Pacemaker Rate Response – Accelerometer
I have had a question into how Rate Response works, which bits are programmable and how that affects the patient.
Now I am going to use the names of some adjustable settings that Medtronic use. Now there are similarly programmable settings on most rate response algorithms but obviously this is manufacturer specific.
Firstly as a side note we use a code in pacemaker language that tells us how many leads the pacemaker has or more accurately how it is set up to work. You may have seen them or heard the terms, we describe pacemakers as DDD or VVI and there are many more. If a device has rate response on we put an R after this code. So a a pacemaker programmed DDD, doesn’t have rate response on. A pacemaker programmed DDDR does have rate response switched on.
What is Rate response?
Rate response is a pacemaker function that is designed to increase the patients heart rate when they exercise.
Take a patient who had a heart rate of 60 but because of disease when they ran up a hill the heart rate didnt increase in the same fashion a healthy heart would. Enter a pacemaker, the pacemaker can use vibration (through a piezo-electric crystal) or impedance (a measurement of electrical ‘resistance’)
at the end of the lead to assume that person is exercising….
This will then gently increase the persons heart rate when they start to exert themselves.
How is this adjustable for different patients?
With many algorithms in the pacemaker world there are lots of settings so it can be tailored for the patient. Here are a few that Medtronic use.
Upper Sensor Rate: Example Setting – 130bpm
When a person is exerting themselves the pacemaker will increase the heart rate up to a maximum of this rate. It will plateau at this number so long as the vigorous exercise continues. The more movement in the pacemaker the more strenuous it will predict your exercise to be and it will adjust your heart rate accordingly up to 130bpm (in this example).
When technicians program this figure we take into account many factors. For Example, the age, health and activity level are all important. Fitter, younger patients will generally require a higher upper sensor rate as their body will likely have periods of very high demand of blood and oxygen but most importantly it is less likely that their heart will have any physiological issues through being paced at higher rates e.g. 180bpm. If a patient has a cardiac history which includes Angina, we will tend to err on the side of caution as pacing the heart at a quicker rate could induce symptoms or put their health at risk.
The ‘speed’ the pacemaker increase its rate by is determined by the Exertion Response this is programmable between 1-5. A 1 would mean that the pacemaker would take its time in increasing your heart rate to the rate it feels your exertion requires. A 5 would mean the pacemaker upped your heart rate very quickly. The normal setting is quite predictably a 3. If a patient felt that it takes a while to get going when they start exercising, we would potentially look at increasing the exertion response.
A similar feature is the ADL Response, this is programmable in the same way with the same figures, but it is in reference to the ADL described below.
ADL (Activities of Daily Living): Example Setting 95 bpm
This is a helpful tool for a pottering around heart rate! Now when you potter around at home your heart rate will increase… but not by very much. Movement is minimal (therefore the vibration on the sensor in the pacemaker will be minimal). The pacemaker will ‘know’ that your exertions are not too strenuous but you are up and about all the same. So the pacemaker will bring the heart rate up to this ‘pottering about’ figure and keep you there so long as mild activity continues.
The level of movement (leading to vibration in the crystal) required to trigger the pacemaker to reach ADL is minimal.
Which brings me on to the final setting I am going to discuss with you right now, The Activity Threshold. In my head I see this as how sensitive your pacemaker is to exercise. Programmable from Low to High with a couple of permutations in between this is how vigorous your activity would have to be before your rate response kicks in. If set to Low then your pacemaker will only have to detect slight movement before rate response kicks in. This level of required exertion increases with each setting all the way up to High, where only vigorous hard exercise would trigger a rate response from the device.
It is important to add that these algorithms do have value in helping patients meet the demands of their lifestyles. However they do not always have the answers if symptoms are due to a health issue that is not treatable with a Pacemaker with Rate Response.
A complete explanation of these topics and more is available in the book Pacemakers Made Easy by Carl Robinson.