Question of the Week

How does an artifical pacemaker know how fast to beat?

Sun, 26th Jul 2009

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Marel Oliver asked:

How does an artificial pacemaker know how fast the heart should beat?


Pacemaker We put this to Cathy Ross, Cardiac Nurse at the British Heart Foundation. 

The role of an artificial pacemaker is to monitor and control the heartís natural rhythm.  Theyíre implanted for many reasons and the way they work will vary according to the reasons for which theyíre implanted.  In the case of the slow heart rate, for example, a certain number of beats per minute will be programmed into the pacemaker according to the needs of the patient.  The pacemaker would then sense through a number of wires the number of beats being delivered naturally by the heartís own pacemaker, which is called the sino-atrial node.  

And it will only interject to deliver another beat if there is a shortfall.  So the pacemaker is there only for the reason for which it has been implanted.  So if itís a slow heart rate and you go running for a bus then the pacemaker will only interject if your heart rate falls below the required minimum level.  Sometimes, doctors will put a maximum level on a pacemaker.  So for some abnormal heart rhythms (if the heart rate went too fast) it could cause fainting or black-outs or possibly even a life-threatening rhythm and then maybe in an overriding upper level that the pacemaker will be set at but thatís quite complex. 

They tend to be implanted with internal defibrillators so they will be the ones that are sensing for life-threatening rhythms so that the person can receive a shock if they require it.


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With earlier generations of pacemakers I think the devices were set to run at a certain rate e.g. 80-90 bpm. This ensured the minimum heart rate did not drop below this, thus preventing bradycardia and possible hypotension. If the heart then went faster of its own accord and "overtook" the pacemaker this was fine because the pacing signal was much less frequent than the endogenous heartbeat and hence there was no problem.

I *think* the newer generations of pacemakers are dual-chamber pacing and sensing and can register what the heart's native rhythm is (by recording from the atrium where the intrinsic pacemaker is located) and work at that frequency,  transmitting the signal to the ventricle; thus any conduction problems within the heart are bypassed.

But don't quote me on this. This QotW is a learning exercise for me too on this occasion!

Chris chris, Thu, 23rd Jul 2009

Alternatively use your i-pod
RD, Fri, 24th Jul 2009

What is it about iPods and electricity!?

Two years back we reported the story of man admitted to hospital with burst ear drums, burns and unconsciousness after using his iPod as a lightning conductor...

Chris chris, Fri, 24th Jul 2009

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