771 Smaller Capacitors Improve the Biphasic Waveform KEVIN RIST, M.D., PH.D., PATRICK J. TCHOU, M.D., KENT MOWREY, M.S., MARK W. KROLL, PH.D.,* and JAMES E. BREWER, M.S.* From the University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania, and *Angeion Corporation, Plymouth, Minnesota Capicitance for Biphasic Waveform. Introduction: Current implantable cardioverter defibrillators (ICDs) use relatively large capacitance values. Theoretical consideratioas suggest, however, that improved defibrillation energy requirements may be ohtained with smaller capacitance values. Methods and Results: We compared the energy requirement for defibrillation in a porcine model using a biphasic waveform generated from two capacitance values of 140 fiF and 85 fsF, Phase 1 reversal of the shock waveform occurred at 65% tilt. Phase 2 pulse width was equal to phase 1. Shocks were delivered through epicardial patch electrodes after IU seconds of induced ventricular fihrillation. The defihrillation threshold (DFT) was determined hy a "down-up" technique requiring three reversals of defibrillation success or failure. The DFT was defined as the average of the values ohtained with all trials starting from the successful sbock prior to the first failure to defibrillate to tbe last successful defibrillation. In eigbt experiments, tbe mea- sured parameters at DFT were as follows. The average stored and delivered DFT energies for the 85 /aF capacitor were 6.1 ± 2.1 and 6.0 ± 2.0 J, respectively, compared to 7.5 ± 1.3 and 7.4 ± 1.3 J for the 140 /iF capacitor (P < 0.04). The phase 1 pulse widths were significantly shorter for the 85 ^F capacitor (5.1 ± 0.8 msec vs 9.2 ± 1.3 msec) and the impedances were lower (54.4 ± 5.8 fi vs 59.9 ± 6.3 0). The mean leading edge voltage was trending higher for tbe 85 /iF capacitor, but tbis difference did not reacb statistical significance (374 ± 63 V vs 326 ± 30 V; P = 0.055). Conclusion: Smaller capacitance values do result in lower energy requirements for tbe bipbasic waveform, at a possibly higber leading edge voltage and a mucb sborter pulse widtb. Smaller capacitance values could represent a significant enhancement of well-established benefits demonstrated witb the bipbasic waveform. (J Cardiovasc Electrophysiol, Vol. 5, pp. 771- 776, September 1994) defibrillation, biphasic waveform, capacitance values Introduction Biphasic waveform defibrillation has been shown to be more effective than monophasic wavefonn deflbrillation in animals'- and humans,^'" and many studies have been done to delineate its prop- erties as a method for terminating ventricular fibril- lation (VF). Despite this, there is an increasing This project was supported by & research grant from Angeion Cor- poration, Plymouth, Minnesota. Address lor correspondence: Patrick J, Tchou, M.D., Head, Car- diac Electrophysiotogy, Department of Cardioiogy/F15, Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, OH 44195- Fax:216-445-9595. Manuscripl received 9 May 1994; Accepted for publication 17 August 1994. awareness that an optimal biphasic waveform, or an optimal means for its delivery, has not been determined. In fact, studies have been published on the effects of varying pulse durations and tilts in animals- "•'- and humans'^ for the biphasic wave- form. However, there is little published data describ- ing the optimal capacitance for the biphasic wave- form. Recently, theories have been developed sug- gesting that a smaller capacitance value can achieve a lower defibrillation threshold (DFT).'* '^ The the- ory proposes that an implantable cardioverter defibrillator (ICD) capacitor should deliver its charge within a time (as manifested by its sbock pulse width) closer to the chronaxie time for hean defibrillation. Present capacitance values of 140 to
Transcript
771
Smaller Capacitors Improve the Biphasic Waveform
KEVIN RIST, M.D., PH.D., PATRICK J. TCHOU, M.D., KENT MOWREY, M.S.,MARK W. KROLL, PH.D.,* and JAMES E. BREWER, M.S.*
From the University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania, and *Angeion Corporation,Plymouth, Minnesota
Capicitance for Biphasic Waveform. Introduction: Current implantable cardioverterdefibrillators (ICDs) use relatively large capacitance values. Theoretical consideratioas suggest,however, that improved defibrillation energy requirements may be ohtained with smallercapacitance values.
Methods and Results: We compared the energy requirement for defibrillation in a porcinemodel using a biphasic waveform generated from two capacitance values of 140 fiF and 85 fsF,Phase 1 reversal of the shock waveform occurred at 65% tilt. Phase 2 pulse width was equal tophase 1. Shocks were delivered through epicardial patch electrodes after IU seconds of inducedventricular fihrillation. The defihrillation threshold (DFT) was determined hy a "down-up"technique requiring three reversals of defibrillation success or failure. The DFT was defined asthe average of the values ohtained with all trials starting from the successful sbock prior to thefirst failure to defibrillate to tbe last successful defibrillation. In eigbt experiments, tbe mea-sured parameters at DFT were as follows. The average stored and delivered DFT energies forthe 85 /aF capacitor were 6.1 ± 2.1 and 6.0 ± 2.0 J, respectively, compared to 7.5 ± 1.3 and 7.4 ±1.3 J for the 140 /iF capacitor (P < 0.04). The phase 1 pulse widths were significantly shorterfor the 85 F capacitor (5.1 ± 0.8 msec vs 9.2 ± 1.3 msec) and the impedances were lower (54.4± 5.8 fi vs 59.9 ± 6.3 0) . The mean leading edge voltage was trending higher for tbe 85 /iFcapacitor, but tbis difference did not reacb statistical significance (374 ± 63 V vs 326 ± 30 V; P= 0.055).
Conclusion: Smaller capacitance values do result in lower energy requirements for tbebipbasic waveform, at a possibly higber leading edge voltage and a mucb sborter pulse widtb.Smaller capacitance values could represent a significant enhancement of well-establishedbenefits demonstrated witb the bipbasic waveform. (J Cardiovasc Electrophysiol, Vol. 5, pp. 771-776, September 1994)
Biphasic waveform defibrillation has been shownto be more effective than monophasic wavefonndeflbrillation in animals '- and humans,^'" andmany studies have been done to delineate its prop-erties as a method for terminating ventricular fibril-lation (VF). Despite this, there is an increasing
This project was supported by & research grant from Angeion Cor-poration, Plymouth, Minnesota.
Address lor correspondence: Patrick J, Tchou, M.D., Head, Car-diac Electrophysiotogy, Department of Cardioiogy/F15, ClevelandClinic Foundation, 9500 Euclid Avenue, Cleveland, OH 44195-Fax:216-445-9595.
Manuscripl received 9 May 1994; Accepted for publication 17August 1994.
awareness that an optimal biphasic waveform, oran optimal means for its delivery, has not beendetermined. In fact, studies have been publishedon the effects of varying pulse durations and tiltsin animals- "•'- and humans' for the biphasic wave-form. However, there is little published data describ-ing the optimal capacitance for the biphasic wave-form.
Recently, theories have been developed sug-gesting that a smaller capacitance value can achievea lower defibrillation threshold (DFT).'* ' The the-ory proposes that an implantable cardioverterdefibrillator (ICD) capacitor should deliver itscharge within a time (as manifested by its sbockpulse width) closer to the chronaxie time for heandefibrillation. Present capacitance values of 140 to
772 Journal of Cardiovascular Electrophysiology Vol. 5. No. 9, September 1994
180 ^F require 7 to 9 msec to deliver 65% of theircharge into a 50 Q load. These times aresigniiicantly greater than the 2 to 4 msec rangeof chronaxie times that have been found.'^ -"
TTie purpose of this study was to investigate theeffects of delivering a biphasic shock pulse withtwo capacitance values, one of which was muchsmaller than the conventional size of 140 fjF.TTie theoretically suggested optimal range of 32 to45 /iF' ' '" capacitance values is not practical todaybecause of high voltage. We therefore evaluatedan 85 /JF capacitance value as a compromisebetween the theoretical range and the conventionalICD values. We hypothesized that, in a porcinemodel, the smaller capacitance value better defibril-lates than does a larger value when using a 65%tilt, biphasic waveform.
Methods
Experimental Preparations and Protocol
Pigs weighing between 20 and 40 kg were anes-thetized with ketamine and xylazine and main-tained under anesthesia with inhaled isofluraiie andnitRius oxide. An intra-iirteriiil line was establishedin the femoral artery for continuous blood pres-sure monitoring as well as blood gas sampling. Alarge-bore intravenous line was also inserted via afemoral vein. Surface electrocardiograms weremonitored in three leads (I. IT. 111). A mid-line inci-sion was made through the sternum and the heiuiwas exposed. The pericardium was opened andformed into a cradle. Two titanium mesh patchelecUtides (CPI Model 0040 [Cardiac Pacemakers,Inc., St. Paul, MN, USA]) were attached to theepicurdium with sutures at the comers. One patchwas placed inferoposteriorly, while the other wasplaced anteriorly over the right ventricular outflowtract. Two cardiac plunge wires were inserted intothe lateral left ventricle for pacing. The electro-gram from the patch leads and the pacing leadswere continuously monitored on a physiologicalrecoixler (PPG Model VR12 |PPG Biomedical Sys-tems, Pleasantville, NY, USA]).
VF was initiated with modulated AC deliveredto the patch electrodes for 3 to 4 seconds. Fibril-lation was allowed to persist for 10 seconds fol-lowing initiation of AC current delivery (CPI ModelECD APU). A test dciibrillation shock was thengiven via a custom-built extemal detibrillator. Ifthe test shock failed to convert fibrillation, a res-cue shock of 20 J was given immediately via a
second external defibrillator. Initial testing estab-lished that such a shock could reliably convert VF.
The custom-built detibrillator was a micro-processor-controlled device capable of deliveringbiphasic. truncated-capacitative dischai'ges of vary-ing tilts (Angeion Corporation. Minneapolis,MN, USA). Two different capacitance values couldbe selected, one having 85 /iF capacitance and theother having 140 //F capacitance. The biphasicwaveforms used in this study have equal first andsecond phase durations. Reversal of phase wasaccomplished by inverting the waveform. The twophases were separated by 1 msec. The device mea-sured the stored capacitor energy, the deliveredenergy, the leading edge voltage, the voltage atphase inversion, the terminal voltage, the pulsewidth of the two phases, and the average imped-ance.
ElectRilytic capacitors have different capacitancevalues depending on the measurement voliage.Therefore, the capacitance viUues in this study weredetermined by chiirging each capacitor to 3(X), 5(X),and 700 V and discharging them into a 50 H non-inductive load. A capacitance value was calculatedfrom measured time constants as defined by thevoltage decay into the load. We found less than a2% difference between the mean charge and theextremes.
The use of experimental animals in this studywas approved by the Institutional Animal Ciire ;uidUse Committee at the University of PittsburghMedical Center and conformed to the position ofthe American Heart Association on Research Ani-mal Use adopted November 11,1984.
DFT Testing
The experiments compared the defibrillationefficiency of the two capacitance values. The firstcapacitance value to be used in each experimentwas alternated between the 85 /^F and 140 fjFcapacitance. The DFT was measured by a mtxiifiedthree-reversal method.-'
The stored energy to start the DFT testing wasbetween 10 and 12 J. This selection was verifiedto reliably terminate VF. When this energy selec-tion could not defibrillate reliably, the patches wererepositioned to obtain reliable VF tennination priorto starting the DFT measurement. If the processof repositioning patches failed to provide a stabledefibrillation environment, the experiment wasabandoned.
After the initial successful shock, the storedenergy was decremented after each succesf ul
Rist, et al. Capacitance for Biphasic Waveform 773
deflbrillation by 1 J. At least 3 minutes was allowedto pass between successive inducfions and termi-nations of VF. BIcKKJ pressure and heart rates wereverified to have retumed to the preshock level priorto the next induction of VF. When a shock failedto defibrillate, the next trial was perfomied by incre-menting the stored energy by 1 J. Successive tri-als were performed by incrementing each storedenergy by I J until a defibrillation success occurred.At this point, the stored enei^y was again decre-mented by 1 J for each subsequent trial until a fail-ure to defibrillate occurred. From this point, thestored energy was incremented in 1 J intervalsfor each successive trial until a successful defibril-lation was accomplished. This protocol could bedescribed as a "down-up-down-up" sequence ofstored energy bounded by unsuccessful and suc-cessful defibrillation. The DFT was defined as theaverage of the values obtained with all trialsstarting from the successful shock prior to the fii-stfailure to defibrillate to the last successful defibril-lation using the protocol above.
After the DFT was obtained with one capaci-tance vaiue. the protcxrol was carried out with theother capacitance value using the same procedure.Once the DFT was obtained with the second capac-itance value, the stability of DFT during the exper-iment was verified by performing the protocol withthe first capacitance value once again. If there wasa > 2 J change in the measured DFTs between thetwo measurements, the DFT was not consideredstable, and the data were rejected.
Statistical Analysis
The mean and SD of all parameters were cal-culated for each of the two waveforms usinggrouped data from eight animals. Statistical com-parisons were peiformed using the Wilcoxon signedranks test. P < 0.05 was considered to be statisti-cally significant.
Results
Ten porcine models were evaluated with the 140fjF (C,4o) and 85 fiF (Cgj) capacitor biphasic wave-forms using the study protocol. We obtained com-plete DFT datasets from eight pigs. We rejectedtest data from two pigs due to unstable defibrilla-tion environments.
The smaller capacitance value waveform pro-duced a significant decrease in the stored energyat DFT by 20%. from 7.5 ± 1.3 J for the C. ^ to6.1 ± 2.1 J for Uie C^^ (P < 0.04). The delivered
energy at DFT was also reduced by 20%, from 7.4± 1.3 J for the C, ,, to 6.0 ± 2.0 J for the C,, (P <0.04). The stored and delivered energies at DFTwere reduced in six pigs and were higher in twopigs.
To detennine the differences between the CR,and the CI Q waveforms, the leading edge voltage,impedance, and phase 1 pulse widths for each deliv-ered biphasic shock were measured. Tlieoreticaily.a smaller capacitance value should have higherleading edge voltages and shorter phase I pulsewidths at a fixed stored energy due to the differ-ence in a capacitor's time constant. As expected,the leading edge voltages were higher for the Cj,,.but not significandy higher (374 ± 63 V comparedto 326 ± 30 V for the C^^, P = 0.055). Similarly,the phase I pulse widths for the C ^ (5.1 ± 0.8msec) were significantly shorter compared to thoseof the C,4,, (9.2 ± 1.3 msec; P < 0.001). We alsofound that the impedance dropped significantly(54.4 ± 5.8 O for the Cg vs 59.9 ± 6.3 il for theC|4,,; P < 0.02). The results are summarized inTable 1 and illustrated in Figures I through 3.
Discussion
The mechanism of defibrillation by the bipha-sic waveform is unclear. ^ It is known, however,that theoretical analysis of the monophasic wave-form shows that capacitance values should bereduced to produce improvements in ICD devices.This has been demonstrated for monophasic wave-forms.-' Therefore, it was reasonable to hypothe-size that this same capacitance reduction advan-tage would carry over to the biphasic wavefonn.
Our results show that, contrary to the usual clin-ical assumption, the energy required for deflbril-lation depends on the capacitor from which it isdrawn. We were not surprised to find that the deliv-ered energy DFTs were reduced. It is well knownthat a more narrow pulse will have lower deliv-ered energy DFTs." However, reducing the pulsewidth, while holding the capacitance constant,results in a significant amount of residual energyleft in the capacitor and does not dramaticallyreduce stored energy. Smaller capacitance values,on the other hand, lower stored energy DFTs byminimizing the undelivered energy. Our resultsshowed an average stored enei^ reduction of 20%.
Smaller capacitor values require higher lead-ing edge voltages than do larger capacitors todeliver the same energy. There may be someconcem that the higher leading edge voltages couldhave a deleterious impact on tlie heart. Our results
774 Journal of Cardiovascular Electrophysiology Vot. 5, No. 9, September 1994
I1—1
o
&
12
9-
6-
7.5 ± 1.3
p<0.04
6.1 ± 2.1
12
- 9
- 6
140 p.F 85 )i.F
Figure 1. Stored energy defibrillation threshold comparison betv>'een the conventional 140 /iF capacitance value and thesmaller 85 fzF capacitance value. fiF ~ microfarad.
Figure 2. Phase J leading edge voltage comparison between the conventional 140 piF capacitance value and the smaller 85IJ.F capacitance value. //F = microfarad.
Rist, et at. Capacitance for Biphasic Waveform 775
140
Figure 3. Shock impedance comparison between the conventional 140 //F capacitance value and the smaller 85 fjF capaci-tance value. IJ.F = microfarad.
sbowed tbat the smaller, 85 ^F bipbasic waveformbad an average of 374 V for tbe leading edge volt-age, which was not statistically significantly dif-ferent from the 326 V for the 140 ^F waveform.This tack of significant difference may, in part,be due to the reduction in energy requirement.However, had this trend been seen with a largersample size, then one might say that tbere was a\5% higher voltage requirement. Human beartshave been endocardially defibrillated with morethan I4(X) V without detected problems. '* Tbere-fore, tbe voltages reported bere do not pose anypotential for detrimental effects due to a possiblyhigher leading edge voltage needed for tbe smallercapacitance values. Tbe finding tbat the DFT (in
joules) is lower for a smaller capacitance value issignificant in this regard, as this lessened tbeincrease in initial voltage to an acceptable range.
The largest components of an ICD are the capac-itor and tbe battery, wbose sizes are directly pro-portional to tbe maximum stored shock energy.Therefore, our demonstration tbat smaller capaci-tors do reduce DFT is important for the continuedevolution to smaller ICD sizes. Smaller capacitorslower DFTs for the tilt-truncated biphasic wave-form, witb possibly higher leading edge voltagesand much sborter pulse widths. Hence, smallercapacitors could represent a significant enhance-ment of well-established benefits represented bythe biphasic wavelbrm over the monopbasic wave-
TABLE 1Comparison of Empirically Derived DFT Parameters for the C]4(, and Cs? Biphasic Waveforms and the Amount of
Change Due to the Smaller Capacitor Wavefonn for the Eight Porcine Model Experiments
140 fiF Capacitance 85 /iF Capacitance Average % Chiinf>eStored energy DFT (J)Delivered energy DFT (J)Leading edge voltage (V)Impedance (S2)Phase t pul.sewidth (m.sec)
DFT = detibriltation ihreshold; /j ± a = meantP < 0.04; $P < 0.02; *P < O.(X) I.
standard deviation; }j,F = microfarad; msec = milliseconds; Si = ohms.
776 Journal of Cardiovascular Electruphysiotogy Vol. 5, No. 9. September 1994
form. Such a trend is well recognized to improvethe usefulness of the ICD and the patient's qual-
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