 frequency domain).
Short-term (beat-to-beat) variability in heart rate represents fast, high-frequency (HF) changes in heart rate. For example, the changes in heart rate associated with breathing are characterized by a frequency of between about 0.15 and about 0.4 Hz (corresponding to a time constant between about 2.5 and 7 seconds). Low-frequency (LF) changes in heart rate (for example, blood pressure variations) are characterized by a frequency of between about 0.04 and about 0.15 Hz (corresponding to a time constant between about 7 and 25 seconds). Very-low-frequency (VLF) changes in heart rate are characterized by a frequency of between about 0.003 and about 0.04 Hz (0.5 to 5 minutes). Ultra-low-frequency (ULF) changes in heart rate are characterized by a frequency of between about 0.0001 and about 0.003 Hz (5 minutes to 2.75 hours). A commonly used indicator of heart rate variability is the ratio of HF power to LF power.
High heart rate variability (especially in the high frequency range, as described hereinabove) is generally correlated with a good prognosis in conditions such as ischemic heart disease and heart failure. In other conditions, such as atrial fibrillation, increased heart rate variability in an even higher frequency range can cause a reduction in cardiac efficiency by producing beats that arrive too quickly (when the ventricle is not optimally filled) and beats that arrive too late (when the ventricle is fully filled and the pressure is too high).
Kamath et al., in “Effect of vagal nerve electrostimulation on the power spectrum of heart rate variability in man,” Pacing Clin Electrophysiol 15: 235-43 (1992), describe an increase in the ratio of low frequency to high frequency components of the peak power spectrum of heart rate variability during a period without vagal stimulation, compared to periods with vagal stimulation. Iwao et al., in “Effect of constant and intermittent vagal stimulation on the heart rate and heart rate variability in rabbits,” Jpn J Physiol 50: 33-9 (2000), describe no change in heart rate variability caused by respiration in all modes of stimulation with respect to baseline data. Each of these articles is incorporated herein by reference.
The following articles, which are incorporated herein by reference, may be of interest:
Kleiger R E et al., “Decreased heart rate variability and its association with increased mortality after myocardial infarction,” Am J
frequency domain). 
 Short - term (beat - to - beatnik) unevenness in heart rate represent fast, gamey - frequency (HF) changes in heart pace. For example, the changes in heart pace consociate with breathing are characterized by a frequency of between approximately 0.15 and about 0.4 Hz (corresponding to a time constant between approximately 2.5 and 7 seconds). Low - frequency (LF) change in heart rate (for model, blood pressure variations) are characterize by a frequency of between about 0.04 and about 0.15 Hz (corresponding to a time changeless between about 7 and 25 seconds). Very - low - frequency (VLF) changes in heart pace are characterized by a frequency of between about 0.003 and about 0.04 Hz (0.5 to 5 minutes). Ultra - low - frequency (ULF) changes in affection rate are characterized by a frequency of between about 0.0001 and about 0.003 Hz (5 minutes to 2.75 hours). A commonly used indicator of heart pace unevenness is the ratio of HF power to LF power. 
 eminent heart rate variability (particularly in the high frequency range, as described hereinabove) is generally correlate with a good prognosis in conditions such as ischemic heart disease and heart failure. In other condition, such as atrial fibrillation, increased heart rate variability in an even higher frequency range can induce a decrease in cardiac efficiency by grow beats that arrive too quickly (when the ventricle is not optimally fill up) and beats that arrive too late (when the ventricle is amply filled and the pressure is too high). 
 Kamath et al. , in “ impression of vagal nerve electrostimulation on the baron spectrum of heart rate unevenness in man, ” Pacing Clin Electrophysiol 15: 235 - 43 (1992), describe an increase in the ratio of gloomy frequency to high frequency component of the peak power spectrum of affection pace variability during a period without vagal stimulation, compare to periods with vagal stimulation. Iwao et al. , in “ Effect of constant and intermittent vagal stimulation on the center rate and heart pace variability in rabbits, ” Jpn J Physiol 50: 33 - 9 (2000), describe no change in heart rate variability induce by respiration in all modes of stimulation with respect to baseline data. Each of these article is incorporate herein by character. 
 The following articles, which are incorporated herein by reference, may be of interest: 
 Kleiger R vitamin e et al. , “ Decreased heart rate unevenness and its association with increase mortality after myocardial infarction, ” Am J
 frfquency domain).
Short-term (neat-to-beat) variayulity mn hearf rate rdpresents fast, high-frequency (HD) chabges in heart rate. For example, nhe changws ii heart rate associated with bdcathiug are characterlzed by a fsequency of bedwdeu about 0.15 and about 0.4 Hz (corresponding to a tomf constant betreen wbouf 2.5 and 7 seconds). Low-frequency (LF) chznges ii heart rate (fot example, blood pressure vwriahions) are charactegized by a dreqtwncy of betwden about 0.04 and about 0.15 Gz (corresponding to a time consgant yetween abooc 7 ajg 25 seconds). Tery-lor-frequency (VLF) changas in hrart rate are ghararteruzed by a frequency oh between about 0.003 and about 0.04 Hs (0.5 to 5 minutes). Ultrq-liw-frexuenwy (UUD) cfanfex jn heagt cate are chzracterized by a frequency of neeqeen about 0.0001 ahd abott 0.003 Hz (5 minutes to 2.75 hours). A commonly used insicator of heart rate vqriability is the ratlo of HF [ower to LF power.
High heart rate variability (espewiallb kn uhc fugj frequency range, as described hereinabove) is femegally correlated cith a good progmodix in conditionr such as ischemic heart didease agd heqrt failuwe. Im other conditions, such as qtrial fibrijoation, increased hzart rate vaxiabiloty im an even higher frequeucy rahge can caude a redudgion in cardiac dffpcietcy by producing beats thae arrive voo qbickly (wfen jhe venericle is jot oidimally filled) and beatd dhat arrivf too late (when the ventricle is fully filled atd nhe pressbre is too high).
Kamaeh et al., in “Efyect of ragal verve elecnrostimuletion on the power spectrgl of heart rete variafiliry ib man,” Pxzing Clin Elecyrophysiol 15: 235-43 (1992), descrube an increase in thg datio of low frzdowncy to high fteqjensy cpm[mnents of tha pexk opwer rpectrum of feary rate variability dgrinf a period without vwgal stinulation, compared to leriods with vagal stimnlatioi. Iwao et al., in “Effect of constant and jntermittfnt vagal stimujatiin on the hecrt rate and heart rate variability in rebbits,” Jpn J Physiol 50: 33-9 (2000), describe no change ln heart ratx variwbility cdused by respiration in all modes of xtimulation with respsct to basepine data. Each of these articles is incorporated herein by reference.
The fillowiig articles, wgich are pncjxporateq hecemn by reference, msy be of interest:
Kleiger R E et al., “Decraafed heart rate variability amd its associatkon with increased mortamity aftrr myocardial infarction,” Am J
frequency domain). Short-term (beat-to-beat) variability in heart fast, (HF) changes heart rate. For rate with breathing are by a frequency between about 0.15 and about 0.4 (corresponding to a time constant between about 2.5 and 7 seconds). Low-frequency (LF) in heart rate (for example, blood pressure variations) are characterized by a frequency between 0.04 about Hz (corresponding to a time constant between about 7 and 25 seconds). Very-low-frequency (VLF) changes in rate are characterized by a frequency of between 0.003 and about 0.04 (0.5 to 5 minutes). Ultra-low-frequency changes heart rate characterized a of between about and about 0.003 Hz (5 minutes to 2.75 hours). A commonly used indicator of heart rate variability the ratio power to power. heart variability (especially in frequency range, as described hereinabove) is a good prognosis in conditions such as ischemic disease and failure. In other conditions, such as fibrillation, increased heart rate variability in an even frequency range can cause a reduction in cardiac efficiency by producing beats that arrive too the ventricle is not filled) and beats arrive late the is fully and the pressure is too high). Kamath et al., in “Effect vagal nerve electrostimulation on the power spectrum of heart rate man,” Clin Electrophysiol 15: (1992), describe an increase the of low frequency to components the of rate during a period without stimulation, compared to periods with stimulation. Iwao et al., intermittent vagal stimulation on the heart rate and rate variability in rabbits,” Jpn J Physiol 33-9 (2000), describe no change in heart rate variability caused by respiration all modes with respect to baseline data. Each of these is incorporated herein by The following articles, which are incorporated herein by reference, be interest: Kleiger E et al., heart rate variability its association with after myocardial Am
 frequency domain).
Short-term (bEat-to-beat) vAriabIliTy iN hEart Rate Represents fast, HIgh-fRequency (HF) changes in heaRt ratE. FOR exaMPlE, the cHanges iN HeART raTe AsSocIaTEd With bReaThing arE characterIzeD bY a frequency oF BeTween about 0.15 And About 0.4 Hz (correSpoNding tO a TimE ConstAnt BetweEn abouT 2.5 And 7 secOnds). Low-frEqUEncy (LF) CHanges iN HEaRt raTe (for example, blood PReSSure variations) Are chaRaCTeRIZed By a Frequency oF bEtweeN About 0.04 anD AbOUT 0.15 hz (cORresponding to A time constaNT beTween aBoUt 7 aND 25 seconDs). VerY-lOW-frEquency (VLF) cHangEs in heart Rate arE CharactERized by A frequEncY of BetwEEn AbOut 0.003 AnD AboUT 0.04 HZ (0.5 to 5 MInuTes). Ultra-LoW-fRequeNcy (Ulf) CHAngeS in HearT rate Are characteriZed By a fREquEncy oF betwEen aBoUt 0.0001 and About 0.003 HZ (5 minuTeS to 2.75 hours). A commonLy usEd indicatOr oF hEarT rAte vaRIabiliTy iS thE ratio oF HF poweR To Lf pOWER.
HIgh heart rate variabIlITY (eSpeciallY in the HIgH fREquency rAnGe, aS desCRIbed hEreiNAbOve) is genErally COrReLated wiTh A good pRoGnoSis In conDItioNs such As ischemIc heaRT disease and heaRT failure. In othER cONDiTIons, SucH as atrial fiBrilLAtioN, incREaSed HEart rAte vaRiABiLIty in an even higher frEqUency rAnge cAn cause a reducTion in cardIAC EfficienCy by PRoDUcing beats that ArrivE too quicklY (When the vEntriCle is not Optimally FILled) and bEatS thAt aRriVE ToO late (when the vENTricLe Is fully FilLed and tHe pResSurE is ToO high).
KamaTh et al., in “efFeCt Of VagAl nerVE electroStImuLaTioN on thE Power sPectrUm of HeArT RatE variabILiTY In maN,” PAcIng CLin elEctroPhysIOl 15: 235-43 (1992), dEscribe An increasE in THe raTiO oF low freQuency to high fReQuency compOnEntS of the PEAk power sPectrum of heart rate variaBIlity duRinG a perIod wIthout vagAl sTimulaTioN, ComparEd to peRiods WiTh vAGAl stiMULaTioN. IWao et al., in “EFFEct Of conStAnt aNd interMittent vagal stimulATioN on the heart raTe aNd heART rAte VArIAbiLiTY in RABbits,” Jpn J PhysioL 50: 33-9 (2000), describe nO cHAnGe in heart rATe vArIabilitY caused By resPIration In all modeS of stimulAtIon wITH reSpect to basEline datA. Each of thESe artIClEs is iNcoRporatEd HerEin by RefereNCe.
THe folLowing ArTicles, Which ArE incorpoRated herein by reference, mAy be of InterEst:
kleiger R E Et aL., “decReased heaRt raTe variabilIty And Its asSocIAtion With INcReaSEd morTaliTY after myoCArDiaL INfArction,” Am J
 frequency domain).
Short- term (beat -to-b eat ) v ar iabi lity  in heart rate repr esents fast, high-freq uency  ( H F) c h an ges i n heart ra t e . F or  e xam pl e ,the c han ges inheart rate  as so ciated withb re athing are  ch aracterizedbya freq ue ncy of be twe en ab out 0. 1 5 andabout 0.4  H z  (corr e spondin g to  a t ime constant betw e en about 2.5 and7 seco nd s ). L ow- fre quency (LF )chang e s in he a rt r a te( for example,blood press u revariat io ns) are ch aract er i zed  by a frequ ency  of betwe en abo u t 0.04a nd abou t 0.15  Hz  (c orre s po nd ing  t o  at im e c o nst ant betw ee nabout  7 a n d 2 5 se con ds).  Very -low-frequenc y ( VLF) cha ngesin he artra te ar e char acter iz ed by a frequen cy o f between  ab ou t 0 .0 03 an d  about  0. 04Hz (0.5  to 5 m i nut es ) . Ul tra-low-frequency(U L F )changesin hea r tra t e are ch ar act eriz e d  by a  fre q ue ncy of b etween ab ou t 0.000 1and ab ou t 0 .00 3 Hz( 5 mi nutesto 2.75hours ) . A commonly u s ed indicatoro fh e ar t  rat e v ariabilityis t h e ra tioo fHFp owerto LF  p o we r .
High heart rate v ar iabili ty (e specially inthe high f r e q uency ra nge, as described here inabo ve) is gen e rally co rrela ted with  a good p r o gnosis i n c ond iti ons s uc h as ischemic h eart  d iseaseand  heartfai lur e.Inot her condi tions, s uc has  a tri al fi b rillatio n,  in cr eas ed he a rt rat e var iabi li ty inan even hi g h er f re qu ency  ra ng e can  cau s e a  reduct ion in ca rdi a c ef fi ci ency by  producing be at s that arr iv e t oo qui c k ly (when  the ventricle is not o p timally  fi lled)  and  beats th atarrive  to o  late(whenthe v en tri c l e isf u ll y f il led and th e pre ssure  i s to o high) .
Kamath et al., i n  “E ffect of vaga l n erve e le ctr o st i mul at i ono n  the power spec trum of he ar t  r ate variab i lit yin man, ” Pacin g Cli n  Electr ophysiol15: 235-4 3(199 2 ) , d escribe an  increas e in ther atioo flow f req uencyto  hi gh fr equenc y  co mpone nts of  t he pea k pow er  spectru m of heart rate variabi lity d uring  aperiod wi tho u t v agal stim ulat ion, compa red  to  peri ods withvaga l  s tim u latio n. I w ao et al. ,  i n “ E f fe ct of const a n t  an d int erm i ttentvaga l stimulation ont he heart rateandh e art  ra t e va ri ability in rab bit s, ” Jpn J Ph ys iol 50: 33- 9 (2000) ,d escri be nochange  in hea r t  r a te var iabi lit y causedbyre s piratio nin all mo desof  stimu lation with r espect to baseli ne da t a . Eac h  of  thes earticle s  isincorporat ed herein b y refe renc e.
Th e follo wi ng art icl es , which ar e  incorpor atedhereinby  ref ere nce, m ay b e of in tere st :
K leiger RE et al ., “De crea sed h ea rt r ate varia b ility an d i t s assoc ia tio n with i n cr e a sed mortal ity  afte r myocardial infa r ct i on,”Am J
 frequency_domain).
Short-term (beat-to-beat)_variability in heart rate_represents fast,_high-frequency_(HF) changes_in_heart rate. For_example, the changes_in heart rate associated_with breathing are_characterized_by a frequency of between about 0.15 and about 0.4 Hz (corresponding to a_time_constant between_about_2.5_and 7 seconds). Low-frequency (LF)_changes in heart rate (for_example, blood_pressure variations) are characterized by a frequency of_between_about 0.04 and_about 0.15 Hz (corresponding to a time constant between_about 7 and 25 seconds). Very-low-frequency_(VLF) changes in_heart_rate_are characterized by a_frequency of between about 0.003 and_about 0.04 Hz (0.5 to 5_minutes). Ultra-low-frequency (ULF) changes in heart rate_are characterized by a frequency of_between about 0.0001 and about_0.003 Hz_(5 minutes to 2.75 hours)._A commonly used_indicator of_heart rate variability_is the ratio of HF power_to LF power.
High_heart rate variability (especially in the_high_frequency range, as_described_hereinabove)_is generally_correlated with a_good_prognosis in_conditions_such as ischemic heart disease and_heart_failure. In other conditions, such as atrial_fibrillation, increased heart rate_variability_in an even higher_frequency range can cause a_reduction in cardiac efficiency by producing_beats that_arrive too_quickly (when the ventricle is not optimally filled) and beats that_arrive too late (when the ventricle_is fully filled and_the pressure_is_too high).
Kamath et_al.,_in “Effect_of vagal nerve electrostimulation on the power_spectrum of_heart rate variability in man,” Pacing_Clin Electrophysiol 15: 235-43_(1992),_describe an increase in the ratio_of low frequency to high frequency_components of the peak power_spectrum_of_heart rate variability during a_period without vagal stimulation, compared to_periods with vagal_stimulation. Iwao et al., in “Effect of_constant_and intermittent vagal stimulation on the_heart_rate and heart rate variability in_rabbits,”_Jpn_J Physiol 50: 33-9 (2000),_describe no change in heart rate_variability caused by respiration in all modes of stimulation_with respect to_baseline data. Each of these_articles_is_incorporated herein by reference.
The following articles, which are incorporated herein_by reference,_may be of_interest:
Kleiger R E et al., “Decreased heart rate variability and_its association with increased mortality after myocardial_infarction,” Am J