In the case of non-invasive measurement of bioelectric
potential from body surface, the influence of the skin impedance has been investigated by using strict equivalent circuits of skin impedances. The relationship between the characteristic of skin impedance and the wave distortion was made clear. Furthermore, in the case of electrocardiograph, the shift potential of indifferent electrode to standard potential was obtained by means of simulation, and the distortions caused by connective resistances of a central terminal and skin impedance were quantitatively explained

Yamamoto, Tatsuma

Yamamoto, Yoshitake

Okayama University Digital Information Repository

Influences of the Skin Impedance as the Interface in Bioelectric Potential Measurement

Okayama University Scientific Achievement Repository

Memoirs of the School of Engineering, Okayama University, Vol. 15·1, November 1980Influences of the Skin Impedance as the Interfacein Bioelectric Potential MeasurementYoshitake YAMAMOTO*and Tatsuma YAMAMOTO*(Received September 10,1980)SynopsisIn the case of non-invasive measurement of bio-electric potential from body surface, the influence ofthe skin impedance has been investigated by usingstrict equivalent circuits of skin impedances. Therelationship between the characteristic of skin imped-ance and the wave distortion was made clear.Furthermore, in the case of electrocardiograph,the shif} potential of indifferent electrode to stand-ard potential was obtained by means of simulation, andthe distortions caused by connective resistances of acentral terminal and skin impedance were quantitativelyexplained.1. IntroductionIn the early state of the development of electrocardiography (ECG)and electroencephalography (EEG), the influences of the skin impedanceto these wave forms were very large. Then, the various contrivancesfor reducing the influence of the skin impedance have been conducted.Nowadays, the influence of the skin impedance became small with thedevelopment of electronic amplifier. However, the accuracy requiredfor the waveforms is progressively increasing. It is necessary tograsp precisely the situation of the waveform distortion by the skinimpedance. Then, the problems of the skin impedance are examined by* Department of Electrical Engineering1718 Yoshitake YAMAMOTO and Tatsuma YAMAMOTOusing the equivalent circuit of skin impedance from a point of viewas the interface for bio-measurement amplifier. The wave form dis-tortions caused by the skin impedance are obtained and examined inthe relationship with the characteristic of skin impedance.2. Equivalent Circuits of Skin Impedance and Interface DesignConsidering the conjunction of biological tissue and instrumentin the bioelectric potential measurement, it is necessary that theinput impedance of an instrument is much larger than the electrodepolarization impedance and than the skin impedance in order to trans-mit correctly the signal. The input impedance of the instrument hasbeen decided by JIS(Japanese Industrial Standard) (1) more than 1 MQfor ECG, 5 Mn for EEG and 2 MQ for EMG. Althogh the input impedanceof preamplifier is able to prepare extremely large, it should belimited up to 10 MO because of the capacity of the input cable andso on. In comparison with this value, the skin impedance is scatter-ing and can become several Mn.cm 2 as the extrapolated value (ZO or R2)of zero frequency (f = 0) (2) The problems of skin impedance becomeespecialy serious in the case of small electrode area.Now, the problems of the skin impedance as the interface of aninstrument is examined by using the equivalent circuit with fixedelements for skin impedance (3) • The parameters of skin impedancesand values of circuit elements prepared are given as shown in Table 1.The frequency characteristics of equivalent circuit is shown in Fig.l.The area of skin i.e. of electrode is assumed as 1 cm 2 and 10 cm2 ,and the impedance (Zoo) obtained as the extrapolated value of infinite,00 100 model mmodel I20E t=10Hz E':'50 ua a 0.5-" ~ 1=10 z'in n IIIx X0.20.1100 200 100 200Rs(kO·cm) Rs(kO·cm)(a) models for lcm2 (b) models for 10cm2Fig.l Vector loci of model equivalent circuit.Influences of the Skin Impedance as the InterfaceTable 1 PARAMETERS OF EQUIVALENT CIRCUIT19Zo f3 f S Values of Equivalent Circuit Elementsm(kn) (Hz) (cm2) 1 2 3 4 5I 200 0.8 20 1 R(kl!) 9.40 44.60 92.00 44.60 9.40C(llF) 21. 39 0.557 0.0865 0.0572 0.0335n 100 0.8 40 1 R(kn) 4.70 22.30 46.00 22.30 4.70C(llF) 21.39 0.557 0.0865 0.0572 0.0335m 200 0.9 2 10 R(kn) 30.38 139.24 30.38C(llF) 9.35 0.571 0.734IV 100 0.85 4.3 10 R(kl!) 3.3 19.7 54.0 19.7 3.3C(llF) 250. 5.35 0.689 0.659 0.502V 30 0.83 15 10 R(kn) 1.13 6.42 14.9 6.42 1.13C(llF) 204. 4.83 0.707 0.551 0.389VI 5 0.8 105 10 R(kl!) 0.235 1.115 2.30 1.115 0.235C(llF) 163. 4.24 0.659 0.436 0.255frequency (f=oo) are assumed as zero. In the case of large valus ofS as model lIT, even three couples of RC element can simulate well askin impedance. Besides these models, two models are prepared as (1)pure resistance model of 200 kQ and (2) Debye type model constructedwith two parallel elements of resistor 200 kQ and capacitor 0.04 pF.When the input impedance of an instrument amplifier is assumedas 1 MQ, the characteristics of signal distortion are shown in Fig.2for skin impedance models as follOWS:(1) pure resistance 200 kQ(2) Debye type 200 kQ, 0.04 pF, S 1(3) Cole-Cole type I (1 cm 2 )(4) Cole-Cole type lIT (10 cm 2 )The influences of electrode polarization impedance have been reportedin detail(4) (5) and are ignored in present examinations. The signaldistortion is varied naturally when the value of Zo is changed.However the differences of the distortion due to the types of modelcan be estimated quantitatively. The skin impedance is the Cole-Cole type and causes the signal distortion as shown in Fig.2(3).The case of the Debye type is shown in Fig.2(2). Both cases havethe characteristic of differentiation which decreases greatly the com-ponent of lower frequency. However, the attenuation characteristicsare different in both cases. In the case of the pure resistance, thedistortion for frequency is absent and flat as shown in Fig.2(1).In the case of the electrode area of ten times, the characteristics20 Yoshitake YAMAMOTO and Tatsuma YAMAMOTO100010010frequency(Hz)(1) Pure Resistance(2) Debye Type(3) Cole - Cole Type 1cm2(4) Cole- Cole Type 10cm2o L-[--'-----...I.------'----L__-L----.L.-_----l.--.Jr0.10.91.00.8(a) gain characteristic-10 ,---,-,---..----,--__---,_,-__---,_---,_ -6:2Vl3: - 4..s::Q.-2o0.1(2) Debye Type(3) Cole -Cole Type 1cm2(4) Cole- Cole Type 10cm210frequency (HZ)100 1000(b) phase characteristicFig.2 Characteristics of signal distortion due tothe several types of model.of gain and phase are shifted to the side of lower frequency and thedistortion becomes lower in spite of same 200 kn as shown in Fig.2(4).It is attributable to lowering the skin impedance because the capaci-tance was assumed to the ten times proportional to the electrode areaconsidering the characteristic of skin impedance. When the electrodearea becomes ten times in the actual skin, the extrapolated resist-ance Zo of zero frequency becomes 1/10. Therefore, the electrode oflarge area has notable merit for the purpose of avoiding the wavedistortion. When the resistance Zo becomes large, the frequencypoint of maximum phase angle is shifted. But, the phase angle becomesInfluences of the Skin Impedance as the Interfacelarger and the gain characteristic goes wrong.3. Influences of Skin Impedance to the Distortion ofElectrocardiography21Many researches have been reported to the influences of skinimpedance to electrocardiography (6) (7). However, the pure resistancemodel as the equivalent circuit of skin impedance was mostly used andwas not precise in the case of examinations for wave distortion dueto the simulation. In this paper, the equivalent circuits of theCole-Cole type are used as the equivalent circuits for skin impedance.The detailed examinations can be executed by the comparison of waveforms which are yielded in the cases of present and absent of circuits.The block diagram of the measurement circuit is shown in Fig.3. Thepotentials (R. L. p. V) induced in each part of body are amplified to1000 times through buffer amplifiers and stored into four channeldata recorder. The distortions of wave forms are examined with theregenerative wave forms from the data recorder. The input impedanceof the electrocardiograph is 1 MQ-l MQ and the connective resistanceof a central terminal is 30 kQ(l).Usualy, the skin impedance of an ankle is larger than the skinimpedance of a wrist. Then, it is supposed that only the left footin Fig.3 has the skin impedance (Zp) of model IT in Table 1. Theinduced wave forms in the cases of present and absent of Zp are cor-respondingly shown in Fig.4. The standared limb leads (I, IT, m)have little difference, the augmented unipolar limb leads (aVR. aVL.1 ch IR IT2 ch illLaVR3 ch aVLFaVF4 ch VVxlOOO DATA SKIN IMPEDANCE ECG RECORDERRECORDERFig.3 Block diagram of measurement circuit of electrocardiography.22 Yoshitake YAMAMOTO and Tatsuma YAMAMOTO.. -~-_.~;~!.reUI.:lOU••.•~,--~_-=Fj-===::-C~ -~~E]~~ 1". ~~~- ;.. _--:~ E---1'-+-:--9. .--=1~~::·i"E:?:"==T"eUI.:lOU'~;.:::::._...__ ..... __ .-l--- . __!:.:.::;:~._.-(e)ZR=OZL = 30krl (10)ZF = 200kl1 (10)_,I,""" , 11'1 ,;~! tiji' : 't; 1\i:1Ii ilii'!ll i 1~: ~ :I' t I! 11 111 .-iF m: 11 l! !r ,iI '~;' ;'1 1'"'' - :' ,t ti! ,Ii I iiit '~liI\ tM:!:" "I •. " jli I !, fO;' II I'" II;' :II fI I ",: :11 !', .'II IT: 1':1" I" ' 11 'ITi~1 i !I II 1.1 !i!III, I ,'II iii:: ii' iH''i!i1U '1 11 j'll; 'I itil fir: :!!~j!Lln hi! 'm [,(d)ZR=OZL = 0ZF'" 100krl (10)'I '[![IIII', II ' "i .. I;.f~WI:ti~'I·! f ! 11.-;! 1~jH1, i i'~ "I u' i :: , II:, IIi ,1;, ,! " 'i-J-! I'/lt'': I;:! ' 'II I I ,:I i:'li Ii' j f 1.111 'I!iii! Iii: !Il! II!I ,Ii i,ill'II ii!1 'I j jil': I'L i 'i ' 'II ! !' :ji -: , :-1 t! ~ I: : I I ' ,~ : 1" .. I~ ! !~ ;:! I -1 ! I!! i I :I I 'i: I! I I: d~iiU' :i/i IM'i1 1 ' ,4 HIi i !:! I , ~;!: ! .TIl;' i :q I ril'Lj' ,i, il'l ' i,'" 11'\'/' I I", Iii, i~: : J i IllY! ! Iflill 'II;' i ' 111'1 il iII"I! ::, i" ! Ii ,I I,II :', :1" I: I(C)ZR=OZL = 0ZF = 200kl1 (10)III I'd ',,, ii': '\1' 'i' 'I',dl i 1'4; t 'il : Iii, 11I Ii! ,i' 'jill h!! '!III i!illli'" i' 'Ill): lil!'1 i'I'II~1 #t1li' :":1it Ii,"" --" :1,; ::,' ',- _ttl +1+''iii'!'" "I: ,'I, I "I, I III!W'l-i!:I-: '";;!"!, ..ft H: :jnlU:Y1I!1!1 Hli fll' Wi 4 ~~!!Iif:1IIl}l H! "I,! 1Iii, ' "'Ii ",',::'1 'rK'1 'i lil'j' i' "'\', '!:I"pi !!I,y,ilhHili! I'I!!I·, I !" Iii] I ,! Il't, i I I I ! Il;!l !I :l i III :I 'I ' I ,,It' I,' 1 , 1TIli'i j,: i i!lil' I.:' I I i IIii ':j I' !II'! ! ~~t, !iI;[!, :;", "I! 'I !' I' I I ":1" IT,'" '" ~_" .. .' I,. . I II I I t(b)ZR=OZL = 0ZF = 100kl1 (1)ill HlII!,llllli!i ,f!tfH '@ij'! , I! -I ii 1~" .! ; j ,\: II L (I 1i ~W," 'rl\:~ ii, i ':' 'iV,:!q I Vj I I ': .H::"lI Ii) i I ill' Iii i : l!III,i liil II ilL I i I: I II :l,; I II i iii' 'II I' 'I II j'!' 'ii" ,'llIt',:fii :1'1 I' ,'I: - I!I! 111Hj~~,,,: ,I 'iI; i l Iii! J I iI!L;;,11111!!.111!ijl: ~l i;,: :d111illl,I, I'm, I Ill! I' :I:l.!mli' i Ii '1'\,11:11" i 'I "":, Iii ii: 'I 'il: , ,I Illililli, id r , j ! I I : III"111'1: liiil I Ii :,1·[ : I" : il I-II11'~ I'" ".J,' I' 'II 'j' iI ~ ii! i , i I: I !"I)!II ql, [Iii ;',: ill: 1:1 1'I i iir :Irlt'!=l~; Ji :ri iTI'~ Hi~ [-Ii ill";(a) ZR = 0ZL = 0ZF =200kl1 (1)p _ pII I ,I IiiiIt Iii, I: i", r' r~;I I 'liP:' II; 'I. t l !I I " ,I!' j J ~ i ': II', ' ,"::',,:I ' 'II" '. "'"II 1,11Ii'li!H!I ! i.1 i !:II'.llllllli1,11'! 'I, ,Ii! 11' I' rl Ii 1,., ';', . ,I I , , 'I ..normalwave formp - pi(f) ZR = 0 (g) ZR = 0 (h)ZR = 0 (i) ZR = 0ZL = 200kl1 (1) ZL = 100kl1 (1) ZL = 200kl1 (10) ZL = 100kl1 (10)ZF = 0 ZF = 0 ZF = 0 ZF = 0(j)ZR=OZL = 200kl1 (10)ZF = 30krl (10)Fig.5 Influences of skin impedance to the augmented unipolar limb leads aVBand the shift of the standard potential.24 Yoshitake YAMAMOTO and Tatsuma YAMAMOTO·aVF) have large difference and the precordial unipolar leads (Vl~ V2~V3~ ••••• VB) have some difference. These are mainly caused that thestandard potentials of Goldberger and Wilson connective electrodesshift by the skin impedance ZF' In the aVR leads of augmented uni-polar limb leads, wave form distortions and the shift of standardpotential induced by the skin impedance of each part are shown inFig.5. Equivalent circuits used to limb electrodes are given inTable 1. The shifts of standard potential p-p' have been recordedwith sensitivity (0.5 mV/IO mm) as same as electrocardiography. Theskin impedance is larger and the area of skin (i.e. electrode) issmaller, the wave form distortion becomes larger. And as well known,the unbalance of skin impedance is larger, the distortion becomeslarger.Schwarzschild et ala (6) have reported about the shift of poten-tial of the connective electrode using the skin impedance model ofpure resistance. Now, further strict examination is proceeded inconsideration of the frequency characteristic of skin impedance.In order to estimate the frequency characteristic, wave forms ofeach limb leads potentials and standard limb leads are expanded tothe Fourier series as follows;RIL $(w),wL I(w),wL:IIL Z(w),wL 11 (w) ,wF L F(w)w:III = L Jl!(w)w} (1)In the Goldberger connective electrode as shown in Fig.6, when theskin impedance is absent, the standard potential XAVRO of a point p~...y = 1R ZR + ry = 1L ZL + ry = 1F ZF + rFig.6 Goldberger's connective electrode (aVR).Influences of the Skin Impedance as the Interfacecan be expressed as follows;L Z(w) + R'(w)w 2(2)25when the skin impedances are presented, the standard potential of apoint p(i.e. p') becomesXAVR = L ~AVR(w)w= LYLZ(w) +YFP'(w)w Y L + Y FTherefore, the shift (p -p') of the standard potential is( 3)L(R'(w) - Z(w»wL /I/(w)w(4)It has been multiplied by the coefficient to the !IT lead of standardlimb leads. Its coefficient is depending to a frequency.The shifts (p -p') of potential for the augmented unipolar limbleads aVL and aVF and for the precordial unipolar leads V are givenrespectively as follows;XAVLO - XAVL L l/(w)Y R - Y F(5)w 2 (yR + YF)XAVFO - XAVF L I(w)Y R - Y Fw 2 (YR + YF)(6)XVO - Xv =L~(W) + Z(w) + p'(w)w 3- LYR~(w) +YLZ(w) +YFR'(w)(7)w YR+YL+YF26 Yoshitake YAMAMOTO and Tatsuma YAMAMOTOIn the case of aVR, when ZL =0 and Zp = model II or model m (Table 1)are assumed, the coefficients of eq(4) expressed by eq(8) have thefrequency characteristic as shown in Fig.7.KL e = (8)If the fundamental component of QRS wave is considered as 10 Hz, thecoefficients are 0.367 for model I and 0.255 for model m. On theother hand, the ratio of QRS wave in the corresponding of m lead inFig.4 and p -p' in Fig.5 are 0.3 in the case of model I(a) and 0.2in the case of model m(c). These coefficients became small valuethan former coefficients because of the high frequency componentsincluded in the waves •. Skin impedances fluctuate considerably due to various factors.Then, it is not sufficient that the examination have been done byonly the special models for skin impedance. However, the strict esti-mation of its influences can be performed by the model of Cole-Coletype instead of the customary model of pure resistance.The wave form distortions for the one channel electrocardiographwere examined in accordance with JIS of electrocardiographs (1) • Theconnective resistances of multichannel electrocardiograph are provid-ed more than 200 kQ by JIS. However, the influences of skin imped-ance are larger than the case of one channel because the connective90o1000----9(m)1009(1) ,/ 60.-,,, 0" Q),,,I," 30.-,,10frequency (Hz)K(()IIIIIIII,,"~."",,,,,----- --_ ... -_...o ----- ------0.10.30.20.10.4Fig.7 Coefficients of the shift of the standard potential.Influences of the Skin Impedance as the Interface 27resistances are connected simultaneously. Then, it is necessary thatthe connective resistances relation to the distortion are set to morelarge value in order to avoid the influences of skin impedance toelectrocardiography. And the input impedance of the amplifier mustbecome large sufficiently. Some of the latest one channel electro-cardiographs have the connective resistance of 60 kQ and input imped-ance of 5 MQ exceeding the values of JIS. Multichannel electrocardio-graphy mostly have the buffer amplifiers to electrodes of four limbleads and biopotential signals whose impedance is transformed aresupplied to the connective resistances. The best method to avoid theinfluences of skin impedance is to provide the buffer amplifiers toall lead electrodes.4. ConclusionsThe investigation of the skin impedance as the interface inbioelectric potential measurement has been performed. Namely, howare the potential wave forms distorted have been definitely shownfrom the relationship between the input impedance of the instrumentand the skin impedance using the each model of equivalent circuitfor skin impedance. Cencerning the leads of the electrocardiographswhose indifferent electrode are constructed with the resistors con-necting the electrodes, the potential differences between indifferentelectrode and standard potential were separated. The wave formdistortions were quantitatively explained. Further, the skin surfaceelectrodes using to bioelectric potential measurement were examinedfrom the standpoint as the interface.References(1) JIS T 1202-1970 Electrocardiographs(2) Y. Yamamoto and T. Yamamoto: Dispersion and Correlation of theparam~ters for skin impedance. Med. & BioI. Eng. & Comput.,Vol.16, No.5, pp.592-594 (1978)(3) Y. Yamamoto and T. Yamamoto: Construction of Equivalent Circuitfor Skin Impedance. Japanese Journal of Medical Electronics andBiological Engineering., Vol.14, No.2, pp.129-132 (1976)(4) L. A. Geddes and L. E. Baker: The relationship between inputimpedance and electrode area in recording the ECG. Med. BioI.,28 Yoshi take YAMAMOTO and Tatsuma YAMAMOTOEng., Vol.4, pp.439-450 (l966)(5) T. Matsuo: Properties of recording electrodes connected throughthe input impedance of bioelectric amplifier. Japanese Journalof Medical Electronics and Biological Engineering., Vol.8, No.5,pp.347-352 (1970)(6) M. M. Schwarzschild et al.: Errors in unipolar limb leads caus-ed by unbalanced skin resistances, and a device for their elimi-nation. Amer. Heart J., Vol.48, pp.235-248 (l954)(7) C. Sato et al.: Recording obstacle, distortion and electro-cardiogram. Gendaiiryo, Vol.4, pp.429-438 (l972) (in Japanese)

Influences of the Skin Impedance as the Interface in Bioelectric Potential Measurement

Abstract

Similar works

Full text

Available Versions

Okayama University Digital Information Repository

Okayama University Scientific Achievement Repository