Note: Descriptions are shown in the official language in which they were submitted.
~30~
BLOOD PRESSURE MONITORING METHOD ~ND ~PPARATUS
ORIGIN OF THE INVENTION
The ln~ention described herein wa~ made ln the
course of work under a Brant or award from th~
5 Department of ~ealth and Human Services.
TECHNICAL FIELD
This invention relates to method and app~ratus
for non-in~a~i~ely ~onitoring blood pres~ure through ~se
of a tran6ducer ~rray of individual pressure or force
senslng elemeDts, ~nd to method and means for
ascertalning the correct transducer hold-down pressure
required for obtaining accurste blood pre~sure
mea~urements.
BAC~GROUND OF THE INVENTION
The contlnuous measurement of blood pressure
by use of srterlal tonometer traQ~ducers i~ well known
~B ~hown in U~S. Pstent Numbers 3,123,06B to R.P.
Bigliano, 3,219,035 to G.L. Pre6sman, P.M. Newgard and
John J. ~ige, 3,880, 145 eO E.F. Bllck, 4,269,193 to the
present i~entor, and 4~423,738 to P. M. New~ard, and in
e~ ~rticle by G.L. Pres~msn and P.M. Newgsrt entltled "A
Tran~ducer for the Contlnuous External Mea~ure~ent of
Arteridl Blood Pressure" (IEEE Tran~. Bio Med. ~lec.,
Apr~l 1963, pp. 73-81).
In a typicfll tonometrlc technique for
~onitorlng blood preseure, a transducer ~hich includea
~n crray of pres~ure ~e~siti~e element~ 15 positioned
o~er a super~lcial artery, ~Dd a hold-down force i6
2 ~l30~Æ53
applied to the tran6ducer BO ~8 to flatten the wall of
the underlylng artery ~lthout occlutlng the artery. The
pres~ure sen~lti~e element6 ln the array typicslly have
at least one dimension ~maller th~n the lumen of the
5 underlyin~ ~rtery ln which blood pressure is measured,
and the transducer i6 positioned ~uch thst ~t lea~t one
of the indi~idual pres~ure-sensitiYe element6 iB oYer ~t
lea~t a portlon of the underlylng artery. The output
from one of the pre~sure ~ensitive elementB iB ~elec$ed
for monltoring blood pre~6ure. In some prior art
arrangements, the pressure sen~iti~e element having the
m~imum pulse amplitude output is selected, ~nd in other
arrsnge~ent~ the element haYing a local minimum of
diastolic or systolic pressure which element i~ within
substantially one artery diameter of the element which
generates the ~a~eform of maximum pulse amplitude i~
selected . , - .
The pre~sure mea6ured by the selected pressure
sensitive element, i.e the element centered over the
artery, wlll depend upon the hold-down pressure used to
press the transducer ~gain6t the skin of the sub~ect.
Although fairly accurate blood pressure measurements ~re
obtained when 8 hold-down pres6ure within 8 rather wide
pre3sure ran8e ls employed, lt has been found that there
exists a sub~t~ntlslly unique ~lue of hold-down
pressure within said range for which tonometrir
measurements are most accurate. Thi6 so-called "correct"
hold down pressure varies among sub~scts. Wlth prior
art tonometrlc type transducerR ~he correct hold-down
pressure often 18 not determlned thereby leading to
inaccur~cles ln the bloot pres~ure ~ea~urements.
SUMMARY ~ND OBJECT~ OF T~E INVENTION
A~ ob~ect of this ln~ention 18 the pro~ision
of an improYed ~onometric type method and appar~tus for
3 ~ 4~3
non-in~a6i~ely monitoring blood pres6ure wl~h a hiph
de8ree of accur~cy.
Another ob~ect of thi3 lnventlon ls the
pro~i6ion of ~uch a blood pressure messuring method and
5 appur~tu6 which includes the use of a transducer h~ving
~n arrsy of ind~idual arteri~l rlders (pressure
se~sltive elsment~) and wherein mesns sre provided for
determining the correct pres6ure required to hold the
tr~nsducer ug~inst the sub~ect to assure ~ccuracy of the
blood pressure readin~s.
The present lnvention includes a tr~nsducer
arr~y for gener~tion of electrical waYeform~ indicati~e
of blood pres~ure ln an ~rt~ry. Using the ~elected
pressure sensing element that 18 determined to be
poæitioned substsntially o~er the center of the
unterlying artery, n set of st least one of the
diastollc pressure, eystolic pressure, or pulse
amplitude pre~sure ~ersus hold-down pressure values o~er
a ran8e of hold-down preBsures between which the
underlying 8 rtery i~ unf 18 ttened and ls occluded is
obtained. ~ polynomial iB fitted to at least one of
the set6 of ~alues from which polynomial the correct
hold-down preBsure 1B tetermined. The hold-down
pressure at the polnt of minimum slope of graphs of ~he
polynom~al6 fitted to the ~ystolic or diastolic ~ersus
hold-down pressure ~alues provides An indication of the
correct hold-down pressure. ~n indication of the
correct hold-down pre~sure using the pulse 0mplitude
meB8Urement~ 16 proYided by loc~ting the polnt where the
slope of the polynomi~ zero or the midpoint of ~
pair of hold-down pre6sures at whith the pul~e smplltude
1~ sub6ts~tiHlly ~isty percent of maslmum pul~e
zmplltude of the ~raph of the polynomial fitted ts the
pulse a~plltude qer~us hold-down pres~ure ~alues. A
~easure of compllance of the underlying ~rtery ls
4 ~l30~53
obtai~ed fro~ the ratio of tbe mini~um 810pe of the
graph of the polynomial fltted ~o one ~f the ~y6tolic or
diastollc pres~ure ~er~us hold-down pre~sure values to
the ~lope of a straight line fitted to a ~ubset of one
of the systolic or tlastolic ~ersus hold-down pressure
Yalues oYer a rsnge of hold-down pressures below wh~ch
flattenlng of the underlying artery beBin6 or it may be
obtained dlrectly from the polynomial coefficlents. The
hold-down presæure st which fla~tening of the underlying
artery be8ins i8 taken as the lowest of the abo~e-
mentlonea pair of hold-down pressures at whlch the pul6e
amplitude is subst2ntially sixty percent of maximum
pulse amplitude of the graph of the polynomial fitted to
the pulse amplltude versus hold-down pressure ~alue6.
BRIEF DESCRIPTION OF THE DRAWINGS
The in~ention, ~ogether with the abo~e and
other ob~ects snd adYantages thereof will be better
understood fro~ the followins description when
considered with the accompanying drawings. It vlll be
understood, howe~er, that the lllustrated embodiments of
the invention are by way of example only and that the
ln~entlon is not limited thereto. The noYel fe~tures
which are believed to be ch~racteristic of the inYention
are set forth wlth particularity in the appended claims.
In the drawlngs, whereln like reference character6 refer
to the same parts in the se~eral ~iews:
Flg. 1 shows the external appearance of a
blood pres6ure transducer csse, typically po itioned
o~er a superficial artery auch a6 the radial srtery, f~r
pro~id$ng ~ continuouB external measurement of arterial
blood pres~ure;
Fig. 2 1~ a schematic dlH~ram lllustratin~
the force balance bet~een thP artery and the multiple
transducer elem~nts tarter$al riders), with the artery
.~3~4~53
wall properly depre~aed ~o giYe accur~te blood press~re
readin~6;
Fig. 3 1~ a combination 6implified ~ertlcal
sectional ~iew taken through the transducer case of Fig.
1 and block dlagram of a system wh$ch may be employed
there~ith ln the practice of thi~ ~n~ention;
Fi~. 4 is & waYeform of human blood pres~ure
versus tlme of the type which may be obtained using ~he
present inve~lon for illustrating syctolic and
10 disRtolic pre66ures and pulse amplitude of the blood
presBure wsve;
Figs. 5A and SB to~ether show a flow chart
for use in explaining o~erall operation of this
lnvention;
15Fig. 6 ~ho~ plots of diastolic pressure snd
pulse amplitude ~er6us hold-down pressure for a typical
~ub~ect; and
Fi~. 7 iB a flow chart showin~ details o~
the computstion of a correct hold-down pressure.
20A typical appllcation of the transducer
array f or arterial tonometry 1B illustrated in Fig. 1
whereln the transducer housiDg, or case, 10 whlch may
ha~e the appearance of ~n ordinary wristwatch c~se, is
held in place over the radial artery ln a human wrist 12
by a band 14. ~ cord 16 extends from the tran~ducer
housl~g 10 through which electrical wiring for the
transducer array~wlthin the housing, together with a
8m811 tube that conDects the housing to an ~ir pressure
~ource, e~end. The wirln~ 18 and tube 20 are 6hown in
Fi~. 3, but not ln Fig. 1.
Reference now i8 made to Fi8. 2 wherein a
diagr~mmQ~lc mechsnic~l model 18 shown which is
representstl~e of fsctor~ to be considered in the
physical syste~. The illu~trated model is that shown in
the ~boYe-mentioned J.S. Eckerle p~tent number 4,269,193
6 1~04~5i3
~hlch ~as ~dap~ed from the G.L. Pre~man and P.M.
Newgard ~rtlcle entitled "A Trsnsducer for the
Continuous External Messurement of ~rter$al Blood
Pressure~. In brlef, an ~rray 22 of lndi~dual pressure
sensiti~e element6 or transdu~ers 22A through 22J which
con~titute the ~rterlal riders, i8 po61tioned ~o that
one or more of the rider~ ~re entirely o~er ~n artery
24. The lndi~idual riders 22A-223 ~re ~mall relatlve to
the diameter of the srtery 24 thus assurin~ that at
least one of the rider~ in it~ entirety ls over the
artery. The skln surface 26 and artery underlying the
transducer must be flattened by flppl~cation of a hold-
down pres~ure to the transducer. One rider overlying
the center of the artery is ldentlfied as the "centered"
srterial rider, from wh~ch arterial rider pressure
readin86 for ~onitoring blood pressure are obtained.
Mean6 for selecting the arterial rlder are disclosed in
the above-mentioned J.S. Eckerle ps~ent and G.L.
Pressman and P.M. Newgard ~rticle. Using the sbove-
ment~oned rider selecting means, rider 22E, for example,may be selected as the ~centered" srterial rider, in
~hich csse the remainder of the riders, here riders 22A-
22D and 22F through 22J comprl~e side plates which ~erve
to flatten the underlylng ~kin snd srtery. Depending
upon the pO8~ tioning of the transducer on the sub~ect, a
different trsnsducer element may be positioned oYer the
center of the artery and thereby be ~elected 3S the
"centered" arterlsl rider.
Superficial arteries, such as the radial
artery, are ~upported fros below by bone which, in Fig.
2 1B illu~trated by ground symbol 28 under ~he srtery~
The w~ll of srtery 24 behs~es ~ubstantlally like a
~embrsne ~n that it transmlts tension force6 but ~ot
bendlnR ~o~snts. The ~rtery wnll re6ponds to the
loading force of the transducer, snd during blood
:: .
7 ~3~53
pressure measurements 8Ct6 as lf it i8 resting on the
firm ba~e 28. The effecti~e ~iffness of an srtery wall
i~ ~mall And differs between 6ub~ects. In prior art
mech~nlcsl models of the physical ~ystem, the effecti~e
stiffDess of the ~rtery wall i6 taken ~s zero, ln which
case the actual hold-down pressure employed iB not
consldered to affect ~ccur~cy of the blood pressure
readingB BO lon~ as the transducer i6 pressed agalnst
the 6kin surface with sufficient force to cause
compre~sion but not occlusion of the underlying ~rtery.
Appllcant ha~ found thst not only are blood pre6sure
readings dependent upon hold-down pressure within the
ran~e of hold-down pressures that the sr~ery is
flsttened but not occluded, but thst most accurate blvod
pres~ure resdings ~re obtained where a hold~down
preasure is selected thst iB subætsntially midway
between the pressure where flattening of the ~rtery
begln6 and the minimum pressure required for occluding
the ~ame. Novel steps in~olved ln computing the correct
hold-down pressure are described ln detail hereinbelow
following completion of the description of the
mechanic~l model of Fi8. 2 snd the oversll system 6hown
in FiB. 3.
With the illu~trsted system, the trRnsducer
csse 10 ~nd mounting str~p 14 together with sir pressure
applied to ~ ~ellows, 54, supply the required
compres6ion force and hold the riders 22A-22J in such a
manner ~hat ~rterisl pressure chsnges are transferred to
ehe riders ~hich oYerlie tbe ~rtery 24.
Di2grAmmat~c~lly thls 18 illustrated by showing the
lndi~idual riders 22~-22J backed by rlder sprlng
me~ber~ 30~-30J, re~pec~i~ely, a rigld ~pring bscking
plste 32, aDd ~ hold-down force generator 36 between the
b~cking plate 32 and the ~ounting strap system 38.
If, without force generator 36, the ooupling
4~53
between the mountlng s~Lrap 8y8tem 38 and sprlng backing
pl~te 32 ~ere infinltely ~tiff ~o restrain the riders
22A-22J rigldly with respect to the bone structure 2B,
the riders would be maintained in a fi~ed positlon
relati~e to the artery. In practice, bowe~er, 6uch a
~y~tem i8 not practic~l, and hold-down fo~ce ~enerator
36, comprising a pneumatic or other 6uitable loadin~
system, is lncluded to keep const3nt the force applied
by the mountin~ ~trap system 38 to riders 22A through
22J. In the mechanical model the 6prlng con~tant) k
(pressure ~er unit of deflection) of the force generator
36 i8 nearly zero. Suitable pneumatic loading systems
are shown and described in the abo~e-reerenced U.S.
Patent number~ 3,219,035, 4,269,193 and the Pressman-
Newgard IEEE article.
In order to insure that the riders 22A through22J flatten the artery and pro~ide a true blood pressure
measurement, they must be rigidly mounted to the backing
plate 32. Hence, the rider ~prings 30A through 30J of
the model ideally are infinitely ri~ld (~pring constant
k .O~ ). It i8 found that as long as the sy~tem
operates in such 8 mBnner that it can be ~odeled by
rider spring6 30A ~hrou~h 30J having a spring constant
on the order of about ten t~mes the value f~r ~he
artery-~kin syBtem~ 80 that the deflection of rider~ 22A
throu~h 22J i6 small; 8 true blood pressure messurement
may be obtained when the correct hold-down pressure is
employed.
The octual physical structure of a prsctical
transducer of a type which may be employed for
transtucer ~rray 22 in the pre~ent ~ystem i 6hown ln
the aboYe-mentioned J.S. Eckerle patent No. 4,269,193.
There, ~ transducer array i8 shown in which the
~ndl~idusl tran~ducer~ (riders) are formed in a thin
~onocrystslline silicon ~ubstrate which 18 made using
~: ,
..
9 ~ 3
integrated circuit technlques. In Fig. 3, to which
reference now i~ made, a slmplified ~ho~ing of
trsnsducer 22 iB shown co~prislng a chlp 40 which
lncludes an array of individual tr~nsducers, not shown.
Electrical conductors 42 connect the indivldual
tr~n~ducers to the wlring 18 for connect~on thereof to a
multiple~er 43.
A6 seen in Fig. 3, ca~e lO compri~es a
gener~lly cylindrlcal, hollow, contsiner having rigid
back snd aide walls 44 and 46, respecti~ely. The
silicon tr~nsducer chip 40 i6 moun~ed within the face 48
of the ca~e ~de~ignated as the front or operati~e face)
ln ~ cyli~drical cup-like tran6ducer housing 50. The
operati~e foce 48 lncludes the silicon transducer chip
40 along with its included lndlvidual transducers and
arterial riders. Chip 40 may be affixed to a
conventional ceramic dual in-line package that i~
plugged into an associated dual in-line socket, neither
of whlch are shown in the drawings. A silicone rubber
filler 52 iB proYided lnside the housing 50 and sround
the dual in-line package and 60cket to provide a good
sesl, pre~ent electrical leakcge between the transducer
circuits and housing 50, and provide a flat fiurface to
press ag~inst the sub~ect. The front face 48 of the
transducer includes the lower face6 of hou~lng 50 and
filler 52.
The transdu~er hou~ing 50 i~ fixed to the
inside of the tranaducer CBBe 10 by me~ns of a cup-like
sllicone rubber bellows 54 whlch i8 sealed ~round the
lower outalde lip of the cup-shaped transducer housing
50, e~tends upwardly inside the outer wall of the
tr~n~ducer case 10~ and is ~ealed to a ring 56, whlch in
turn iB fl~ed and se~led to the inside back of the
trsnsducer cs~e lO. A chamber i8 formed $nside the
bello~ ~hich iB connected to on ~ir pressure source 58
\ I
lo ~30A~53
through tube 20. A pres6ure controller 58A may be
lncluded ln the pre~sure ~ource. Slnce the flexible
bellows 54 1~ ~ealed both to the ~ran~ducer hou~ing 50
and the inside of the transducer case 10, flir under
presxure from source 58 pneumstically losd~ the
operatlYe face 48. With ~he trsnsducer ~trapped to the
subject'~ wri6t, the hold-down force Fl exerted by the
transducer array a8s~nst the skin of the 6ubject is
ad~ustsble by control of the ~ir pres6ure. tIn the
diagrammatic ~echanical model 6hown ln Fig. 2 the hold-
down force Fl i8 generated by hold-down force generator
36.)
As ~oted above, electrical signals related to
pressure 6ensed by the indi~idual transducers 22A-~2J of
transducer 22 are supplied a~ inputs to an analog
multlplexer 43. From the multipleser, the signals are
digitized by ~n analog-to-digital (A-D) converter 60,
and the digitized slgnals sre 6upplied to B digital
computer 62 hs~ing memory 62A and a clock 62B. Other
information, ~uch a6 the sub~ects name, sex, weight,
hei~ht, s~e, arm/wrist dimensions, and the like, may be
fiupplied to the computer throu~h 8 keyboard 64. Output
from the computer i~ supplied to data dlsplay and
recorder means 66 which may include a recorder, cathode
ray tube monltor, a sQlid state di6play, or the like.
If desired, at least a portion of the ~isual display may
be included in transducer case 10. In fact, all of the
components ~hown ln Figure 3 may be lncluded in the case
lO witbout depar~ing from the prlnciples of thi6
lnventlon. Obviously, the computer output may be
supplied to a printer, un oudible slarm, or the ~ike, a~
desired. ~1BO~ an output from the computer iB ~upplied
o~er llne 68 to the pressure controller for control of
the transducer hold-down pressure.
In Fi8. 4, to whicb reference now 16 msde, the
1 1 ~30~53
signsl waYeform of the output from one of the pre sure
sensitive elemen~ 22A through 22J which overlles ~rtery
24 i8 ehown. Other element~ of the ~ransducer arrsy
which oYerlie the ar~ery ~ill have wAveforms of almilar
shape. ~ith a correct hold down pressure and correct
selection of the ~centered" arterial rider (i.e. the
rlder ~ubstantislly centered o~er the artery~ the
~aveform i~ representstiYe of the sub~ect'6 blood
pressure ~ithiD the underlying artery. Systolic,
diastollc and pulse ampl~tude pressures are indicated on
the wa~eform, wherein pulse amplitude ls the difference
between the ~ystolic and diastolic pressures for a giYen
heartbeat.
OVERALL SYSTEM OPERATION
Fi~s. 5A and 5B, together ahow a flow ch~rt of an
algorithm for gener~l, overall, operation of the blood
pressure monitorlng system. Some of the operstions
indicated there~n are under control of computer 62
responsive to programming instructions contained in
memory unit 62A. Obviously, one or more progr~mming
~tep~ may be in~olved in the actual implementation of
the lndicated operations. Since the progrsmming of ~uch
6tepB iB ~ell within the skill of the s~ersBe
programmer, a complete program listin~ is not required
~nd i8 not included herein.
Preparation for monitoring is begun at START
~tep 100 ~t which time ~ystem power 1B turned on or a
re~et operation læ per~ormed, by mesn6 not shown, and
counter~, regi~tere, tlmera, and the like ln computer 62
ore lDitialized. At ~tep 102 information concerning the
~ub~ect, ~uch A~ the sub~ect'6 name, ~e~, weight,
height, ~ge, arm ond/or ~rist dimensiona, ~nd the like,
is entered into the compute~ memory thrsu~h U8e of the
keyboard 64. Ne~t, at 6tep 104, ~ nominsl hold-down
pres~ure (~-D.P.) i8 applied wherein ~ir under pressure
12
~3C)~53
from source 58 ls 6upplied to the transducer. For
example, 8 hold-down pressure of ~ay 120 mmHg may be
supplied to the transducer~ which pressure ser~es to
extend the bellows 54 whereby operati~e face 48 extends
out~rdly a 6hort d~stance from the bottom of the case
lO. The tr~nsducer i8 attached to the ~ub~ect at ~tep
106 at a location wherein ~ centraily locsted transducer
element, ~uch ~s element 22E of transducer srray 22
o~erlies the center of artery 24. Of course, the exact
posltion of the transducer srrsy relati~e to the
underly~ng artery generally 1~ not ~isually apparent to
the subject, or operator, and repo.sitioning Qf the
transducer may be required to properly position tne
same .
~ith the transducer attached to the subject,
step 108 ls enterea (select centered transducer element)
at which point the transducer element which overlies.the
center of the artery is identified. The location of
the selected trsnsducer element is displsyed at step
llO. If desired, the exact transducer element 22A
through 22J selected at step 108 may be di~plsyed at
6tep llO. Alternati~ely, a l~near array of, say, three
lights may be provided wherein energization of the
center light indicates that a centrally located
transducer element wa6 Belected st Btep 108.
Illumlnation of elther of the end li~hts would lndicate
thnt NoYement of the transducer to the right or left is
required for proper posltioning of the transducer array
relati~e to the underlyln~ artery. ~8 noted above,
Yelection step 108 may lnclude means for selecting the
one pressure sensiti~e element ~hat has a local mlnimum
of at least one of the diastollc and ~ystolic pre~sures
that ~B ~i~h~n substantially one artery diameter of the
o~e pressure sensitl~e element which generates the
wa~e~orm of ~aximum pulse amplitude, as disclosed in
'~ : . '- ': '' '
13
U.S. P~tent Number 402S9,193. Processes whlch ~ay be
~mployed 1~ ~election ~ep 108, including identifying
6y~tolic and diastol~c pressure~, pul6e ~mplitude,
~s~im~, loc&l minima, ~nd the like, from the transducer
outputs sre resdily ~mplemented u6ing diglt~l computer
62.
After the trsnsdueer ele~ent directly o~er ~h~
artery 16 ~elected and it~ location dlsplayed, decision
step 112 i8 entered to determine whether sr not the
selected element 18 nesr the center of the tran6ducer
~rr~y. If it i8 not, the transducer i6 repositioned on
the subJect at 6tep 114, and ~tep 108 is reenterPd. The
proce6~ is repeated until the transducer is properly
loc~ted on the subject.
When tec~6ion step 112 i6 affirmative, the
hold-do~n pre~sure to u6e i6 computed ~t 6tep 11~. An
a$firmatlve decl~ion at 6tep 112 may require the
operator to actuate an "Adjust Pressure" but~on, or the
like, to enter ~tep 116 from 6tep 112. No~el
algorithms which may ~e used in computing the correct
hold-down pre6sure (step 116) are shown in Flg. 7 snd
described below. For present purpoæes it will be
under~tood that ~ correct hold-down pressure for
accur~te blood pre~ure monitoring i6 computed at step
116, follo~in~ ~hlch, at step 118, the computed hold-
down pressure i6 set by control of pre6sure con~roller
58A by the computer 62.
Data obtained and u6ed to compute the correct
hold-down pre6sure (step 116) may be u~ed in the
calculatlon of an ~ccur~cy index ~t Etep 119, which
lndex ~l~ply co~prises ~ measure of the complisnce o~
the underlyi~g crtery 24. Generally, ~he more
compli~Dt, or le86 ~tlff, the underlying artery, the
better will be the ~ccurecy of the blood pres6ure
~eHsurement~. Means for computing the compliance and
i
14 ~3~S3
accuraoy index are deRcribed ln deta~l hereinbelow. For
pre6ent purpo~es, lt ~111 be understood that the
accuracy indeY m~y be cslculated ~t s~ep 119 (Fi~. 5B)
~nd the ~lue thereof stored for di6plsy at B later step
5 with blood pressure messurements.
With the transducer properly positioned on the
sub~ect and the correct hold-do~n pressure supplled
thereto~ the syste~ ie ln condition for obtaining
accurate blood pres~ure readlngs. At step 120 an
~ndication that the system i6 operati~e i6 provided, as
by di6pl~y of the ~ords "Readings Val~dn. ObYiously,
other d~spl~ys, such 8S a 8reen indicator light, ~ay be
employed for indic~ting the opersting state of the
~y6te~.
From the output from the 6elected tran~ducer
element, systolic and diastolic pres6ure ~alues toge~her
with pulse amplitude values are readily determined in
step 122. Also, pulse rate i6 readily calculated by
determinlng the time between 6ucces6ive diastolic or
6y6tolic pressures. At 6tep 124, ~slue6 calculsted
and determined ln step6 119 and 122 are displayed
and/or recorded nlong with the actual waveform.
ObYiously, the Yalues whlch are calculated and di6played
depend upon the use to be made of the blood pre6sure
monitor, a dl~play of all of the ~alues not being
requlred in ~any lnst~Dce6. For example, the blood
pressure waveform could ~e recorded without cslculaeion
~nt di~play o~ any of the ~lues identlfied ln ~teps 119
and 122.
After the value6 ldenti$ied in step 124, such
a~ systollc Dnt/or dla~tolic pres6ure, 3re displayed,
step 126 i8 entered ~erein the 6ystem ~alt~ for the
next heart~eat cycle. Dia~tolic or systolic pressure
point~ 0ay be used to ldenti~y reference point6 ln the
35 hesrtbe~t cycle6. Deci6ion ~tep 128 then ls entered at
~ 4~53
~hich time 6 timer ln computer 62 18 tested to determine
~hether or not it ha~ reached ~ predetermined time "M",
where M 1B a tlme period of, 8~y, 30 minu~e6. If the
el~psed tl~e exceeds the predetermined time period, M,
the deci6ion i6 affirmatiYe, the timer 1~ re~et at step
130, hold-down pressure i~ reduced to appro~im~tely 120
~mH8 at step 131, and step 116 i6 reentered for
recomputstion of ~he correct hold-down pressure and
resetting thereof, if required. Periotic checking and
resettlng of hold-down pressure helps to ~ssure long-
term accuracy of the blood-pressure readings.
If the predetermined time period has not been
exceeded, 6tep 132 is entered for qelection of the
centr~l transducer element, which step is the ~ame as
step 108 described abo~e. Decision step 134 then ~s
entered in which the selected transducer element
deter~ined ln ~tep 108 1~ compAred to that determined in
step 132. If there ls no change in the 6elected
transducer element, step 120 is reentered indicsting
thst the readln~s are ~alid. However, if there has bPen
a change in the ~elected transducer element, ~uch that
decl~lon step 134 ls ~ff~rmati~e, then step 136 is
entered wherein the newly-determined eelected transducer
is displayed. Thi~ ~tep corresponds to step llO wherein
either the actusl ~electad trsn~ducer element iB
identified, or arrow6 or llghts indlcate the direction
tha~ the tran~ducer needs to be mo~ed to recenter the
~ame over the underlying artery. A ~arnlng is l~sued at
~tep 138 indlcatlng to the ~ub~ect or operator that
movement of the trsn~ducer relatiYe to the artery hs~
taken place. If deslred~ the transducer then m~y be
reposltioned and the process researted. If the
tran~tucer i~ not repositloned, end ~he process 18 no~
termlnsted and restarted, step 120 i~ reentered for
contlnuatlon of the monltoring process, but now u ing
. .
16 iL;304~5~
the output from the newly selected tran~ducer element.
DET~RMINATION OF OLD-DOWN PRESSUP~E
1) Di~stolic pressure vs ~old-Down Pres~ure Method
Reference now 18 made to Fig. 6 wherein plots
of diastolic pres6ure and pulse smplitude ver~us hold-
down pres6ure are shown which wlll facllitste an
under6tanding of novel meAn~ for determinin~ correct
hold-down pressure for accurs~e blood pressure
measurement6. The method for obt~ining the d~ta points
in this plot wlll be described below. A third-order
polynomisl i~ fitted using, for example, least squares
techniques to the Fig. 6 series of diastolic pressure
points to pro~ide a curve 140 which has the typical
shspe 6hown reg~rdless of physical characteristics of
the sub~ect.
A thlrd-order polynomisl fitted to the
measured data may be written aR follows:
Pm - aO + 8lPh ~ a2Ph ~ a3ph (1)
whereln:
Pm ' measured diastolic pressure,
Ph - hold-down pressure, and
aO, al, e2~ and a3 are coefficients of the
polynomisl.
For hold-down pressures between zero and Pl, the
uDderlyin~ artery remains unflsttened, and the measured
pre88Ure iB pri~arily dependent upon the hold-down
pressure snd secondsrily upon the intraarterial
pressure, Pa. The graph of the polynomial ls a
relatively ætrai8ht line o~er this range. Up to
pre~ure Pl the effecti~e spring const~nt of the artery,
uslng the mechanical model of the system shown in FiR.
2, i6 large.
Between hold-down pressures Pl and P2 the
hold-do~n pres6ure ls 8rest enough to part~lly flstten
~ . ~
. . . - .
~04~53
the underlyiDg artery, but no~ 8reat enough to occlude
lt. Experiment hss shown thBt ~DoBt accurate blood
pre3sure mea6urement~ are sbtained when a hold-down
pre6sure that i~ Rub~tsnti~lly mitway between presRures
5 Pl and P2 ls employed. Between pressures Pl and P2 the
effectiYe ~pr$n~ coDstant of the artery using the
olechan~cal model of Fig. 2, i6 relDtively small.
At hold-down pressures greater than P2, the
underlying artery i6 completely occluded, and the
effectiYe spring con~tant of the underly~n~ artery is
a~in relatl~ely l~rge. Consequently, the me~sured
pres~ure i6 again ~ubstantially independent of the
intraarterlsl pre~ure, Pa, and the curYe ls
sub~tantially 8 strai8ht line above pressure P2. As
~een in Fig. 6, the ~lope of curve 140 i~ lowest between
pressure~ Pl and P2 where the underlying ~rtery is
flatte~ed but not occluded. A~ noted above,
~ubstantislly the center, or midpoint, of this region of
lowest slope, bet~een Pl and P2, i6 the corr ct hold-
down pressure for obtainin~ accura~e blood pressuremeasurement6. This midpoint also substantially
coincldes wlth the point ~here the slope of the grsph of
the polynomial (equation 1) is minimum. Therefore, the
correct hold-do~n pressure value iB readily determlned
25 by locating the mini~um slope point using coefficients
of the graph 140 of the polynomial fitted to the
di~stolic pre~sure point~. In particulsr:
p3 , ~2 ~2)
B3
where: P3, the polnt of mlnimum ~lope, is the correct
hold down pressure, and
~ 2 ana a3 are the roefficlents of the second and
third degree terms of the third-order polynomial.
- ~
18 ~ 53
For curYe 140 of Fl~. 6, a2 ~~ 0.084760 and
a3 . 0.00014431 whereby, ~rom equat~on (1) a correct
hold-down prçssure of approxlmately 196 mm~g i~
indlcated.
52) Systolic Pressure vs Hold-Down Pressure Method
Instead of using a plot of diastolic pressure
~s hold-down pressure to determ~ne the correct hold-down
pressure, a plot of systolic pressure versus hold-down
pressure points msy be employed. The method is the same
a~ that described sbo~e except that a third order
polynomlal i6 fitted to the series of systolic pres~ure
points, and equatlon (2) $s appliea to the re~ultant
polynom i81 to pro~ide an lndioation of the correct hold-
down pres6ure.
153) Pul~e Amplltude Y8 Hold-Down Pressure Method
A. A third method of determining the correct hold-
down pressure to u~e in monitoring blood pressure
lnYol~es the use of the plot of pulse smplitude vs hold-
down pressure point3 shown in Fig. 6. As with the plot
of dlastolic pre6sure ~s hold-down pressure, a third
order polynomial is fitted to the series of pulce
amplitude polnt~ which results in a generally ln~erted
U-shaped cur~e 1~2. It has been found that value~ of
hold-down pressure corre~ponding roughly to pressures Pl
~nd P2 shown in Fi8. 6 msy be iound by taking the
pre~sures where the pul~e amplitude i~ 0.6 times the
maxlmum pulse amplltude on the graph of the polynomi~l,
142 . In Fi 6, these 0.6 maximum hold-down pressure
points ore ldentified as pll ~nd P2~. The correct hold-
down pressure, P3', i8 the Yalue substantlally midwaybet~een these points. Uslng thl~ method, the correct
hold-to~ press~re IB
- ~ .
: ' .
- . :
- - ' .
.~ ' ~ .'' :', ' .
. ~ . ~'-:
~30~5;3
2 (3)
For cur~e 14~ of F~8. 6 ~ correct hold-down pres~ure of
~pproYimately 195 mmHg B indicated by Eq. (3). For the
~ub~ect fro~ ~hich the diastolic snd pulse amplitude
5 plot8 of Fig. 6 were obt~lned, the correct hold-down
pressure determined u8ing equa~ion~ (2) ~nd (3) differ
by only one mmHg. If "correct" hold-down pressure
~alue~ celculated u~ing the three abo~e-described
methods agree within approsimstely 10 mmHg, then
~ub~tantially correct blood pre~sure measurements ~re
obtaiDed usin~ 6ny one of 6cid calculated values.
B. In a variation of ~he pulse amplitude VB H-D.P.
method, the pressure, P3', may be found directly from
polynomial coefficients. For example, if a 6econd-order
polynomisl is fltted to the pulse amplitude ~s N-D.P.
data, ~ith coefficients aO, al, and a2, then P3' is
gi~en by
-al (4)
2~2
This corre~poud~ to the m~ximum of the polynomial.
Si~ r expressions may be used for third ~nd hi~her-
order polynomials.
ARTERY ,COMPLIANCE - ACCURACY INDE:R CALCULATIONS
The accuracy of tonometric methods of the
pre~ent type for measuring blood pressure i8 dependent
upon the compllance, or ~tlf~ness, of the underlying
artery; the ~ccuracy of measurement decreasing with
incressed stiffness of the artery. A messure of the
stlffnecs, or co~pliance, of the srtery may be obtained
usln~ lnfor~Dtion contained in the diastollc (or
.. ..... .. , , , _ _ __ _
2~ 13~53
systollc) pre66ure curve 140 and the pulse amplitude
curYe 142 of Fi~. 6. In particulsr, a meaeure of
complisnce of the artery i8 provided by the ratlo of the
slope,S2, of the tia~tolic tor Rystolic) pres~ure curve
at the correct h~ld-down pres6ure P3, and the slope, Sl,
thereof bet~een zero snd ~old-d~wn pressure Pl.
From the abo~e, lt will be aeen that a measure
of compliance, C, of the underlying srtery ia
C ~ S2 (5)
~s noted ~bo~e, pressure~ Pl' and P2', equal
to 0.6 of the maximum pulse amplitude value of ~raph
142, zubstantially correspond to hold-down preæsures Pl
snd P2. Slope Sl ia determined by fittlng a straight
l~ne 144 to the messured diastolic ~ersus hold-down
pressure points for hold-down preRsures less than Pl.
As noted sbo~e, flattening of the underlyi~g artery does
not beg~n until hold-down preasure Pl iB reached. Slope
S2 at the correct hold-down pressure P3 is re~dily
obtainsble using coefficients of the polynomial
20 (equation 1) identlfylng curYe 140. In particular,
S2 ~ Bl ~ (6)
If the mea6ure of compliance, C, i6 Bmall
compared to unlty (aay C<0.3) the srtery ~tiff~ess is
relatl~ely low, ~nd lntr~arterial blood pressure wlll be
measured relatiqely ~ccurately by the present tonometric
method. Lar8er v~lues of C, appro~ching unity, inticate
that t~e artery atiffnes6 ls relati~ely large, which may
le~d to relati~ely lnaccurate measurements. The
compllance psrame~er obtained from equation (5) i8
. . ' ' '
'
.
`: , :
- 21
~L3~ ;3
calculated at ~tep 119 after the polynomials for the
~y~tolic, di~stolic ~nd pulse a~plitude qersus hold-down
pressure curYes are determined, and the calculated
compliance parameter ~alue i~ 6tored for ti~play ~t step
124. The ~alue it~elf may be diæplayed, or indicstion6
that arCur~cy i8 "goodn, "fsir~, ~poorn, or the like,
may be displ~yed, dependent ~pon the calculsted value.
Reference now i~ made to tbe flow ehart of
Fig. 7 wherein deteils of ~tep 116 of Fi8. 5A for
computing hold-down pressure are shown. A~ no~ed
sbove, when deci~ion step 112 of Fig. SA i6 affirm~tive,
indicsting thst the selected trsnsducer element i~ near
the center of the tran~ducer array, the correct hold-
dow~ pressure iB computed at step 116. As seen in Fi~.
7, thi~ ~tep (step 116) ~ncludes waiting for the next
hesrtbeat at Btep 150, following which the centered
transducer element 15 ~elected at gtep 152. At 3tep 154
syætolic snd dis6tolic pre sures are determined from the
blood pressure measurement6 obtaiDed from the selected
trsnsducer element, or rlder, snd the systolic and
diastolic pressure ~slue~, along with the hold-down
pressure employed, are stored in computer memory 62A.
It will be recalled that at step 104 (Flg. 5A) a nominal
hold-do~n pressure of approsimately 120 mm~g was
Z5 applied. Therefore, the first sy~tolic snd diastolic
pressure values are obtained using the nominal, 120
mmHg, hold-down pre~sure.
~ t step 156 the hold-down pres~ure is
lncreased by ~n incrementsl amount of, 6ay, 5 mmHg.
Decision ~tep 158 i~ then entered to determine whether
or ~ot ~he hold-down pressure i~ greater than, say, 300
mmHg. If the dn~wer is negati~e, ~tep lS0 i6 reentered
whereupo~ another ~et of sy~tolyr snd diastolic
pressure6 are obtained end ~tored for thls lsrger hold-
down pressure.
.
,
.. . . .
,
22
~()4~53
After an entire set of ~y6tolic and dlastolicpre~ure ~alue& have been obtained for a range of hold-
down pre~sure6 betwee~ 120 and 300 mmH~, decis~on ~tep
158 is affir~tiYe and ~tep 160 i~ entered wherein a set
of pulse ~mplitude v~lues are calculated by subtraction
of d$~stolic pre~sure from the a~sociated ~ystolic
pres~ure Yalue. The pulse amplitude ~alues Are stored
in memory along with the ~e~ of systolic aDd diastolic
pressure ~alues.
At ~tep 16Z, a polyDomial fit (typically a
third-order fit~ is computed for each of the syætolic,
diAstolic and pul~e amplitude ~ersus hold-down pressure
~ets of dst~ obtained at ~teps 154 and 160. The shape
of curve 140 ln Fig. 6 is representative of the shspe
obtsi~ed for both disstolic and systolic pressure ~s
hold down pre6sure plots, and cur~e 142 in Fig. 6 has a
~hape that 1~ representati~e of pulse amplitude versus
hold-down pre6sure plots. The constants and
coefficlent6 of the polynomials obtained at ~tep 162 are
stored for u~e in step 164 where a correct hold-down
pressure i8 calculated for each polynomial.
As described above, the correct hold-down
pressure using the 8y6tolic ~nd diastolic versus hold-
down pres~ure points i6 obtained using equatlon (2)
which locates the point of minimum ~lope of the plot of
the thlrd-order polynomial fltted to the points. In
p~rticular, the hold-down pressure is taken ~6 the
negatl~e of the coefficient o the ~econt degree term
di~lded by three time~ the eoeffic$ent of the third
degree term of the polynomial.
Uslng the set the pul~e amplltude ~ hold-down
pressure point~, the correct hold-down pres~ure i8
deter~inet b~ first calculatlng a pul~e amplitude Yalue
substanelally equal to slxty percent of maximum pulse
empl~tude on the graph of the polynomial fitted to the
- .
.
: ; :
.
, : :
.
23 ~.304~i3
polnt~. The two hold-do~n pre~sure ~alue~ along the
graph ~t ~hich the pul6e ~mplltude ~alues ~re
sub6tantlally equal to ~aid BiXty percent of ~axi~um
pul6e amplitude are identified, and the mean ~lue of
the~e two hold-down pre66ure ~lues i6 taken as the
correct hold-down pre~sure. AlternatiYely, the maximum
of the polynomial may be determlned directly from the
polynom~al coefficients, which maximum value i6 tQken 8S
the correct hold-down precsure.
At decision ~tep 166 the three (or four) hold-
down pressure ~alues calcul~ted in 6tep 164 ~re comp~red
to determine if the Yalues substantially Q~ree, e.g., if
they agree within, ~Ry, 10 mmHg of each other. If the
correct hold-down values do subs~al~tlslly agree, the
deci~ioD ~s affirmati~e and step 118 (F$g. 5A) is
entered where the hold-down pressure i5 set within the
range of computed ~alue~. If they do not ~ubstantially
agree, the hold-down pressure 18 reduced to ~ low ~alue,
say 120 ~mHg, at step 168 snd step 150 is reentered for
redetermination of a correct hold-down pressure ~alue.
Although the operation of the blood-pressure
monltoring system is belie~ed to be apparent from ~he
sbove-de~cription, ~ brief descrlption thereof now will
be pro~lded. After turning on or resettlng the 6ystem
(6tep 100) infor~ation regarding the sub~ect $s eDtered
into computer memory 62A through keybo~rd 64 (~tep 102).
A hold-down pressure of apyroximately 120 mmHg is
supplied to the transducer ~hrough tube 20 from pre~sure
~ource SB (s~ep 104) sfter ~hich the transducer i~
~ttached to the ~ubJect (step 106). Outputs from the
tr~nsducer elements 22A through 22J are digltized at
anslog to digital con~erter 60 ~nd are supplied to
digital co~puter 62 for proce6~ing. Us$ng outpu~6 from
eac~ of the trsnsducer ele~ent~ 22A through 22J, the
transducer element thst i8 sub6tant~ally centered oYer
' :
453
24
the underlylDg artery 1B selected as thst element from
whl h blood pree~ure mes~urement6 are to be obtalned
(step 108). A method of ~electing the centered
transducer element employing analog circuitry ifi shown
ln U. S. Pateht 4~26~,193, which method iR readlly
implemented digit~lly u6ing digital computer 62. The
locat~on of the selec~ed transducer element i~ diRplayed
(~tep 11~) and if the selected element 18 not nesr the
center of the ~rray, the array may be repositloned
(steps 112 and 114) and the process of selectin~ the
centered element i6 repested.
With the transducer properly positioned on the
subject, the correct hold-down pressure to empl~y for
obtaiDl~g accur~te blood pressure measurements is
determi~ed (step 116). Four dlfferent methods of
computing the correct hold-down pressure are disclosed,
one or more of which may be employed in a monitoring
~ystem. The first, second, and third methods use
~y6tolic pressure, diastolic pressure, and pulse
amplitude ~ersus hold-down pressure mea6urements,
respectl~ely, which sre obt~ined o~er hold-down
presQureR which rsnge from a pressure where the
underlging artery i6 unflattened to a pressure where it
iB occluded. The fourth method is a ~sristion of the
third, using a formula of polynomial coefficie~ts rather
thsn the aforementioned 60% polnt6.
After 8 heartbeat ~step 150, Fig. 7) the
tran~ducer e~e~ent centered over ~he underlying artery
le selected (step 152) ~nd systolic and dia~tolic
preasure~ are determined from the blood pres~ure
waYe~orm (Flg. 4) from the eelected transducer element.
These maximum and minimum pre6sure points sre readily
determlned by digital computer 62 UB~ n8 known
progr~mming methots. The sy6tolic and diastolic
~ressures sre 6tored in computer memory 62A together
'' .. . ' . ' ' -
- ,' :
~ 4S3
wlth the associated hold-down pre~sure. The hold-down
pressure then i~ lncrea6ed ~n incremental amount (s~y by
5 mmHg) ~ ætep 156, and, if the hold-down pre sure i8
le6s thsn about 300 mmHg, the ~y6tollc ~nd di~stolic
pressure ~alues for the new hold-down pres~ure are
obtained aDd stoTed in computer memory 62A. If the
hold-down pressure iB ~reater than 300 mmHg (~tep
158),whlch i6 ~ pressure greater than that required for
occlusion of the underlyin~ srtery, the ~cquisition o
systolic and di~tollc pre6sures ~er~us hold-down
pressure ~alues is 6topped, and pulse amplitudes for the
collected dat~ ~re calculsted and stored. As ~een in
Fig 4, pulse ampl~tude simply comprises the difference
ln systolic and diastolic pressures for a given cycle of
the blood pressure wa~eform.
Third order polynomials using a le~st ~quares
method, for example, are fitted to the ~ystolic,
disstolic and pulse amplltude ~er~u~ hold-down pressure
~slue6 (6tep 162). From the~e cur~es, correct hold-down
2f) pressure6 are c~lculated (step 164). From the ~ystolic
and di~Etolic pre6sure cur~es, the point of mlDimum
~lope i~ determined to provide an indication of correct
hold-down pressure. This minimum slope point is located
substsnti~lly midwsy between hold-down pressures at
which flattenlng of the underlying artery begins, and at
which the artery is occluded. From the pulse amplitude
curve, hold-down pres~ure Yalue~ at which the pulse
amplltude ls si~ty percent of maximum are determlned9
which ~Alues sub~tantislly equal hold-down pres~ure~ at
30 which flattenln~ of the underlylng artery be8in~ and ~t
which the artery ~ occluded. Hold-down pre~sure at the
midpoint between the two si~ty percent point6 i8 tAken
a~ the po~ne at which ~he hold-dovn pre~6ure i8 correct.
Also, correct hold-do~n pressure i6 determlned from
coefflcients of the polynomial fitted to the pul~e
.
~ ,
26 ~30~3
~plitude ~alue6 using, for esample, equ~tlon ~4).
If the plural~ty of "correct" hold-down
pre6aure8 calculated at step 164 a8ree to vithln, 6~y,
10 mm~g, then the hold-do~n pre~Bure ~8 Bet at a ~alue
~ithln the calculated ran3e of ~alues (6tep 118) and an
lndlcatton that blood pre~fiure resdin8s now sre ~alid 1B
pro~lded at ~tep 120. The blood pre6sure va~eform from
the aelected tran~ducer element ~ay be di6played or
recorded (step 124), ~nd the systolic and diastolic
pressures, as ~ell BB pulse amplitute and pulse rate m~y
be obtained $rom the blood pre6sure waveform (~tep 122)
~nd dl~plsyed at step 124.
~ t the ne2t heartbeat (step 126), the el~psed
tlme of operstlon is eompared to a predetermined time
perlod, M, ~uch as 30 ~inu~e6, and if M minutes have not
elapsed, the process of selecting the central transducer
element iB performed (atep 132). If there i~ a change
in the selected trsnsducer element from that which was
18 t-determined, the newly selected ele~ent is displayed
(~tep 136) ~Dd e ~arnln~ lssued (step 138). The
transducer oay be repositloned ln response to the
w~r~n~, or the monitoring proce6s may be con~lnued
without reposltlonlng of the transducer, but u61ng the
newly-~elected tr~Dsducer ele2ent. If, at atep 134,
there hss been no change ln the selected transducer
element, the oonltorl~ process contlnue~ without
l~susnce of ~ ~arnlDg.
If, ~t declslon step 128, the respon6e 1B
~f~lrmstiYe lndlcatlng thst M ~inutes haYe elapsed, ~he
tlmer 1B reBet ~t step 130, tbe hald-down preseure i~
decrecsed to ~ub~taDtially 120 ~8 ~t step 131~ and
correct kold-do~n pres~ure to be u~cd 1~ recomputet by
reentry Into tep ~16.
~t ~tep 119 ~n accuracy lndex may be calculated,
vhich then i8 d~played at 6tep 124. The accuracy indes
2~ 53
18 tsken ~ the ratl~ of the ~lope of the ~y6tolic (or
ti~ollc~ pressure ~erau~ hold-down curve ~here the
~lope ~B ~lnioum, to the alope of 8 6traight llne fitted
to the cur~e befor~ fl~te~in~ of the artery be~in6.
The ~inl~um slope of the cur~e 16 readi?y deter~ined
from coefficients of the polyno~l~l fitted tc the curve,
ond the slope of the strai~ht llne i~ determlned by
fitting a atr~ight llne to the lnitlal portlon of the
cur~e. The hold-down pres~ure Pl' at ~hlch flsttening
of the underlying artery begins i8 determlned from the
lo~er BiYty percent o~ ~a~imum pulse smplitude point of
the pulse amplltude YersUs hold-down pressure curYe 142.
The ~nvent~on ha~in8 been described in detail
1D accordance ~ith re~uirements of the P~tent St~tutes,
varlou6 other ch3nge6 und modific~tions will 6uggest
themsel~es to those skilled this ast. For example, not
all ~our of the abo~e-described methods of determlning
correct hold-dowD pres~ure Deed be employed 6ince the
four methods 8enera~1y result in hold-down pres~re6
that Are 6ubstantially the same. If only one of the
methods i6 employed, then &teps 166 snd 168 of Fig. 7
would be ellminated from the process. Also, once the
transducer i6 properly positloned ~nd the correct hold-
do~n pre~sure determlned aDd applled to the transducer,
theD the waYe~orm fro~ the ~elected transtucer element
~ESy be di6pl~yed, recorded, or the like, ond/or any
de~lret value derl~ed there~rom may be ti6pl~yed,
recorded, or the l~ke. ~ny de6ired use ~y ~e ~ade of
the bloDt ~r2~sure ~cYe~orm, the in~entlon not being
l~mited ts an~ partlcular uae. ~1 BO ~ ~n a10~ c~rcult
~esns ~ be employed for proces~lng the blood pres~ure
w~Yeform ln ~lsce of the ~llustrated dlgital proce~lng
meaD~. Flhall~, tronsducera of ~arioua conatructions
(Includ~n~ ~lngle-element tran6ducer6, capaci~lYe,
fiber-optlc, plezoresl~tl~e ~nd other types of force or
2&
3~5i3
pre~sure tren6ducers) may be used to obtEIln pre~sure
waveforms from the sub~ect. It ls lntended that the
sbove and other such chsnges and modiication~ shall
~all w~thin the ~plrlt and scope of the invention
defined ln the appended claims.
:
