probe, facilitates the choice of configuration by automatically checking the impedances and stimulation thresholds of the left ventricle for the different stimulation polarities selected. The results include the stimulation threshold, the relative impact of the results on device longevity, and the impedance of the probes for each stimulation polarity tested. The results of the phrenic nerve stimulation threshold test may also be included.
The test results can be used to determine the appropriate VG stimulation polarity. They can also be used to determine the amplitude and pulse duration settings to ensure threshold control, reduce energy usage, and thereby optimize device longevity.
- The VectorExpress VG automatic test: practical details
The VectorExpress VG automatic test performs a pacing threshold search to determine the LV pacing amplitude threshold at the selected pulse duration for each selected LV pacing polarity.
The test varies the amplitude of the test stimulations to find the lowest amplitude that excites the left ventricular myocardium. The device evaluates threshold control by observing the timing of RV events detected following a stimulated LV event to determine whether threshold control is achieved with the test amplitudes.
If the left ventricle responds to a test pacing stimulus, the result is “Entrained.” If no response is detected, the result is “Disenrained.”
If the results are inconclusive or too many intrinsic events occur, the VectorExpress test is terminated on the current LV pacing polarity and moves on to the next one.
A pacing threshold search is performed for each selected LV pacing polarity at an initial test amplitude of 2.5 V. The search process then varies depending on whether this test amplitude of 2.5 V is above or below the pacing threshold.
If the 2.5 V test amplitude is greater than the stimulation threshold, the device lowers it in increments of 0.25 V until it is considered to be below the threshold or the minimum test amplitude of 0.25 V is reached.
If the 2.5 V test amplitude is below the stimulation threshold, it is increased to 6.0 V. If the 6.0 V test amplitude is below the stimulation threshold, the test indicates that the threshold is above 6.0 V for the VG stimulation polarity. Otherwise, the device lowers the test amplitude in increments of 0.5 V until a test amplitude is considered to be below the threshold or reaches 3 V.
The last test amplitude above the threshold corresponds to the VG stimulation threshold for this stimulation polarity.
The 16-vector test takes 2 to 3 minutes. It is possible to exclude stimulation polarities that caused phrenic nerve stimulation.
Left ventricular pacing amplitude
The main objective of device adjustment is to enable permanent biventricular pacing. Adjusting the pacing amplitude should optimize the device’s lifespan while maintaining a sufficient safety margin. Epicardial left ventricular pacing thresholds are often higher and more variable than right ventricular pacing thresholds. One study found threshold values twice as high for left ventricular leads as for right ventricular leads. It is not always necessary to maintain a margin corresponding to twice the threshold, as this can lead to premature wear if the threshold exceeds 2 volts. In some patients, it is necessary to adjust the pacing amplitude and pulse duration as closely as possible in order to achieve left ventricular capture without phrenic stimulation.
The pacing amplitude also influences the probability of anodic capture. The cathode at the tip of the left ventricular lead is generally smaller than the anode. The reduced size of the cathode explains the high current density. High stimulation amplitudes can result in a current density high enough to capture the tissue near the anode.
In a triple-chamber pacemaker, the RV ring is often used as the anode for LV pacing. High-amplitude pacing can cause right ventricular anodic capture, resulting in triple-point pacing: left ventricular and right ventricular cathode + right ventricular anode. Anodic capture occurs more frequently when the LV pacing configuration includes a true right ventricular bipolar lead (lead ring) rather than an integrated bipolar lead (with the distal coil as the anode), probably because the smaller size of the ring allows for a higher current density. The electrocardiographic appearance is often slightly different from the traditional biventricular appearance. A 12-lead electrocardiogram, rather than a single lead from the programmer, is usually necessary to make the diagnosis.
However, anodic capture can significantly alter the analysis of the tracings when performing the left ventricular threshold test.
The hemodynamic effect of this type of pacing, which consists of increasing the number of pacing sites, could be beneficial. The clinical impact remains to be demonstrated. This anodic capture generally requires high pacing amplitudes, which has an adverse effect on the device’s lifespan.
