When using the term sensing in talking about pacemakers, we mean the ability of the pacemaker to “see” a signal of the heart’s own rhythm. As the heart’s myocardium depolarises, it generates an electrical output. This signal travels up the pacemaker lead in contact with the myocardium, and reaches the IPG. In the event that the IPG receives an electrical signal, the programming contained within it knows not to generate its own electrical stimulus.
This trace example shows 3 true beats, and lots of artefact, the same as the kind you might find on a 12 lead ECG. You know what represents a true beat, because you have the power of cognitive reasoning, but pacemaker software, as advanced as it may be, is still just software and as such, may interpret this incorrectly;
Oversensing causes the IPG to refrain from pacing. One missed beat wouldn’t cause many problems, but a pause of 10 seconds would lead to a syncopal event, and make the implantation of the device a waste of time, and a danger to the patient, so programming to “cut out” noise and only register true beats is in the patient’s best interests.
If, for example, if the sensitivity of the device software is set to 7mV, it will sense everything FROM 7mV upwards, but not below.
As is demonstrated above, this can result in undersensing, wherein genuine beats are missed.
In order to avoid this, the pacemaker is given a safety margin or 1/3rd of the measured value. The voltage output of a heart beat, whilst likely to vary, is highly unlikely to be less than 1/3 of its averaged value, hence setting the safety margin in such way will eliminate the majority of the artefact and still allow the device to sense the true beats.
The Student Physiologist 