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Biomedical subjects

L A Geddes

Publications and source records attributed to L A Geddes.

At least 37 records · Page 2Linked to original sources
PubMed1995

The pumping and left ventricular unloading capabilities of the ventricular synchronous skeletal-muscle ventricle.

The pumping and left ventricular unloading capabilities of the left ventricular, ventricular synchronous skeletal-muscle ventricle were determined in nine anesthetized dogs ranging in weight from 20.7 to 31.8 kg. The ventricular synchronous skeletal-muscle ventricle consists of the left rectus abdominis muscle wrapped around a 4-mil-thick polyethylene pouch (wrapped volume 80 to 100 ml) connected to the left ventricular apex with no valve and to the aorta via a prosthetic heart valve. The rectus muscle is timed to contract tetanically and relax during left ventricular ejection. This arrangement provides a high precontraction pressure for the rectus muscle and a high muscle capillary blood flow during skeletal muscle relaxation. The timing signal for initiation of the train of stimulating pulses (40/sec) was derived from the ventricular electrogram. The delay for the stimulus train determines the preload for the rectus muscle and along with the stimulus train duration determines ventricular synchronous skeletal-muscle ventricle stroke volume, which was measured by electric impedance. With unconditioned rectus muscles (70 to 120 gm) and with a pumping ratio of 1:3, ventricular synchronous skeletal-muscle ventricle stroke volume average 26.1 ml, which provided an average output of 876 ml/min. The normalized ventricular synchronous skeletal-muscle ventricle output was 35.6 ml/min per kilogram of body weight. In a typical resting dog (and man), the normalized cardiac output is 70 ml/min per kilogram. Therefore the ventricular synchronous skeletal-muscle ventricle is capable of pumping 52% of the cardiac output (with a pumping ratio of 1:3). The optimum train delay from the apex of the ventricular electrogram ranged from 10 to 100 msec. The left ventricular ejection period averaged 309 msec, and this determines the time available for the rectus muscle to contract and relax. Evidence for unloading the left ventricle is shown by the reduced left ventricular diastolic pressure and stroke volume for the postassisted beats.

Animals↗
PubMed1994

Comparison of the resistance to infection of intestinal submucosa arterial autografts versus polytetrafluoroethylene arterial prostheses in a dog model.

PURPOSE: Prosthetic graft infection represents a most challenging complication to the vascular surgeon. Although expanded polytetrafluoroethylene (ePTFE) grafts have an acceptable patency rate, especially in the large-diameter arterial location, bacterial contamination of this material usually requires surgical removal of the graft. METHODS: We compared the resistance of large-diameter ePTFE grafts and grafts constructed of small intestinal submucosa (SIS) to deliberate infection with Staphylococcus aureus. Eighteen dogs were divided into two equal groups, and the infrarenal aorta was replaced with either ePTFE or SIS graft material. One hundred million S. aureus organisms were deposited directly on the graft at the time of surgery, and the dogs were observed for 30 days. RESULTS: One dog with an ePTFE graft died of hemorrhage from anastomosis site at 21 days. Of the remaining eight dogs with ePTFE grafts, four had positive culture results from the removed graft material, and all had histologic evidence for persistent infection. These dogs also had chronic fever, and the average white blood cell count at day 30 was 15,600/mm3. All nine dogs with SIS grafts had patent grafts, were afebrile after the first week, had an average white blood cell count of 11,500/mm3 at 30 days (p value = NS), had negative culture results, and had the histologic appearance of graft remodeling with collagen that was free of active inflammation. CONCLUSIONS: We conclude that large-diameter arterial SIS grafts are more resistant to persistent infection with S. aureus than ePTFE grafts in this dog model of deliberate bacterial inoculation.

Anastomosis, Surgical↗
PubMed1994

A selective, non-ischemic, non-pharmacological left ventricular failure animal model.

Selective left ventricular failure was induced in 13 acute anesthetized, closed chest dogs ranging in weight from 18-26 kg. Failure was induced by passing a single, high intensity pulse of current from a defibrillator connected to a left ventricular catheter electrode and a left chest electrode. The intensity of the myocardial damaging shock was related to the predicted current required for transventricular defibrillation, based on heart weight. Thermodilution cardiac output, left ventricular pressure, impedance stroke volume, the cardiac electrogram, and lead II ECG were recorded, along with the pressure impedance (volume) loop, which is a measure of stroke work. It was found that the cardiac output decreased with increasing current intensity. Immediately following the high current shock, cardiac output, and stroke work decreased. In some animals, with a moderate intensity shock, there was a transient increase in cardiac output, followed by a decrease. In the five animals that were monitored continuously for 4 hours, the average percent reduction in cardiac output at this time was 42.5% for an average current overdose ratio of 5.39. The energy setting on the defibrillator to obtain this range of reduction in cardiac output was 175-350 joules. The method described herein is easily applied to the closed chest animal and will allow evaluation of the pumping capabilities of cardiac augmentation techniques, such as dynamic cardio-myoplasty and the skeletal muscle ventricle.

Animals↗
PubMed1994

Two unsung heroes of closed-chest cardiac pacing: Green and McWilliam.

In the latter half of the last century, cardiorespiratory arrest was not uncommon due to the anesthetic used (chloroform). Animal studies showed that the heart could be paced. One physician (Green) resuscitated 5 of 7 cardiac arrest patients with single stimuli applied to body surface electrodes. Another (McWilliam) issued a proposal for adoption of closed-chest pacing and provided information on the procedure. The following historical footnote describes these pioneering events in the history of cardiac pacing.

Animals↗
PubMed1994

Stroke volume with dynamic cardiomyoplasty during ventricular fibrillation in the acute dog.

The objective of this study was to determine the pumping capability of dynamic cardiomyoplasty during induced ventricular fibrillation. In this acute study of 6 dogs, the pumping capability of the unconditioned left latissimus dorsi (LD) muscle (141 to 292 gm), wrapped around both ventricles, was investigated during induced ventricular fibrillation. Left-ventricular and femoral artery pressure, the ECG and aortic root flow velocity were monitored. Prior to inducing ventricular fibrillation, the ability of the unconditioned LD muscle to augment stroke volume (SV), was quantified as the area under the aortic flow-velocity record. The ventricles were then fibrillated and, after 10 sec, rhythmic 250 msec trains (1/sec) of stimuli (40/sec) were delivered to the thoracodorsal nerve to contract the LD muscle tetanically. In no case could dynamic cardiomyoplasty produce the same SV as when the ventricles were beating normally. In one animal, the SV attained two percent of the normal SV by 5 contractions; in another, the SV reached one percent by 25 contractions. In the remaining animals, the SV varied around 20% of the prefibrillation SV. By 90 contractions, the stroke volume was 10% of the prefibrillation value. The progressive decrease in SV was likely a consequence of LD muscle ischemia and fatigue, since the latissimus dorsi muscle provided low blood flow during the period of fibrillation.

Animals↗
PubMed1993

Porosity of porcine small-intestinal submucosa for use as a vascular graft.

The porosity of a vascular graft material has been suggested as a major factor affecting the rate and degree of neovascularization of newly implanted grafts, with higher porosities generally associated with better performance. The objective of this study was to determine the water porosity of a new vascular graft material, small-intestinal submucosa (SIS), and to compare the values to those reported for other common vascular graft materials. In addition, the porosity of SIS was investigated with respect to applied pressure and applied uniaxial tension. Both rectangular, flat specimens and tubular specimens of SIS were subjected to static water pressure, and water was collected as it passed through the SIS material. SIS has a typical porosity of 0.52 mL/min.cm-2 at an applied pressure of 120 mm Hg. Although porosity appeared to be unaffected by uniaxial tension, it increased in proportion to applied pressure at a rate of 4.8 x 10(-3) mL/min.cm-2/mm Hg. These low porosity values and the past success of SIS as a vascular graft material suggest that high-porosity materials are not required for implant success.

Animals↗
PubMed1993

Directional porosity of porcine small-intestinal submucosa.

Small-intestinal submucosa (SIS) has been shown to be a promising biomaterial for vascular graft applications. This study examines the directionality property of SIS porosity using 35 SIS specimens from 13 pigs. In addition, the effects of the weight of the donor pig, pre-conditioning of 13 additional SIS specimens, and the duration of the test of five additional SIS specimens on such porosity are reported. The porosity from serosal to mucosal direction was found to be four times greater than the porosity in the opposite direction. The weight of the donor pig was not found to be an important factor in SIS porosity. Preconditioning served to increase the average serosal porosity index at 120 mm Hg static water pressure from 2.99 to 8.33 mL/(min cm2). The porosity in the mucosal direction was not affected by preconditioning. Porosity in both directions decreased with increasing test duration. The directionality property of SIS porosity may be an important factor in its success as a vascular graft. The term 'porosity' is used throughout this article, but current standards also refer to the term 'permeability' to describe the passage of liquid through a vascular graft.

Animals↗
PubMed1993

The ventricular-synchronous, skeletal-muscle ventricle: preliminary feasibility studies.

The two requirements for the optimal use of skeletal muscle wrapped around a pouch used to pump blood are:(1) a low pouch diastolic pressure (to assure a high muscle capillary blood flow; and (2) a high pouch precontraction pressure (PCP) (to assure a forceful muscle contraction). Both requirements are satisfied with the pumping method described herein. This new type of skeletal-muscle ventricle (SMV) consists of a rectus abdominis muscle wrapped around a pouch connected to the left ventricular apex (with no valve) and to the aorta via a one-way valve. Consequently, the pressure in the SMV pouch is always equal to left ventricular pressure. The high PCP is obtained by stimulating the rectus muscle to contract at the desired left ventricular pressure. The R (or P) wave of the cardiac electrogram initiates a delayed train of stimuli to cause the rectus muscle to contract tetanically and expel blood from the pouch. We have designated this pumping configuration the ventricular-synchronous SMV (VS-SMV). In this study, eight acute anesthetized dogs were used. The muscles were unconditioned and among the items investigated were the importance of the delay (d) between the R (or P) wave and the onset of the stimulus train, the optimal stimulus frequency and train duration, the VS-SMV output with different ratios of VS-SMV to left ventricular contractions, unloading of the left ventricle, Frank-Starling curves for the VS-SMV, pressure-volume loops for the VS-SMV with and without contraction of the rectus muscle, and washout characteristics of the VS-SMV. It was found that a stimulus frequency of 40/sec, and a train duration of 250 msec is optimal. It was also found that choice of the proper delay from the R or P wave provided maximal augmentation in stroke volume, typically 20 to 40 mL. Pumping with a ratio of 1:2 provided VS-SMV outputs ranging from 20 to 140 mL/min per kg of body weight. With this same pumping ratio, cardiac output increased by 28% and the post-VS-SMV contraction, left ventricular stroke volume was reduced. The Frank-Starling curves showed that PCPs on the order of aortic pressure are needed for the most forceful muscle contraction. With this new pumping configuration, the left ventricle resembles an atrium which delivers blood to the VS-SMV that is stimulated to contract when the desired PCP is reached. Studies were also conducted in which the VS-SMV outlet (valve end) was closed and filling and emptying occurred through the left ventricle.(ABSTRACT TRUNCATED AT 400 WORDS)

Abdominal Muscles↗
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↗
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

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, and I was determined alternately for each current waveform at each of several pulse durations (d). Strength-duration curves for sensation were obtained using d of 0.01-50 ms. The threshold peak current (Ir) for a rectangular pulse of duration d was compared to the threshold peak current (Icd) for a capacitor-discharge pulse of duration d, where d was the time constant; that is, the time required for the current to decrease to 1/e, or 37% of its peak value. Chronaxie, the pulse duration at which I 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 than Ir of equivalent duration. Chronaxie for the capacitor-discharge pulse was, on the average, twice that for the rectangular pulse (p less than 0.01). Moreover, the ratio Icd/Ir increased with decreasing d. 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.

Adult↗
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

The impedance of a spherical monopolar electrode.

The impedance of a monopolar electrode immersed in an environmental volume conductor consists of two parts; the impedance of the active electrode-electrolyte interface, and the resistance of the environmental conductor. Two studies were carried out to quantitate these components. First, impedance-frequency data were collected for five spherical stainless-steel electrodes (ranging from 0.473 to 1.11 cm in diameter) immersed in 0.9% saline (p = 70 omega-cm). Impedance measurements were made from 100 Hz to 100 kHz and two sets of data were obtained; one before and one after each electrode was polished with fine emery paper. At low frequency, the measured impedances were high and varied with electrode surface preparation. However, above a transition frequency, the impedances were resistive, independent of the electrode surface preparation, and equal to rho/2 pi d as predicted from the theory. This study indicates that the low frequency impedance of a monopolar electrode is dominated by the impedance of the electrode-electrolyte interface. Above a transition frequency, the resistance of the environmental conductor dominates, the value of this resistance depending on the electrode geometry and the resistivity (rho) of the environmental conductor. A second study was conducted, to examine the effect of the distance to the indifferent electrode. A frequency (100 kHz) above the transition frequency was used and impedance data were collected for various distances between the monopolar and indifferent electrodes. The measured resistance increased asymptotically as the distance between the electrodes was increased. When the indifferent electrode diameter was at least 10 times the diameter of the spherical monopolar electrode, the measured resistance was within 5% of the value predicted for an indifferent electrode at infinity.

Electric Impedance↗
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↗