Closed-chest cardiac stimulation with a pulsed magnetic field.

Magnetic stimulators, used medically, generate intense rapidly changing magnetic fields, capable of stimulating nerves. Advanced magnetic resonance imaging systems employ stronger and more rapidly changing gradient fields than those used previously. The risk of provoking cardiac arrhythmias by these new devices is of concern. In the paper, the threshold for cardiac stimulation by an externally-applied magnetic field is determined for 11 anaesthetised dogs. Two coplanar coils provide the pulsed magnetic field. An average energy of approximately 12 kJ is required to achieve closed-chest magnetically induced ectopic beats in the 17-26 kg dogs. The mean peak induced electric field for threshold stimulation is 213 V m-1 for a 571 microseconds damped sine wave pulse. Accounting for waveform efficacy and extrapolating to long-duration pulses, a threshold induced electric field strength of approximately 30 V m-1 for the rectangular pulse is predicted. It is now possible to establish the margin of safety for devices that use pulsed magnetic fields and to design therapeutic devices employing magnetic fields to stimulate the heart.