Configuration PACING
The arsenal of left ventricular leads offered by Medtronic includes monopolar, bipolar and quadripolar leads. The pacing polarity is fully programmable in the left ventricle using various selections of bipoles and according to lead specificity: quadripolar (4-pole), bipolar or not (two or one electrodes). Quadripolar leads provide 16 pacing polarities, while unipolar leads and bipolar leads provide 1 or 4 pacing polarities respectively.
Unipolar leads
The single electrode of a unipolar lead is called «LV tip». If a unipolar left ventricular lead is implanted, only one programming configuration is available on the new generation of defibrillators: DistLV tip to RV coil. The ICD cannot be integrated in the stimulation circuit. In a three chamber pacemaker however, unipolar stimulation between the tip electrode and can is possible, the only electrode of the lead being the active electrode and the can being the neutral electrode.
Bipolar leads
The two electrodes are called «LV tip» and «LV ring or LV anode». If a bipolar left ventricular lead is implanted, four programming options are available with the new generation of defibrillators: LV tip to RV coil; LV ring to RV coil; LV tip to LV ring (LV bipolar) and LV ring to LV tip. LV Thresholds are often higher with the LV ring to RV coil configuration. The tip electrode (cathode) is generally used as the active electrode, the RV coil or ring VG (anode) being used as a neutral electrode.
Quadripolar leads
The 4 electrodes are called LV1, LV2, LV3 and LV4, LV1 being the most distal electrode. Sixteen different configurations can be proposed.
Automatic test «VectorExpress »: general concept
This test is available on the device compatible with a quadripolar leads and facilitates the choice of a stimulation configuration by automatically measuring the impedance and the stimulation thresholds for left ventricular pacing in all possible different polarities. The results include the threshold simulation, the relative impact of the results on the longevity of the device and lead impedance for each polarity of stimulation tested. The results of the phrenic nerve stimulation threshold test can also be included.
The test results can be used to determine the most suitable LV stimulation polarity. They can also be used to set the amplitude and the pulse duration, to ensure the appropriate control of the threshold test, to reduce the energy used and therefore to optimize the device longevity.
Automatic test «VectorExpress »: practical modalities
The Automatic test «VectorExpress » determines the LV pacing threshold for each LV pacing polarity for the selected pulse duration. The test varies the amplitude of stimulation to find the lowest threshold that effectively entrains the left ventricle. The device estimates the LV threshold by observing the right ventricular-coupled event detected after a pacing in the LV. If a right ventricular event is detected during the threshold test, the result is "entrained". If no right ventricular response is detected, the result is a "loss of entrainment". If the results are inconclusive or too many intrinsic events occur, the test is aborted with the actual specific pacing polarity and the test progress to the next pacing polarity.
A stimulation threshold test is performed for each selected polarity LV pacing amplitude at an initial value of 2.5 V. The search process then varies depending on whether this test amplitude of 2.5 V is above or below the LV threshold.
If the test amplitude of 2.5 V is higher than the LV threshold, the device lowers the amplitude by increments of 0.25 V until the LV threshold value or the minimum amplitude test (0.25 V) is reached.
If the test amplitude of 2.5 V is lower than the LV threshold, the stimulus amplitude is increased to 6.0 V. If the test amplitude of 6.0 V remains lower than the stimulation threshold, the test indicates that the threshold is greater than 6.0 V for that polarity. Otherwise, the device lowers the amplitude by increments of 0.5 V until the test LV threshold or the minimum amplitude of 3 V is reached.
The last test amplitude test entraining the RV defines the threshold for that LV pacing polarity.
The test of the 16 possible vectors takes 2 to 3 minutes. It is possible to exclude the polarities known for causing phrenic nerve stimulation.
Amplitude of left ventricular pacing
The main goal when programming a CTR device is to ensure permanent biventricular capture. The adjustment of the stimulation amplitude is expected to optimize the device lifespan while maintaining an adequate safety margin. Epicardial left ventricular pacing thresholds are often higher and more variable than right ventricular pacing thresholds. A recent study found threshold values 2 times higher for left ventricular leads than for the right ventricular leads. However, it is not always necessary to select a high safety margin (corresponding to two times the threshold) because this is accompanied by premature use of the battery in case of threshold exceeding 2 volts. Moreover, in some patients it is necessary to adjust precisely the stimulation amplitude and pulse duration in order to obtain a left ventricular capture with no phrenic stimulation.
The amplitude of LV stimulation also affects the probability of anodal capture. The cathode located at the extremity of the left ventricular lead is generally smaller than the anode. The reduced size of the cathode creates a high current density. The existence of high amplitudes of stimulation can result in a current density sufficiently high to capture the tissue close to the anode. In a triple chamber stimulator, the RV ring electrode is often used as anode for LV pacing. A high amplitude of stimulation may result in right ventricular “anodal” capture and in a triple points stimulation: right ventricular cathode and left ventricular + right ventricular anode. The occurrence of anodal capture is more common when the LV pacing configuration includes a true bipolar right ventricular lead (ring electrode) rather than an integrated bipolar lead (the right anode being the RV coil), probably because the size of the ring is smaller and allows for higher current density. The electrocardiographic aspect is often slightly modified compared to the traditional biventricular appearance. A 12-lead electrocardiogram, rather than the single derivation ECG on the programmer is usually required to make the diagnosis. Although anodal capture can significantly alter the analysis of the left ventricular threshold test, the hemodynamic effects of this type of stimulation might be favorable (by increasing the number of stimulation sites). However, its clinical impact remains to be demonstrated. Anodal capture generally requires high amplitudes of stimulation, which has a negative effect on the device longevity.
