Every pacemaker has a metallic “Reed switch” . Putting this switch on and off is possible with an external magnet .The circuitry is such that , switching on makes sensing function null and void.It functions as asynchronous mode.In other words pacemaker converts to a mandatory pacing mode .All sensing related issues are immediately removed.
Note : This switch can be activated by any strong magnetic filed applied externally .And removing the magnet disconnects the circuit.
When a magnet is applied the pacemakers changes the mode in the following way.
- VVI to VOO (Paces only ventricle without sensing)
- AAI to AOO (Paces only Atria)
- DDD to DOO (Paces both ventricle and atrial with a fixed AV interval )
Purpose of magnet application .
Essentially it may be called as a safety mechanism to prevent external sensing in strong electrical fields in case the need arises.
Indirectly , it may aid us in detecting end of life of battery as well
During elctrocautery and related procedures application of magnet will help.
What is magnet rate* ?
- The moment you apply the magnet the pacing rate changes
- This is variable with each make and preset.
- End of life magnet rate will be different . It can be 65 to 85 fixed depend upon the make.
- Some pace makers fire initial magnet rates with 3-6 beat fast run and later revert to steady baseline rate.(One should not be confused )
* Always check with the pacemaker manual for the exact response.
When I put a magnet over nothing happened .What is the inference ?
- Magnet is not placed properly
- Pacemaker battery is totally dead.
- Very rarely some pacemakers have magnet function turned off
What happens during magnet application in ICD ?
- There is no change in ICD mode.
- All anti tachycardia functions are immediately suspended. (A major use in an unusual runaway inappropriate ICD shock situation )
Is there any risk of applying magnet ?
Since magnet removes the sensing function , interfering a cardiac rhythm which is dependent on sensing can be problematic.Similar situation arises in MRI scans and other magnetic fields.
Hence , application of magnet in a patient as a part of pacemaker trouble shooting who has no pacing spikes is rarely a problem While one should not do it without supervision of a learned cardiologist.
What is smart magnet ?
Each pacemaker and ICD is interrogated with the respective programmer.As such , there is no cross brand programmer available.This makes it difficult for patient( as well as physicians) to call for help in case of malfunction.
The solution is to make standard universal analyser and programmer .This requires cooperation between various stake holders.Meanwhile as temporary relief a magnet which can community two way with basic functional switches can be developed .
There is a need for universal smart magnet with constant interaction between device and magnet ..
Since , the generation next generation human heart is going to be wired and deviced in a complex manner, we need to know the basics about these issues.
Final message
Magnet application is akin to novice’s pacemaker analyzer. Every cardiac care unit must have one in their shelf. It aids us to diagnose over-sensing as a cause for pacemaker malfunction.(* please note , it has little role in all other pacemaker issues !) .In an emergency it can help stop inappropriate ICD shocks.(More importatnly It gives time to call an expert ! )
Reference
Dr.S.Venkatesan MD 