Fixed rate pacemakers do not contain sensing circuits and are not
influenced by biological or external electromagnetic signals. A demand
pacemaker must be sensitive to the natural heart rhythm and therefore
contains sensing circuits. The presence of a sensing circuit makes the
pacemaker theoretically sensitive to other biological or electromagnetic
signals which, if they closely mimic the cardiac signal, may suppress the
pacemaker. In practice this is rarely a problem because the pacemaker
electronics are well shielded by a metal case and special circuits are
included to detect and remove unwanted electrical activity.
However, in the hospital there are electrical machines which could
interfere with pacemakers. Of these some kinds of surgical equipment pose a
serious risk and it is obviously important that the surgeon is informed about
his patient’s pacemaker so that proper care can be taken. Outside hospital,
electromagnetic signals from electromagnets, spot welding machines,
anti-theft devices, metal detectors, radio and television transmitters and
faulty microwave ovens very occasionally influence the working of a
pacemaker. In public places warning notices are usually displayed near to
such devices but at home or at work extra care must be taken. Specific
advice depends on the nature of the pacemaker and each manufacturer gives
detailed instructions as to what sources of electromagnetic interference must
be avoided in the case of their particular pacemakers. Additional advice may
be given by the doctor at the pacemaker clinic.