Basic concepts of LV pacing
In this lesson we will cover technical aspects of LV pacing, implantation of an LV lead is discussed elsewhere.
Configuration of left ventricular stimulation
The various programming options available enable the selection of a LV stimulation polarity associated with a reliable and durable myocardial capture with the least amount of power necessary in order to maximize the longevity of the device, without stimulating the phrenic nerve. The programming of the stimulation amplitude must optimize the battery life while preserving a sufficient safety margin. The LV epicardial stimulation threshold is often higher (twice as high in one study) and more variable than the RV stimulation threshold. It might not be possible to obtain a safety margin as high as twice the capture threshold since, when >2.0 V, the batteries will likely be prematurely depleted. In some patients the stimulation amplitude and pulse width must be meticulously programmed to capture the left ventricle without phrenic nerve stimulation. The influence of the LV stimulation configuration on the quality of response to cardiac resynchronization remains to be determined and rarely represents a selection criterion.
The armamentarium offered by the various device manufacturers includes uni-, bi- and quadripolar LV leads. The stimulation polarity is programmable in the left ventricle by choosing a quadripolar (4 electrodes) or non-quadripolar (1 or 2 electrodes) lead. If a unipolar LV lead is implanted, a single configuration (LV tip à RV ring or RV coil) might be programmable depending on the manufacturer and type of device. With most defibrillators, the RV coil is used instead of the RV ring. With CRT-P, the RV ring (proximal electrode of a bipolar lead) is used. With some devices and, depending on the manufacturer, the pulse generator may participate in the stimulation vector, offering the programming of LV tip à pulse generator (can) as second choice of stimulation configuration. If a bipolar LV lead is implanted, between 3 and 6 configurations are available depending on the manufacturer and type of device. The threshold is often higher with LV ring à RV coil configuration and the distal LV electrode (cathode) is preferentially used as the active electrode, while the RV coil or LV ring (anode) are used as the indifferent electrode. If a quadripolar LV lead is implanted, between 12 and 17 configurations are available depending on the manufacturer and the type of device. This multitude of choices allows an optimization of the capture threshold and a lower likelihood of phrenic nerve stimulation. With a wide spacing of the electrodes, the activation sequence can be varied, for example by choosing a distal (apical) versus proximal (basal) electrode. This choice may have a positive impact on the response rate, despite the paucity of criteria objectively predictive of the quality of response to therapy. St. Jude Medical makes the only devices, which offer a choice of dual site LV stimulation, as an increase in the number of stimulated sites is likely to decrease the activation dyssynchrony.
The pulse strength influences the likelihood of anodal capture. The cathode at the tip of the LV lead is usually smaller than the anode, explaining the high current density at that site. A strong pulse may create a high enough current density to capture the tissue near the anode. The RV ring of CRT-P is often used as anode to stimulate the left ventricle. A high-amplitude stimulation may cause an anodal RV capture resulting in triple-point stimulation: LV and RV cathodes + RV anode. Anodal capture is more likely when the LV stimulation configuration includes a true RV bipolar lead (lead ring) instead of an integrated bipolar lead (since the anode is the distal coil), probably because the small size of the ring promotes a higher current density. The electrocardiographic morphology is often slightly modified compared with the usual BiV appearance. A 12-lead electrocardiogram, instead of the single channel of the programmer, is usually needed for the diagnosis. On the other hand, an anodal capture may considerably modify the analysis of the tracings during the LV threshold test. The hemodynamic effect of this type of stimulation, which increases the number of stimulated sites, might be positive, though the clinical results remain to be seen. An anodal capture usually requires strong stimulation pulses, which shorten the devices’ life expectancy.