Electrical dose for ventricular defibrillation of large and small animals using precordial electrodes.

Electrical ventricular defibrillation of heavy subjects (over 100 kg body weight) is uncommon for the human or any animal species. This paper reports trans-chest ventricular defibrillation of subjects ranging in weight from 2.3 to 340 kg using conventional defibrillation current (heavily damped sine wave) of 0.3-30 ms duration. It was found that a body weight-to-electrical-shock strength relationship exists and can be expressed in terms of either electrical energy or peak current. For the duration of current pulse used clinically (3-10 ms), the relationship between energy requirement and body weight is expressed by the equation U = 0.73 W(1.52), where U is the energy in W.s and W is the body weight in kilograms. The current relationship is I = 1.87 W(0.88) where I is the peak current in amperes and W is the body weight in kilograms. The energy dose is somewhat more species and weight dependent and ranges from 0.5 to 10 W.s/kg (0.23-4.5 W.s/lb). The data obtained indicate that the peak current dose is virtually species and weight independent and is therefore a better indicator than energy for electrical defibrillation with precordial electrodes. In the duration range of 3-10 ms, the electrical dose is very nearly 1 A/kg of body weight (0.45 A/lb).

Energy dose for ventricular defibrillation of children.

Transthoracic direct current shock is the recommended treatment for ventricular fibrillation in children as in adults. To determine the appropriate energy dose, data were collected from 71 defibrillation attempts in 27 children. Sixty-three of the 71 shocks (89%) were successful in terminating fibrillation. Fifty-seven shocks were within 10 watt-seconds (w-sec) above or below an energy dose of 2 w-sec/kg of body weight. Fifty-two (91%) of these were effective and five (9%) were ineffective. In every case, fibrillation was ultimately terminated by a shock of 4 w-sec/kg or less. The results of these studies suggest that an energy dose of 2 w-sec/kg (or approximately 1 w-sec/lb) is adequate to defibrillate most children weighing under 50 kg. We currently use 2 w-sec/kg and double the energy dose if the first defibrillation attempt is unsuccessful.

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.

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.

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, 4.1 cm in diameter, were sutured bilaterally to the optimal stimulation site. The relationship between inspired volume and stimulus intensity was determined using a 0.8-s burst of stimuli (60/s) with a pulse duration of 0.1 ms. Using the same electrodes, the threshold current for producing ectopic beats was determined for single pulses ranging from 0.1 to 10 ms duration. In all dogs, the current required to produce an ectopic beat increased greatly as the pulse duration decreased. At 0.1 ms, the safety factor for electroventilation was calculated to be 25.8.

The electrical dose for direct ventricular defibrillation in man.

The threshold electrical energy for direct ventricular defibrillation was measured in 100 patients whose hypothermic hearts were fibrillated for cardiac operations. In 93 cases 10 joules or less was sufficient, and in 48 of these cases 5 joules or less defibrillated the ventricles. Because a shock of 10 joules defibrillated the heart of most of our patients, we recommend an initial shock of 5 to 10 joules rather than the 20 joules used more commonly. Until the safety margin between defibrillation threshold and damage threshold is established for direct defibrillation, use of shocks with adequate but not excessive strength may avoid unnecessary damage to the myocardium. When hearts refibrillate after defibrillation, it is unnecessary to use higher energy settings for subsequent defibrillation attempts. Instead, an antiarrhythmic drug should be administered and another shock of the same intensity that defibrillated the first time should be applied.

Optimal stimulus duration for extracranial cortical stimulation.

Motor cortex stimulation is a new testing area in which either electrical or magnetic fields are used to produce transcranial stimulation of the underlying brain. The resulting signals, recorded from the spinal cord, peripheral nerves, or muscles, are monitored by electromyographic and evoked potential techniques. This report considers the issues in effective stimulation with minimal noxious sensation. Short duration pulses are optimal for electrical fields, and electrodeless stimulation with magnetic fields also makes use of short duration pulses.

Motor evoked potentials in the dog: effects of global ischemia on spinal cord and peripheral nerve signals.

Motor evoked potentials (MEPs) in cats, rats, and humans have been reported. They appear promising as a test of central nervous system function, and they are sensitive not only to mechanical injury but also to ischemia. In mechanical trauma, the peripheral nerve response is much more sensitive to damage than the cord response, with a lower threshold and an earlier disappearance. We are reporting that the MEP can also be produced in the dog and that, under conditions of cardiac arrest induced by fibrillation, the peripheral nerve response disappears first at about 30 seconds and then the spinal cord response disappears at about 10 to 13 minutes. The late disappearance of the spinal cord response raises serious questions about its role as an adequate injury monitor. The most useful warning feature of the spinal cord response is an increase in amplitude during the critical first 2 minutes of arrest. Latency changes in the cord and peripheral nerve response did not seem as useful as amplitude changes in terms of providing adequate detection of injury. We also evaluated the peripheral nerve signals to determine whether they are partially volume-conducted weak muscle responses, and evidence substantiates their nonmuscle origin.

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.

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.

Inspiration produced by bilateral electromagnetic, cervical phrenic nerve stimulation in man.

Eddy-current stimulation of both phrenic nerves at the base of the neck in human subjects was carried out to provide inspiration resulting from tetanic diaphragm contraction. The inspired volume obtained was in excess of spontaneous tidal volume.

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.

Ability of the Lapicque and Blair strength-duration curves to fit experimentally obtained data from the dog heart.

The objective of this study was to determine the ability of the empirical Lapicque and theoretically derived Blair expressions for excitation to fit experimentally obtained threshold current values to evoke a ventricular extrasystole using rectangular-wave stimuli applied to the dog heart. The data points were fitted to both expressions and the ability of each to predict the measured values was determined. The Levenberg-Marquardt (L-M) algorithm was used to fit the Lapicque and Blair expressions. The Lapicque data were also fitted to the linear charge-duration expression of Weiss (W). It was found that the ratio of the predicted to measured current was slightly different from one 0.95 (L-M) and 1.06 (W) for the Lapicque and 0.92 (L-M) for the Blair expression. Thus, there appears to be little difference between the ability of the expressions to fit the same experimentally obtained data. The L-M Lapicque fit is best for the short durations range; the Weiss-Lapicque fit overestimates in the short duration range and underestimates near chronaxie. The L-M Blair fit is best for the short duration range and poor for the durations near the membrane time constant.

Small intestinal submucosa as a small-diameter arterial graft in the dog.

Autogenous saphenous vein, human umbilical vein, modified bovine collagen, Dacron, and PTFE have been used as small-diameter arterial grafts with moderate success. We tested autogenous small intestine submucosa as a small-diameter arterial graft in both a carotid and femoral artery (mean ID 4.3 mm) of 18 dogs (total of 36 grafts). All dogs received aspirin and warfarin sodium for the first 8 weeks after surgery. Graft patency was evaluated by Doppler ultrasound techniques and angiography. Two grafts ruptured and 5 grafts occluded by 21 days after surgery. One graft became occluded at 14 weeks. Fifteen dogs were sacrificed at periodic intervals until 48 weeks after surgery. Patent grafts had no evidence of infection, propagating thrombus, or intimal hyperplasia. Graft aneurysmal dilation occurred in 4 grafts (11%). The grafts were composed of a dense organized collagenous connective tissue with no evidence of endothelial cell growth on the smooth luminal surface. Three dogs are alive at 76 to 82 weeks after surgery. Overall, graft patency was 75%. Graft patency after cessation of anticoagulation therapy was 92.3% (12 of 13 grafts). We conclude that autogenous small intestinal submucosa can be used as a small-diameter arterial graft in the dog and is worthy of further investigation.

Small intestinal submucosa as a vascular graft: a review.

Continuing investigations of vascular graft materials suggest that unacceptable graft complications continue and that the ideal graft material has not yet been found. We have developed and tested a biologic vascular graft material, small intestine submucosa (SIS), in normal dogs. This material, when used as an autograft, allograft, or xenograft has demonstrated biocompatibility and high patency rates in aorta, carotid and femoral arteries, and superior vena cava locations. The grafts are completely endothelialized at 28 days post-implantation. At 90 days, the grafts are histologically similar to normal arteries and veins and contain a smooth muscle media and a dense fibrous connective tissue adventitia. Follow-up periods of up to 5 years found no evidence of infection, intimal hyperplasia, or aneurysmal dilation. One infection-challenge study suggested that SIS may be infection resistant, possibly because of early capillary penetration of the SIS (2 to 4 days after implantation) and delivery of body defenses to the local site. We conclude that SIS is a suitable blood interface material and is worthy of continued investigation. It may serve as a structural framework for the application of tissue engineering technologies in the development of the elusive ideal vascular graft material.

The effect of skeletal muscle ventricle pouch pressure on muscle blood flow.

Skeletal muscle powered assist ventricles (SMV) are being investigated in animal studies as a treatment for heart failure. Muscle fatigue is almost always dependent upon muscle capillary blood flow. This study examined the relationship between SMV intrapouch pressure and blood flow to the circumferential muscle in a working SMV with a mock circulation. The unconditioned rectus abdominis muscle was used to create an in situ SMV in five dogs. Muscle blood flow was measured by both the radioactive microsphere and the electromagnetic flow probe method as the pouch pressure was varied between 10 and 70 mmHg and as the SMV was stimulated to contract at a rate of 20 min-1. The correlation coefficient for the two methods was 0.908. At pouch pressures of 10, 40, and 70 mmHg, the respective blood flow values were 22.60 +/- 2.50 (1 SEM), 12.20 +/- 2.10, and 4.40 +/- 0.74 ml min-1 (p less than 0.05). When they were corrected for muscle weight, the mean blood flow values at these same pouch pressures were 0.28 +/- 0.03, 0.15 +/- 0.03, and 0.05 +/- 0.01 ml min-1 g-1, respectively (p less than 0.05). SMV output was measured for each pouch pressure that was tested. Pouch output, expressed as ml min-1, was 458 +/- 20 (1 SEM) at an SMV diastolic pouch pressure of 10 mmHg, 309 +/- 22 at a pouch pressure of 40 mmHg, and 103 +/- 6 at a pouch pressure of 70 mmHg.(ABSTRACT TRUNCATED AT 250 WORDS)

Measuring heart muscle and nerve potentials with slow-response galvanometers.

The use of slow-response galvanometers to record the time course of heart muscle and nerve action potentials in the mid-1800s is discussed. The method that was used to chart these bioelectric events, which was developed by E. Lenz in 1854, is described. Also described is the rheotome, the device that allowed both the stimulation and the subsequent sampling of nerve and heart muscle action potentials.

Biomedical engineering: Baylor Medical College-preparing for the space age, 1956-1974.

The establishment of a six-week summer program entitled ;Classical Physiology with Modern Instrumentation', and its contents are described. The program was designed to train physicians and basic life scientists in the fundamentals of electronics and to train engineers of all specialities in basic medical physiology. During the period 1957-1974, over 1000 trainees from all over the world completed the course.