G. Mouchawar

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 thant 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 12kJ is required to achieve closed-chest magnetically induced ectopic beats in the 17–26kg dogs. The mean peak induced electric field for threshold stimulation is 213 Vm−1 for a 571 μs damped sine wave pulse. Accounting for waveform efficacy and extrapolating to long-duration pulses, a threshold induced electric field strength of approximately 30 Vm−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.

Thermodynamic model for electrostatic-interaction chromatography of proteins

A thermodynamic model derived by Record et al. [M. T. Record, Jr., Biopolymers, 14 (1975) 2137 and M. T. Record, Jr., C. F. Anderson and T. M. Lohman, Q. Rev. Biophys., 11 (1978) 103] from Wymans linkage theory [J. Wyman, Adv. Protein Chem., 19 (1964) 223] using Mannings condensation model [J. Manning, J. Chem. Phys., 51 (1969) 924] was extended to electrostatic interaction chromatography. Mixed, electrostatic and hydrophobic interactions of a model protein, ovalbumin were characterized by ion and water release.

Ability of the Lapicque and Blair strength-duration curves to fit experimentally obtained data from the dog heart

The ability of the empirical Lapicque and theoretically derived Blair expressions for excitation to fit experimentally obtained threshold current values to evoke a ventricular extrasystole using rectangular-wave stimuli applied to the dog heart is determined. The data points were fitted to both expressions, and the ability of each to predict the measured values were determined. The Levenberg-Marquardt (L-M) algorithm was used to fit the Lapicque and Blair expressions. The Lapicque data were also fitted to the linear charge-duration expression of Weiss (W). The ratio of the predicted to measured current was 0.95 (L-M) and 1.06 (W) for the Lapicque and 0.92 (L-M) for the Blair expression. Thus, there appears to be little difference between the ability of the expressions to fit the same experimentally obtained data. The L-M/Lapicque fit is best for the short-duration range; the W/Lapicque fit overestimates in the short duration range and underestimates near chronaxie. The L-M/Blair fit is best for the short-duration range and poor for durations near the membrane time constant.<<ETX>>

Inspiration produced by bilateral electromagnetic, cervical phrenic nerve stimulation in man

Eddy-current stimulation of both phrenic nerves at the base of the neck in human subjects was carried out to provide inspiration resulting from tetanic diaphragm contraction. The inspired volume obtained was in excess of spontaneous tidal volume. In a practical application of the method to provide artificial respiration, the trains of stimuli would be applied rhythmically. The train rate determines the respiratory rate. The frequency of the pulse in the train needs to be 25/s or slightly higher and the duration of each train should be long enough so that the inspired air velocity will fall to zero; typically requiring about 0.5-0.7 s.<<ETX>>

Energy considerations in the magnetic (eddy-current) stimulation of tissues

The magnetic field energy and Joule heating required in a coil to induce an electric field pulse are calculated for different geometries of circular coils with rectangular cross section. For a fixed target distance away from the coil, the magnetic field energy takes on a broad minimum when the coil outer radius is between two and five times the target distance. Joule heating in the coil decreases as the radius of the coil increases. Increasing annular width results in a small reduction in field energy and coil Joule heating, and thin coils (small height) reduce the field energy but increase Joule heating. A reasonable compromise between efficiency and coil size results in a coil with an outer diameter that is twice the distance between the coil surface and the underlying tissue to be stimulated and height and annular width that are 0.2 and 0.6 that of the mean radius, respectively. The use of a pair of coplanar coils results in improved efficiency, but placing the coil perpendicular to the skin surface requires additional energy for stimulation. >

Magnetic (Eddy-Current) Electroventilation in the Dog

Magnetic (eddy-current) stimulation of the inspiratory motor nerves in the neck of the anesthetized dog was achieved. Using a 10-turn coil wound around the base of the neck and a train of pulses (25/s), inspiration was produced by tetanic contraction of the inspiratory muscles. The volume of air inspired increased with an increase in the voltage applied to the capacitor that was discharged repetitively into the coil. In this 10-dog study, the maximum inspired volume was in excess of the spontaneous tidal volume. In a second study, breathing was captured by repeating the stimulus trains at a rate in excess of the spontaneous breathing rate. Oxygen consumption was measured during spontaneous breathing and with captured breathing. The oxygen uptake with magnetic electroventilation was, on the average, 75% higher than with spontaneous breathing. However body temperature did not increase. Although the neck coil was not critical in placement, its field of stimulation was larger than needed to stimulate the phrenic and accessory motor nerves.

Magnetic Stimulation of the Heart and Safety Issues in Magnetic Resonance Imaging

Our group at Purdue University has been studying the physiological effects of pulsed magnetic fields for several years. The initial work was directed toward cardiac pacing with a pulsed magnetic field. Our motivation was the development of a non-invasive and relatively pain-free method for cardiac stimulation. We were the first to induce cardiac ectopic beats with pulsed magnetic fields in the closed-chest dog [1,2]. Unfortunately, the large energies required to stimulate the heart preclude the development of a portable magnetic cardiac pacemaker.