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Biomedical subjects

S J Whistler

Publications and source records attributed to S J Whistler.

10 recordsLinked to original sources
PubMed1986

A hierarchical approach to the treatment of ventricular tachycardias.

Electrical management of ventricular tachycardias with an implanted device is greatly complicated by reduced hemodynamic tolerance to the tachycardia with increasing rate, and by the risk of accelerating the tachycardia into fibrillation. Pacing (extrastimuli, bursts), low-energy cardioversion, and high-energy defibrillation therapies are all useful in safely treating a ventricular tachycardia. A implantable device is described which allows the classification of up to four different arrhythmias each having their own detection criteria (including high rate, sudden onset, rate stability and sustained high rate). For each classified arrhythmia, up to four therapies may be programmed to be sequentially delivered. This scheme allows for increasing on the aggressiveness of the therapy based on elapsed time, increased rate, or both; and provides a large amount of flexibility for tailoring the desired therapies to individual patients.

Electric Countershock↗
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

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↗
PubMed1979

Indirect measurement of mean blood pressure in the normotensive and hypotensive horse.

Indirect mean arterial pressure was obtained from the tails of 78 anaesthetised and unanaesthetised normotensive and hypotensive horses. Data were obtained to determine the optimum cuff width in relation to tail circumference. A cuff that is too wide under-estimates and a cuff that is too narrow over-estimates blood pressure. With the optimum cuff width, which is slightly in excess of about one-fifth of the tail circumference, indirect mean blood pressure was obtained consistently in normotensive and hypotensive animals with mean blood pressures of 40 mmHg and frequently in animals with pressure as low as 30 mmHg.

Animals↗
PubMed1978

Temporal stability and precision of ventricular defibrillation threshold data.

Over 200 measurements of the minimum damped sinusoidal current and energy for transchest electrical ventricular defibrillation (ventricular defibrillation threshold) were made to determine the stability and precision of threshold data in 15 pentobarbital-anesthetized dogs. Threshold was determined by repeated trials of fibrillation and defibrillation with successive shocks of diminishing current, each 10% less than that of the preceding shock. The lowest shock intensity that defibrillated was defined as threshold. In three groups of five dogs each, threshold was measured at intervals of 60, 15, and 5 min over periods of 8, 5, and 1 h, respectively. Similar results were obtained for all groups. There was no significant change in mean threshold current with time. Owing to a decrease in transchest impedance, threshold delivered energy decreased by 10% during the first hour of testing. The standard deviations for threshold peak current and delivered energy in a given animal were 11% and 22% of their respective mean values. Arterial blood pH, Pco2, and Po2 averaged change of pH, PCO2 and PO2 were not significantly different from zero. The data demonstrate that ventricular defibrillation threshold is a stable physiological parameter that may be measured with reasonable precision.

Animals↗
PubMedDate not supplied

Evaluation of the operating internal resistance, inductance, and capacitance of intact damped sine wave defibrillators.

A method is developed for determing actual values of circuit elements in a damped sine wave (Lown waveform) defibrillator solely from measurements of the output, using two or more power resistors and a storage oscilloscope. If a defibrillator containing capacitance C, inductance L, and internal resistance RI, is discharged into increasing 5- to 100-ohm resistive loads R, it is shown for underdamped output waveforms that â = RI/2L + R/2L and ĉ = CRI + CR, where â = pi[t2tan(pit1/t2)], ĉ = 2â/[â2 + (pi/t2)2], t1 = time from onset to peak, and t2 = time from onset to first zero crossing of the output waveform on the oscilloscope trace. Linear plots of â vs R are constructed for seven defibrillators, and values of RI and L computed as intercept/slope and 1/(2 slope) respectively. C is given by the slope of a linear plot of ĉ vs R. Delivered energy is accurately predicted as stored energy X R/(RI + R).

Electric Conductivity↗