Search PubMed⌕ Search

Biomedical subjects

L A Geddes

Publications and source records attributed to L A Geddes.

At least 145 records · Page 8Linked to original sources
PubMed1980

Therapeutic indices for transchest defibrillator shocks: effective, damaging, and lethal electrical doses.

Although prospective studies of defibrillator shock overdose cannot be performed in man, the therapeutic indices of various defibrillating current waveforms can be measured in animals. We determined the ratios TD50/ED50 and LD50/ED50 (where TD50 = median "toxic" or damage-inducing dose, ED50 = median effective or defibrillating dose, and LD50 = median lethal dose) as measures of the therapeutic index for damped sine wave defibrillator shocks in dogs. Death of an animal and/or any degree of cardiac damage found by gross or microscopic examination were defined as harmful effects of shock, analogous to drug toxicity. In terms of peak current, the ED50, TD50, and LD50 were 1.1, 5.8, and 24 amperes/kg.; the therapeutic indices were TD50/ED50 = 5 for morphologic damage and LD50/ED50 = 22 for death. In terms of delivered energy the ED50, TD50, and LD50 were 1.5, 30, and 470 joules/kg.; the therapeutic indices were TD50/ED50 = 20 for damage and LD50/ED50 = 320 for death. These data indicate a reasonable margin of safety for damped sine wave defibrillator shocks in dogs, and are consistent with reported incidences of suspected shock-induced damage in humans.

Animals↗
PubMed1980

Dependence of defibrillation threshold upon extracellular/intracellular K+ concentrations.

The effect of increasing extracellular potassium concentration (Ko) upon electrical ventricular defibrillation threshold was investigated in pentobarbital anesthetized dogs treated with intravenous potassium chloride. Defibrillation threshold fell duirng potassium intoxication. The percent decrease in defibrillation threshold was linearly related to the logarithm of Ko and to the potassium equilibrium potential (Ek), calculated from measured extracellular and intracellular potassium concentrations of ventricular muscle. In dogs supported by left ventricular bypass in order to maintain the circulation during potassium intoxication, the values of Ko and Ek required for spontaneous, K+ induced defibrillation (electrical defibrillation threshold = zero) were 16.6 mEq/L and -46 mV compared to the normal values of 3.9 mEq/L and -84 mV. Changes in defibrillation threshold related to changes in Ek may be significant events in digitalis intoxication and in myocardial anoxia during prolonged fibrillation.

Animals↗
PubMed1980

Bipolar catheter defibrillation in dogs using trapezoidal waveforms of various tilts.

The choice of defibrillating waveform is critical in determining the size, battery life, and effectiveness of an automatic implantable defibrillator (AID). The trapezoidal (truncated exponential) waveform is well suited for the AID and its use can be optimized by the selection of appropriate values of pulse duration and tilt. The purpose of this study was to determine the dependence of the threshold peak current (the minimum peak current necessary to defibrillate the ventricles) on pulse duration and tilt for a bipolar catheter electrode configuration. Successive fibrillation-defibrillation trials were performed in 30 dogs anesthetized with sodium pentobarbital (30 mg/kg). The defibrillating pulse was applied via a bipolar-electrode catheter positioned such that the electrodes were located in the right ventricle at the apex and in the superior vena cava. The threshold peak current was determined in each dog for trapezoidal waveforms with 80%, 65%, 50%, and less than 5% tilt and with pulse durations of 2, 5, 10, 15, and 20 milliseconds. From a total of 600 threshold peak-current values, a strength-duration curve was derived for each value of tilt. The threshold peak current dose (peak current divided by body weight) increased with increasing tilt and decreasing duration. The threshold average current dose (average current over the duration of the defibrillating pulse divided by body weight) was IAV = 0.26 + 0.47/d, where d is the pulse duration in milliseconds and IAV is the average current in amperes per kilogram. If catheter apparent impedance is known, the minimum capacitance and output voltage necessary for defibrillation can be inferred from the strength-duration curves. From these data one can quantitatively assess the effect of trapezoidal waveform shape on the design criteria for the AID.

Animals↗
PubMed1980

INfluence of interposed ventilation pressure upon artificial cardiac output during cardiopulmonary resuscitation in dogs.

This study was conducted to determine the effects of high pressure interposed ventilations during cardiopulmonary resuscitation (CPR). Cardiac output was measured by a modified indicator dilution technique in eight anesthetized, intubated mongrel dogs. Positive pressure ventilations (12/min, 80% O2) were interposed after every five chest compressions (performed at 62/min) by a mechanical chest compressor (Thumper). On repeated trials in the same animal, ventilation pressures from 10--50 cm of H2O were tested in randomized sequence, while the technique of chest compression was held constant. Arterial blood gases immediately after resuscitation were monitored. Increasing ventilation pressure had surprisingly little effect on cardiac output curing CPR, although blood gases were profoundly altered. For ventilation pressures of 10, 20, 30, 40, and 50 cm of H2O, producing mean tidal volumes 23, 38, 61, 83, and 94 ml/kg; cardiac output remained nearly constant, averaging 21, 25, 23, 26, and 24 ml/min . kg. Corresponding mean postresuscitation pH was 7.24, 7.41, 7.51, 7.56, and 7.53; PCO2 was 41, 26, 18, 16, and 15 torr. The postresuscitation arterial oxygen tension was greater than 100 torr at all ventilation pressures except 10 cm of H2O. Interposed ventilations of pressure and volume more than adequate to prevent acidosis during CPR did not impair artificial cardiac output. If anything, cardiac output was slightly improved by more forceful ventilation.

Animals↗
PubMed1980

A new technique for repeated measurement of cardiac output during cardiopulmonary resuscitation.

The authors have developed a method for measurement of cardiac output during CPR with ventricular fibrillation. The method avoids the problems encountered when conventional techniques are used under the conditions of very low cardiac output. The method consists of injecting 5% saline as the indicator into the left ventricular and detecting its appearance in the descending aorta by withdrawing aortic blood through an electrically calibrated conductivity cell. The adequacy of indicator mixing has been verified by obtaining dilutions curves simultaneously from the brachial and femoral arteries. Cardiac output can be determined even when output is as low as 7 ml/min . kg during CPR with ventricular fibrillation. Repeated determinations can be made as often as every min. This method offers promise as a practical research tool which can also be used with dye indicators.

Cardiac Output↗
PubMed1980

The effect of newer antiarrhythmic drugs on defibrillation threshold.

This study was conducted to determine the effects of clofilium phosphate and bretylium tosylate on ventricular defibrillation threshold. Dogs were anesthetized with pentobarbital and subjected to repeated fibrillation-defibrillation episodes. Defibrillation thresholds were determined at 15-min intervals, using underdamped 5--6 msec sinusoidal current shocks, from 30 min before drug injection to 120 min after injection. Eight dogs were given clofilium phosphate (0.34 mg/kg, iv). Another 10 dogs were given bretylium tosylate (10.0 mg/kg, iv). Both drugs lowered defibrillation threshold from 15--90 min after injection. The maximum clofilium effect was a 31% decrease in threshold current and a 54% decrease in threshold energy. The greatest decrease in defibrillation threshold produced by bretylium was 16% for current and 31% for energy. These drug induced changes in defibrillation threshold are of potential clinical benefit if they occur in human subjects at doses which are effective for control of ventricular arrhythmias.

Animals↗
PubMed1980

The meaning of the point of maximum oscillations in cuff pressure in the indirect measurement of blood pressure--part ii.

When measuring blood pressure indirectly, oscillations in the cuff pressure are observed. The cuff pressure for which these oscillations reach a maximum and its relationship to the true mean arterial pressure was investigated using a simple one-dimensional theoretical model of the cuff-arm-artery system. Results from this model indicate that the cuff pressure for maximal oscillation is strongly dependent on compression chanber air volume, pulse pressure, and arterial elasticity. Parallel experimental studies indicate general agreement with the theoretical model. The cuff pressure for maximal oscillations appears to provide a reasonable estimation of the true mean arterial pressure provided compression chamber air volume is kept small.

Animals↗
PubMed1980

Energy and current requirements for ventricular defibrillation using trapezoidal waves.

The threshold energy and current required for ventricular defibrillation was determined in dogs ranging in weight from 6.4 to 38 kg and in ponies ranging in weight from 40 to 101 kg. Trapezoidal waves, 10 ms in duration, with 10%, 50%, 70%, and 90% tilt were applied to transchest electrodes. For all values of tilt, the energy and current required increased with body weight. The energy dose (joules per kilogram of body weight) was higher for the heavier animals, whereas the current dose (peak amperes per kilogram of body weight) was essentially the same for dogs and ponies. In both species and for all four waveforms, the percent success increased with increasing energy and current dose. For all four waveforms and for both species, the threshold average current required for defibrillation was between 0.38 and 0.48 A/kg, indicating that average current may be a convenient unit to compare the efficacy of different waveforms.

Animals↗
PubMed1980

Indirect mean arterial pressure in the anesthetized dog.

Using the oscillometric method, indirect mean arterial pressure was measured in 14 pentobarbital-anesthetized dogs (8-24 kg) with mean arterial pressure ranging from 23 to 160 mmHg. The effect of cuff size, in relation to forelimb circumference was investigated. Indirect mean arterial pressure agreed with direct mean arterial pressure when the cuff width was about 40% of the forelimb circumference.

Anesthesia↗
PubMed1979

Elevation of ventricular defibrillation threshold in dogs by antiarrhythmic drugs.

Effects of antiarrhythmic drugs upon the threshold delivered energy (TDE) and threshold peak current (TPC) for electrical ventricular defibrillation by damped sinusoidal shocks were investigated in 25 pentobarbital-anesthetized dogs. TDE and TPC were increased by the three antiarrhytmic drugs tested. Bolus injections produced a transient rise and continuous infusions produced a steady rise in difibrillation threshold. The maximal percent elevations in mean defibrillation threshold during the 60 minutes after intravenous drug treatment in groups of n = 5 dogs were: (formula: see text). Accordingly, individuals receiving antiarrhythmic drugs who nonetheless fibrillate may require greater electric shock strength for defibrillation.

Animals↗