Understand and Estimate Pacemaker Battery Life

• by brady
• 2023-03-21 20:49:00
• Batteries & Leads
• 233 views
• 2 comments

Understand and Estimate Pacemaker Battery Life

This post is only for people who are interested in the technical details of how the pacemaker works.

This is a followup of a previous thread (ref 1). Useful information is given in a post by piglet22 (ref 2), in which he supplied the battery capacity of a pacemaker which is 500mA-hours. (Abbott’s batteries are: 500mA-hrs to 1200mA-hrs, ref 3)

I) Battery Capacity (C)

It is useful to change the unit of the battery capacity into a form useful for pacemaker. The time unit instead of hour, we use one year. We have

C is 500mA-hour or about 60 microA-yr.

That is the battery can supplied a current of 60 micro ampere continuously for 1 year.

II) How much current (or energy) is used in pacing the heart?

Assume  we have the following pacemaker settings:

Base rate 60 bpm or 1 pacing pulse per second (typical value),

Pacing frequency 100% (this is an upper estimate),

Threshold voltage 1V (most people have 0.5 to 1V, so this is an upper estimate) ,

Pacing pulse voltage with 2X safety margin that is 3V,

Pulse width 0.4ms

(0.3ms to 0.4ms is the width, which is most efficient that is corresponding to the lowest energy needed to trigger the heart. This is obtained from the so called  strength-endurance  curve in the literature,),

Impedance 500 ohm (typical value)

The heart depolarizes due to the action of ions, so it is best to convert pacing voltage to current, which supplied the needed charges to the heart for polarization and depolarization. The relationship between voltage and current is current is equal to voltage divided by impedance. So

The pacing pulse current I is then

I = ~6 mA with a pulse width 0.4ms

At 60bpm that means there is 1 pacing pulse of width 0.4ms per second.

Battery does not like to talk about pulse current, but like to use time average current.

So we spread out the 6mA current from 0.4mA over 1 second and we get a steady state current (or called DC) of

I =~2.4 microA (time averaged current used to make the heart contracts)

This is for a pacing spike of 3V @0.4ms

III)  Current needed to run the pacemaker electronics

This is known as house keeping current. Abbott said, it needed 7 microA for one of its pacemaker (Ref 3). This is about 3 times more than the average pacing current of 2.4 microA given above. That is the electronins sucks up more current that what it needs to pace the heart. Thus, Reducing the pacing voltage will not save as much battery life as we thought it would be.

So the total current needed to make the whole pacemaker work is sum of pacing current and house keeping current:

2.4+7=9.4 micrA

IV) Battery Life

The battery capacity is 60 microA-yrs and the pacemaker needs 9.4 microA  to work. Therefore the battery life is given by

60/9.4=6.4 years for 0.5 A-hr battery

If we were to use the 1.2 A-hr battery, the battery life would have been 15.4 years. 

In short, to calculate battery life, the key parameter is the house keeping current. The rest is easy.

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Ref 1 “BATTERY LIFE EXTENSION CALCULATION – NEED HELP” 

Ref 2  piglet22 - 2023-03-21 12:05:53

I program the sort of devices used in pacemakers and use a lot of the supporting devices.

It's going to be next to impossible to calculate anything meaningful about battery/cell lifetime.

There are going to be devices like operational amplifiers to measure any residual waveforms, there may be memory chips, communication chips for Bluetooth or RFID/NFC communications.

The crucial component will be the central processor or microcontroller that does all the work and will hold some very sophisticated code (program).

Most certainly, the designers will be aiming to minimise the power consumption of all the components.

A typical amplifier might draw 1 to 2 microamps (uA). The battery/cell, probably lithium of some sort, might have a capcity of 500 milliamp hours. i.e.it will supply 500-mA for one hour or 1-mA for 500 hours.

Our amplifier will run for 28 years at 1-mA or 14 at 2-mA.

The processor will draw more power than that, but you can see that the components are going to be in the right ball park for lifetimes in the order of 10 years or so.

The only real way to find out is put a meter on it and measure the current and voltage, but that's not going to happen, unless of course the pacemaker does the measurements internally.

There are specialised chips for just that purpose under the name of fuel gauges, but I suspect that the processor chip would have that built in.

I don't think it would be possible to make a meaningful estimate of battery/cell life using what the pacemaker outputs because there are going to be a lot of other things going on.

I'm going to have a bash at putting a simple ECG machine togther so I might be able to throw a bit more light on it.

Ref 3 “Optimizing Device Longevity”, Abbott

https://www.cardiovascular.abbott/content/dam/bss/divisionalsites/cv/pdf/guides/TI-Optimizing-Device-Longevity-Rev-D-May-2017.pdf

“The typical housekeeping current for a St. Jude Medical pacemaker is approximately 7 microamps, and the typical pacemaker battery has a capacity of 0.5 to 1.2 amp-hours.”