Types of medical devices

Pacemakers and Intra-cardiac Defibrillators (ICDs) 

How they work

The heart produces electrical signals or action potentials, which need to be reliably detected by pacemakers’ and ICDs’ sensing leads. The heart has its own natural pacemaker, called the Sino-Atrial Node (SA). Pacemakers and ICDs are needed when problems arise affecting the heart’s natural cardiac cycle.

Pacemakers are designed to stimulate the heart when either the heart’s own pacemaker paces at an inappropriate rate or the heart's electrical conduction system prevents the propagation of electrical impulses from the native pacemaker to the lower chambers of the heart. Pacemakers are needed to solve a number of problems with the heart's cardiac cycle and therefore have to be programmable and to function in a number of different modes.

ICDs are designed to detect and treat ventricular fibrillation of the heart by sensing the heart’s electrical activity. ICDs deliver a shock to the heart that is designed to stop the heart’s activity, allowing it to restart itself at the correct pacing.

Susceptibility to interference

The electrical sensitivities of the atria and ventricles vary, but are both very low. The atrial sensitivity for electrical stimulation is 0.25-1.6 mV and ventricular is 0.75-4.0 mV, both of which are so low that EMFs realistically can induce potentials in sensing leads. The sensitivity setting varies and is dependant on the patient not the device itself. The sensitivity to external interference varies greatly between devices; however, it is clear that unipolar sensing is more sensitive than bipolar. Unipolar sensing has only one lead attached to the atrium or ventricular wall, as opposed to bipolar, which has two leads, attached to both.

The devices themselves are usually tested to EMC standards that are designed to prevent damage to the device itself, and not the influence of external fields. Once implanted, the sensing of the devices can be tuned to that which is required by the patient. The more sensitive the setting of the device the more susceptible to interference the implanted system can be. Therefore, it is not only the model of device that is important; it is also the clinical configuration (including lead location and number, and sensitivity setting of the device).

Similarities and differences

Interference from power lines or any other source of electromagnetic fields to an implanted device occurs through signals induced in the sensing leads.  Implanted Cardioverter Defibrillators (ICDs) and implanted pacemakers have very similar sensing leads and very similar detection circuitry within the device.  Where they principally differ is in the therapy delivered.  Thus, they are expected to be very similar in their levels of immunity to interference, though the consequences of any interference could differ.  This is confirmed in that the various CENELEC Standards on interference make essentially the same provisions for ICDs and pacemakers, and the manufacturers generally group them together in a single category of “implanted heart device” when giving information about interference.  Therefore, in terms of understanding the likelihood of interference, it is appropriate to treat the two devices as a single group.

Other active devices

Other active implantable devices include:

Passive implants

Passice implants are artificial joints, and the various plates, nails, screws etc used to repair bones.  None of these are affected by electric and magnetic fields.