George P. Graber

Electrophysiological control of ventricular rate during atrial fibrillation.

Thirteen anesthetized canine subjects (17–29 kg) were used to demonstrate that mild cervical left vagal stimulation could control ventricular rate effectively during atrial fibrillation (AF). Two studies are presented. The first study used six subjects to demonstrate the inverse relationship between (manually applied) left vagal stimulation and ventricular excitation (R wave) rate during AF. As left vagal stimulation frequency was increased, ventricular excitation rate decreased. In these studies, a left vagal stimulus frequency of 0–10 per second reduced the ventricular excitation rate from > 200/min to < 50/min. The decreasing ventricular excitation rate with increasing left vagal stimulation frequency was universal, occurring in all 26 trials with the six subjects. This fundamental principle was used to construct an automatic controller for use in the second study, in which seven subjects were used to demonstrate that ventricular rate can be brought to and maintained within a targeted range with the use of an automatic (closed‐loop) controller. A 45‐minute record of automatic ventricular rate control is presented. Similar records were obtained in all seven subjects.

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.

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.

A reduced skin-irritation defibrillation electrode

A new type of disposable external defibrillation electrode has been developed to reduce the skin irritation commonly associated with defibrillation and synchronized cardioversion. This design employs an impedance gradient to reduce the proportion of current delivered to the electrode periphery. The temperature distribution under the new electrode was compared to that of several other types of commercially available electrodes after repeated high-energy biphasic defibrillation discharges to domestic swine. Skin temperature distributions were acquired using non-invasive thermography. Histological examination of erythematous sites excised 24 hours after defibrillation quantified the associated skin damage. Thermographic results indicate that the new impedance-gradient electrode produces lower maximum temperature increases than electrodes without increased perimeter impedance. Likewise, histological results demonstrate that the new electrode design produces less skin damage than traditional uniform impedance electrodes.

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.