Kirk S. Foster

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.

AN ADAPTIVE NOISE REDUCTION STETHOSCOPE FOR AUSCULTATION IN HIGH NOISE ENVIRONMENTS

Auscultation of lung sounds in patient transport vehicles such as an ambulance or aircraft is unachievable because of high ambient noise levels. Aircraft noise levels of 90-100 dB SPL are common, while lung sounds have been measured in the 22-30 dB SPL range in free space and 65-70 dB SPL within a stethoscope coupler. Also, the bandwidth of lung sounds and vehicle noise typically has significant overlap, limiting the utility of traditional band-pass filtering. In this study, a passively shielded stethoscope coupler that contains one microphone to measure the (noise-corrupted) lung sound and another to measure the ambient noise was constructed. Lung sound measurements were made on a healthy subject in a simulated USAF C-130 aircraft environment within an acoustic chamber at noise levels ranging from 80 to 100 dB SPL. Adaptive filtering schemes using a least-mean-squares (LMS) and a normalized least-mean-squares (NLMS) approach were employed to extract the lung sounds from the noise-corrupted signal. Approximately 15 dB of noise reduction over the 100-600 Hz frequency range was achieved with the LMS algorithm, with the more complex NLMS algorithm providing faster convergence and up to 5 dB of additional noise reduction. These findings indicate that a combination of active and passive noise reduction can be used to measure lung sounds in high noise environments.

Interactions of magnetic resonance imaging radio frequency magnetic fields with elongated medical implants

Magnetic resonance imaging (MRI) is a well established diagnostic technique, one from which all patients should be able to benefit, including those with implanted medical devices. This paper describes an experimental and numerical study of the temperature rise near the ends of wires by the radio frequency (rf) field in MRI. These wires simulate long wires which may be part of a medical implant. Temperature rise as a function of time was measured for wires of different lengths and diameters in a phantom exposed to the 64 MHz rf field inside a MRI body coil. For wires with no or thin insulation, the maximal rise was about seven times background for a wire length of 20 cm; wires with longer and shorter lengths exhibited less temperature rise. The temperature rise was greater for wires with thicker insulation. Computer simulations using a quasistatic model are in reasonable agreement with the measurements on shorter wires. It is concluded that medical implants with long conducting lead wires may result in pot...

Safety Factor for Precordial Pacing: Minimum Current Thresholds for Pacing and for Ventricular Fibrillation by Vulnerable‐period Stimulation

Temporary cardiac pacing by means of rapidly applied, precordial electrodes can be of great value in selected patients suffering cardiac arrest or profound bradycardia, post‐defibrillation atrioventricular block, or digitalis intoxication. This study provides data pertaining to the safety of precordial pacing measured as the ratio of the minimum current required to induce ventricular fibrillation (VF) to the minimum current required to pace the ventricular myocardium. Single rectangular pulse stimuli having durations of 1–50 ms were evaluated. In 6 anesthetized dogs, a cutaneous electrode was centered over the shaved apex beat area of the left chest and paired with a larger electrode sutured to the right chest wall. Using a specially fabricated, synchronized, high‐energy pulse generator, the “most vulnerable” time in the cardiac cycle at which the least current was required to induce VF with a single shock was identified. This current was compared to the much lower current required to pace the heart in the diastolic interval. While the current required to produce either fibrillation or pacing decreased as pulse duration increased, the safety factor (i.e., the ratio of fibrillation current to pacing current) remained nearly constant, averaging 12.6 for all pulse durations examined. That is, on the average, a stimulus of any given duration between 1 and 50 ms required at least 12 times the current required for pacing to produce ventricular fibrillation. We conclude that in normal canine hearts, the risk of inducing VF during precordial pacing is small.

Rapid Sample Processing for Detection of Food-Borne Pathogens via Cross-Flow Microfiltration

ABSTRACT This paper reports an approach to enable rapid concentration and recovery of bacterial cells from aqueous chicken homogenates as a preanalytical step of detection. This approach includes biochemical pretreatment and prefiltration of food samples and development of an automated cell concentration instrument based on cross-flow microfiltration. A polysulfone hollow-fiber membrane module having a nominal pore size of 0.2 μm constitutes the core of the cell concentration instrument. The aqueous chicken homogenate samples were circulated within the cross-flow system achieving 500- to 1,000-fold concentration of inoculated Salmonella enterica serovar Enteritidis and naturally occurring microbiota with 70% recovery of viable cells as determined by plate counting and quantitative PCR (qPCR) within 35 to 45 min. These steps enabled 10 CFU/ml microorganisms in chicken homogenates or 102 CFU/g chicken to be quantified. Cleaning and sterilizing the instrument and membrane module by stepwise hydraulic and chemical cleaning (sodium hydroxide and ethanol) enabled reuse of the membrane 15 times before replacement. This approach begins to address the critical need for the food industry for detecting food pathogens within 6 h or less.

The Rectification Properties of an Electrode-Electrolyte Interface Operated at High Sinusoidal Current Density

The rectifying properties of a stainless steel/saline electrode-electrolyte interface were investigated using sinusoidal current (100, 1000, and 10 000 Hz) of increasing current density. The series-equivalent resistance, capacitance, and impedance were determined versus current density. At each frequency, with increasing current density, it was found that above a critical current density, the capacitance increased, the resistance and impedance decreased then increased. The current density for these transitions were higher as the frequency was increased. There was a threshold (peak-to-peak) current density for rectification, being 3.2, 30 and 390 mA/cm2 for 100, 1000, and 10 000 Hz, respectively. The most sensitive indicator of approaching rectification was the increase in capacitance of the electrode-electrolyte interface with increasing current density.

Choice of the optimum pulse duration for precordial cardiac pacing: a theoretical study.

In precordial pacing with skin‐surface electrodes, the goal is to excite the ventricles with minimal stiniuiation of overlying tissues. A theoretical analysis is presented to identify the relevant factors; the two most important are electrode location and pulse duration. Using the basic law of stimulalion, we developed a model which indicates that the optimum pulse duration for the closed‐chest pacing pulse is one that is long with respect to the membrane time constant of cardiac muscle. Current‐versus‐duration curves are presented for pacing and pain, based on experimentally obtained data. For pacing with minimum pain, the optimum stimulus duration was found to be about 10 ms.

The effect of elevated intracranial pressure on the vibrational response of the ovine head.

Although potentially fatal increases in intracranial pressure (ICP) can occur in a number of pathological conditions, there is no reliable and noninvasive procedure to detect ICP elevation and quantitatively monitor changes over time. In this experimental study, the relationships between ICP elevation and the vibrational response of the head were determined. An ovine animal model was employed in which incremental increases in ICP were elicited and directly measured through intraventricular cannulae. At each ICP increment, a vibration source elicited a flexural response of the animals head that was measured at four locations on the skull using accelerometers. Spectral analysis of the responses showed changes in proportion to ICP change up to roughly 20 cm H2O (15 mm Hg) above normal; a clinically significant range. Both magnitude and phase changes at frequencies between 4 and 7 kHz correlated well (γ>0.92) with ICP across the study group. These findings suggest that the vibrational response of the head can be used to monitor changes in ICP noninvasively.

Comparison of rectangular and exponential current pulses for evoking sensation

There exists a paucity of quantitative information comparing the stimulating currents for different waveforms. Thus, the objectives of this study was to compare the threshold peak current (I) for sensation using rectangular and exponential; that is, capacitor-discharge, cathodal pulses of equivalent pulse duration (d). In 10 human subjects, stimuli were applied to the skin of the forearm, andI was determined alternately for each current waveform at each of several pulse durations (d). Strength-duration curves for sensation were obtained usingd of 0.01–50 ms. The threshold peak current (Ir) for a rectangular pulse of durationd was compared to the threshold peak current (Icd) for a capacitor-discharge pulse of durationd, whered was the time convalue. Chronaxie, the pulse duration at whichI is twice the infinite-duration current asymptote (i.e., the rheobase), was calculated for each waveform and subject using the Weiss-Lapicque expression for excitability.Icd was found to be always higher thanIr of equivalent duration. Chronaxie for the capacitor-discharge pulse was, on the average, twice that for the rectangular pulse (p<0.01). Moreover, the ratioIcd/Ir increased with decreasingd. These results indicate that these two waveforms are not equivalent on the basis of an equal-charge requirement for excitation, particularly at the short pulse durations. Furthermore, they suggest the need of a better expression to describe the excitability characteristics of tissues.

Electroventilation in the baboon

The optimum sites for chest-surface electrodes and the relationship between the inspired volume of air and stimulus intensity were determined in six pentobarbital-anesthetized baboons. The optimum stimulation sites were identified using hand-held, trans-chest stimulating electrodes. Thereafter, conducting adhesive electrodes 4 cm in diameter were placed over each of the optimal sites. The relationship between inspired volume and stimulus intensity was determined with the forelimbs restrained and unrestrained using an 800-msec burst of 60/sec stimuli with a pulse duration of 20 microseconds. Two optimum stimulation sites were identified, one in the anterior axillary region, and another just lateral to the xiphoid process. The maximum volume of inspired air was obtained when the stimulating electrodes were placed at the anterior axillary sites. With the forelimbs restrained or unrestrained, an increased stimulus intensity resulted in an inspired volume well in excess of the spontaneous tidal volume of the animal. These studies indicate that, with chest-surface electrodes at the optimal locations, electroventilation can produce inspired volumes greater than spontaneous tidal volume in anesthetized baboons. With electrodes in the xiphoid region a smaller maximum inspired volume was obtained because of the stimulation of expiratory (abdominal) muscles.