Evaluation of the operating internal resistance and capacitance of intact trapezoidal waveform defibrillators

Transcription

1 Purdue University Purdue e-pubs Weldon School of Biomedical Engineering Faculty Publications Weldon School of Biomedical Engineering 1980 Evaluation of the oerating internal resistance and caacitance of intact traezoidal waveform defibrillators Charles F. Babbs Purdue University, SJ Whistler A Geddes Follow this and additional works at: htt://docs.lib.urdue.edu/bmeubs Part of the Biomedical Engineering and Bioengineering Commons ecommended Citation Babbs, Charles F.; Whistler, SJ; and Geddes, A, "Evaluation of the oerating internal resistance and caacitance of intact traezoidal waveform defibrillators" (1980). Weldon School of Biomedical Engineering Faculty Publications. Paer 94. htt://docs.lib.urdue.edu/bmeubs/94 This document has been made available through Purdue e-pubs, a service of the Purdue University ibraries. Please contact eubs@urdue.edu for additional information.

2 Clinical Device Note Evaluation of the oerating internal resistance and caacitance of intact traezoidal waveform defibrillators C. F. Babbs, M.D., Ph.D. S. J. Whistler, B. S..A. Geddes, M. E., Ph.D. Biomedical Engineering Center, Purdue University, West afayette, Indiana, USA. (MEDICA INSTUMENTATION, Vol.14, No.1, 1980, ) The technique described ermits determining values that may be useful in reventive maintenance rograms. Key words: defibrillation, ventricular fibrillation, waveform. We have reviously described a simle method for determining the values of internal resistance, inductance, and caacitance of a damed sine wave defibrillator solely from measurements of the outut waveform using two or more ower resistors and a storage oscilloscoe (Babbs et al. 1978). Measurement of these values may be useful in reventive maintenance, since a change might indicate equiment aging or imending failure. This clinical device note resents a similar method alicable to traezoidal waveform defibrillators. The outut circuit of a traezoidal waveform defibrillator can be reresented by a caacitor C, a series internal resistance s, a arallel internal resistance, the subject or load resistance, and switching elements S 1, and S 2 (Fig. 1). The arallel internal resistance is large with resect to the load and is included to ensure roer oeration of the switching elements. To initiate the defibrillating current ulse, the charged caacitor is switched so that it discharges through the internal resistances and the load. After a redetermined duration, d, the exonential discharge is arrested by a short-circuiting switch S 2, in arallel with the subject. 1

3 Fig.1. Traezoidal waveform defibrillator outut circuit. If the initial current value is i i and the final nonzero current value is i f, then d i f ii ex ; eqc where equivalent resistance = eq s. earrangement of this exression gives the time constant,, of the circuit: d eqc. ln i / i i f This exression indicates that if the ratio of initial to final current and the ulse duration are measured on a storage oscilloscoe, then a lot of the calculated variable d/ln(i i /i f ) as a function of eq will yield a straight line with sloe equal to the caacitance, C, in the defibrillator. For any resistive load, the ratio of initial to final current is equal to the ratio of the initial to final outut voltage and is easily determined from the recorded waveform. 2

4 It is necessary, however, to know the values of the internal series and arallel resistances, s, and, to calculate eq. These may be determined by investigating the relationshi of the calculated time constant of the ulse for the oen-circuit condition ( for = ) to the calculated time constant of the ulse for various measured loads. In articular, s s. eq s Simlifying and rearranging, one obtains the exression s s 2, which is a linear function of the load resistance,. Therefore, if the defibrillator is first discharged into the oen air to obtain and then discharged into differing known resistive loads, a lot of the calculated value / ( ) as a function of will be a straight line with sloe ( s + )/( ) 2 and intercet s /. Then, solving for s, and in terms of the sloe and intercet, one may obtain s = intercet(1 + intercet)/ sloe, and = (1 + intercet)/sloe. These are the values of series and arallel internal resistances of the defibrillator shown in Fig. 1. To find the caacitance, it is merely necessary to calculate the equivalent resistance, eq s, for each load and determine the caacitance as C = eq, the best estimate of C being given as the sloe of the vs. eq grah for several values of load resistance. 3

5 Fig. 2. To: Plot of time constant ratio vs. load resistance. Intercet = 7 x 10-3 ; sloe= 2.02 x Bottom: Plot of time constant vs. equivalent resistance. Sloe= 19.2 msec/100. 4

6 We alied this analysis to a low-energy traezoidal waveform defibrillator in our laboratory. The defibrillator was discharged in the oen-circuit mode and into known resistive loads from 15 to 100, while the initial amlitude, final amlitude, and duration of each ulse were recorded using a storage oscilloscoe. The sloes and intercets of the time constant lots (Fig. 2) were determined by least-squares linear regression. Using the conversion factor, 1 sec = 1 Farad x 1, the calculated values of caacitance and arallel internal resistance were 192 F and 498. Corresonding rated values for these comonents were 200 F and 500. The calculated series internal resistance was 3.5. As the use of traezoidal waveform defibrillators becomes more widesread, this technique may become increasingly useful to clinical engineers in establishing reventive maintenance rograms. eference Babbs, C. F., and S. J. Whistler, Evaluation of the oerating internal resistance, inductance, and caacitance of intact damed sine wave defibrillators. Med. Instrum. 12: Grahical Summary 5

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