9 Estimation of Beat-To-beat Changes in Stroke Volume From Arterial Pressure-A Comparison of Two...

8/9/2019 9 Estimation of Beat-To-beat Changes in Stroke Volume From Arterial Pressure-A Comparison of Two Pressure Wave

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Research Paper ClinicalAutonomic Research 1 9 9 9 , 9 : 1 8 5 - 1 9 2O b j e c ti v e : T h e a i m o f t h is s t u d y w a s t o c o m p a r e b e a t - t o -b e a tc h a n g e s i n s t r o k e v o l u m e ( S V ) e s t im a t e d b y t w o d i f fe r e n t pres -sure w a v e a n a l y s i s t e c h n i q u e s d u r i n g o r t h o s t a t i c s t r e s s te s t in g :p u l s e c o n t o u r a n a l y s i s a n d M o d e l f l o w , ie, s i m u l a t i o n o f a t h r e e -e l e m e n t m o d e l o f ao r t ic i n p u t i m p e d a n c e .M e t h o d s : A r e d u c t i o n i n S V w a s in t r o d u c e d i n e i g h t h e a l t h yy o u n g m e n ( m e a n a ge , 2 5 ; ra n g e , 1 9 - 3 2 y ) b y a 3 0 - m i n u t eh e a d - u p t i l t m a n e u v e r . I n t r a b r a c h i a l a n d n o n i n v a s i v e f i n g e rpressure w e r e m o n i t o r e d s i m u l ta n e o u s l y . B e a t - t o - b e at c h a n g e si n S V w e r e e s t i m a t e d f r o m i n t r ab r a c h i a l p r e s s u re b y p u l s e c o n -t o u r a n a l y s i s a n d M o d e l f l o w . I n a d d i t i o n , t h e r e l a t i v e d i f f e r -e n c e s i n M o d e l f l o w S V o b t a i n e d f r o m in trabrach ia l pressurea n d n o n i n v a s i v e f i n g e r p r e s s u r e w e r e a s s es s ed .R e s u l ts : B e a t - t o - b e a t c h a n g e s i n M o d e l f l o w S V f r o m i n t r a -b r a c h i al p r e s s u r e w e r e c o m p a r a b l e w i t h p u l s e c o n t o u r m e a -s u r e s . T h e r e l a t i v e d i f f e r e n c e b e t w e e n t h e t w o m e t h o d sa m o u n t e d t o 0 .1 - 1 % ( m e a n S E M ) a n d w a s n o t d e p e n d e n to n t h e d u r a t i o n o f t i l t. T h e d i f f e re n c e b e t w e e n M o d e l f l o w ap-p l i e d t o i n t r a b r a c h i a l p r e s s u r e a n d f i n g e r p r e s s u r e a m o u n t e dto - 2 . 7 - 1 . 3 % ( p = 0 . 0 4 ) . T h i s d if f er e n c e w a s n o t d e p e n d e n to n t h e d u r a t i o n o f t i lt o r l e v e l o f a r t e r ia l pressure.

C o n c l u s i o n s : B a s e d o n d i f fe r e n t m a t h e m a t i c a l m o d e l s o f t h eh u m a n a r te r i al s y s te m , p u l se c o n t o u r a n d M o d e l f l o w c o m p u t es i m i l a r c h a n g e s i n S V f r o m i n t r a b r a c h i a l p r e s s u r e d u r i n g o r -t h o s t at i c s t re s s te s t in g i n y o u n g h e a l t h y m e n . T h e m a g n i t u d eo f t h e d i f f e re n c e i n S V d e r i v e d f r o m i n t r a b r a c h i a l a n d f i n g e rpressure m a y v a r y a m o n g s u b je c ts ; M o d e l f l o w S V f r o m n o n i n -v a s i v e f i n g e r p r e s s u r e t r a c k s f a s t a n d b r i s k c h a n g e s i n S V d e r i v e df r o m i n t r ab r a c h i a l pressure.Keywords:aort ic impedance, bloo d pressure, Finapres, head -up t i l t ,pulse contour , s imulat ion, s troke volume.

E s t i m a t i o n o f b e a t - t o - b e a tc h a n g e s i n s t r o k e v o l u m e f r o ma r t e r i a l p r e s s u r e : a c o m p a r i s o no f t w o p r e s s u r e w a v e a n a l y s ist e c h n i q u e s d u r i n g h e a d - u p t i l tt e s t in g in y o u n g , h e a l t h y m e nW i l b e r t T . J e l l e m a , M . D . , 1 B e n P . M . I m h o l z ,P h . D . , 1 H a n s O o s t i n g , M . S C . , 3 K a r e l H .W e s s e l i n g , M .S C., a n d J o h a n n e s J .v a n L i e s h o u t , P h . D . 1Dep artme nts of 1 In terna l Medicine,2TNO-BioMedica lIns trumentat ion, and aCl in ica l Epidemio logy andB ios ta t is t i cs , Academic Med ica l Cen te r , Am s te rdam,The Ne the r lands

P l e a s e a d d r e s s c o r r e s p o n d e n c e t o D r . W . T . J e l l e m a ,A c a d e m i c M e d i c a l C e n t e r , U n i v e r s it y o f A m s t e r d a m ,D e p a r t m e n t o f I n t e r n a l M e d i c i n e , R o o m F 4 - 2 5 7 , P . O .B o x 2 2 7 0 0 , 1 1 0 0 D E A m s t e r d a m , T h e N e t h e r l a n d sT e l : 31 ( 0) 2 0 5 6 6 5 9 8 1 F a x : 3 1 ( 0 ) 2 0 6 9 1 9 6 5 8E - m a i l : w . j e l l e m a @ a m c . u v a . n lReceived 10 March 1999 ; accepted 24 Ma y 199 9

F o r s e v e r a l y e a r s p r o l o n g e d h e a d - u p t i lt t e s t i n g h a s b e e nu s e d a s a t o o l i n t h e c l i n ic a l e v a l u a t i o n o f p a ti e n t s p r e s e n t i n gw i t h p o s t u r a l d i z z i n e s s o r r e c u r r e n t n e u r o c a r d i o g e n i c s y n -c o p e [ 1]. T h e i n d u c t i o n o f v e n o u s p o o l i n g w i t h s u d d e na r t e r i a l h y p o t e n s i o n a n d c o n s e q u e n t s y n c o p e f o r d i a g n o s t i cp u r p o s e s h a s c r e a t e d a n e e d f o r c o n t i n u o u s m o n i t o r i n g o fb l o o d p r e s s u r e a n d l e f t v e n t r i c u l a r s t r o k e v o l u m e ( S V ) [ 2] .I n v a s i v e b l o o d p r e s s u r e m o n i t o r i n g p r e d i s p o s e s su b j e c ts t of a i n t i n g d u r i n g o r t h o s t a t i c s t re s s [ 3 ,4 ] , a n d t h e r e f o r e n o n i n -v a s iv e m o n i t o r i n g i s r e c o m m e n d e d [ 1].

T h e n o n i n v a s i v e F i n a p r e s d e v i c e ( N e t h e r l a n d s O r g a n i z a -t i o n f o r A p p l i e d S c i e n ti f ic R e s e a r c h , B i o m e d i c a l I n s t r u m e n -t a ti o n, T N D - B M I , A m s t e r d a m ) h a s b e e n sh o w n t o p r o v id ea r e l i a b l e a l t e r n a t i v e f o r c o n t i n u o u s i n t r a - a r t e r i a l m e a s u r e -m e n t s i n a v a r i e t y o f s i t u a t io n s [ 5 ], i n c l u d i n g s u d d e n c h a n g e si n b l o o d p r e s s u r e i n d u c e d b y p r o l o n g e d h e a d - u p t i lt te s t i n g[ 6 ,7 ] . F o r t h e n o n i n v a s i v e e s t i m a t i o n o f b e a t - t o - b e a t c h a n g e si n S V d u r i n g o r t h o s t a t i c s t r e s s , i m p e d a n c e c a r d i o g r a p h y [ 8 ]a n d p u l s e c o n t o u r [ 9 ] a r e t h e t e c h n i q u e s c l i n i c a l l y f e a s i b l e .H o w e v e r , S V f r o m i m p e d a n c e c a r d i o g r a p h y c o r re l at e s m o d -e r a t e ly w i t h t h e r m o d i l u t i o n [ 1 0 ], a n d m u l t i p l e a s s u m p t i o n sa b o u t c h e s t d i m e n s i o n s a n d c o m p o s i t i o n h a v e t o b e m a d e .I n a d d i t io n , p u ls e c o n t o u r m e t h o d s a s s u m e a or t ic d i m e n -s i o n s a r e c o n s t a n t , a l t h o u g h t h e s e p r o p e r t i e s a l t e r w i t hc h a n g i n g d i s t e n d i n g p r e s s u r e s [ 1 1 ] .

R e c e n t l y , a p r e s s u r e w a v e a n a l y s i s t e c h n i q u e h a s b e e ni n t r o d u c e d t h a t e s t im a t e s c o n t i n u o u s S V f r o m a r t e r ia l p r e s -s u r e b y s i m u l a t i n g a n o n l i n e a r , t h r e e - e l e m e n t m o d e l o f a o r t i ci n p u t i m p e d a n c e : t h e M o d e l f l o w t e c h n i q u e [ 1 2 ] . T h i s t e c h -n i q u e , i n c o n t r a s t t o p u l s e c o n t o u r m e t h o d s , t a k e s i n t oa c c o u n t t h e v a r i a t i o n i n a o r t i c p r o p e r t i e s w i t h c h a n g e s i nd i s t e n d i n g p r e s s u r e s [ 1 2 ] .

T h e a i m o f t h is s t u d y w a s t o c o m p a r e b e a t - t o - b e a t c h a ng e si n S V a s i n t r o d u c e d b y o r t h o s t a t i c s t re s s in h e a l t h y y o u n gm e n e s t i m a t e d b y p u l s e c o n t o u r a n a l y s is a n d M o d e l f l o w .E a r l ie r , c h a n g e s w e r e r e p o r t e d i n S V o b t a i n e d b y p u l s ec o n t o u r a n a ly s is o f t h e i n t r a b r a c h i a l a n d f i n g e r p r e s s u r ew a v e f o r m d u r i n g p r o l o n g e d h e a d - u p t i lt t e s t in g [ 6 ]. I n t h ep r e s e n t s t u d y a r t e r i a l p r e s s u r e r e c o r d i n g s w e r e r e a n a l y z e dt o c o m p a r e c o n t i n u o u s S V o b t a i n e d w i t h t h e M o d e l f l o wt e c h n i q u e w i t h p u l s e c o n t o u r m e a s u r e s . F u r t h e r m o r e , t h er e l a t i v e d i f f e r e n c e s i n M o d e l f l o w S V o b t a i n e d f r o m i n t r a -b r a c h i a l a n d n o n - i n v a s i v e f i n g e r p r e s s u r e w e r e q u a n t i f i e d .

M a t e r i a l s a n d m e t h o d sE i g h t n o r m o t e n s i v e , h e a l t h y m a l e v o l u n t e e r s w e r e i n v e s t i -g a t e d . S u b j e c t c h a r a c t e r i s t i c s a r e s u m m a r i z e d i n T a b l e 1 .

0959-9851 9 1999 Lippincott Wil l iams & W ilkins 185


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J e l l em a e t a l.

Ta ble 1. Subject characteristics an d ave rage values and range of heart rate (HR) and m ean arterial pressure (MAP ) during head-up t i l tSubject Age Height Weight HR (bpm) MAP (mm Hg)no. (yr) (cm) (kg) (range) (range)

1 32 182 77 80 80( 6 7 - 9 3 ) ( 6 1 - 9 7 )2 22 181 72 70 81( 5 5 - 9 5 ) ( 6 2 - 9 9 )3 30 197 86 80 80( 6 3 - 1 0 2 ) ( 6 2 - 9 3 )4 22 183 72 74 74( 4 7 - 8 9 ) ( 4 0 - 9 7 )5 26 186 80 82 83( 5 9 - 1 0 4 ) ( 5 6 - 1 0 0 )6 24 187 76 75 78( 4 7 - 9 7 ) ( 6 0 - 9 4 )7 19 185 73 83 83( 5 5 - 9 8 ) ( 5 9 - 9 9 )8 2 2 1 8 5 75 83 80( 6 3 - 1 0 2 ) ( 6 2 - 9 3 )Mean 25 186 76 78 80SD _+4 _+5 _+5

Mean, av erag e di fference; SD, standard deviat ion; bpm , beats p er minute.

W ri t t en inform ed consent was obta ined before enro l lment .Th e p ro tocol was approved by the m edica l e th ica l comm i t t eeof the Academic Medica l Cente r and conforms to the pr inc i -p les out l ined in the Dec la ra t ion o f He l s inki of the W or ldMedical Association.P r o t o c o lTh e experimen tal procedure was s tarted at 9.00 A.M. in aroom wi th a cons tan t ambient t empera ture of 22 ~ C . F ingerand intrabrachial pressures were measured simultaneously.After a 30-m inute supine res t period the subjects were t i l tedup to 70 ~ in 2 to 3 seconds us ing a man ual ly driven t i l ttable with foot suppo rt . B lood pressure and heart rate werecont inuous ly moni tored before and dur ing the subsequentper iod o f head-up t il t. Th e t est was t e rmina ted b y re turn ingthe subjec t to the ho r izonta l pos i t ion a t the com ple t ion ofthe 30 -m inute h ead-up t i lt , a t the request of the subject ,or when sys tol ic blood pressure decreased more than 20m m H g .M e a s u r e m e n t sIntra-arterial blood pressure was measured thro ugh a Teflon(DuPont , Wi lmington , DE) cannula wi th a l ength of 11centim eters. After local anesthesia w ith 1% lidocaine, thecannula was inse rt ed in to the brachia l a r t e ry of the n ond om i-nant a rm by the Se ld inger t echnique . Via a 70-cent imete rlong polye thylene tube the cannula was connec ted to thetransducer of a commercial ly avai lable Oxford MedilogMa rk I I sys tem (Romulus Techn ology Ltd . , Uxbr idge ,Uni ted Kingdom) and pos i t ioned on the an te r ior ax i l l a ryborder at the second intercos tal space, a t heart level . Thedynamic performance of the invas ive sys tem was measuredby applying 100 m m H g pressure s teps with 10 mil l isecondrise t ime while recording the responses on a high speede lec tronic s t rip char t recorder (Mode l T A 4000; Gou ld Inc . ,C leve land, OH ) . Th e resonance f requency of the ca the te r-manomete r sys tem ranged f rom 14 to 30 her t z .

Finger pressure was measured with a Finapres model 5(Netherlands Organ isat ion fo r Appl ied Scienti fic Research,Biomedical In s t rum entat ion ). Finapres is based on the arte-r ial volume -clamp m eth od of Pefif iz [13] and th e Physiocal[7] (physiological ca libration) criteria o f Wesseling et al .[14] . The c uf fwas appl i ed to the mid-pha lanx o f the middlefinger of the dom inan t arm. Th e pressure t ransdu cer andthe f inger cuf f were p osi t ioned at heart level . In the f irs tminute of head-up t i lt t he pos i tions of the f inger cuf f andintra-arterial pressure transd ucer were chec ked for possiblehyd rostatic level errors and, if necessary, readjusted. In theFinapres device, a built-in expert system (Physiocal [7]) wasin operat ion to es tabl ish and maintain a proper volumeclamp setpoint [14]. Intrabrachial and finger pressure signalsand an event marker were recorded s imul taneous ly on aSanborn the rm opape r wr i te r for d irec t inspec t ion and on afour channe l FM tape recorder (Bel l and Howel l , mode l TI )for off- l ine evaluat ion.S t r o k e v o l u m e c o m p u t a t i o nPulse contour ana lys i s : th i s method computes changes inleft ventr icular s t roke vo lum e fro m the pulsat ile systolic area.SV is com pute d as: SV = A~ / Zao, where SV is the pulsecontou r s troke volume o f the hear t , As, the a rea under thesystolic porti on o f the pressure w ave, an d Zao the characteris-t ic imp edance o f the aorta . H owev er, the characteris tic prop -erties of the ao rta are pressure d epen den t and v ary with age[11]. In addi t ion, the p ulse wave veloci ty increases with age,causing peripheral ref lect ions to return to the heart duringsystolic eject ion, dis turbing the m odel . W e, therefore, usedthe improved meth od ofW esse l ing e ta l . [15], who develop eda correct ion formu la us ing mea n arterial pressure to co rrectfor pressure-depen dent propert ies of the arterial impedan ce,and heart rate to correct for early reflect ions coming fromthe per iphery , the degree of cor rec tion depending o n theage of the subject [15]. Mathem atical ly: Zao = a / [b+ (c MAP) + (d HR)] w here MA P is me an arterial pressure,

186 Cl inical Autonomic Research 1999, Vol 9 No 4


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T I M E ( M I N )F i g u r e 1 . R e l a t i v e c h a n g e s i n s t r o k e v o l u m e ( S V ) d u r i n g a 3 0 - m i n u t e h e a d - u p t il t in e i g h t s u b je c t s . V a l u e s r e f l e c t t h e a v e r a g e s o f a 1 0 - s e c o n dp e r i o d a r o u n d e a ch m i n u te o f t il t. Th e f i r s t su b j e c t to l e r a te d t il t te s t i n g fo r l e ss th e n 4 m i n u te s . B o l d l i n e s , M o d e l f l o w S V d e r i v e d fr o m t h e f i n g e rp r e s s u r e c u r v e ; t h i n l i nes , M o d e l f l o w S V d e r i v e d f r o m i n t ra b r a c h i a l p r e s s u re c u r v e ; d a s h e d l i n e s, p u l s e c o n t o u r S V d e r i v e d f r o m t h e i n tr a b r a c h ia lp r e s s u r e c u r v e .

HR is heart rate , and a , b, c , and d age-dependent parame-ters, respectively.

Pulse c onto ur SV fro m radial ar tery pressure correlatedto thermodilut ion cardiac output (CO) with a regress ionslope close to 1 (r = 0.94) [16]. Th e s tandard deviat ion forthe d i f fe rence be tween the two methods was 11% (0 .5 L/min) under the adverse condi t ions of open-heart surgery.Fur the rmore , pu l se contour SV f rom noninvas ive f ingerpressure compared to inert gas rebreathing CO in heal thysubjects pro duce d a scat ter of comparab le ma gni tu de [17].

Mo de l f low: Thi s meth od com putes an aor t i c f low wave-form from an arterial pressure s ignal us ing a n onl inear, three-e lement mode l of the hum an aor t ic input impedance [12] .Integrat ing the aort ic f low waveform per beat provides SV.CO i s com puted by m ul t ip ly ing SV and hear t rat e.

T h e m o d e l o f h u m a n a o r ti c in p u t i m p e d a n ce [ 1 8 ] d e -scribes the behavior of the aorta in opposing eject ion ofb lood by the l e f t vent r ic l e and the reb y the re l a tion be tweenaort ic pressure and aort ic inflow [19]. The three elementsof the m ode l represent the major proper t i e s of the aor t a

C l i n ic a l A u t o n o m i c R e s e a r c h 1 9 9 9 , V o l 9 N o 4 1 8 7


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TIME (SEC)Figure 2. Individual beat-to-beat changes in SV during the last minute before tilt back. B o l d l i n e s , Modelflow SV derived from the fingerpressure curve; t h i n l i n e s , Mod elflow SV derived from intrabrachial pressure curve; d a s h e d l i n e s , pulse contour SV de rived from the intrabrachialpressure curve.

a n d a r t e r i a l s y s t e m : a o r t i c c h a r a c t e r i s t i c i m p e d a n c e , a r t e r i a lc o m p l i a n c e , a n d p e r i p h e r a l v a s c u l a r r e si s ta n c e [ 1 8 ] . T h em a j o r d e t e r m i n a n t s o f t h e s y s to l i c i n f l o w a r e t h e a o r t i c c h a r -a c t e ri s ti c i m p e d a n c e a n d a r t e ri a l c o m p l i a n c e ; t h e s e e l e m e n t sa r e d e p e n d e n t o n t h e e l a s t ic p r o p e r t i e s o f t h e a o r t a . T h et h i r d e l e m e n t , p e r i p h e r a l v a s c u l a r r es i st a n c e, i s n o t a m a j o rd e t e r m i n a n t o f sy s t o li c i n f l o w [ 1 2 ], i s t i m e - v a r y i n g , a n d i sc a l c u l a te d f o r e a c h h e a r t b e a t a s t h e q u o t i e n t o f m e a s u r e da r t e r i a l p r e s s u r e a n d c o m p u t e d M o d e l f l o w C O .

T h e e l a s ti c p r o p e r t i e s o f th o r a c i c h u m a n a o r t a w e r e s t u d -

l e d b y L a n g e w o u t e r s e t a l . [ 1 1 ] . T h e y f o u n d t h a t t h e e l a s t i cb e h a v i o r o f th e a o r t a v a r i e d n o n l i n e a r l y w i t h c h a n g i n g d i s -t e n d i n g p r e s su r e s ; t h e c r o s s - s e c t io n a l a r e a o f t h e a o r t a ( A )i n c re a s e s w i t h a o r t i c p r e s s u re i n a n o n l i n e a r m a n n e r : a t l o w e rp r e s s u r e s t h e a r e a i n c r e a s e s q u i c k l y , a t h i g h e r p r e s s u r e s t h ea r e a i n cr e a se s s l o w ly . T h e y d e s c r i b e d t h e r e l a t io n o f c r o s s-s e c t io n a l a r e a to p r e s s u r e ( P ) b y a n a r c t a n g e n t e q u a t i o n ,m a t h e m a t i c a l l y e x p r e s s e d a s :

A ( P ) = A ~ 0 . 5 + 1 a r c t a n P - P o P 1188 C l in ica l Autonom ic Research 1999, Vo l 9 No 4


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Beat-to-beat s troke volume from ar ter ial pressureA ( P ) , a o r t i c c r o s s - s e c t i o n a l a r e a f o r a n y p r e s s u r e P ; A m = ,

P 0, a n d P l , a ge a n d g e n d e r d e p e n d e n t p a r a m e t e r s , r e s p e c -t ive ly .

V a l u e s o f t w o o f t h e m a j o r m o d e l e l e m e n t s ( a o r t ic c h a r a c -t e r i s t i c i m p e d a n c e a n d a r t e r i a l c o m p l i a n c e ) c a n b e d e r i v e df r o m t h e p r e s s u r e - a r e a e q u a t i o n b y a l g e b r a i c m a n i p u l a t i o n :t h e d i m e n s i o n o f c o m p l i a n c e i s d e f in e d a s a c h an g e i n v o l u m e( d V ) d i v i d e d b y a c h a n g e i n p r e s s u r e ( d P ) . A s s u m i n g t h a ta o r t i c l e n g t h ( 1) i s c o n s t a n t , c h a n g e s i n v o l u m e ( V = m r 2 .1o r 7 r.A .1 ) a r e p r o p o r t i o n a l t o c h a n g e s i n c r o s s - s e c t i o n a l a r e a( d A ). C o n s e q u e n t l y , c o m p l i a n c e ( C ) is c o m p u t e d b y C =d A / d P a n d a o r t i c c h a r a c t e ri s t ic i m p e d a n c e ( Z 0 ) c a n b e d e -r i v e d f r o m t h e s t a n d a r d f o r m u l a : Z 0 = ~ v / p /A . C , w i t h p ,d e n s i t y o f b l o o d .

W i t h t h e a r t er i a l p r e s s u r e w a v e f o r m , s u b j e c t g e n d e r , a n da g e a s i n p u t , t h e M o d e l f l o w s o f t w a r e c o m p u t e s v a l u e s o ft h e m o d e l e l e m e n t s C w a n d Z 0 f o r e ac h n e w p r e s s u r e s a m p l et a k e n a t 1 0 0 h e r t z . M o d e l f l o w a p p l i e s t h e v a l u e s f o r A m~ x,P 0, a n d P 1 f r o m a b u i l t -i n d a t a b a s e . I n s t a n t a n e o u s v a l u e s o fC w a n d Z 0 a r e u se d i n t h e m o d e l s i m u l a t i o n r e s u l t in g i n t h ec o m p u t a t i o n o f a n a o r ti c f lo w w a v e f o rm .

I n c a r d i a c s u r g e r y p a t i e n t s, M o d e l f l o w C O ( r a di a l a r t e r yp r e s su r e ) s h o w e d a d e q u a t e t r a c k i n g o f b o l u s t h e r m o d i l u t i o nC O o v e r a ra n g e o f 3 .1 t o 6 .9 L / m i n f o r s e v e r a l h o u r s[ 1 2 ] . I n m e c h a n i c a l l y v e n t i l a t e d p a t i e n t s w i t h s e p t i c s h o c k ,M o d e l f l o w c a r d i a c o u t p u t r e f l ec t s t h e r m o d i l u t i o n c a r d i a co u t p u t o v e r a r a n g e o f 4 . 1 t o 1 8 . 2 L / r a i n [ 2 0 ]. U n d e r t h e s ec i r c u m s t a n c e s , M o d e l f l o w a p p l i e d t o t h e n o n i n v a s i v e f i n g e rp r e s s u r e a c c u ra t e l y tr a c k s c h a n g e s i n t h e r m o d i l u t i o n c a r d i a ci n d e x f o r s e v e ra l h o u r s ; t h e o v e r a l l d is c r e p a n c y b e t w e e n b o t hm e a s u r e m e n t s w a s 0 . 1 4 L . m i n 1 . m -2 [ 2 1 ]. M o d e l f l o w S Va s o b t a i n e d f r o m f i n g e r p r es s u r e i n h e a l t h y s u b j e c t s s u b j e c t e dt o o r t h o s t a t i c s t r e s s t r a c k s t h e r m o d i l u t i o n - b a s e d e s t i m a t e o fS V w i t h a s m a l l ( 3 + 8 m l ) o f f s e t d u r i n g 7 0 ~ p a s s iv e h e a d -u p t i l t [ 2 2 ] .Data analys isT h e i n t r a b r a c h i a l a n d f i n g e r p re s s u r e s ig n a ls a s r e c o r d e d o nm a g n e t i c t a p e w e r e A / D c o n v e r t e d o f f - l i n e a t a s a m p l i n gr a t e o f 1 0 0 h e r t z . B e a t - b y - b e a t s y s t ol ic , m e a n , a n d d i a s to l i cb l o o d p r e s s u r e s w e r e r e c o r d e d . M e a n a r t e r i a l p r e s s u r e w a so b t a i n e d a s t h e i n t e g r a l o f p r e s s u r e o v e r o n e b e a t d i v i d e db y t h e c o r r e s p o n d i n g b e a t i n t e rv a l . I n s t a n t a n e o u s h e a r t r a t ei n b e a t s / m i n w a s c o m p u t e d a s t h e i n v e r se o f t h e i n t e r b e a t i n -te rva l .

A p u l s e w a v e r e c o n s t r u c t i o n t e c h n i q u e w a s a p p l i e d to t h ef i n g e r p r e s s u r e s i g n a l t o r e c o n s t r u c t a b r a c h i a l a r t e r y b l o o dp r e s s u r e c u rv e . T h i s t e c h n i q u e c o r r e c ts f o r t h e p h y s i o l o g i cp u l s e s h a p e a n d p r e s s u r e l e v e l d i f f e r e n c e s b e t w e e n t h e b r a -c h i a l a rt e r y a n d f i n g e r [ 2 3 ]. A p p l i c a t i o n o f p u l s e w a v e r e c o n -s t r u c t i o n h a s b e e n d e m o n s t r a t e d t o i n c r e a s e t h e c o m p a r a b i l -i t y o f M o d e l f l o w S V f r o m i n t r a b r a c h i a l a n d f i n g e r p r e s s u rei n p a t i e n t s w i t h a v a r y i n g d e g r e e o f v a s c u l a r d is e a s e [ 2 4 ] .

A c h a n g e i n f i n g e r c u f f p o s i ti o n c a n a l te r t h e s h a p e o ft h e a r te r ia l w a v e f o r m u s e d f o r t h e S V c o m p u t a t i o n . T h e r e -f o r e , o n l y d a t a o b t a i n e d a f t e r th e f i n g e r c u f f r e a d j u s t m e n tw e r e u s e d i n t h e c o m p a r i s o n a n d t h e S V b a s e l in e v a l u e w a sd e f i n e d a s t h e 1 0 - s e c o n d p e r i o d a r o u n d t h e f ir s t m i n u t e o f

t i l t - u p ( 5 5 t o 6 5 s ) . I n a b s e n c e o f a r e fe r e n c e S V m e a s u r e -m e n t , c h a n g e s i n S V a r e e x p r e s s e d a s p e r c e n t a g e c h a n g e sf r o m b a s e l i n e .

S t r o k e v o l u m e m e a s u r e m e n t s d u r i n g h e a d - u p t il t w e r ea n a l y z e d i n t w o w a y s . F i r s t , t o c o m p a r e t h e l o n g - t e r m ( 3 0r a i n ) a p p l i c a b i l i t y a n d t o p e r m i t f o r s t a t is t i c a l a n a ly s i s , v a l u e sw e r e a v e r a g e d o v e r 1 0 - s e c o n d p e r i o d s a r o u n d e a c h m i n u t eo f t i l t - u p ( 1 1 5 t o 1 2 5 s , 1 7 5 t o 1 8 5 s , e t c . ) a n d e x p r e s s e da s p e r c e n t a g e c h a n g e s f r o m b a s e l i n e . S e c o n d , t o e n a b l e i n -s p e c t i o n o f t h e s u d d e n a n d f a s t c h a n g e s i n S V a s o b s e r v e di n t h e l a t e s t s t a g e s o f ti l t , p e r c e n t a g e c h a n g e s d u r i n g t h el a s t m i n u t e p r i o r t o t i l t - b a c k a r e p r e s e n t e d o n a b e a t - t o -b e a t b a s i s .Statistical ana lysisI n d i v i d u a l d a t a a r e p r e s e n t e d a s m e a n d i f f e r e n c e + s t a n d a r dd e v i a t i o n ( S D ) . G r o u p r e s u l t s a r e e x p r e s s e d a s m e a n d i f f e r -e n c e + s t a n d a r d e r r o r o f t h e m e a n ( S E M ) . I n o r d e r t o as se sst h e r a n d o m d i f f e r e n c e s b e t w e e n s u b j e c t s a n d t o c o r r e c t f o rt h e u n e q u a l n u m b e r o f d a ta p o i n t s b e t w e e n s u b j ec t s, a g e n -e r a l m i x e d m o d e l o f a n al y s is o f v a r i a n c e w a s u s e d ( B i o m e d i -c a l P r o g r a m s , B M D P , U n i v e r s i t y o f C a l i f o r n i a , L o s A n g e l e s ,C a l i f o r n i a ) . S e q u e n t i a l a n a l y s i s ( W a l d t e s t [ 2 5 ] ) w a s a p p l i e dt o a s s e s s s y s t e m a t i c d i f f e r e n c e s i n S V a n d t o i d e n t i f y a p o s s i -b l e t i m e d e p e n d e n c y . A p v a l u e l e s s t h a n 0 . 0 5 w a s c o n s i d -e r e d s i g n i f i c a n t .

Resu l t sS u b j e c t c h a r a c te r i st i c s a n d h e a r t r a t e ( H R ) a n d m e a n a r te r i alp r e s s u r e (M A P ) r a n g e d u r i n g t h e h e a d - u p t i lt m a n e u v e r ar ep r e s e n t e d i n T a b l e 1 . I n s i x o f t h e e i g h t s u b j e c ts h e a d - u pt i l t h a d t o b e t e r m i n a t e d w i t h i n 3 0 m i n u t e s : t w o s u b j e c t se x p e r ie n c e d p r e s y n c o p a l s y m p t o m s ( w e a k n es s a n d a b d o m i -n a l d i s c o m f o r t ) w i t h o u t a fa l l i n b l o o d p r e s s u r e a f te r 4 a n d2 1 m i n u t e s a n d w e r e t i l t e d b a c k o n r e q u e s t ; a f t e r 1 2, 1 8 ,2 3 , a n d 2 9 m i n u t e s a fa l l i n s y s t ol i c b l o o d p r e s s u r e g r e a te rt h a n 2 0 m m H g d e v e l o p e d i n t h e o t h e r f o u r s u b j e c t s w i t hp r o m p t r e c o v e r y u p o n t i l t b a c k [ 6 ].Stroke volum e j% m intrabrachial pressure: pulse contourand Mode l flowD u r i n g a 3 0 - m i n u t e h e a d - u p t i l t , p e r c e n t a g e c h a n g e s i np u l s e c o n t o u r S V f r o m i n t r a b r a c h i a l p r e s s u r e w e r e c l o s e l yf o l l o w e d b y M o d e l f l o w S V i n a l m o s t a l l s u b j e c ts ( F i g s. 1a n d 2 , T a b l e 2 ) . T h e m e a n S V d i f f e r e n c e b e t w e e n t h e t w ot e c h n iq u e s a m o u n t e d to 0 .1 + 1 . 0 % ( m e a n + S E M ) . T h i sd i f f e re n c e w a s n o t d e p e n d e n t o n t h e d u r a t i o n o f t il t.Mod elf low s troke volume from intrabrachial an dfinger pressureB e f o r e a p p l i c a t i o n o f t h e p u l se w a v e r e c o n s t r u c t i o n t h e d i f -f e r e n c e b e t w e e n M o d e l f l o w S V f r o m i n t r a b r a c h i a l a n d f i n g e rp r es su r e a m o u n t e d t o - 0 . 8 + 2 . 8 % ( m e a n + S E M ) . W i t hp u l s e w a v e r e c o n s t r u c t i o n t h e d i f f e r en c e w a s - 2 . 7 + 1 . 3 %( i n t r a b r a c h i a l v s f i n g e r p r e s s u r e p = 0 .0 4 ) . T h e d i f f e r e n c ei n S V w a s i n d e p e n d e n t o f t h e d u r a t i o n o f ti l t a n d l e v e l

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J e l l e m a e t a l .T a b l e 2 . I n d i v i d u a l d i f fe r e n c e s b e t w e e n p u l s e c o n t o u r a n a l y s i s a n d mo d e l s i mu l a t io n a p p l i e d t o t h e i n t r a b ra c h i a l p r e s s u r e w a v e

D a t a p o i n t s D a t a p o i n t s A V G S D ( % )S u b j e c t ( 1 0 s a v g ) w h e n Me a n S D ( % ) ( b e a t s ) d u r i n g l a s t d u r i n g l a s t m i n . o f

no . t i lt - up dur i ng HU T min . t i lt - up t i lt - up1 2 2.1 -+ 1.8 85 0.4 3.12 28 0.2 -+ 1.0 72 1.0 0 .63 19 4.5 _+ 2.7 86 3.3 _+ 4.34 10 2.7 _+ 1.2 68 -1 .6 _+ 4.95 1 6 - 7 . 3 3 . 2 8 6 - 9 . 0 + 3 . 06 28 0.7 _+ 2.1 82 0.2 _+ 4.27 28 0.3 1.1 90 1.1 3.58 2 2 - 2 . 4 2 . 0 1 0 2 - 1 . 3 _+ 9 .1

Me a n , a v e r a g e d i f fe r e n c e f o r e a c h s u b j e c t; S D , s t a n d a r d d e v i a t i o n ; H U T , h e a d - u p t i lt .

of arterial pressure (range MAP: 40 -1 00 m m Hg). Th emagni tude of the difference in SV varied among subjectsup to 7% (subject 2 in Table 3) . St i l l , Modelf low SV fromnon-invasive finger pressure tracked fast and brisk changesin SV derived from intrabrachial pressure in all subjects(Fig. 2).

D i s c u s s i o nP r e s su r e w a v e a n a l y s i s a p p l i e d t o a p e r i p h e r a l a r t e r i a lp r e s s u r e s i g n a lTheoret ical ly, aort ic pressure is the preferred waveform forthe co mp utat io n of SV in bo th pressure wave analysis tech-niques. H ow eve r, aortic pressure is not rou tine ly availablein clinical practice, and perip hera l arterial pressure resemblesaortic pressure sufficiently to compute SV [12]. With pulseco nto ur analysis, the percen tage changes in pulsatile systolicarea o f the a ort ic wave form were s imilar to changes in theperip herally derived pulsatile systolic area, even thou gh abso-lute values were different [26]. In addi t ion, the computedflow wavefo rm in M odelflow was dis torted because the pe-ripheral pressure wave was distorte d; however, th e area unde rthe com puted f low waveform ( i e , s t roke volume) w as affectedonly m inimal ly [12].P r e s su r e w a v e a n a l y s i s : p u l s e c o n t o u r v er su s M o d e l s q o wA study comparing both techniques appl ied to radial ar terypressure in cardiac surgery pat ients (m ean age, 58 y) dem on -s t ra t ed a reduc t ion in the SD of the d i f fe rence in C O f rom12% w i th pul se conto ur to 8% wi th M ode l f low, in re ference

to the rm odi lu t ion CO [12] . Thi s i s in cont ras t to the f indingof the present s tudy in young, hea l thy voluntee rs repor t ings imilar changes in SV with bo th techn iques from intrabra-chial pressure recordings (Figs. 1 and 2) . Th is com parabi l i tyof SV t racking can be a t t r ibu ted to the y oung age of thesubjects inves tigated: in th e three -eleme nt mod el of aort icinput im pedance , the pr inc ipa l e l ement tha t de te rmines f lowin young adults is aortic characteristic impedance. Withincreasing ag e, arterial com plian ce diminishes [27 ] and be-comes the do mina nt e l em ent in the mo de l . Langewoute rs e ta l. [11] fou nd that th e hu m an arterial com pliance decreasesnonl ine arly when arterial pressure r ises . The refore, co nsider-a t ion of the nonl inear decrease in com pl iance becomes moreimpor tan t wi th inc reas ing age . Th e M ode l f low techniqueaccounts for this physiological decrease with mo re precis ionthan p ulse con to ur analysis [12]. In the presen t investigation,the pe r forman ce of Mod e l f low was equiva lent to pul se con-tour analys is . Th is m ay be at t r ib uted to the fact that changesin compl iance a re not a major cont r ibut ion to Mode l f lowSV computa t ion in young adul t s .M o d e l f l o w a p p l i e d t o i n t r a b r a c h i a l a n d f i n g e r p r e ss u r eMo delflow SV es t imat ion f rom the noninvasive f inger pres-sure s ignal is a techn ique th at can easi ly be perform ed durin gprolon ged head-u p t i l t tes ting, a l lowing b eat- to-beat SVmo nitorin g thro ugh ou t the inves tigation. How ever, f ingerarteries are affected by contrac t ion and di latat ion in relat ionto psychological an d physical (heat, cold, blo od los s, or-thostasis) stre ss [28]. Th us, finger arterial pressure de pen dson several factors aff ecting peripheral b loo d flow. Effects

T a b l e 3 . I n d i v id u a l d if f e r e n c e s i n Mo d e l f lo w S V d e r i v e d f r o m i n t ra b r a c h i a l a n d f in g e r p r e s s u r eD a t a p o i n t s D a t a p o i n t s

S u b j e c t ( 1 0 s a v g ) w h e n Me a n S D ( % ) ( b e a t s ) d u r i n g l a s tno . t i lt - up dur i ng HU T min . t i lt - up

AV G _+ SD (%)dur i ng l as t min . o f

t i l t -up1 2 -3 .8 -+ 0 .2 85 6 .3 _+ 4 .22 2 8 - 7 . 1 3 . 7 7 2 - 1 2 . 4 7 . 33 19 -5 .2 3 .0 86 5 .4 _+ 4 .94 1 0 - 5 . 5 + 1 . 9 6 8 1 1 . 7 1 55 1 6 0 . 5 5 . 4 8 6 - 5 . 2 1 26 28 -3 .1 -+ 4 .6 82 0 .7 4 .77 28 -2 .1 __+ 3.1 90 -3 .1 -+ 6.58 22 4 .6 2 .0 102 4 .9 6 .3

Me a n , a v e r a g e d i f fe r e n c e f o r e a c h s u b j e c t ; S D , s t a n d a r d d e v i a t i o n ; H U T , h e a d - u p t ilt .

1 9 0 C l i n ic a l A u t o n o m i c R e s e a r c h 1 9 9 9 , V o l 9 N o 4


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Beat - to -b ea t s t roke vo lurne j~o m ar ter ia l pressureof these pheno men a a re , however , reduced by the bui l t - inPhysiocal a lgori thm in Finapres and rel iable t racking ofintra-arterial measurements by noninvasive f inger pressurehas been de mo nstrated u nde r ci rcumstances of low arterialpressure [6] and in pat ients with b oth h yperten s ion andvascular disease [26]. This study clearly demonstrates thatModelflow SV from noninvasive f inger pressure can t rackfas t and brisk changes in s imultaneous intrabrachial re-cordings in heal thy yo ung m ale subjects, a l tho ugh the mag-ni tude of the d i f fe rence in SV m ay vary amon g subjec ts .

Recent ly Vooge l e t a l . [24 ] demon s t ra ted tha t pu l se waverecon struct ion with age and pressure level correct ion s ignif i-cant ly inc reased the comparabi l i ty of Mode l f low SV f romintrabrachial and f inger pressure in pat ients with a varyingdegree of vascular disease (- 7 .5 + 17% to 4.1 + 12%,p < 0.05). I t i s wel l -kno wn that pulse shape and pressurelevels obtain ed fro m th e f inger differ from those o f thebrachial artery. Owing to pulse wave reflections and thegradual dec l ine of MAP f rom the hear t toward the pe r iphery ,the f inger pressure w ave becom es dis torted [30]. Th ese physi-o logic phenomena [27] and the i r cor rec t ion become moreimp orta nt w ith increas ing age and degree o f atherosclerosis[ 2 4 ] . In the present s tudy we fou nd a smal l di fference be-tween SV from intrabrachial pressure and f inger pressureregardless of pulse wave recon struct ion. Ho weve r, our s tu dyindicates th at in heal thy you ng m en the t racking o f int ra-brachia l -de r ived Mod e l f low SV by f inger pres sure - -d er ivedMod el f low SV may be improved by pul se wave recons t ruc-t ion (SE M decreased fro m 2.8% to 1.3%) at the cos t of aslightly increased average difference (difference from -0.8%t o - 2 . 7 % w i t h re c o n st r uc t io n ) .

Conclus ionsA l imitat ion of this s tudy is the absence of a referencetechnique to d e te rmine bea t - to-bea t SV, and we a re the re foreunable to com me nt on the absolu te changes in SV wi th b othtechniques during head-up t i l t tes t ing. In heal thy subjects ,va l idat ion of a new techniqu e to m oni tor S V is t roublesomebecause a reference m easurem ent requires a pulm on ary arterycatheter ( thermodilut ion) or is di ff icul t to obtain withchanges in po sture over longer periods of t ime (Do pplerul t rasoun d echocardiography) [31,32]. In addi t ion, s tandardCO techniques are discont inuous and del iver values for SVaveraged over m any heartbeats . These m ethods d o n ot reflectthe considerable beat- to-beat f luctuat ions in SV, presenteven in the recumbent pos i t ion [ 3 2 ] , as found wi th thetechniques discussed in this article.

Bea t - to-bea t pul se contou r and impedance ca rd iographySV have been compared dur ing head-up t i l t by Schondorfe t a l . [ 3 3 ] . They conc luded tha t bo th pul se contour andimpedance SV ca nnot be used in terchangeably dur ing head-up t i l t bu t can be regarded as equivalent descriptors of apop ulat io n response to head -up t i lt . In addi t ion, a comp ari-son of Mod e l f low and Dop ple r u l t rasound SV [34] dur ingorthostat ic s t ress in he al thy you ng m en showed equ al assess-m ent o f beat- to-be at changes in S V during sup ine rest .

In conclus ion, we compared beat- to-beat changes in SVest imated by two different pressure wave analys is tech-n i q u e s - p u l s e c o n t o u r an aly si s a n d M o d e l l o w - - i n h e a l t h yyou ng m en dur ing or thos ta t i c s tres s. Both t echniques ap-pl ied to intrabrachial pressure t racked percentage changesin SV wi th comparable pe r formance . Fur the rmore , f ingerpres sure - -d er ived M ode l f low SV can be used to t rack bea t-to-beat SV changes from basel ine com pared to s imu ltaneousintrabrachial recordings . However, prudence is cal led forwhen in te rchanging f inger pres sure - -der ived Mode l f lowvalues for int rabrachial-derived values in the individualsubject.

Acknow ledgmen tsThis work was suppor ted in pa r t by a grant f rom the Du tchHear t Fo unda t ion (NR 94.140) and the Rui t inga-van Swie-ten Foundat ion (W.T. Jel lema).

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9 Estimation of Beat-To-beat Changes in Stroke Volume From Arterial Pressure-A Comparison of Two...

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