Babbs, Charles F

Geddes, LA

Whistler, SJ

English

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Purdue UniversityPurdue e-PubsWeldon School of Biomedical Engineering FacultyPublications Weldon School of Biomedical Engineering1980Evaluation of the operating internal resistance andcapacitance of intact trapezoidal waveformdefibrillatorsCharles F. BabbsPurdue University, babbs@purdue.eduSJ WhistlerLA GeddesFollow this and additional works at: http://docs.lib.purdue.edu/bmepubsPart of the Biomedical Engineering and Bioengineering CommonsThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Recommended CitationBabbs, Charles F.; Whistler, SJ; and Geddes, LA, "Evaluation of the operating internal resistance and capacitance of intact trapezoidalwaveform defibrillators" (1980). Weldon School of Biomedical Engineering Faculty Publications. Paper 94.http://docs.lib.purdue.edu/bmepubs/941   Clinical Device Note    Evaluation of the operating internal resistance and capacitance of intact trapezoidal waveform defibrillators   C. F. Babbs, M.D., Ph.D.  S. J. Whistler, B. S.  L.A. Geddes, M. E., Ph.D.   Biomedical Engineering Center, Purdue University, West Lafayette, Indiana, USA.  (MEDICAL INSTRUMENTATION, Vol.14, No.1, 1980, pp. 67-68)    The technique described permits determining values that may be useful in preventive maintenance programs.  Key words: defibrillation, ventricular fibrillation, waveform.   We have previously described a simple method for determining the values of internal resistance, inductance, and capacitance of a damped sine wave defibrillator solely from measurements of the output waveform using two or more power resistors and a storage oscilloscope (Babbs et al. 1978). Measurement of these values may be useful in preventive maintenance, since a change might indicate equipment aging or impending failure. This clinical device note presents a similar method applicable to trapezoidal waveform defibrillators.  The output circuit of a trapezoidal waveform defibrillator can be represented by a capacitor C, a series internal resistance Rs, a parallel internal resistance Rp, the subject or load resistance RL, and switching  elements S1, and S2 (Fig. 1). The parallel internal resistance Rp is large with respect to the load and is included to ensure proper operation of the switching elements. To initiate the defibrillating current pulse, the charged capacitor is switched so that it discharges through the internal resistances and the load. After a predetermined duration, d, the exponential discharge is arrested by a short-circuiting switch S2, in parallel with the subject. 2     Fig.1. Trapezoidal waveform defibrillator output circuit.   If the initial current value is  ii  and  the final nonzero current value is if, then  CRdexpiieqif ;  where equivalent resistance = LpLpseqRRRRRR .  Rearrangement of this expression gives the time constant,  , of the circuit:   fieqi/ilndCR  .  This expression  indicates that if  the ratio  of  initial to final current and the pulse duration are measured on a storage oscilloscope, then a plot of the calculated variable  d/ln(ii/if)  as a function of Req will yield a straight line with slope equal to the capacitance, 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 output voltage and is easily determined from the recorded waveform. 3   It is necessary, however, to know the values of the internal series and parallel resistances, Rs, and Rp, to calculate Req . These may be determined by investigating the relationship of the calculated time constant of the pulse for the open-circuit condition ( for RL = ) to the calculated time constant of the pulse for various measured loads. In particular,  LpLpspseqpsRRRRRRRRRR .  Simplifying and rearranging, one obtains the expression  L2ppsps RRRRRR,  which is a linear function of the load resistance, RL.   Therefore, if the defibrillator is first discharged into the open air to obtain  and then discharged into differing known resistive loads, a plot of the calculated value  / (  ) as a function of RL will be a straight line with slope (Rs + Rp)/(Rp)2 and intercept Rs/Rp. Then, solving for Rs, and Rp in terms of the slope and intercept, one may obtain  Rs = intercept(1 + intercept)/ slope,  and  Rp = (1 + intercept)/slope.   These are the values of series and parallel internal resistances of the defibrillator shown in Fig. 1.  To find the capacitance, it is merely necessary to calculate the equivalent resistance,  LpLpseqRRRRRR ,  for each load and determine the capacitance as C = ·Req , the best estimate of C being given as the slope of the  vs. Req graph for several values of load resistance.  4        Fig. 2. Top: Plot of time constant ratio vs. load resistance. Intercept = 7 x 10-3 ;  slope= 2.02 x 10-3 -1. Bottom: Plot of time constant vs. equivalent resistance.  Slope= 19.2 msec/100 .  5   We applied this analysis to a low-energy trapezoidal waveform defibrillator in our laboratory. The defibrillator was discharged in the open-circuit mode and into known resistive loads from 15 to 100 , while the initial amplitude, final amplitude, and duration of each pulse were recorded using a storage oscilloscope. The slopes and intercepts of the time constant plots (Fig. 2) were determined by least-squares linear regression. Using the conversion factor, 1 sec = 1 Farad x 1 , the calculated values of capacitance and parallel internal resistance were 192 F and 498 . Corresponding rated values for these components were 200 F and 500 . The calculated series internal resistance was 3.5 .  As the use of trapezoidal waveform defibrillators becomes more widespread, this technique may become increasingly useful to clinical engineers in establishing preventive maintenance pro- grams.   Reference  Babbs, C. F., and S. J. Whistler, 1978. Evaluation of the operating internal resistance,  inductance, and capacitance of intact damped sine wave defibrillators. Med. Instrum. 12: 34-37.    Graphical Summary   6        

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

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

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