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M D Ewy

Publications and source records attributed to M D Ewy.

6 recordsLinked to original sources
PubMed1977

Effectiveness of direct current defibrillation: role of paddle electrode size.

Myocardial necrosis from repeated direct current defibrillation discharges is less when the same stored energy is delivered by paddle electrodes that are larger than those presently available on the majority of commercial defibrillators. The present study was undertaken to determine if the larger 12.8 cm. diameter paddle electrodes are as effective as the standard 8.0 cm. diameter paddle electrodes in defibrillation. Ventricular fibrillation (VF) was induced in 45 dogs and each was allowed to remain in ventricular fibrillation for progressively longer time intervals before defibrillation was attempted. With the 12.8 cm. diameter paddle electrodes, the longest duration of VF sucessfully terminated was 1.22+/-1.05 minutes when the 8.0 cm. paddle electrodes were used (p less than 0.02). Ventricular fibrillation was terminated during the first minute with the 12.8 cm. diameter electrode in 88 per cent of trials as compared with a 71 per cent success rate with 8.0 cm. diameter paddle electrodes (p less than 0.04). When the success rates during the first minute of VF for both sizes of paddle electrodes were ploted against the measure transthoracic impedance, a high correlation cofficient (r=-0.94) was found. This study suggest that 12.8 cm. diameter paddle electrodes are more effective for defibrillation of subjects in the 13.5 kilogram (29 to 69 pound) weight range than are paddle electrodes that are only 8.0 cm. in diameter.

Animals↗
PubMedDate not supplied

Transthoracic impedance to direct current discharge: effect of repeated countershocks.

The effect of repeated countershocks on transthoracic apparent impedance to direct current (dc) defibrillator discharges was studied. Repeated dc countershocks result in a progressive decrease in transthoracic apparent impedance that is dependent upon the time interval between countershocks. This decrease was significantly greater in the group of animals shocked at 3-min intervals compared to the groups shocked at 15-sec intervals (P less than 0.001) or at 1-min intervals (P less than 0.005). Since lowered impedance results in higher delivered current for the same energy setting on a defibrillator, this observation may help to explain the enhanced effectiveness of repeated countershocks in defibrillation. Plots of simultaneous current against voltage during transthoracic dc discharge revealed that the current lagged slightly behind voltage during the rising phase of the recording, but that current and voltage were nearly simultaneous during the falling phase. This effect appears to be similar to an ionization phenomenon in that the effective impedance asymptotically approaches a lower value with increasingly applied voltage. This might explain why transthoracic impedance is highest at low energy countershocks and decreases with higher energy countershock.

Animals↗
PubMedDate not supplied

Disposable defibrillator electrodes.

The transthoracic impedance to direct-current defibrillation discharge of the half-sinusoidal waveform was compared using recently marketed disposable defibrillator electrode pads (SAF-D-FIB and DEFIB-PADS) with electrode paste as the interfaces between the defibrillator paddle electrode and chest wall. Twenty-four mongrel dogs with an average weight of 17.3 kilograms were used. Half were shocked with the defibrillator meter setting at 100 watt-seconds (mean delivered energy, 59 watt-seconds) and half at 400 watt-seconds (mean delivered energy, 205 watt-seconds). Each animal received six shocks with both paste and one of the sets of disposable pads. The sequence of shocks was changed in alternate animals. At a meter setting of 100 watt-seconds, the mean impedance using SAF-D-FIB was 59 +/- 6 ohms compared to 46 +/- 6 ohms with paste (p less than 0.001), while that encountered with DEFIB-PADS was 57 +/- 5 ohms compared to 50 +/- 5 ohms with paste (p less than 0.01). At settings of 400 watt-seconds, the impedances encountered were also significantly higher with the disposable electrode (p less than 0.01). The output of many defibrillators in use today is inadequate for consistent defibrillation of adult patients weighing more than 50 to 80 kilograms. Since a minimal peak current per unit of body weight is required for ventricular defibrillation and since a higher transthoracic impedance results in a lower delivered peak current, one should use the paddle electrode-chest wall interface that results in the lowest impedance to defibrillator discharge. The impedance encountered with disposable electrodes is significantly higher than that encountered with electrode paste. Therefore, we do not recommend the use of these disposable electrodes for defibrillation or elective cardioversion.

Animals↗