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Biomedical subjects

J D Bourland

Publications and source records attributed to J D Bourland.

At least 19 recordsLinked to original sources
PubMed2000

Review of patient safety in time-varying gradient fields.

In magnetic resonance, time-varying gradient magnetic fields (dB/dt) may stimulate nerves or muscles by inducing electric fields in patients. Models predicted mean peripheral nerve and cardiac stimulation thresholds. For gradient ramp durations of less than a few milliseconds, mean peripheral nerve stimulation is a safe indicator of high dB/dt. At sufficient amplitudes, peripheral nerve stimulation is perceptible (i.e., tingling or tapping sensations). Magnetic fields from simultaneous gradient axes combine almost as a vector sum to produce stimulation. Patients may become uncomfortable at amplitudes 50%-100% above perception thresholds. In dogs, respiratory stimulation has been induced at about 300% of mean peripheral nerve thresholds. Cardiac stimulation has been induced in dogs by small gradient coils at thresholds near Reilly's predictions. Cardiac stimulation required nearly 80 times the energy needed to produce nerve stimulation in dogs. Nerve and cardiac stimulation thresholds for dogs were unaffected by 1.5-T magnetic fields.

Animals↗
PubMed2000

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.

Animals↗
PubMed1999

Morphology-guided radiosurgery treatment planning and optimization for multiple isocenters.

This work merges two distinct fields, 3D morphology and ionizing radiation dosimetry, to solve the problem of 3D-treatment planning and optimization in stereotactic radiosurgery. In Leksell Gamma Knife radiosurgery, dose delivery is based on the unit "shot," a dose distribution approximately spherical in shape. Multiple shots, or isocenters, are used in Gamma Knife treatment to deliver a conformal dose to an irregular radiosurgical target. The medial axis transformation, or skeleton, of the target, which uniquely characterizes the target volume and shape, is used to determine the optimal shot positions (isocenters), sizes (collimator helmet size and dosimetric weight), and the total number of shots that will deliver a conformal dose distribution to the target. The skeletonization approach reduces a complicated 3D-optimization problem to 1D searching with potential savings in computation time and mathematical complexity. In addition, optimization based on target shape replicates and automates manual treatment planning. This approach makes the process easily understandable. The relationship between skeleton discs and the dose distributions they predict is discussed. Results of optimal plans and corresponding dose distributions are presented. This approach is generally applicable to other types of multi-isocentric stereotactic radiosurgery techniques.

Bone and Bones↗
PubMed1999

Physiologic effects of intense MR imaging gradient fields.

The strength duration relationship for peripheral nerve stimulation by MR imaging pulsed gradient magnetic fields was measured in 84 human subjects. The data were fitted to the hyperbolic strength-duration relationship: dB/dt=b(1 + c/d), where b is rheobase, c is chronaxie, and d is duration, and dB/dt is reported as the maximal value on the axis of the bore. For sensation threshold, average (b,c) (15 T/s, 0.37 ms) for the y-gradient and (26 T/s, 0.38 ms) for the z-gradient coil. The dB/dt intensity to induce a sensation which the subject described as uncomfortable was about 50% above the sensation threshold. Experiments with dogs showed that the cardiac stimulation by pulsed magnetic gradient fields is exceedingly unlikely.

Abdominal Muscles↗
PubMed1997

Automatic external defibrillators for public access defibrillation: recommendations for specifying and reporting arrhythmia analysis algorithm performance, incorporating new waveforms, and enhancing safety. A statement for health professionals from the American Heart Association Task Force on Automatic External Defibrillation, Subcommittee on AED Safety and Efficacy.

These recommendations are presented to enhance the safety and efficacy of AEDs intended for public access. The task force recommends that manufacturers present developmental and validation data on their own devices, emphasizing high sensitivity for shockable rhythms and high specificity for nonshockable rhythms. Alternative defibrillation waveforms may reduce energy requirements, reducing the size and weight of the device. The highest levels of safety for public access defibrillation are needed. Safe and effective use of AEDs that are widely available and easily handled by nonmedical personnel has the potential to dramatically increase survival from cardiac arrest.

Algorithms↗
PubMed1997

Ventricular defibrillating threshold: strength-duration and percent-success curves.

The term defibrillation threshold is usually understood to mean the shock intensity just enough to defibrillate a specified cardiac chamber (atria or ventricles). With the advent of so many different types of defibrillator, it is important to be able to specify the defibrillation threshold, which has frequently been described by the classical strength-duration curve. Another method of representing defibrillation plots the percent-successful defibrillation against shock-strength area. The mechanism of defibrillation is discussed, and the concepts of the strength-duration curve and percent-success against shock-strength curves are compared. Because defibrillation is associated with a time-varying spectrum of cellular excitability, a given shock strength will not always achieve defibrillation, and this produces the sigmoid shape for the curve that relates percent-successful defibrillation to shock strength. Therefore it is important to recognise two concepts: first, there is a family of strength-duration curves for defibrillation, each curve representing a given percent-successful defibrillation, and, secondly, there is a family of percent-success against shock-strength curves, one for each pulse duration. Canine ventricular defibrillation data are used to bring these two concepts together. Most importantly, the concepts adduced in the paper apply to transventricular, intracardiac and transchest defibrillation; the only difference in these applications is a scale factor that represents electrode location with respect to the heart.

Animals↗
PubMed1996

Maintenance of atrial fibrillation in anesthetized and unanesthetized sheep using cholinergic drive.

Atrial fibrillation (AF) was induced electrically and the duration of AF was measured in six isoflurane-anesthetized sheep (weight range 54.5-72.7 kg), and in five unanesthetized sheep (weight range 60-75 kg). In the anesthetized sheep, AF was induced by direct electrical stimulation of the right atrium with a catheter electrode and the duration of AF was determined. Intravenous neostigmine (10 micrograms/kg IV) was administered and the duration of AF was again measured. Then cholinergic drive was increased by bilateral electrical vagal stimulation; AF was induced and the duration of AF was measured. In the anesthetized animals with no neostigmine or vagal stimulation, 34% of the episodes of AF lasted 10 seconds, 11% lasted 20 seconds, and only 1% lasted 200 seconds. However, in one anesthetized animal AF was sustained for 4,800 seconds with no drug or vagal support. The administration of neostigmine alone in 3 anesthetized animals more than doubled the average duration of AF. In the animals with vagal stimulation (after neostigmine), AF persisted throughout stimulation, but ceased shortly after vagal stimulation was terminated at 2,220, 4,500, and 3,840 seconds. The AF frequency ranged from 325-750/min. The unanesthetized sheep were lightly sedated with a small dose (200 micrograms/kg IM) of xylazine to make them less sensitive to environmental noise; then AF was induced and its duration was timed. After these measurements, neostigmine was administered (30 micrograms/kg IM) and cholinergic drive was produced reflexly by intravenous injection of 60-2,000 micrograms of phenylephrine. AF was electrically induced at the time of maximum reflex slowing in heart rate. For the control (no drug) studies, 64% of the AF episodes lasted 10 seconds, 20% lasted 20 seconds, and only 2% of the episodes lasted as long as 140 seconds. When phenylephrine was injected after neostigmine to provide increased cholinergic drive, the duration of fibrillation depended on the dose of phenylephrine. In a 60-kg sheep, the duration of AF increased from 1 second with an intravenous dose of 60 micrograms to 700 seconds with an intravenous dose of 2,000 micrograms. However, there was a considerable range in responsiveness to the reflex cholinergic drive provided by the intravenous phenylephrine; for example a single intravenous 500-micrograms dose produced AF ranging from 190-540 seconds among the sheep. The duration of AF was most controllable in the anesthetized sheep, following neostigmine administration and with bilateral vagal stimulation. In the unanesthetized sheep, AF could generally be sustained for more than the duration of the half-life (about 4 minutes) of phenylephrine following neostigmine. However, there was a large variation in the duration of AF among the animals for the same dose of phenylephrine. This study identifies two methods (direct vagal stimulation and reflex vagal stimulation) for providing the cholinergic drive needed to sustain AF in the adult sheep. The duration of AF is sufficiently long to enable the measurement of electrical atrial defibrillation threshold.

Anesthesia, General↗
PubMed1994

Static field conformal stereotactic radiosurgery: physical techniques.

PURPOSE: Lesions in the head that are irregularly shaped or large present challenges for radiosurgical treatment by conventional techniques. Single, large circular fields may treat normal tissue volumes. Multiple shot or multiple isocenter treatment plans provide better conformation to the target than a single field, but may be difficult to plan and/or treat. As an alternative to these conventions, we are developing static field, conformal stereotactic radiosurgery. In this technique a finite number of fixed, shaped, linear accelerator fields are used to irradiate the target. METHODS AND MATERIALS: Computer simulations were performed for a four-path arc and fixed field techniques and evaluated with dose distributions and dose volume histograms. Beam geometries are defined with a 3-D treatment planning system with beam's eye view capabilities. Equipment for treatment delivery has been designed, including a head frame/support stand assembly and a method for manufacturing the required custom collimators. RESULTS: Isodose distributions and dose volume histograms show that beam geometries with seven or more fields provide target dose distributions equivalent to the arc treatment, but with small increases in peripheral dose. Dose homogeneity across the target volume increases as the solid angle of irradiation is increased. For a hemispherical target, the four-path arc and shaped, static fields provide equal target coverage while the shaped fields treat a smaller high-dose volume. CONCLUSION: Shaped, static fields are an alternative to single isocenter arc radiosurgery and result in smaller volumes at high dose. This smaller volume could translate into sparing for normal adjacent tissues that would otherwise be treated.

Computer Simulation↗
PubMed1993

The importance of timing muscle contraction in dynamic cardiomyoplasty.

This acute dynamic cardiomyoplasty (CMP) study used ten dogs (weight range 21-32 kg) and was designed to determine the importance of the train of stimuli initiation time when applied to the thoracodorsal nerve, which innervates the latissimus dorsi (LD) muscle that is wrapped around the ventricles. Using the P wave of the cardiac electrogram to trigger a special delay circuit, the stimulus train could be initiated from the apex of the R wave to any time throughout and at the end of the isovolumic period, signaled by opening of the aortic valve. The cardiac electrogram (which contained the R wave), left ventricular pressure (LVP), aortic flow velocity (AFV), beat-by-beat stroke volume (SV), femoral artery pressure, and the envelope of the stimulus train were recorded as the onset of the stimulus train was varied from the R wave to the end of the isovolumic period with a pumping ratio of one LD contraction for every seven ventricular contractions. In four dogs there was a pronounced increase in the augmentation in LVP, AFV, and SV when the stimulus train was initiated later than 40 msec after the first peak of the R wave. In five dogs the augmentation in LVP, AFV, and SV was not as clearly apparent, although all of these dogs exhibited an optimal train delay. Data were not obtained on one dog due to an anomalous LD muscle blood supply. For all of the dogs, the optimum train delay from the R wave averaged 58 msec (range 40-80 msec). The average augmentation in SV was 26% (range 13%-45%). The same muscle-wrap tightness was used in all dogs. In one dog, the muscle-wrap tightness was varied, and by tightening the wrap the SV augmentation increased from 17% to 27%. For all dogs the range of augmentation in SV (13%-45%) perhaps represents variations in muscle-wrap tightness, which may be a major uncontrolled factor in dynamic CMP.

Animals↗
PubMed1993

Detection of ventricular tachycardia and fibrillation using coronary sinus blood temperature: a feasibility study.

This study investigated the potential of coronary sinus blood temperature to detect ventricular arrhythmias. A rapid-response, thermistor-tipped catheter placed in the coronary venous system of anesthetized dogs was used to record the blood temperature during periods of induced bradycardia, tachycardia, and ventricular fibrillation. A second catheter was used to measure blood temperature in the aortic arch during these same episodes. A pulsatile component of venous blood temperature, typically 40 m degrees C in amplitude, was well correlated with the cardiac cycle, while another, slightly larger, pulsatile component was well correlated with respiration. The cardiac component peaked during ventricular systole, and the respiratory component peaked during expiration. As compared with sinus rhythm, the cardiac signal diminished during bradycardia and tachycardia and nearly disappeared during asystole and ventricular fibrillation. The baseline component of venous blood temperature rose during periods of tachycardia and fibrillation, while respiration proved to be an important factor in the baseline temperatures. The presence of small, cyclic, thermal variations in the coronary venous system was verified, and the concept of measuring metabolic activity to assess ventricular function was substantiated. These studies show promise that this concept could be incorporated into medical devices that use these temperature signals for diagnosis of ventricular arrhythmias.

Animals↗
PubMed1993

The evaluation of five specialized support surfaces by use of a pressure-sensitive mat.

This study used two full-size pressure-sensitive mats to evaluate five patient support surfaces. The mats, containing 1,536 and 2,340 pressure measurement sites, respectively, made it possible to quantify the entire interface pressure distribution of each support surface. Measurements for groups of 64 and 32 subjects were obtained in supine positions of 0 degree, 30 degrees, at maximal head elevations (50 degrees-60 degrees), and at lateral recumbency. Several statistical descriptors were calculated to characterize the interface pressure distributions. The support surfaces tested included: a powered-air overlay (ACUCAIR Continuous-Air-flow System), a low-airloss bed (FLEXICAIR MC3 Low-Airloss Therapy), a continuous lateral-rotation low-airloss bed (RESTCUE Dynamic Air Therapy), an air-fluidized bed (CLINITRON II Air-Fluidized Therapy), and an integrated bed (prototype Advance 2000 Bed). An analysis of variance revealed that the interface pressure was significantly lower and weight-bearing area was significantly higher on the five surfaces, when compared to the standard hospital mattress.

Beds↗
PubMed1992

Instrumentation for the breath-by-breath determination of oxygen and carbon dioxide based on nondispersive absorption measurements.

This paper describes the development and evaluation of instrumentation for the breath-by-breath determination of oxygen and carbon dioxide in respiratory gases. The method is based on nondispersive absorption and uses the 145-nm absorption band for detection of oxygen and the 4.3-micron band for detection of carbon dioxide. A xenon discharge lamp with a sharp band at 147 nm was chosen as the source for the determination of oxygen, and a carbon dioxide discharge lamp with a sharp band at 4.3 micron was chosen for determination of carbon dioxide. A vacuum photodiode was used as the detector for oxygen, and a photoconductive cell with a built-in interference filter was used for detection of carbon dioxide. Plots of absorbance (A) vs concentration (C, %) were linear for oxygen and were nonlinear for carbon dioxide. Typical least-squares calibration equations were A = 0.020C + 0.02 for oxygen (0-100%) and A = 0.0012C2 + 0.050C + 0.008 for carbon dioxide (0-8%). Comparisons of computed (y) vs prepared (x) values for the concentrations given above were linear for both gases, yielding y = (1.00 +/- 0.01)x - 0.13 +/- 0.73 for oxygen and y = (1.07 +/- 0.02)x - 0.04 +/- 0.06 for carbon dioxide. The standard deviations were 1.2% at 50% oxygen and 1.5% at 4% carbon dioxide. Records are presented to illustrate breath-by-breath monitoring of these gases in a healthy subject.

Absorption↗
PubMed1992

Electrode recovery potential.

In some instances the same electrodes are used for stimulation and then for recording a bioelectric event immediately after the stimulus. However, after the current pulse there remains an electrode potential that decays quasiexponentially. We have designated this falling potential the electrode-recovery potential. This study investigated the recovery potentials of single electrodes of rhodium, stainless steel, platinum and platinum-iridium in contact with 0.9% saline at room temperature (25 degrees C) over a current density ranging from 0.1 to 100 mA/cm2 using a constant-current pulse. In all cases, with increasing current density, there was a decrease in the time for the electrode potential to fall to one half of the immediate post-stimulus value. Above about 20 mA/cm2 the decrease in recovery time was smooth with increasing current density. Below 20 mA/cm2, the recovery time was slightly irregular. The shortest recovery times were for platinum and platinum-iridium. The largest decrease in recovery time with increasing current density was for stainless steel, which decreased 10 fold from 0.1 to 100 mA/cm2. The recovery time for rhodium decreased about three-and-one half fold over the same current density range. It was found that the waveform of the recovery potential is not a simple exponential because the Warburg and Faradic components of the electrode-electrolyte interface are current-density dependent. In general, for all current densities studied (0.1-100 mA/cm2), there was a sudden initial fall in electrode potential with cessation of current flow, followed by a very gradual nonexponential decrease in potential.

Electric Conductivity↗
PubMed1992

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.

Animals↗
PubMed1992

Faradic resistance of the electrode/electrolyte interface.

A new method is used to measure the direct-current (Faradic) resistance of a single electrode/electrolyte interface. The method employs a constant-current pulse and a potential-sensing electrode. By choosing a sufficiently long pulse duration, the voltage between the test and potential-sensing electrode exhibits a three-phase response. In the steady-state phase, the voltage measured is equal to the current flowing through the electrode Faradic resistance and the resistance of the electrolyte between the test and potential-sensing electrode. By measuring this latter resistance with a high-frequency sinusoidal alternating current, the voltage drop in the electrolyte is calculated and subtracted from the voltage measured between the test and potential-sensing electrode, thereby allowing calculation of the Faradic resistance. By plotting the reciprocal of the Faradic resistance against current density and fitting the data points to a third-order polynomial, it is possible to determine the zero-current density (Faradic) resistance. This technique was used to determine the Faradic resistance of electrodes (0.1 cm2) of stainless-steel, platinum, platinum-iridium and rhodium in 0.9 per cent NaCl at 25 degrees. The zero current Faradic resistance is lowest for platinum (30.3 k omega), slightly higher for platinum-iridium (47.6k omega), much higher for rhodium (111k omega) and highest for type 316 stainless-steel (345k omega). In all cases, the Faradic resistance decreases dramatically with increasing current density.

Electric Impedance↗
PubMed1992

The chronaxie for myocardium and motor nerve in the dog with chest-surface electrodes.

The chronaxie (i.e., the duration for a stimulating current having twice the rheobasic, or minimum, value) was determined for ventricular myocardium in 12 pentobarbital-anesthetized dogs. Current was applied transthoracically via chest-surface electrodes located at the optimal axillary site for producing inspiration by stimulation of the phrenic nerve (electroventilation). In four dogs the chronaxie for motor-nerve was determined using electrodes at the same location. After using hand-held electrodes to identify the optimal stimulation site for electroventilation, 4.1 cm diameter electrodes were applied bilaterally to the optimal site on the thorax. In 12 dogs, the threshold current for producing ventricular ectopic beats was determined for single rectangular current pulses ranging from 0.1-10 ms in duration. From these data, strength-duration curves were determined and the average chronaxie for ventricular myocardium was found to be 1.82 ms. In four dogs the relationship between inspired volume and maximum stimulus intensity was determined using a 0.8 s burst of stimuli (60/s) with pulse durations ranging from 20-500 microseconds. From these data, strength-duration curves for current were constructed and the average chronaxie for motor-nerve was found to be 0.17 ms. The results of this study show that, because of the differing chronaxies, the current required to produce inspiration with short-duration stimuli is much less than that required to evoke an ectopic heart beat.

Animals↗