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Biomedical subjects

L A Geddes

Publications and source records attributed to L A Geddes.

At least 55 records · Page 3Linked to original sources
PubMed1992

Small intestinal submucosa as a superior vena cava graft in the dog.

Autogenous spiral vein grafts and ePTFE have been used for reconstruction of the superior vena cava with moderate success. We tested autogenous small intestine submucosa as a superior vena cava interpositional graft in nine dogs. All dogs received aspirin and warfarin sodium for the first 8 weeks after surgery. Graft patency was evaluated by serial venography. One dog died from excessive anticoagulation. Eight dogs were sacrificed at periodic intervals until 72 weeks after surgery. Patent grafts had no evidence of thrombosis, aneurysm, or stenosis. The grafts consisted of dense, organized collagenous connective tissue with a complete endothelial cell layer on the luminal surface. Two dogs are alive at 28 and 34 months after surgery. Graft patency was 89% (eight of nine grafts). We conclude that autogenous small intestine submucosa can be used as a superior vena cava graft in the dog and is worthy of further investigations.

Animals↗
PubMed1992

Use of impedance ratio for the continuous measurement of stroke volume of a valveless pouch used as a cardiac-assist device.

A technique is described for measuring the volume of a valveless compressible plastic pouch and its volume change when used as a cardiac-assist device. The method employs measuring the pouch impedance at high frequency with sleeve electrodes at both ends of the pouch. The use of an adequately high frequency eliminates the electrode impedance and the impedance measured is that of the resistance of the electrolyte in the pouch. By equating the compressible pouch to two truncated cones with their bases adjacent, an equation is derived that relates pouch impedance to volume. It is shown that by plotting the stroke volume ejected (delta V) versus the ratio of systolic (RS) to diastolic (Rd) impedance, the resulting relationship is independent of the resistivity of the fluid in the pouch. Validation tests were made with a 100 mL pouch filled with solutions having resistivities of 60, 102, 145, and 192 omega-cm. The method described herein permits calibration of the volume change of a valveless pouch used as a circulatory-assist device and in use the calibration will not be affected by a change in packed-cell volume which changes resistivity.

Calibration↗
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↗
PubMed1992

Ventricular fibrillation produced by stimulation of external transthoracic electrodes--an experimental study.

The threshold for ventricular fibrillation was determined in 12 pentobarbital anesthetized dogs using transthoracic electrodes located at the optimal axillary electroventilation sites. Electroventilation is the name used to designate inspiration produced by stimuli applied to body surface electrodes. The optimal stimulation site for electroventilation was first determined using hand-held electrodes. Then electrodes, 4.1 cm in diameter, were sutured bilaterally to the optimal anterior axillary stimulation site. The threshold current for producing ventricular fibrillation was determined using single pulses ranging from 0.1-10 msec in duration delivered during the vulnerable period of the cardiac cycle. Fibrillation was produced in all dogs with the 10- and 5-msec pulse durations, in 11 dogs with 0.3-msec, in 6 dogs with 0.2-msec, and in 1 dog with 0.1-msec pulse duration. In all dogs, the current required to produce ventricular fibrillation increased greatly as the pulse duration was decreased. The current required for fibrillation was much in excess of that required to produce one tidal volume. With the longer duration pulses, the ratio was about 80. With the 8 microseconds duration pulses used for electroventilation the estimated ratio is about 800.

Animals↗
PubMed1992

Output power and metabolic input power of skeletal muscle contracting linearly to compress a pouch in a mock circulatory system.

Output power and metabolic input power values were determined for unconditioned canine latissimus dorsi (two), gastrocnemius (seven), and triceps (three) muscles contracting linearly to cause compression of a doubly valved pouch in a hydraulic model of the circulation. The motor nerves to the muscles were stimulated tetanically with 450 msec trains of 0.1 msec pulses having a frequency of 50/sec. The muscles were contracted 10, 20, 30, and 40 times per minute and pouch output in milliliters per minute was measured directly for each muscle at each contraction (train) rate. The output power in milliwatts was determined by two methods: (1) by using the pouch output and the pressure rise imparted to the stroke volume (average power) and (2) by using the pressure-volume loop. Metabolic input power in milliwatts was determined from the oxygen consumption in milliliters per minute of the working muscle. It was found that as the pouch output was increased, the pouch output power and the metabolic input power both increased. The average power output was slightly less than that computed from the pressure-volume loop. The mean output power values, when pumping at L liters per minute, were 0.62 L (average) and 0.75 L mW/gm (pressure-volume loop) for the latissimus dorsi muscles; 0.83 L (average) and 1.16 L mW/gm (pressure-volume loop) for the gastrocnemius muscles; and 0.55 L (average) and 0.66 L mW/gm (pressure-volume loop) for the triceps muscles. The percent efficiency of energy conversion ranged from 9.2% to 17.8% for the latissimus dorsi muscles, from 5.1% to 19.5% for the gastrocnemius muscles, and from 10.5% to 27.3% for the triceps muscles. However, it should not be concluded that one muscle type is better than another on the basis of percent efficiency because efficiency does not take endurance into account. An important observation in this study relates to the large output obtained with the three linearly contracting muscle types. All were capable of pumping in excess of 1.5 L/min. A second observation relates to the absence of fatigue, although determination of endurance was not an objective in these studies.

Animals↗
PubMed1992

A new method for measuring the Faradic resistance of a single electrode-electrolyte interface.

A new method is described for measuring the Faradic resistance of a single electrode-electrolyte interface. The method employs a test (monopolar) electrode, a potential-sensing electrode and a large reference (indifferent) electrode, along with a constant-current source capable of providing a step function of current. The method was used to measure the Faradic resistance of a 0.1 cm2 platinum electrode in contact with saline (p = 150 ohm-cm) at room temperature. It was found that for both a positive and negative current pulse, the Faradic resistance decreased almost hyperbolically with increasing current density. When the reciprocal of the Faradic resistance (Gf) was plotted versus current density and the data were fit to a polynomial curve, the results showed that for the positive pulse Gf = 0.009 + 0.05J - 0.0001J2; (SEE = 0.117); for the negative pulse, Gf = 0.007 + 0.067J - 0.0001J2; (SEE = 0.028); where Gf is in millisiemens and J is in mA/cm2 for this 0.1 cm2 electrode. These relationships permit estimating the Faradic resistance (Rf) for zero current density. For the positive pulse, Rf = 111 kilohms and for the negative pulse Rf = 143 kilohms. The method is applicable to the measurement of the Faradic resistance of a wide variety of metal electrodes.

Electric Conductivity↗
PubMed1991

The polarization impedance of common electrode metals operated at low current density.

The objective of this study was to characterize the polarization impedance (resistance and capacitance) of several common metal/0.9% saline interfaces operated at low-current density and to thereby provide a useful reference for those wishing to calculate the impedance of such electrodes. The series-equivalent resistance (R) and capacitive reactance (Xc) of stainless steel, platinum, silver, MP35N, palladium, aluminum, rhodium and copper electrodes, all having a surface areas S = 0.005 cm2 and all in contact with 0.9% saline, were measured as a function of frequency (100 Hz to 20 kHz) at low-current density (0.025 mA/cm2). For all the metals tested, both R and Xc decreased with increasing frequency and the relationships were linear on a log-log plot. That is, R and Xc exhibited power-law behavior (R = A/f alpha and Xc = B/f beta). However, it was not generally true that A = B and alpha = beta = 0.5 as stated in the Warburg low-current density model. Furthermore, the Fricke constant phase model in which alpha = beta and phi = 0.5 pi beta was found not to be applicable in general. In particular, the constraint that alpha = beta was a good approximation for most of the metals tested in this study, but the constraint that phi = 0.5 pi beta did not hold in general. Although the Warburg low-current density model provides a useful conceptual tool, it is not the most accurate representation of the electrode-electrolyte interface. The Fricke constant phase model is a better representation of electrode behavior, but it also may not be valid in general. We have found that a better representation is provided by the general power-law model R = A/f alpha and Xc = B/f beta, where A, B, alpha, and beta depend on the species of both the metal and electrolyte and A and B depend, in addition, on electrode area. Using this model and the data presented in this study, the impedance of an electrode-electrolyte interface operated at low-current density may be calculated as formula see text where S is the surface area of the electrode in cm2.

Aluminum↗
PubMed1991

Use of electrical impedance for continuous measurement of stroke volume of a skeletal muscle-powered cardiac assist device.

This study describes the use of electrical impedance Z to continuously measure the stroke volume SV of a skeletal muscle-powered ventricle (SMV). An SMV was constructed surgically in four anaesthetised dogs. The rectus abdominis (two dogs) or latissimus dorsi (two dogs) muscle was wrapped around a compressible pouch, the ends of which were connected to a saline-filled (0.9 per cent) mock circulation. The motor nerves to the muscle were stimulated to produce tetanic contractions at a rate of 10 min-1. Z was measured between brass sleeve electrodes within the end conduits of the pouch. To derive a simple expression relating pouch volume V to Z, the pouch was represented as two truncated cones with their bases joined. For V ranging from 53 to 103 ml, the relationship between Z and 1/square root of V was nearly linear; i.e. Z = m(1/square root of V) + b. Impedance-derived stroke volume SV (delta Z) was calculated using this linear approximation and the impedance measured just before and after muscle contraction. The stroke volume SV (EM) ejected by the pouch during muscle contraction was measured with an electromagnetic flowmeter. The linear regression coefficients ranged from 0.99 to 2.55; the correlation coefficients ranged from 0.90 to 0.98. In general, SV(delta Z) tracked SV(EM) very well, although SV(delta Z) tended to overestimate SV(EM).

Animals↗
PubMed1991

Artificial respiration in the dog by percutaneous, bilateral, phrenic nerve stimulation.

Artificial respiration was produced in 11 anesthetized dogs using trains of short duration stimuli (1 msec with a frequency of 35/sec), applied to needle electrodes placed bilaterally at the base of the neck. The tips of the needles were in close proximity to the phrenic nerves. In all cases, the inspired volume increased with an increase in stimulus intensity. Typically, it required 5 to 10 volts (peak) to produce an inspired volume equal to spontaneous tidal volume. The maximum inspired volumes ranged from 1.27 to 4.31 times the tidal volume.

Animals↗
PubMed1991

History of magnetic stimulation of the nervous system.

The use of a time-varying magnetic field to induce a sufficiently strong current to stimulate living tissue was first reported by d'Arsonval in 1896. Since then, there have been many studies in what is now called magnetic stimulation. This paper traces the history of this field from d'Arsonval to its present use in neurophysiology.

Animals↗
PubMed1991

Comparison of canine skeletal muscle power from twitches and tetanic contractions in untrained muscle: a preliminary report.

Power output and blood flow were determined in dogs for four muscles (gastrocnemius, latissimus dorsi, rectus abdominis, and triceps) to determine effects of choice of muscle, tetany or twitch rates, force loading of the muscle, and blood flow on muscle power output. Total power for a 20-Kg dog was greatest for triceps at 0.77 watts (W) and least for rectus at 0.22 W; power per gram was greatest for gastrocnemius at 5.77 mW/g. Muscle perfusion of latissimus and rectus is greatly decreased by overstretching of the muscle. Overstretching also produces severe, persistent, power loss in latissimus and rectus muscles. Gastrocnemius and triceps tolerate stretching much better. We conclude that power can be improved without causing muscle fatigue by choice of muscle, choice of electrical stimulation parameters, linear geometry for contraction of the muscle, and matching the force load to each individual muscle.

Abdominal Muscles↗
PubMed1991

Guidelines for energy-efficient coils: coils designed for magnetic stimulation of the heart.

Magnetic stimulation of the heart requires high magnetic field energy and results in considerable Joulean dissipation. Energetically efficient and mechanically robust coils were designed for magnetic stimulation of the canine heart. Circular coils with rectangular cross section oriented either parallel or perpendicular to the skin were employed. A reasonable compromise between coil size and energy efficiency is achieved when the outer radius is twice the target depth. Nearly optimal efficiency is obtained by coils with annular width and height of 60% and 20% of the radius, respectively. A coplanar pair of coils parallel to the skin surface requires less energy and provides a more localized stimulus than a single coil. Placing a single coil perpendicular to the skin provides stimulus localization comparable to that of a coplanar pair; however, a single coil requires approximately 4 times the energy. The large coil currents required for cardiac stimulation exert Lorentz forces on the conductors that may exceed their tensile strength, emphasizing the need for an adequate support structure. Coils fabricated along the preceding guidelines were used to stimulate dog hearts repeatedly, requiring magnetic fields with energy greater than 12,000 J, without coil failure. Although the principles for coil design described herein were applied to a cardiac stimulator, they can also be applied to coils for other tissues, resulting in less coil heating, better stimulus localization and less power consumption.

Electric Conductivity↗
PubMed1990

Optimization of pulse train duration for the electrical stimulation of a skeletal muscle ventricle in the dog.

The optimal means of electrically stimulating a skeletal muscle to contract around a fluid-filled pouch (i.e., a skeletal muscle ventricle [SMV]) has not been determined. A SMV was made from the latissimus dorsi muscle in five dogs and the rectus abdominis muscle in five dogs, and each SMV was electrically stimulated via the motor nerve(s) to contract around a fluid-filled pouch, which was connected to a mock circulatory system. The pulse train duration (PTD) was varied from 100 ms to 800 ms in 100 ms increments to determine the effect of this variable upon SMV output. The pulse width of the electrical stimulus was kept constant at 100 microseconds and the pulse frequency was maintained at 50 s-1. For SMV contraction rates of 20, 30, and 40 min-1, the optimal PTD was 400 ms for both muscles. The peak output was 710 ml min-1 for the rectus SMV and 556 ml min-1 for the latissimus SMV. For an SMV contraction rate of 10 min-1, the optimal PTD was 800 ms for the rectus SMV and 600 ms for the latissimus SMV. Use of less than an optimal PTD caused reductions in SMV output of 25-50%. Although SMVs made from rectus abdominis and latissimus dorsi had similar values for the optimal PTD, the maximum SMV output was usually greater with the rectus abdominis in this acute study with untrained muscles. We conclude that PTD is an important variable to control, which can markedly affect results when studying the potential use of skeletal muscle power for cardiac assistance.

Animals↗
PubMed1990

Optimum stimulus frequency for contracting the inspiratory muscles with chest-surface electrodes to produce artificial respiration.

Electroventilation is the term used to describe the production of inspiration by applying a train of short-duration pulses to chest-surface electrodes. Studies were conducted in the dog to determine the optimum frequency to produce a smooth air flow in the trachea. It was found that a stimulus frequency of 25/sec or slightly higher meets this criterion. To illustrate that electroventilation can be carried out for a prolonged period using these parameters, electroventilation was applied continuously for 5 hours in a dog without a noticeable change in blood pressure.

Animals↗
PubMed1990

Comparison of three methods of electrical stimulation for converting skeletal muscle to a fatigue resistant power source suitable for cardiac assistance.

Twelve dogs were sorted into 3 equal groups, and the in-situ right latissimus dorsi muscle of each dog was stimulated via its motor nerve for a period of 6 weeks. The resulting isotonic contractions were used to pump fluid in an implanted, 2-chambered, compressible pouch system. Three methods of electrical stimulation were used: (a) continuous 2 sec-1 single pulses that caused muscle twitching, (b) a 250 msec train of pulses (36 sec-1) that caused tetanic muscle contractions and was repeated every 2 sec for 15 min followed by a 15 min period of rest, and (c) alternating 15 min periods of the above 2 stimulation methods to cause alternating twitch and tetanic contractions. The 2 sec-1 twitch stimulation and the combined twitch/tetanic stimulation methods resulted in a 100% conversion to fatigue-resistant fibers within 6 weeks. Standardized muscle function tests were performed weekly. With the twitch stimulation (Method 1), the time to fatigue increased from 9 to 116 min (p less than 0.001), but fluid pumping ability of the muscle decreased substantially from 0.25 to 0.14 liters min-1 (p less than 0.05). With the intermittent tetanic stimulation (Method 2), the fatigue resistance increased only slightly from 7 to 11 minutes (p = NS), and pumping ability was unchanged. With the combined (twitch-tetanic) stimulation (Method 3), the time to fatigue increased from 9 to 107 min (p less than 0.001), and the pumping ability did not significantly change from 0.20 to 0.22 liters min-1 (p = NS). These results suggest that a combined electrical stimulation method which produces both twitches and tetanic contractions can achieve rapid fiber conversion and increased fatigue resistance without loss of muscle strength.

Adaptation, Physiological↗
PubMed1990

Electrical properties of metallic electrodes.

The series equivalent resistance R and capacitance C of metal/saline electrode/electrolyte interfaces were measured as a function of frequency (100 Hz-20k Hz) and current density (0.25 to 1000 A m-2) for eight typical electrode metals. For each of the metals tested, R decreased and C increased as the current density was increased above a critical value (with the exception of silver and MP35N at frequencies above 1 kHz for which R increased and C decreased slightly). With the exception of copper, the current density linearity limit (for 10 per cent decrease in R or 10 per cent increase in C) increased with increasing frequency and, in most cases, the current density linearity limit for 10 per cent increase in C was slightly less than that for 10 per cent decrease in R. Among the metals tested, copper and aluminium had the lowest current carrying capability and rhodium had the highest current-carrying capability. The current carrying capabilities of 316 SS, platinum, silver and MP35N, were intermediate and similar. With increasing current density, an increase in the electrode/electrolyte capacitance was the most sensitive indicator of the current-carrying linearity limit.

Electric Conductivity↗