Cell constant of the tetrapolar conductivity cell.

The factors that specify the cell constant k of the tetrapolar conductivity cell are investigated. An equation to predict the value of k from the geometry of the cell is derived and validated by measuring the cell constant for different geometries in solutions of known resistivity. The constant-current method for tetrapolar resistivity measurement is used.

A pressure-sensitive mat for measuring contact pressure distributions of patients lying on hospital beds.

The authors describe a novel system for sensing and displaying the distribution of contact pressure caused by a patient's lying on a hospital bed. The system includes a flexible, pressure-sensitive mat, electronics to activate the mat, a small computer to process data, and a color video display. The present prototypes can sense pressure at 1,536 discrete locations in a rectangular grid of 24 x 64 nodes, each node representing an area of 4 cm2.

Accuracy and precision of computer-simulated tissue temperatures in individual human intracranial tumours treated with interstitial hyperthermia.

Accurate knowledge of tissue temperature is necessary for effective delivery of clinical hyperthermia in the treatment of malignant tumours. This report compares computer-predicted versus measured intratumoral temperatures in 11 human subjects with intracranial tumours, treated with a conceptually simple 'conductive' interstitial hyperthermia system. Interstitial hyperthermia was achieved by the use of parallel arrays of implanted, electrically heated catheters.

Impedance cardiography by use of a spot-electrode array to track changes in cardiac output in anesthetized dogs.

Transthoracic impedance cardiography is a noninvasive method to determine changes in cardiac output on the basis of the cardiac-induced impedance change measured across the thorax. In this report, we describe a new, easily applied, tetrapolar spot-electrode configuration for use in canine transthoracic impedance cardiography. The array is a convenient alternative to use of the traditional circumferential band-electrode array which, in the dog, is prohibitive because of the extensive skin preparation required.

The use of an electrically activated valve to control preload and provide maximal muscle blood flow with a skeletal-muscle ventricle.

A new method for optimally loading a skeletal muscle-wrapped pouch to act as a blood pump is described. The method takes advantage of the fact that the high preload pressure required for a forceful contraction needs to be present for only a short time. By using an electrically controlled valve to delay pouch filling until just before muscle contraction, pouch diastolic pressure can be kept low, which in turn maintains a high muscle capillary blood flow.

Stroke volume and the three phase cardiac output rate relationship with ventricular pacing.

Knowledge of how stroke volume (SV), and hence cardiac output (CO), changes with ventricular pacing rate (R) constitutes a key aspect of sensor driven, variable rate pacemakers. It has been established that the relationship between CO and pacing rate exhibits three phases for rest and constant exercise. At low rates (phase 1), CO increases with increasing R; with additional rate increase (phase 2), CO either remains constant or increases slightly; and above some critical rate, CO decreases (phase 3).

A surrogate arm for evaluating the accuracy of instruments for indirect measurement of blood pressure.

A surrogate arm, containing an artificial artery, has been developed for evaluating the accuracy of automatic, non-invasive, blood-pressure measuring devices. The surrogate arm produces all of the events associated with the auscultatory and oscillometric methods of measuring blood pressure. The surrogate arm includes a flat artificial artery in a plastic cylindrical chamber, 30 cm in circumference and 25 cm long, that contains an air/water mixture that allows matching the compliance of the human arm.

The use of electrically stimulated skeletal muscle to pump blood.

Electrically stimulated skeletal muscle can be used in many ways to pump blood. The important physiological characteristics of skeletal muscle for use in a cardiac assist role are presented. The force developed by a twitch and tetanic contraction are quantitated and the considerations in the choice of the stimulus applied to the innervating motor nerve are discussed.

Prevention of reperfusion injury in surgically induced gastric dilatation-volvulus in dogs.

Canine gastric dilatation-volvulus (GDV) is a naturally acquired condition of large-breed dogs primarily and is associated with high mortality. The clinical course suggests that reperfusion injury may be important in the pathogenesis of GDV. To evaluate the role of xanthine oxidase and iron-dependent lipid peroxidation (which are purported mechanisms of reperfusion injury) in the pathogenesis of GDV-related mortality, we created experimental GDV in 21 dogs.

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.

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.

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.

Tracking cardiac output by a saline dilution technique using esophageal catheter electrodes.

This paper presents preliminary results of a study in which saline indicator dilution curves were obtained by measuring impedance dilution curves using a tetrapolar catheter-based electrode placed in the esophagus of anesthetized dogs. Cardiac output (CO) was calculated from the area under the impedance-derived saline dilution curve, and compared to CO calculated from the saline dilution curve obtained by a cylindrical external conductivity cell placed in an arterio-venous shunt. The results revealed an approximately two-third overestimation in CO determined using the esophageal electrodes.

Existence of a strength-duration curve for spinal cord motor evoked potentials in cats.

A new technique has been developed to estimate the chronaxie of fibers carrying action potentials that are responsible for short latency motor evoked potentials (MEPs). In a 6-cat study, electrical stimuli were applied to the exposed motor cortex, and spinal cord potentials (at the vertebral level of T9/10 and L2/3) were recorded with needle electrodes using signal averaging. From a plot of MEP amplitude versus stimulus current amplitude for stimuli of 70, 100, 200 and 500 microseconds duration, it was possible (by extrapolation and using the linear relationship between charge and duration) to estimate chronaxie for the first 2 prominent peaks in MEP recordings.

Small intestinal submucosa as a large diameter vascular graft in the dog.

Autogenous saphenous vein and synthetic materials, such as Dacron and expanded polytetrafluoroethylene, have been used extensively as vascular grafts with moderate success. Improved success rates for vascular graft surgery may be possible if superior graft material was available. We tested the use of autogenous small intestinal submucosa (SIS) as a large diameter (10 mm) vascular graft in the infrarenal aorta of 12 dogs.

The inductorium: the stimulator associated with discovery.

The inductorium, or induction-coil stimulator, introduced in the late 1830s, provided a means for obtaining repetitive stimuli; thereby allowing the discovery of new properties of excitable tissues. Major physiologic discoveries with the inductorium were tetanic contraction of skeletal muscle, inhibition of the heart, the role of vasomotor nerves, and cortical localization. In the hands of clinicians, it was used to diagnose neuromuscular disease and to produce artificial respiration with body-surface electrodes.

Case studies on the effect of exercise and hot water submersion on intracardiac temperature and the performance of a pacemaker which varies pacing rate based on temperature.

The effectiveness of using blood temperature change as an indicator to automatically vary heart rate physiologically was evaluated in 3 patients implanted with Model Sensor Kelvin 500 (Cook Pacemaker Corporation, Leechburg, PA, USA) pacemakers. Each patient performed two block-randomized treadmill exercise tests: one while programmed for temperature-based, rate-modulated pacing and the other while programmed without rate modulation. In 1 pacemaker patient and 4 volunteers, heart rates were recorded during exposure to a hot water bath.

A skeletal muscle ventricle made from rectus abdominis muscle in the dog.

This study examined the ability of a skeletal muscle-powered assist ventricle to augment cardiac output in 10 dogs with experimentally induced heart failure. Heart failure was induced with the use of the beta-blocking agents atenolol and propranolol. A "skeletal muscle ventricle" was then surgically constructed by wrapping the rectus abdominis muscle, with an intact neurovascular supply, around a double open-ended compressible pouch.

The safety factor for electroventilation measured by production of cardiac ectopy in the anesthetized dog.

The safety factor of electroventilation (ie, the ratio of the current required to produce an ectopic beat to the current required to produce an inspired volume of 225 ml, which is approximately twice tidal volume) was determined in 12 pentobarbital-anesthetized dogs using transthoracic electrodes positioned at the optimal electroventilation site. The optimal stimulation site for electroventilation was first determined using hand-held, stimulating electrodes. Then electrodes, 4.

Scatchard analysis of methane sulfinic acid production from dimethyl sulfoxide: a method to quantify hydroxyl radical formation in physiologic systems.

A major impediment to the confirmation of free radical mechanisms in pathogenesis is a lack of direct, chemical evidence that oxygen centered free radicals actually arise in living tissues in quantities sufficient to cause serious damage. This investigation was conducted to validate the use of dimethyl sulfoxide (DMSO) as a quantitative molecular probe for the generation of hydroxyl radicals (HO.) under physiologic conditions.

Effect of oxygen concentration on the formation of malondialdehyde-like material in a model of tissue ischemia and reoxygenation.

This study was conducted to explore the functional relationship between oxygen concentration during tissue reoxygenation after ischemia and the extent of postischemic lipid peroxidation, an indicator of reoxygenation injury. Excised rat liver or kidney tissue was rendered ischemic for 1 h at 37 degrees C, minced into 1 mm3 fragments, and then reoxygenated for 1 h in flasks of buffered salt solution containing various amounts of oxygen. Production of malondialdehyde-like material (MDA) was measured to indicate lipid peroxidation.

Bolus dose response characteristics of single chain urokinase plasminogen activator and tissue plasminogen activator in a dog model of arterial thrombosis.

Tissue plasminogen activator (t-PA) and single chain urokinase-plasminogen activator (scu-PA) are relatively "fibrin-specific" thrombolytic drugs with short plasma half lives of 6-8 minutes. Most treatment regimens with these agents utilize a bolus injection followed by continuous drug infusion, usually combined with anticoagulant therapy. The purpose of this study was to establish the dose-response characteristics for scu-PA and t-PA, when given as a single intravenous bolus injection, in a dog model of arterial thrombosis.