Aorta Simulator - ACS

11 Electrokinetic Potentials i n a Left Ventricle/Aorta Simulator EUGENE FINDL and ROBERT J. KURTZ

Downloaded by UNIV OF NEW ENGLAND on February 11, 2017 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0038.ch011

ARK Research, Farmingdale, N.Y. 11735

There a r e two terms in e l e c t r o c h e m i s t r y t h a t sound alike but c o v e r different a s p e c t s o f the field, electrode kinetics and electrokinetics. The first term is used t o d e s c r i b e e l e c t r o d e r e a c t i o n rates. The second term (our s u b j e c t h e r e i n ) i s used to d e s c r i b e interfacial electrochemical phenomena o b s e r v e d when an electrolyte and a solid s u r f a c e move w i t h r e s p e c t t o each o t h e r . Electrokinetic phenomena a r e classically d i v i d e d i n t o f o u r c a t e g o r i e s , i.e., e l e c t r o p h o r e s i s , e l e c t r o - o s m o s i s , s e d i m e n t a t i o n p o t e n t i a l s and streaming potentials. There are however, s e v e r a l lesser known electrokinetic effects, namely motoelectric effects, Ueda effects and a c o u s t o - e l e c t r i c effects. Of t h e s e electrokinetic phenomena, s t r e a m i n g p o t e n t i a l s , under t u r b u l e n t f l o w c o n d i t i o n s , are t h e o r i z e d to c o n t r i b u t e significantly t o the electrical signals attributed t o mammalian hearts. I t is w e l l known t h a t b l o o d f l o w s t h r o u g h the h e a r t and c e r t a i n b l o o d v e s s e l s under t u r b u l e n t flow c o n d i t i o n s ( 1 ) . T h e r e f o r e , based upon the premise t h a t s t r e a m i n g p o t e n t i a l s o f significant magnitude can be g e n e r a t e d by such f l o w c o n d i t i o n s , we d e c i d e d t o i n v e s t i g a t e the possibility that streaming p o t e n t i a l s c o n t r i b u t e a s u b s t a n t i a l part o f the p o t e n t i a l s seen in an electro-cardiogram (EKG). The work of Miller and Dent(2) is o f particular interest in this regard. They p r e s e n t e d e x p e r i m e n t a l e v i d e n c e , b o t h i n v i t r o and i n v i v o ( w i t h d o g s ) , t h a t s t r e a m i n g p o t e n t i a l s a r e the cause o f a t l e a s t the Τ wave p o r t i o n o f the EKG. 0 t h e r s ( 2 - 6 ) have r e p o r t e d the e x i s t e n c e of s t r e a m i n g p o t e n t i a l s i n c a r d i o - v a s c u l a r components. 180

Sawyer; Electrochemical Studies of Biological Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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Our a p p r o a c h t o t h e p r o b l e m o f d e m o n s t r a t i n g t h a t t h e EKG i s a t l e a s t p a r t i a l l y d u e t o s t r e a m i n g p o t e n t i a l s has been t o f i r s t demonstrate i n v i t r o , w i t h m e c h a n i c a l l e f t v e n t r i c l e s i m u l a t o r s , t h a t an EKG l i k e s i g n a l c a n b e g e n e r a t e d . Second, to d e m o n s t r a t e t h a t EKG l i k e s i g n a l s c a n be g e n e r a t e d i n v i v o u s i n g a m e c h a n i c a l , p u l s a t i l e , h e a r t pump. Some o f t h e e x p e r i m e n t a l r e s u l t s o f o u r f i r s t step are presented h e r e i n .


Experimental

Apparatus

Our l a t e s t l e f t v e n t r i c l e s i m u l a t o r (LVS III) i s shown o n F i g u r e 1. £Two o t h e r m o d e l s , L V S I and LVS II, were o f s i m i l a r d e s i g n . ] A dc m o t o r d r i v e n cam m o v e s a s h a f t t h a t d i s p l a c e s a r u b b e r diaphragm. As t h e d i a g r a m i s d i s p l a c e d i n w a r d , f l u i d i n t h e " l e f t v e n t r i c l e " chamber i s f o r c e d o u t , t h r o u g h a o n e way r u b b e r v a l v e a n d i n t o a distensible balloon (aorta). During the v e n t r i c u l a r c y c l e , when c o m p r e s s i o n o c c u r s , t h e " a o r t a " expands due t o t h e o u t w a r d f l o w o f f l u i d . When t h e diaphragm i s brought back t o i t s s t a r t i n g p o s i t i o n , f l u i d from the " a o r t a " flows back i n t o the "left v e n t r i c l e " c h a m b e r t h r o u g h a s e c o n d one way v a l v e . S t r o k e v o l u m e s w e r e v a r i e d b e t w e e n 20 a n d 50 m l b y v a r y i n g t h e f l o w r e s t r i c t i o n c a u s e d by t h e c h e c k valves. E l e c t r o d e s ( A g / A g C l ) were p l a c e d a t v a r i o u s l o c a t i o n s on t h e " l e f t v e n t r i c l e . " The e l e c t r o d e s were p l a c e d o u t o f t h e f l o w i n g s t r e a m s t o m i n i m i z e moto-electric effect artifacts. [ M o r e w i l l be s a i d about t h i s problem area l a t e r . ] Coaxial c a b l e l e a d s were a d a p t e d t o c o m p r e s s i o n f i t t i n g s to connect the e l e c t r o d e s to the readout instrumentation. S i m u l a t o r e l e c t r o d e s w e r e n u m b e r e d a s shown o n F i g u r e 2. The L V S I I v e r s i o n h a d a n a d d i t i o n a l e l e c t r o d e (#6) p l a c e d i n t h e body o f t h e L V S . W a t e r b a t h e l e c t r o d e s ( F i g u r e 3) w e r e l a b e l e d A , B , C a n d D. T h e s e were l o c a t e d a s f o l l o w s : A, n e a r one v a l v e ; B, 2-6 cm f r o m A ; C , 2-6 cm f r o m D; D, near the other v a l v e . The s i m u l a t o r s w e r e made i o n i c a l l y c o n d u c t i v e by d r i l l i n g many s m a l l h o l e s i n t o t h e l e f t v e n t r i c l e chamber and t h e v a l v e h o l d e r s e c t i o n s . Bulk t r a n s f e r of f l u i d from the L V S * s and the w a t e r b a t h i n t o w h i c h t h e y w e r e s u b m e r g e d was p r e v e n t e d by a c o a t i n g o v e r the h o l e s . Several d i f f e r e n t c o a t i n g s were e v a l u a t e d . Among t h e



Figure I. Left ventricle simulator


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FiNDL AND KURTZ "*5

"

^

1

^

I

^

183

*4

&


-®


\

\

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Detail of electrodes inside tank Figure 3.



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m a t e r i a l s were a t h i n l a t e x , a c e l l u l o s e acetate r e v e r s e o s m o s i s membrane, c o l l a g e n , c o l l o d i o n and a cellulose battery separator material. Of t h e s e , c o l l o d i o n a n d t h e c e l l u l o s e membranes a p p e a r e d t o be t h e m o s t s a t i s f a c t o r y . The o t h e r s w e r e e i t h e r too r e s i s t i v e or too d i f f i c u l t to mechanically a t t a c h to the L V S ' s . T h e l e f t v e n t r i c l e was i m m e r s e d i n a w a t e r bath to simulate the "aqueous environment" o f a mammalian b o d y . F i g u r e 3 i l l u s t r a t e s how f o u r A g / A g C l e l e c t r o d e s were a t t a c h e d t o t h e b a t h . A T e k t r o n i x 751^ dual channel, storage o s c i l l o s c o p e w i t h 7A22 d i f f e r e n t i a l a m p l i f i e r s was used to monitor the e l e c t r o k i n e t i c potentials. I n p u t i m p e d a n c e was i n c r e a s e d t o l O * ^ ohms b y means o f s o l i d s t a t e v o l t a g e f o l l o w e r s . I n some t e s t s , i n a d d i t i o n to the p o t e n t i a l s generated, we a l s o m o n i t o r e d t h e " l e f t v e n t r i c u l a r " pressure, u s i n g a s t r a i n gauge t r a n s d u c e r . Test

Results

Examples o f the e l e c t r o k i n e t i c p o t e n t i a l s g e n e r a t e d b y t h e s i m u l a t o r s i s shown o n F i g u r e s 4, 5i 6 a n d 7. A l l t e s t s were r u n a t a m b i e n t temperature, u s i n g v a r i o u s c o n c e n t r a t i o n s of NaCl as the e l e c t r o l y t e . F i g u r e k i l l u s t r a t e s the waveforms o b t a i n e d from v a r i o u s c o m b i n a t i o n s of e l e c t r o d e s u s i n g the LVS I I . The u p p e r t r a c e s a r e t h e e l e c t r o k i n e t i c p o t e n t i a l s while the lower t r a c e i l l u s t r a t e s the l e f t ventricular pressure. Electrokinetic p o t e n t i a l s were m e a s u r e d h a v i n g t h e following v e r t i c a l deflection scale factors, Electrode Pair 1-.2+

l-,3 l-,5 1-.6+

2-,5 2-,6+

+

+

+

Scale (mv/div)

50 200 200 20 20 200 200 20 50

Electrode Pair 3 - A

+

3-.5 3-.6 ^-,5

+

+ +

k~,6

+

5-,

6

+

Scale (mv/div)

20 200 200 200 200 20


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T h e L V S was n o t i m m e r s e d i n t h e w a t e r b a t h . Pulse r a t e was 66 p u l s e s p e r m i n u t e ( P / M ) . The e l e c t r o l y t e was s a l i n e h a v i n g a r e s i s t i v i t y o f 3 x l C P ohm cm. Peak v e n t r i c u l a r p r e s s u r e was 320 mm H g . F i g u r e 5 i l l u s t r a t e s the waveforms o b t a i n e d from t h e LVS I I I u s i n g i t s v a r i o u s electrode combinations. I n t h i s c a s e , t h e p u l s e r a t e was 76 P/M a n d t h e s a l i n e h a d a r e s i s t i v i t y o f 1,040 ohm cm, w h i c h i s a b o u t t h a t o f m a m m a l i a n t i s s u e . S t r o k e v o l u m e was « 50 m l . Figure 6 i l l u s t r a t e s the t e s t r e s u l t s obtained w i t h the LVS III immersed i n t h e water b a t h . The t o p t r a c e shows t h e p o t e n t i a l s a s m e a s u r e d b y e l e c t r o d e s ( A , B , C , D ) immersed i n the b a t h . The b o t t o m t r a c e shows t h e s i m u l t a n e o u s p o t e n t i a l o f e l e c t r o d e s 1 a n d 2, i n s i d e t h e L V S . Figure 7 i l l u s t r a t e s the effect of stroke volume on t h e a m p l i t u d e o f t h e s i g n a l . Pulse rate was 72 P/M f o r a l l p h o t o s . S t r o k e v o l u m e was v a r i e d by c h a n g i n g t h e l e n g t h o f t h e p i s t o n p u s h i n g the LVS III diaphragm. A s e r i e s o f t e s t s was made u s i n g t h e L V S III to determine the e f f e c t of e l e c t r o l y t e c o n d u c t i v i t y on s i g n a l v o l t a g e l e v e l . The r e s u l t s a r e shown o n T a b l e 1. Table

1

Effect of Electrolyte Conductivity Signal Voltage Level Electrolyte Conductivity (ohm-i.cm !) 3

1.1·10" 2.0.10-3 9.1-10-Jf 2

2.9ΊΟ-7 1.5·10"7 I.O.IO-7

3.0-10-f 2.2·10"5 1.4-10-5

Signal Peak

7mvT 0.5 3 5

on

Voltage Level Average (mV) 0.12 0.60 0.87

12 18 20

2.40 2.95 3.82

40 44 47

7.88 9.24 10.86

T h e o r e t i c a l l y , streaming p o t e n t i a l i s inversely proportional to conductivity. Figure 8 i l l u s t r a t e s that our experimental r e s u l t s approximate the t h e o r e t i c a l v a l u e s o v e r the range 10~2 t o 10"5 o h i r r ^ c n T . 1


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Figure 7. Effect of stroke volume on signal amplitude. Electrodes 4+,5— of the LVS HI. Average stroke vol umes are given below photos.

10

-3

Ί

\ c 10" Experiment al

The o r e t i c a Y

10"

2 Average

4

6

Tulsatile

Potential

S

10

12

(Millivolts)

Figure 8. Effect of electrolyte conductivity on pulsatile electrokinetic potential (electrode 4 and 5; LVS III)


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Discussion H i s t o r i c a l l y , streaming p o t e n t i a l s were f i r s t described by Quincke i n 1859.(2.) Other 1 9 century i n v e s t i g a t o r s included Z o l l n e r ( 8 ) , Edlund(£) Haga(lO) and Clark(11). Helmholtz(12) added a t h e o r e t i c a l basis f o r streaming potential> incorporating h i s famous double l a y e r theories into the explanation. Smoluchowski r e f i n e d Helmholtz's theories i n her t r e a t i s e published i n 1921. Most recent e f f o r t i n the f i e l d has been concerned with the determination of zeta p o t e n t i a l s by using streaming p o t e n t i a l measurements. With but few exceptions( 11^,1^, 1Λ) the experi mental e f f o r t described i n the streaming p o t e n t i a l l i t e r a t u r e has dealt with laminar flow through c a p i l l a r y tubes or porous plugs. The region of turbulent flow has been l a r g e l y ignored u n t i l recently. Kurtz, F i n d l , Kurtz and Stormo(15) extended the region of streaming p o t e n t i a l measurements well i n t o the turbulent region, u t i l i z i n g tubing up to 3.8 cm i n diameter. Further they extended the basic streaming p o t e n t i a l r e l a t i o n s h i p s well into the turbulent flow region. t h


f

Laminar region 2

Ε = 8[DcL/nd k]u Turbulent

region

Ε - [0.0^(pA)°- ][DCL/nd - k]u 7 5

1

2 5

1

7 5

where d = tubing diameter u = f l u i d velocity D = d i e l e c t r i c constant ζ = zeta p o t e n t i a l Ε = streaming p o t e n t i a l μ = f l u i d v i s c o s i t y k = f l u i d conductivity ρ = f l u i d density L = electrode spacing The major d i f f e r e n c e between the two r e l a t i o n ships i s that i n the turbulent region, as compared to the laminar region, the streaming p o t e n t i a l increases as a function of v e l o c i t y to the 1.75 power rather than l i n e a r l y . Further, the p o t e n t i a l diminishing e f f e c t of enlarging tubing diameter i s much l e s s i n the turbulent region, i . e . , d ~ i n laminar versus d " * * i turbulent. Overall, the streaming p o t e n t i a l increases much more r a p i d l y i n the turbulent region than would have been expected using extrapolations from the laminar flow region. 2

1

2

n



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There are two other e l e c t r o k i n e t i c e f f e c t s that might be capable of generating p o t e n t i a l s such as were i l l u s t r a t e d . These are moto-electric effects(l6,12,18) and Ueda effects(1£,20). Unlike streaming p o t e n t i a l s , these two e f f e c t s are due to the i n t e r a c t i o n between an electrode and an e l e c t r o l y t e moving r e l a t i v e to i t . [Streaming p o t e n t i a l s on the other hand, are due to the r e l a t i v e motion between an e l e c t r o l y t e and a s o l i d surface, generally an i n s u l a t o r . The p o t e n t i a l i s measured by a p a i r of electrodes which are not located i n the moving e l e c t r o l y t e . ] Moto-electric e f f e c t s are r e a d i l y d i s t i n g u i s h able by the slow response of t h i s p o t e n t i a l to changes i n f l u i d v e l o c i t y . Further, reversal of f l u i d d i r e c t i o n does not cause immediate reversal of p o l a r i t y of the p o t e n t i a l as i s the case with streaming p o t e n t i a l s . In general, moto-electric e f f e c t s simply cause a s h i f t i n baseline and do not contribute to p u l s a t i l e p o t e n t i a l s . Ueda e f f e c t s are e s s e n t i a l l y due to rapid vibratory movement of an electrode-electrolyte i n t e r f a c e . This v i b r a t i n g motion r e s u l t s i n a sinusoidal, a l t e r n a t i n g p o t e n t i a l being developed. It i s t y p i c a l l y caused by a mechanical v i b r a t i o n of the e l e c t r o l y t e , such as caused by banging a water bath with a hard object, or by r a p i d l y c l o s i n g a valve causing a water hammer i n a pipe. [ U l t r a sonic acousto-electric e f f e c t s such as those described by Yeager et al.(20,21,22) are of too high a frequency to be factors i n our experimental results.] We have investigated the p o s s i b i l i t y that Ueda e f f e c t s were contributing to the p o t e n t i a l s generated by the LVS*s. When e l e c t r o l y t e s of high r e s i s t i v i t y (> lO^Qcm) were used, h i t t i n g the LVS with a metal object d i d produce measurable sinusoidal voltages. With e l e c t r o l y t e s of lower r e s i s t i v i t y , the e f f e c t was l e s s s i g n i f i c a n t . The Ueda e f f e c t does not account f o r the p u l s a t i l e p o t e n t i a l s measured i n the water bath, but i t d i d contribute a "sinusoidal noise" signal due to wave motions i n the bath. A factor that indicates that i t was indeed streaming p o t e n t i a l s that we measured was that the wave shape was dependent upon the rubber check valves. These valves were i n d i v i d u a l l y cast i n our laboratory by hand, using a s u r g i c a l latex. Each had a character of i t s own. As a r e s u l t , the opening and closing c h a r a c t e r i s t i c s of each



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Wave form parison

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differed. This resulted i n a noticeable variation i n s i g n a l waveform. I t i s much more r e a s o n a b l e t o assume t h e s e v a r i a t i o n s i n s i g n a l waveforms a r e due t o f l o w v a r i a t i o n s a n d t h u s s t r e a m i n g p o t e n t i a l s r a t h e r t h a n due t o a c o u s t i c e f f e c t s c a u s i n g Ueda potentials. As a f i n a l i n d i c a t i o n t h a t t h e p o t e n t i a l s m e a s u r e d were i n d e e d s t r e a m i n g p o t e n t i a l s , F i g u r e 9 i l l u s t r a t e s a c o m p a r i s o n b e t w e e n t h e wave s h a p e o f e l e c t r o d e s 4 and 5 and the flow o f blood i n t o the human a o r t a ( i ) . N o t e t h e c l o s e s i m i l a r i t y o f wave form. The p o t e n t i a l s m e a s u r e d u s i n g t h e L V S * s w i t h n o r m a l s a l i n e , were a n o r d e r o f m a g n i t u d e l o w e r than those t y p i c a l l y obtained i n v i v o , using mammals. I t was n o t o u r o b j e c t i v e t o d u p l i c a t e in vivo surface-electrolyte conditions or i n vivo potential levels. However, i t i s f e l t t h a t such l e v e l s c a n be a t t a i n e d i n v i t r o , u s i n g b l o o d a s an e l e c t r o l y t e , c o l l a g e n l i n e d plumbing and p u l s a t i l e blood flow conditions as occur i n v i v o . I n summary, i t h a s b e e n shown t h a t p u l s a t i l e flow of s a l i n e e l e c t r o l y t e s generates electrok i n e t i c p o t e n t i a l s remarkably s i m i l a r to i n vivo EKG's. This fact, i n conjunction with p r i o r research^ 2), i n d i c a t e s that the present assumption t h a t EKG p o t e n t i a l s a r e d u e s o l e l y t o m u s c l e a c t i o n p o t e n t i a l s n e e d s t o be r e - e x a m i n e d . Acknowledgments The a s s i s t a n c e o f L i n d a Stormo a n d S i d n e y Golden o f our research s t a f f i n the conduct o f t e s t s and p r e p a r a t i o n o f t h i s paper i s g r a t e f u l l y acknowledged. Abstract Several left ventricle/aorta mechanical s i m u l a t o r s were f a b r i c a t e d t o e v a l u a t e t h e p o s sibility o f g e n e r a t i n g EKG like electrical signals by electrokinetic methodology. The s i m u l a t o r s produced p u l s e d t u r b u l e n t flows, s i m u l a t i n g mammalian h e a r t pumping c o n d i t i o n s . EKG like s i g n a l s were g e n e r a t e d by t h e m o t i o n o f t h e electrolyte through the simulators.


11. FINDL AND KURTZ Literature


Cited

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