Effect of pentobarbital anesthesia on ventricular defibrillation threshold in dogs

The effect of pentobarbital anesthesia upon the minimal voltage and current required for electrical ventricular defibrillation (the defibrillation threshold) was investigated in dogs. Threshold current, energy, and charge in five dogs averaged 2 per cent, 13 per cent, and 6 per cent less under surgical levels of pentobarbital anesthesia than thresholds in the same animals in the awake, unanesthetized state. In dogs given sufficient pentobarbital to produce apnea and supported by mechanical ventilation, threshold current, energy, and charged averaged 3 per cent, 17 percent, and 2 per cent less than comparable awake values. These differences were far from statistically significant. In a second study, five dogs were kept for 8 to 10 hours at a surgical level of anesthesia with pentobarbital sodium. Defibrillation threshold current, determined at hourly intervals, did not drift outside +/-10 per cent limits. Arterial blood gas measurements revealed a stable, compensated metabolic acidosis in all animals (pH 7.36 +/- 0.06, pCO2 33 +/- 4 mm. Hg, pO2 71 +/- 9 mm. Hg). These data support the validity of defibrillation studies using animals anesthetized with pentobarbital and indicate the stability of the defibrillation threshold under controlled experimental conditions.     

Irradiation-hyperthermia in canine hemangiopericytomas: large-animal model for therapeutic response

Results of irradiation-hyperthermia treatment in 11 dogs with naturally occurring hemangiopericytoma were reported. Similarities of canine and human hemangiopericytomas were described. Orthovoltage X-irradiation followed by microwave-induced hyperthermia resulted in a 91% objective response rate. A statistical procedure was given to evaluate quantitatively the clinical behavior of locally invasive, nonmetastatic tumors in dogs that were undergoing therapy for control of local disease. The procedure used a small sample size and demonstrated distribution of the data on a scaled response as well as transformation of the data through classical parametric and nonparametric statistical methods. These statistical methods set confidence limits on the population mean and placed tolerance limits on a population percentage. Application of the statistical methods to human and animal clinical trials was apparent.     

Fundamental criteria underlying the efficacy and safety of defibrillating current waveforms

The efficacy (threshold average current density) and safety factor (overdose current density for a 50 per cent decrease in myocardial contractility) were determined for the rectangular, trapezoidal and damped sine wave defibrillating current waveforms using a new blood-perfused, isolated, working canine heart in an isoresistive and isotonic volume conductor. This preparation permits the achievement of defibrillation with the heart in as uniform a current density field as possible and eliminates myocardial deterioration due to the traditional loss of coronary perfusion during ventricular fibrillation. Since the heart is isolated, it is not subject to any neural influences. It was found that, for a given pulse duration, the threshold average current density (efficacy) values were very similar among the three current waveforms. It was also found that the safety factors for the three current waveforms ranged from 3·2 to 5·3, the conventional 4–6 ms damped sine wave having a safety factor of 5·0±0·9 (1SD).     

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.     

A dose-response curve for the negative bias pressure of an intrathoracic pressure regulator during CPR

An intrathoracic pressure regulator (ITPR) is a device that can be added to the external end of a tracheal tube to create controlled negative airway pressure between positive pressure ventilations. The resulting downward bias of the airway pressure baseline promotes increased venous return and enhanced circulation during CPR and also during hypovolemic shock. In the present study, we exercised a mathematical model of the human cardiopulmonary system, including airways, lungs, a four chambered heart, great vessels, peripheral vascular beds, and the biomechanics of chest compression and recoil, to determine the relationship between systemic perfusion pressure during CPR and the value of baseline negative airway pressure in an ITPR. Perfusion pressure increases approximately 50% as baseline airway pressure falls from zero to -10 cm H2O. Thereafter perfusion pressure plateaus. Negative bias pressures exceeding -10 cm H2O are not needed in ITPR-CPR.