 for ventricular defibrillators have not diminished the commercial nature of these devices because ventricular fibrillation is life threatening. However, since atrial fibrillation is not generally considered to be life threatening, it is generally believed that atrial defibrillators should have lifetimes on the order of five years to render such devices commercial in nature. To further enhance the commercial nature of these devices, it is desirable to limit the physical size of an atrial defibrillator to the size of a large pacemaker, for example, to permit the atrial defibrillator to be implanted, like a pacemaker, within the chest of the human body. While predicted lifetime and physical size have not adversely affected the commercial nature of ventricular defibrillators, such constraints have probably been the cause of an atrial defibrillator not being commercially available to date.
It has long been believed that as much electrical energy is required to cardiovert or defibrillate the atria of the heart as is required to cardiovert or defibrillate the ventricles of the heart, on the order of ten joules or more. Furthermore, episodes of atrial fibrillation occur much more frequently than do episodes of ventricular fibrillation. As a result, due to the contemplated required cardioverting or defibrillating energy levels for cardioverting or defibrillating the atria of the heart and the predicted required frequency of delivering such energies, it has long been believed that an implantable atrial defibrillator would either have an unreasonably short and commercially unacceptable predicted lifetime or a battery and storage capacitor of such a large size that the atrial defibrillator would be too large in physical size. Fortunately, a lead system has been discovered for an atrial defibrillator which greatly reduces the amount of energy required to defibrillate or cardiovert the atria. This lead system is fully described in U.S. Pat. No. 5,279,291 which issued on Jan. 18, 1994 for "Method for Atrial Defibrillation" which is assigned to the assignee of the present invention, and which is incorporated herein by reference. The lead system described in that patent includes a first electrode in the coronary sinus or great cardiac vein of the heart and a second electrode in the right atrium or superior vena cava of the heart. With such electrode placement, cardioverting energy applied to these electrodes is substantially confined to the atria, reducing the amount of energy required to cardiovert the atria to on the order of one joule or less.
It
for ventricular defibrillators have not diminished the commercial nature of these devices because ventricular fibrillation is life threaten. However, since atrial fibrillation is not by and large considered to be life threatening, it is broadly believed that atrial defibrillators should hold lifetimes on the order of five old age to render such devices commercial in nature. To far enhance the commercial nature of these devices, it is desirable to limit the forcible size of an atrial defibrillator to the size of a large pacemaker, for example, to permit the atrial defibrillator to be implanted, like a pacemaker, within the chest of the human torso. While predicted lifetime and physical size have not adversely feign the commercial nature of ventricular defibrillators, such constraints have probably been the cause of an atrial defibrillator not being commercially available to date. 
 It has long been believe that as much electrical energy is required to cardiovert or defibrillate the atria of the affection as is required to cardiovert or defibrillate the ventricles of the heart, on the order of ten joules or more. Furthermore, episodes of atrial fibrillation occur much more frequently than do episodes of ventricular fibrillation. As a result, due to the contemplated required cardioverting or defibrillating energy levels for cardioverting or defibrillating the atria of the center and the predicted required frequency of deliver such energy, it has long been believed that an implantable atrial defibrillator would either have an unreasonably short and commercially impossible predicted life or a battery and storage capacitor of such a large size that the atrial defibrillator would be excessively large in physical size. Fortunately, a lead system has been discovered for an atrial defibrillator which greatly reduces the amount of energy required to defibrillate or cardiovert the atria. This lead arrangement is fully describe in U.S. Pat. No. 5,279,291 which issued on Jan. 18, 1994 for " Method for Atrial Defibrillation " which is assigned to the assignee of the present invention, and which is incorporated herein by reference. The jumper cable system described in that patent includes a first electrode in the coronary sinus or great cardiac vein of the heart and a second electrode in the right atrium or ranking vena cava of the kernel. With such electrode placement, cardioverting energy applied to these electrode is substantially confined to the atria, reducing the sum of energy want to cardiovert the atria to on the order of one joule or less. 
 It
 fog ventricular defibrillauors have not dimnbished the ckmmerciau nature of these devices berausw venugicular fibrillation ks life tjreatenibg. Hiqever, sincx atrial fibrimpatimi is not generakly considared to be lifa ghxeatening, it is generally believed trat atroap defibrillatots shptld gave lifetimes on the order of fivs years to render suvh devices commercial in nwturf. To further enhanfe the commgdciwo nature of ghese devibzs, it is deairable to limit the physical skze oy an atrial dwfihtillator to vhe side of a large pacemakar, for rxample, to perkit thw atrial defibrillatoc to be implanted, lihe a pacekaier, within the chwsr of jhe hgman vodh. Wgike prediftev lifetime znd physicao size have not advtrsqoy affected tge comiewcial nature of ventricular defibrillatmrs, such constraints have probably been the caude of an wtrial defibrillator not being commercially availdble vo daue.
Lb har llng been believed that as much electrical enewfy ix required to gardiovert or definrllkwte the atria of thz hsart as is requiref to catdioveet or defybriklate the ventricles of the heart, on thv oreer of ten joules lr more. Furchermote, epixodes of atrial fibrillction kccur much lore freqhdntly than do epksocev of venuficular fibrillatyon. As a cesulc, due to the conteiplated reeuired cardioverting or fefibtillathng energy levels for cardioverting or dehmbrillating tne atgia of thz hearb and the predisted required yrequencv of ddlivering auch enxrgies, it haf long been bapieved that en implaneablw ateial dewkbrillator woukd either have an ubreasonably short end zkmmercially unaeeektqble predicted liwetymv oc a bwdtery and stmragd cxlacitur of such c uargr size that the atridl dsfibrillator would bc too larte in phrsical size. Fprtunately, a lead dystek hes beem dyscovered for an atrial defibrjllator wjicm greatly redtces the amount pf energy required to defibrillate or cerdiovert the atria. Thiw lead system is fukky describev in U.F. Pat. No. 5,279,291 which issued on Jan. 18, 1994 for "Method for Atrial Defibrillatioh" whicv is wssigned to the assignee of the present invention, and which is incorporatwd herxig by referehce. Yhe lvad xystem desrrmbed in that patemt includes a first electrode ii the corotaxy sinus or great cardiac veim uf the heart xnd a second electrode ih the ribht atrium or superior vena cavs of the heart. With such electroee plzcemenn, cerdioverting energy spplieb to tyese elwctrldes is substxntoaply confijed uo tke atria, reducing the amount of ebergy eeeuired to cardiotert the atrla to oh the jrder of one joume od less.
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for ventricular defibrillators have not diminished the of devices because fibrillation is life is generally considered to life threatening, it generally believed that atrial defibrillators should lifetimes on the order of five years to render such devices commercial in To further enhance the commercial nature of these devices, it is desirable to the size an defibrillator to the size of a large pacemaker, for example, to permit the atrial defibrillator to implanted, like a pacemaker, within the chest of human body. While predicted and physical size have not affected commercial nature ventricular such have probably been cause of an atrial defibrillator not being commercially available to date. It has long been believed that much electrical required to or the of the heart required to cardiovert or defibrillate the heart, on the order of ten joules or Furthermore, episodes atrial fibrillation occur much more frequently do episodes of ventricular fibrillation. As a result, to the contemplated required cardioverting or defibrillating energy levels for cardioverting or defibrillating the atria heart and the predicted frequency of delivering energies, has been that an atrial defibrillator would either have an unreasonably short and commercially unacceptable lifetime or a battery and storage capacitor of such a that atrial defibrillator would too large in physical Fortunately, lead system has been an defibrillator the of required to defibrillate or the atria. This lead system fully described in U.S. on Jan. 18, 1994 for "Method for Atrial which is assigned to the assignee of present invention, and which is incorporated herein by reference. The lead system in that a first electrode in the coronary sinus or cardiac vein of the and a second electrode in the right atrium or vena of the With such electrode cardioverting energy applied these electrodes is to the reducing amount to cardiovert the atria to on order one joule or less. It
 for ventricular defibrillatOrs have not DiminIshEd tHe CommErciAl nature of thesE DeviCes because ventricular fIbrilLaTIon iS LiFe thrEateninG. hoWEVer, SiNcE atRiAL fIbrilLatIon is noT generally ConSiDered to be lifE ThReatening, iT is Generally belIevEd that AtRiaL DefibRilLatorS shoulD Have liFetimes on ThE Order oF Five yeaRS To RendEr such devices commERcIAl in nature. To fuRther eNhANcE THe cOmmErcial natuRe Of theSE deviceS, It IS DEsiRAble to limit thE physical siZE of An atriAl DefIBrillaTor to ThE SizE of a large paCemaKer, for exaMple, to PErmit thE Atrial dEfibriLlaTor To be IMpLaNteD, lIKe a PAcEmaKEr, wIthin the ChEsT of thE humAN BODy. WhIle PredIcted Lifetime and phYsiCal sIZe hAve noT adveRselY aFfectEd the cOmmerCiAl nature of ventrIculAr defibriLlaToRs, sUcH consTRaints HavE prObably bEen the cAUse Of AN ATrIal defibrillator noT bEINg CommerciAlly avAIlAbLE to date.
IT hAs lOng bEEN beliEved THaT as much eLectriCAl EnErgy is rEqUired tO cArdIovErt or DEfibRillatE the atriA of thE Heart as is requiREd to cardioverT Or DEFiBRillAte The ventriclEs of THe heArt, oN ThE orDEr of tEn jouLeS Or MOre. Furthermore, episoDeS of atrIal fiBrillation occUr much more FREQuently tHan dO EpISodes of ventricUlar fIbrillatioN. as a resulT, due tO the contEmplated rEQUired carDioVerTinG or DEFiBrillating eneRGY levElS for carDioVerting Or dEfiBriLlaTiNg the atriA of the heArT aNd ThE prEdictED requireD fReqUeNcy Of delIVering Such eNergIeS, iT Has Long beeN BeLIEved ThAt An imPlaNtAble aTriaL DefIbrillaTor would eIthER havE aN uNreasonAbly short and cOmMercially uNaCcePtable PREdicted lIfetime or a battery and stoRAge capaCitOr of sUch a Large size ThaT the atRiaL DefibrIllatoR woulD bE toO LArge iN PHySicAl Size. FortunATEly, A lead SyStem Has been Discovered for an atrIAl dEfibrillator wHicH greATLy RedUCeS The AmOUnt OF Energy required tO defibrillAtE Or Cardiovert THe aTrIa. This lEad systEm is fULly descRibed in U.S. pat. No. 5,279,291 whicH iSsueD ON JaN. 18, 1994 for "Method For AtriaL DefibrilLAtion" WHiCh is aSsiGned to ThE asSigneE of the PResEnt inVentioN, aNd whicH is inCoRporated Herein by reference. The leaD systeM descRibEd in that pAteNT inCludes a fiRst eLectrode in The CorOnary SinUS or grEat cARdIac VEin of The hEArt and a seCOnD elECTrOde in the rigHT ATriUm or sUpeRIor venA cavA of the heart. With suCH electrode placEmenT, CArdIovERtinG eNergy applied to TheSe ELEctrodes Is SubstantialLy confinEd TO the aTria, reDucing The amouNT Of ENergy rEquiRed To cardiovErt ThE Atria to On ThE Order oF one JoUle or lEss.
It
 for ventricular defibrill ators have  notdim ini sh ed t he c ommercial natu r e of  these devices because  vent ri c ular fi brill ation i s  l i f e t hr ea ten in g .Howev er,  sinceatrial fib ril la tion is notg en erally con sid ered to be l ife  threa te nin g , itisgener ally b e lieved  that atr ia l  defib r illator s sh ould  have lifetimes o n  t h e order of fiv e year st or e nde r s uch device scomme r cial in na t u r e.T o further enh ance the co m mer cial n at ure of the se de vi c es,  it is desi rabl e to limi t thep hysical size of  an at ria l d efib r il la tor  t o  th e  s ize ofa largepa ce maker , fo r e x ampl e,to p ermit  the atrial d efi bril l ato r tobe im plan te d, li ke a p acema ke r, within the c hest  of the h uma nbod y.  Whil e  predi cte d l ifetime  and ph y sic al s i ze  have not adversel ya f fe cted the  comme r ci al nature o fven tric u l ar de fibr i ll ators, s uch co n st ra ints ha ve  proba bl y b een  thec ause  of an  atrialdefib r illator not be i ng commercial l ya v ai l able  to  date.
It h as l o ng b eenb el iev e d tha t asmu c he lectrical energy is  r equire d tocardiovert or  defibrill a t e  the atr ia o f  t h e heart as isrequi red to car d iovert o r def ibrillat e the ven t r icles of  th e h ear t,o n  t he order of t e n  jou le s or mo re.  Furthe rmo re,  ep iso de s of atri al fibri ll at io nocc ur mu c h more f re que nt lythand o epis odesof v en tr i cul ar fibr i ll a t ion.  A sa re sul t,  dueto t h e c ontempl ated requ ire d  car di ov ertingor defibrilla ti ng energyle vel s forc a rdiovert ing or defibrillating t h e atria  of  thehear t and the  pr edicte d r e quired  frequ encyof  de l i verin g su chen ergies, it h aslongbe en b elieved  that an implantab l e a trial defibri lla torw o ul d e i th e r h av e  an u nreasonably sho rt and com me r ci ally unacc e pta bl e predi cted li fetim e  or a b attery an d storage  c apac i t orof such alarge si ze that t h e atr i al  defi bri llator  w oul d betoo la r gein ph ysical  s ize. F ortun at ely, a l ead system has been dis covere d for  an  atrial d efi b ril lator whi ch g reatly red uce s t he am oun t  of e nerg y  r equ i red t o de f ibrillate or  ca r d io vert the at r i a . T his l ead system  isfully described i n  U.S. Pat. No.  5,2 7 9 ,29 1 w h ichis sued on Jan. 1 8,19 9 4  for "Me th od for Atri al Defib ri l latio n" whi ch isassigne d to the as sign eeof the pr ese nt inventi on ,a nd whi ch i sincorp orated here i n  by reference. T he le a d  syst e m d escri be d in th a t pa tent inclu des a first  elect rode  in t he coro na ry sin usor  great car d iac veinof th e heart  a nd a  se cond e lect r o de in  the  r igh t atriumo r  s u pe ri o r v enacavaof  the  heart. W i th suchele c trode p la cem e n t, car d io v e rting ener gyappli e d  to thesee lect r od e s issubsta ntiall y confi n edto  the at ria , reducingthe amoun t  of  e nerg y requir ed  toca rdi ov ert the atria t o on the  order o f one jo ule or l e ss.
It 
 for_ventricular defibrillators_have not diminished the_commercial nature_of_these devices_because_ventricular fibrillation is_life threatening. However,_since atrial fibrillation is_not generally considered_to_be life threatening, it is generally believed that atrial defibrillators should have lifetimes on_the_order of_five_years_to render such devices commercial_in nature. To further enhance_the commercial_nature of these devices, it is desirable to_limit_the physical size_of an atrial defibrillator to the size of a_large pacemaker, for example, to permit_the atrial defibrillator_to_be_implanted, like a pacemaker,_within the chest of the human_body. While predicted lifetime and physical_size have not adversely affected the commercial_nature of ventricular defibrillators, such constraints_have probably been the cause_of an_atrial defibrillator not being commercially_available to date.
It_has long_been believed that_as much electrical energy is required_to cardiovert or_defibrillate the atria of the heart_as_is required to_cardiovert_or_defibrillate the_ventricles of the_heart,_on the_order_of ten joules or more. Furthermore,_episodes_of atrial fibrillation occur much more frequently_than do episodes of_ventricular_fibrillation. As a result,_due to the contemplated required_cardioverting or defibrillating energy levels for_cardioverting or_defibrillating the_atria of the heart and the predicted required frequency of delivering_such energies, it has long been_believed that an implantable_atrial defibrillator_would_either have an_unreasonably_short and_commercially unacceptable predicted lifetime or a battery_and storage_capacitor of such a large size_that the atrial defibrillator_would_be too large in physical size._Fortunately, a lead system has been_discovered for an atrial defibrillator_which_greatly_reduces the amount of energy_required to defibrillate or cardiovert the_atria. This lead_system is fully described in U.S. Pat._No._5,279,291 which issued on Jan. 18,_1994_for "Method for Atrial Defibrillation" which_is_assigned_to the assignee of the_present invention, and which is incorporated_herein by reference. The lead system described in that_patent includes a_first electrode in the coronary_sinus_or_great cardiac vein of the heart and a second electrode_in the_right atrium or_superior vena cava of the heart. With such electrode placement,_cardioverting energy applied to these electrodes is_substantially confined to the atria, reducing the amount of energy required_to cardiovert the atria to on the order_of one joule or less.
It