Quadripolar Pacemaker Leads in Left Ventricular or Biventricular Pacing
I just wanted to explain quadripolar pacemaker leads in simple terms as a few people on the forum have them implanted as part of their pacemaker therapy.
Quadripolar leads have four electrodes (Fig 1.) compared to bipolar left ventricular leads that have two. Quadripolar leads are leads used to pace the left ventricle and give us more options and flexibility.
Why are Four Electrodes Helpful?
As with most inventions in life, quadripolar leads are a solution to a problem or more accurately limitations in left ventricular lead placement. To understand how quadripolar leads are helpful we first need to look at traditional bipolar left ventricular leads and how they are positioned.
Positioning the Left Ventricular Lead
A left ventricular lead is ‘wedged’ in to one of the cardiac veins that runs on the outside of the left ventricle. This method means that the destination of the lead is determined by a person’s venous anatomy and everybody has slightly different veins.
Studies have shown that there is a sweet spot for where we want to ‘pace’ the left ventricle that gives the best chance of improvement in symptoms (Fig. 2.). We want to get at least one of the leads electrodes in this sweet spot but this is not always possible or straight forward if the veins are not ideal.
The majority of these leads have bends in them to help them get wedged in the vein. These leads are usually most secure when they are wedged as far as they will go in to the ‘target’ vein (Fig 3.)
If we look at this example we can see that the lead has had to be wedged so far into the vessel (to make it secure) that the electrodes have really missed the sweet spot. This is one limitation of these leads that quadripolar leads can help.
Quadripolar Leads Can Help
A picture tells a thousand words and Fig 4. really illustrates how more electrodes can help us to guarantee at least one or two electrodes are in the sweet spot.
This is actually just one really benefit and there are several.
A potential complication of left ventricular pacing is diaphragmatic pacing or phrenic nerve stimulation. This is where the pacemaker causes the patient to have hiccup like sensations that they did not have before the device was implanted. It comes about because left ventricular leads are
positioned very closely to the left phrenic nerve. One of a pair of nerves that control diaphragmatic contraction. If you apply electrical current to these nerves the diaphragm will contract – hiccup! This can happen during left ventrcular pacing where the electrical current delivered to this region by the pacemaker, excites the phrenic nerve too! The proximity of the phrenic nerve to the LV lead can be seen in Fig 5.
In this example showing a quadripolar lead we can see that we have four electrodes to chose from that are in the ‘sweet spot’. If two of them are causing diaphragmatic pacing we still have two other electrodes to choose from. It would be a shame to move such a beautifully positioned lead because it was causing hiccups. Quadripolar lead = more electrodes = more options.
There is one more benefit I just want to highlight, battery longevity. Device battery longevity is affected by a few factors;
- Circuit impedance, the resistance to the electrical current being delivered by the pacemaker
- Output pulse (in Volts), the energy delivered by the device to make the heart muscle contract.
- How often the pacemaker is pacing the heart (pacing percentage).
Well in pacemakers with left ventricular leads we are usually aiming to pace this chamber 100% of the time, so we would not be willing to compromise on this figure. The two other factors are of importance – impedance and output pulse are affected by the pacing circuit being utilised. For reasons I will not go in to in this post (you can look at Pacemakers Made Easy for more information) the best combination between a high impedance and a low output pulse will help prolong battery life.
Providing all the quadripolar lead’s electrodes are in favourable positions, we have up to 16 possible pacing circuits we can choose the best one to prolong battery, bipolar leads to not offer us such luxury when it comes to pacing options.
Hopefully this makes sense and if you do want more information the book ‘Pacemakers Made Easy’ is free on this website!
Thank you for reading,
P.S. As a side note to the left ventricular pacing sweet spot there is now studies and evidence to support pacing at the longest QLV but that will have to wait for a later post!