When high defibrillation voltages are applied, the ensuing muscle-contractions can create large accelerations, as evidenced by the sight of patients “jumping” on the table (or being levitated), which requires that the accelerations induced are larger than the acceleration of gravity (9.8 m/s2). Basing that observation, a group of researchers, including Dr. Halperin, developed a defibrillation waveform geared to reduce skeletal muscle contraction, and thereby reduce the pain encountered during defibrillation. A combined waveform dubbed “rHFAC+B” was created, in which a low-voltage Kilohertz range pulse (rHFAC) is applied prior to the application of the standard short (5 msec) biphasic (B) defibrillation pulse. The role of the rHFAC pulse is to slowly compress the skeletal muscles, which makes these muscles not react (“tetanizes them”) when the Biphasic pulse is applied afterwards. The Biphasic pulse rapidly compresses the myocardial muscle, restoring heart rhythm, while patient motion, and therefore pain, is considerably reduced.
A 2nd generation prototype system for executing the tetanizing pulses prior to the Biphasic pulse was constructed. The rHFAC waveform is completely controlled by a Tetanizing Unit generator, which was built so it can be placed underneath an R-series Zoll Medical (Chelmsford, MA) defibrillator. The two chassis are connected by a high-voltage cable, so that the Tetanizing unit triggers the Zoll generator to fire once it completes delivering the rHFAC pulse, with the Tetanizing Generator chassis outputting the combined waveform to the defibrillation pads.
The research group has also worked on the development of an external defibrillation system that operates safely in the MRI environment. A prototype MRI-conditional defibrillator was developed, which utilizes a commercial Zoll Medical (ZM) bi-phasic defibrillation generator. Because this commercial generator emits strong RF interference (RFI) at the 1.5 Tesla MRI’s operating frequency, a low-pass (LP) filter was constructed to remove this RFI. Commercial Zoll disposable multi-functional defibrillation pads (electrodes) are used with the system. The long cable between the LP filter and the pads is specifically constructed to allow a patient to be scanned inside the MRI with the defibrillation pads connected to their chest, and situated below the MRI surface coils. This is performed in order to remove the necessity of mounting pads after a cardiac arrest had occurred, which would delay the administration of defibrillation.