Predicting the relative efficacy of shock waveforms for transthoracic defibrillation in dogs.

STUDY OBJECTIVE: Previous work has shown that a passive membrane model using a parallel resistor-capacitor circuit is capable of predicting optimal waveforms for transvenous defibrillation. This study tested the ability of that model to predict optimal waveforms for transthoracic defibrillation. METHODS: This study was divided into 3 parts, each of which determined transthoracic defibrillation thresholds (DFTs) in 6 dogs for several different waveform shapes and durations. For each part, strength-duration relationships were determined from both experimental and model data and then compared with test model predictions. Part 1 DFTs were determined at various durations for 3 different monophasic waveforms-the ascending ramp, descending ramp, and square waveform. Part 2 DFTs were determined for 3 biphasic waveforms. Phase 1 was a 30-ms ascending ramp, and phase 2 was an ascending ramp, a descending ramp, or a square waveform. Part 3 DFTs were determined for 3 biphasic waveforms with very short second-phase durations. Phase 1 was a 30-ms ascending ramp, and phase 2 was a descending ramp. RESULTS: For part 1, the model was able to predict the relative defibrillation efficacy of the 3 monophasic waveforms ( P < .05). For parts 2 and 3, the model was able to predict the biphasic waveforms with the lowest DFTs. These predictions were based on the criterion that the model response at the end of the second phase should return to or slightly pass the model response value at the beginning of the first phase. CONCLUSION: The resistor-capacitor model successfully predicted the relative defibrillation efficacy of several different waveforms delivered transthoracically.

Transmembrane potential changes caused by monophasic and biphasic shocks.

Transmembrane potential change (DeltaVm) during shocks was recorded by a double-barrel microelectrode in 12 isolated guinea pig papillary muscles. After 10 S1 stimuli, square-wave S2 shocks of both polarities were given consisting of 10-ms monophasic and 10/10-ms and 5/5-ms biphasic waveforms that created potential gradients from 1.1 +/- 0.3 to 11.9 +/- 0.4 V/cm. S2 shocks were applied with 30, 60- to 70-, and 90- to 130-ms S1-S2 coupling intervals so that they occurred during the plateau, late portion of the plateau, and phase 3 of the action potential, respectively. Some shocks were given across as well as along the fiber orientation. The shocks caused hyperpolarization with one polarity and depolarization with the opposite polarity. The ratio of the magnitude of hyperpolarization to that of depolarization at the three S1-S2 coupling intervals was 1.5 +/- 0.3, 1.1 +/- 0.2, and 0.5 +/- 0.2, respectively. DeltaVm during the shock was significantly greater for the monophasic than for the two biphasic shocks. The prolongation of total repolarizing time (TRT) was significantly greater for monophasic (119.8 +/- 19.1%) and 10/10-ms biphasic (120.5 +/- 18.2%) than for 5/5-ms biphasic (113.0 +/- 12.9%) waveforms. The dispersion of the normalized TRT between instances of hyperpolarization and depolarization caused by the two shock polarities was 7.4 +/- 7.1% for monophasic, 3.0 +/- 4.1% for 10/10-ms biphasic, and 2.8 +/- 3.1% for 5/5-ms biphasic shocks (P < 0.05 for monophasic vs. biphasic). Shock fields along fibers produced a larger DeltaVm and prolongation of TRT than those across fibers. We conclude that 1) a change in shock polarity causes an asymmetrical change in membrane polarization depending on shock timing; 2) the 5/5-ms biphasic waveform causes the smallest DeltaVm, prolongs repolarization the least, and causes the smallest polarity-dependent dispersion; and 3) the changes in transmembrane potential and repolarization are influenced by fiber orientation.

Conversion of the beckman liquid phase sequencer to a gas-liquid phase sequencer.

As an effective aid to extend the microsequencing capabilities the Beckman protein/peptide sequenator Series 890C has been successfully converted to a gas-liquid system, in which coupling buffer 25% trimethylamine was employed as a gas, and heptafluorobutyric acid as a liquid. The system has been found to be efficient for microsequencing (less than 100 pmol). The details of mechanical, plumbing, and other minor changes are described in this paper along with the results of sequencing proteins and peptides, directly and from blots.