A Leadless Dual Chamber Pacemakers Disadvantage: Shorter Battery Life Than Leaded PMs

• by brady
• 2023-04-03 20:21:25
• Batteries & Leads
• 265 views
• 1 comments

This post is for technically oriented readers who are interested in the engineering aspect of the pacemaker. But if you are considering a leadless pacemaker, especially a leadless dual chamber pacemakers, you may be interested to read on.

Leadless pacemakers (Micra and Nanostim) have been available for more than 10 years. The design did not start from scratch and was based on the design of leaded pacemaker ( https://www.medicaldesignandoutsourcing.com/how-medtronic-fit-a-battery-into-a-tiny-pacemaker/). Thus, the knowledge gained from the leaded pacemakers was carried over to the leadless. However, until recently, these leadless pacemakers could only be used in the ventricle and to my knowledge, they have few users when compares to leaded pacemakers. It is not hard to understand why. Obviously, even for patients who only need a leadless ventricle pacemaker now would like to know a leadless atrium pacemaker would be available in the future when they need one, else they would need to re-implant a leaded dual chamber pacemaker and also take out the leadless. This is a situation no one would like to be in. This would defeat the purpose of having a leadless pacemaker In the first place. 

To have a leadless dual chamber pacemakers, the atrium pacemaker must be able to communicate with the ventricle pacemaker, which turns out to be a dauting task. To work as a pair, the atrium pacemaker will need to let the ventricle pacemaker know when to start it's timer, signifying either a heartbeat has been detected (sensed) or initiated (paced). For 10 years, the industry had not been able to offer a solution. Wireless is an obvious choice, but it would be hard to implement and also would subject to heart muscle blockage and external interference. Then lately, a solution emerged: communicate through the heart muscle. This is not as easy as it looks. The same electrode that is used for pacing the heart is now also being used to send and receive  signals  to and from the ventricle. Thus the communication signal voltage must be small compared to the smallest pacing signal to avoid inadvertently pacing the heart and yet large enough to overcome the attenuation of the heart; and also stay above the noise level. The solution was first tested in pigs and it worked. Now it is being tested in human. How robust and reliable this communication channel is is being tested. (https://www.sciencedirect.com/science/article/pii/S2452302X18302213)

I am one of the early users of this new leadless dual chamber pacemakers. Since the implant 6 months ago, they have been working correctly in me as well as in several hundreds clinical trial participants, some for as long as a year now according to the company engineer.

(The smallness of the leadless pacemaker has an interesting application: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6926214/)

Atrium is smaller than the ventricle and also it has a thinner wall. As a result, the atrium PM is smaller in size than the ventricle PM. This means it has smaller battery with smaller capacity. Thus, though the leadless PMs have advantages but they also have disadvantages and one of them is shorter battery life than the leaded dual chamber pacemakers.

Because of the shorter battery life for the atrium pacemaker, I had asked my EP what would happened when the battery died. The answer was he would implant another one next to it or retrieve the dead one. This is similar to the leaded pacemakers when a lead is not working: either place a new lead next to the old lead or extract the non-working lead.

With the disadvantage of the leadless atrium pacemaker having a shorter battery life, the leadless dual chamber pacemakers does offer an option: upgradable that is one can have only one pacemaker now then upgrade to dual chamber pacemakers  later on, because the implant entry is from the femoral vein with a ~1cm width opening and leaves no scar afterwards. If one does not need a pacemaker for both chambers, why implant a pacemaker that the heart does not need? For example, a bradycardia who's has no AV block and who only needs a pacemaker for the atrium. Some of the clinical trial participants are  patients who had upgraded from a ventricular pacemaker to dual chamber. The option is somewhat different for the leaded pacemakers. When I needed only a leaded atrium pacemaker, my EP wanted me to implanted a leaded dual chamber pacemakers, saying that just in case if I needed a ventricular pacemaker in the future. Obviously, I didn't want to re-implant again, better to put both in now, he said. This would mean take out the generator and then put in another lead.

Everytime after a followup, I asked for a copy of the printout, which is not as extensive as the leaded PM's printout, but it contains all the parameters that the PM used. These printouts show all the parameters that have been adjusted. I put them in an excel table. I am in a clinical trial with frequent followups. The sizable data can reveal more how the pacemaker operates. (I also used Apple Watch to record my heart rate. The table and the heart rate record let me know how the heart performs after parameter adjustments.)

Below I will share my pacemaker parameters measured during several followups.

Some observations could be drawn from these data.

1) In my case, the atrium battery life is about 60% of the leaded PMs which have a typical 12 years of battery life.

2) They are very sensitive devices: The average current that needs to power the PM and to pace the heart are only microamperes;  the voltage they measured are 0.1 milli volts; the pacing pulse is very narrow 0.2-0.4 milli seconds. Thus noise could interfere the PM’s operation and is also why MRI with pacemakers is a concern because the RF pulse is kilo watts and the magnetic field is teslas.

Because it takes only micro amps to run the pacemaker and to pace the heart, this is why a lithium type (mixed with other elements) battery with a capacity of 0.5 A-hour can power the PM for more than 10 years.

3) The pacing current is comparable to the current needed to run the PM, thus reducing the pacing voltage and make the pacing pulse narrower will lengthen the battery life. In my case for the atrium from 2.5V@0.4ms to 1.25@0.2ms has lenthen the battery life from 5 to 7.2 years.

4) The increment or gap between two neighboring units for the threshold voltage and the pacing voltage is 0.25V. Because of this uncertainty if the threshold voltage is 0.5V, and is stable, one cannot set the pacing voltage to 0.75V, resulting a waste of precious energy and battery life.

5) The minimum energy needed to pace the heart occurs with a pulse width of 0.2-0.4ms and my PMs are set to those values.  And my engineer was proactive to set the atrium window to 0.2ms giving me a longer battery life. (The minimum energy needed to pace the heart is contained in the "Strength Endurance Curve"; too wide a pulse width is a waste of energy and too narrow a width would cause threshold voltage to increase; this results in a minimum.)

6) After implant, the pacing voltages were set at 2.5V even though the threshold voltages were only 0.5V, which was a great waste of battery life. At that time, I was not educated enough to object.

7) The atrium threshold voltage is 0.5V, yet the pacing voltage is set at 1.25V correspondsing to 2.5X safety margin, which is higher than the typical 2X safety margin. It seems that that the pacing voltage could be further reduced to 1V to prolong  battery life. 

8) Why the ventrical pacing  frequency decreases from 8% to <1% has no explanation.

9) For bradycardias, who require infrequent ventrical pacing, the ventrical battery life is comparable with leaded PMs.

10) For the leadless dual chamber PMs, on my followup printout sheet stated that the electrode structure is bipolar. The bipolar structure makes it difficult to see pacing spikes on the EKG graphs (disscussed in one of my posts.)