BIOMEDICALAPPLICATIONS,DEVELOPMEN A ARTERIA APPLANATIOTONOMETE DETECTI - PDF document

BIOMEDICALAPPLICATIONS,DEVELOPMEN A ARTERIA APPLANATIOTONOMETE DETECTINARTERIA PRESSUR AN VOLUMJIA-JUNdepartmen University Taiwadepartmen Universit o Scienc ABSTRACTraditional arterial tonometry permits noninvasive and continuous recording of the arterial pressure waveform, by applanating a superficial artery supported by a bone. In the paper, we present an arterial tonometer to simultaneously register the blood pressure waveform and the arterial time-varying volume. The tonometer consisted mainly of a chamber filled with a conductiveflexible diphragm in 323in many applications. Although the faithful bloodpressure measurements can be obtained by thismethod, the negative effects on the patient may, inmany cases, prevail over the benefits of achievingaccurate results from such a method [5-6].Noninvasive methods for the arterial bloodpressure measurefinent include predominantly theauscultation [4,7], the oscillometry [8-10], and thetonometry [1, 11-14]. The auscultatory method, basedon the Korotkoff sound, is the most extensively usedone for monitoring a patient's arterial blood pressurenoninvasive, it only provides a measurement ofsystolic and diastolic pressures on an intermittentbasis, and it does not provide the entire blood pressurewaveform on a continuous basis. Furthermore,employment of this method often yields inaccurateresults. Moreover, the rate at which blood pressure canbe recorded is restricted by the inflation and deflationrate of the occlusive cuff. Consequently, true beat-to-beat continuous blood pressure monitoring is unlikelyby means of this method.Most of currently commercialized blood pressuremonitors apply the oscillometric method to detect auser's blood pressrue in homecare environment. In themethod, occlusive cuff pressure oscillationscorresponding to vessel volume pulses demonstrate aspecific pattern when the occlusive cuff pressure isgradually decreased from above systolic to belowdiastolic values. The oscillation amplitude and the cuffpressure are then utilized to determine the meanarterial, systolic and diastolic pressures. It is now, onthe whole, accepted that a maximum cuff pressureoscillation takes place when the occlusive cuff pressureis the same as the mean arterial pressure of the subject[15-16]. The systolic pressure is the one where theoscillation amplitude is identical with a systolic ratioof the maximum oscillation in the period of the highcuff pressure, while the diastolic pressure is the onewhere the oscillation amplitude is equal to a diastolicratio of the maximum oscillation in the period of thelow cuff pressure. This method can provide the userswith the heart rate and the discrete pressure values,such as the systolic and diastolic pressures. Amongdifferent methods for measuring blood pressure, boththe auscultation and the oscillometry are the mostprevalent at the present time.A major advantage of using the arterial tonometryis to record continuous blood pressure waveform. Inaddition to the tonometry, more than a few methodshave been proposed in previous literature formeasuring entire blood pressur waveform. Thosethe optical method [2, 19], and the impedanceplethysmography [1, 20-21].One type of arterial tonometers incorporates anVol. 16 No. 6 December 2004array of individual transducer elements placed directlyon the patient's tissue overlying an artery or bloodvessel from which blood pressure is to be determined[22]. The elements directly sense the mechanical forcesin the tissue with which each of them is in contact. Theelements of the array are dimensioned and spaced apartfrom each other such that a plurality of these elementsare required to envelop the entire diameter or width ofthe underlying artery. The size of each element isdesigned to cover only a small fraction of the diameterof the underlying arery. The pressure of the arrayagainst the tissue is increased to suitably applanate theunderlying artery without producing occlusion. Theblood pressure within the artery is then conductedthrough the vessel wall and the overlying tissue to thetransducers.An obvious drawback to such sensor-arraytonometers includes the use of the discrete elements. Ithas been found that with such tonometers a continuouscontour of the tissue stresses under the array is notyielded [7, 12]. As well, in the previous methods nocompensation means is provided for the motion artifactthat may influence the forces translated to the sensorsfrom the artery. As a result, it is needed to provide anmeasurement of arterial blood pressure. Such atonometer preferably eliminates the requirement forhigh-resolution sensor technology and has the ability toregister the pressure within vessels.Therefore, the purpose of the study is to make aneffort in developing a new arterial tonometer that iscapable of not only measuring the subject's bloodpressure waveform, but also recording the change inthe arterial volume. In addition, the new tonomter isexpected to have ability to assess the mechicalcompliance of the vessel being monitored.2. METHODS2.1. Construction of the TonometerAn arterial applanation tonometer modified fromthe previous study was constructed for thedetermination of arterial blood pressure and change invessel volume [ 11 ]. The arterial tonometer (W X L X H= 2.0 X 2.0 X 1.5 cm) was principally fabricatedusing acrylic materials (plexiglass), and wasminiaturized such that it could be directly placed overa superficial artery, such as a radial artery. As shownin Fig. 1, the arterial tonometer consists of a hollowchamber that allows operators to perfuse certain kinddirectly contact the superficial artery, and a pressuresensor used to detect the underlying arterial pressure.Four electrodes (alloy of gold and copper) were in-30- BIOMEDICAL ENGINEERING-APPLICATIONS,BASIS &COMMUNICATIONSparallel disposed across the tonometer's chamber todetect the variation of the fluid distribution inside thechamber. For the purposes of convenience, there weretwo outlets connected to the chamber. One wasdesigned to allow the perfusion of the conductive fluid,and the other to get rid of the bubbles that possiblytake place during perfusion procedure. Table 1summarizes the structural characteristics of theproposed tonometer.When applied, the arterial tonometer was requiredto make an appropriate contact with a superficial arterythrough the flexible diaphragm. To achieve this, weutilized currently a strip bandage to immobilize thetonometer such that a maximal swing in output voltagebecame apparent. To improve the sensitivity of thetonometer, the short axis of the elliptical opening onthe cover was required to be centralized over theunderlying artery of interest.The change in the chamber pressure wascontinuously monitored with a pressure sensor (NPI-12, Lucas, USA) that was proximally connected to thetonometer chamber. The distance between the sensorand the chamber was kept as short as possible, in orderto reduce the distortion of the pressure waveformduring transmission2.2 Tonometric PrincipleIn the study, the variation of the arterial volumewas assessed using the impedance plethysmography[1]. In this method,a constant-current source wasprovided and connected to the tow peripheralelectrodes. As the current passed through theconductive fluid (heresaline) between the twoelectrodes, a voltage difference between the two innerelectrodes was then induced. In addition, the voltagedifference was varied with the fluid distribution in thechambe.The electricresistance, R, of a rectangularcompartment that corresponds to the space between thetwo electrodes can be represented by the followingequation,R=JULS(1)where p is the resistivity of the fluid in thecompartment, L is the length and S is the cross-sectionarea of the compartment. Based on the fact that thevolume of the compartment,Vmi,is the product ofLand S, the resistance can then be alternativelyrexpressed as:R = PSzVml(2)Vd = IRpIL2324(3)VmlFuther, according to Ohm's Law,the voltagedifference Vd across the compartment can be obtainedbywhere I is the current uniformly passing throughthe compartment.It is clear from(3) thatVdis inversely proportionalto the compartment volume. That is, as p and L aremaintained and a constant-current source is applied,the larger the volume, the smaller the voltagedifference between the two inner electrodes.2.3 Block Diagramof ArterialPressure andVolume MeasurementThe arterial tonometer was designed to measuretwo vascular variables, the arterial pressure and thechange in vessel volume. Fig. 2 illustrates the blockdiagram of the variable measurement that can beessentially divided into the pressure-sensing andvolume-sensing circuits. For the volume measurement,a constant-current source generating a 50 kHzsinusoidal waveform was injected into the two outsideAir OutletWet"Inlet(b)Fig. 1(a) Dimensional view and(b) sectional view ofthe proposed arterial tonometer, showing four wireelectrodes arranged in parallet inside thetonometer's chamber and a flexible diaphragm.-31- 325Table 1 Structuralfeaturesof theproposed arterial tonometer.ItemSizeWeightComponent separabilityCoupling mediumFixationDescription2.0 (W) X 2.0 (L) X1.5 (H) cm5 g (excluding the pressure sensor)Vol. 16 No. 6 December 2004Most components of the tonometer are designed to be separable and the flexiblediaphragm is designed to be interchangeable.A compliant diaphragm is used to couple the varying vessel volume.A bandage is used to fasten the tonometer on the wrist by an appropriate force.electrodes, and the induced voltage difference betweenthe two inner electrodes inside the tonometer'schamber was sensed and amplified by means of adifferential amplifier. Because the output signal of thethrough the sinusoidal waveform with a frequency of50 kHz, it was necessary to demodulate the siganl toobtain the original change in vessel volume. For thearterial pressure measurement, the strain-gaugepressure sensor converted the chamber pressure to avarying resistance. Here, a Wheatstone bridge circuitwas designed to yield a zero output when the chamberpressure is equal to the atmospheric pressure. Theoutput signal of the bridge circuit was first high-passfiltered and then properly amplified.2.4 Data and Statistic AnalysisThe femoral arterial blood pressure was measuredwith a catheter-tipped pressure transduce, and theradial arterial blood pressure and vessel volume changewere measured with the proposed tonometer for eachpatient during catheterization surgery. These threeanalog variables were digitized by a 12-bit converter,respectively, with a sampling frequency of 200 Hz, andrecorded using the MP 100 Manager (BIOPAC SystemInc., USA). Subsequent data analysis was performedwith a personal computer.Regresssion analysis of linear relationshipbetween two variables (for instance, invasive pressureand noninvasive pressure) was performed with asoftware program provided by SigmaPlot (SPSS Inc.,USA). Curve-fit methods were applied to find outwhat kind of function can best delineate therelationship between the arterial pressure and vesselvolume [23], if necessary. Correlation coefficientscalculated from the linear regression were used todetermine how closely the two variables related to.3. CALIBRATIONtonometer's chamber and a sphygmomanometer wereboth connected to an air cuff by means of Y-shapedtubing. The cuff pressre was gradually inflated by anair pump at an incremental rate of 10 mm Hg per step.Both the output voltage of the pressure-sensing circuitand the pressure reading of the sphygmomanometerwere recorded simultaneously, and the output voltagewith respect to the cuff pressure are plotted and shownin Fig. 3. It is obvious that the relationship of theoutput voltage to the cuff pressure (or the chamberpressure) with an intercept of 1.04 voltage and a slopeof 0.02 voltage/mmHg is extremely linear, because ahigh corelation coefficient of 0.998 was obtained.3.1 Pressure Calibration Fig. 2 Block diagram of the arterial pressure andTo calibrate the pressure measurement, the vesselvolme measurement.-32- BIOMEDICAL ENGINEERING-APPLICATIONS,BASIS &COMMUNICATIONS3.2 Volume CalibrationFig. 4 demonstrates the schematic diagram to helpexplain the volume calibration method in the study.As shown in Fig. 4(a), the arterial tonometer is just intouch with the underlying artery, with the assumptionthat the tissue, mostly skin, between the tonometer'sdiaphragm and the superficial artery is thin and can beneglected. Fig. 4 (b) illustrates that when the artery isexpanded, a part of the artery pushes the flexiblediaphragm and intrudes into the tonometer's chamber.In the study, we assumped that the cross-section of thearteryremainsto be circular as it either dilates orcontracts. According to the geometric relationship, theintruded vomue of the artery, Vin (corresponding to theABCD), can be determined with the followingformula,V„=( 2rd-d2-0r2)L(4)020 40 60 80Chamber Pressure [mmHg]100 120326Fig. 3 Relationship between the output voltage ofthe pressure-sensing circuit and the chamberpressure.where r is the radius of the artery, d is the distancefrom the original position (D) of the diaphragm to theartey wall (C), 0 is the half of the opening angle that isequal to cos-'[(r-d)/d] , and L is the width of theflexible diaphragm as shown in Fig. 2.We here selected three cylinder tubes havingdifferent diameters to simulate the arteries. Once weknowed, by means of a precision meter, the distancebetween the tube wall and the origial position of theflexible diaphragm, the intruded volume can be readilyyielded. By applying different-sized cylindrical tubes,sensing circuit to the change in the fluid volume in thevoltge varies positively and linearly with the amount ofchange in vessel volume. Furthermore, all correlationcoefficients calculated from these regression lines are0.968, 0.911 and 0.884 for the tubes with a diameter of3.95, 2.90 and 1.60 mm, respectively.4. TESTING RESULTSThe feasibity of the arterial tonometer wasvalidated in collaboration with the Department ofCardiology, Hsin Chu Hospital, Taiwan. Duringcatheterization surgery, the blood pressure in thefemoral artery was measured with a catheter-tippedpressure transducer, and at the same time, the radialarterial blood pressure and vessel volume changemeasurements were performed with the proposedtonometer.Fig. 6 displays one typical recording of arterialvariables, including the invasive blood pressure (IBP)waveform of the femoral artery by the catheterization,the noninvasive blood pressure (NIBP) waveform andthe change in vessel volume of the radial artery by thecurrent tonometer. It is found that there is a smallreflection in the descending portion of the radialpressure waveform, while no reflection occurs in thefemoral pressure curve. As shown in the figure, themaximum of the femoral blood pressure is up to 190mm Hg, but that of the radial pressure is about 160 mmHg. Obviously, there is a quite distinction between thetwo blood pressure maximums, partially due to thechange in vessel volume varies principly with theblood pressure of the radial artery, and incorporatesmore high frequency components in the volumewaveform than the blood pressure curve.Fig. 7 shows the relationship between the invasiveand noninvasive arterial blood pressures. In theascending portion of the blood pressure, thenoninvasive measurement with the arterial tonomertercorrelates well to the invasive measurement with thecatheterization, and a correlation coefficient of 0.92 isachieved (upper panel). Compared to the ascendingportion, a smaller correlation coefficient (r = 0.89)between the descending portions of the blood pressurestipped pressure sensor is found (middle panel). Forcomparison of the full blood pressure waveforms withthe invasive and noninvasive techniques, a correlationup to 0.932 is obtained (lower panel).5. DISCUSSIONThe arterial applanation tonometer developed inthe study has been miniaturized to cope with actualapplications. It was also validated to measure two-33- 327WVol. 16 No. 6 December 20040 D1=3.95 mmo D2=2.90mm D3=1.60mm- Regression line0.0 0.53.03.5L1 : y=3.647x-1.569 r-0.968L2y=4.056x-0.011 r=0.911L3 : y=4.385x-0.245 r-0.8841.0 1.52.0 2.5Volume [uL](b)Fig. 4 Schematic diagram for explaining how tocalculate the partial volume of the artery intrudinginto the tonometer's chamber,when(a) the arterialwall just contacts the flexible diaphragm and (b)the partial arterial volume has intruded into thechamber.vascular variables, the blood pressure and the vesselvolume change, from a superficial radial artery of apatient under catheterization surgery. As we know thatmost tonometers designed in previous studies merelyallow the pressure measurement [12, 24]. On the otherhand, the testing results have shown that the presenttonometer permits not only to record the underlyingradial blood pressure but also the change in the arterialvolume.To register an accurate arterial pressure waveformwith a tonometer, three conditions should be taken intoaccount. Theyare: (a) the tonometer should beproperly immobilized upon the artery of interest, (b)the artery must be superficial to eliminate as much aspossible the effects of the subcutaneous tissue betweenFig. 5 Relationship between the output voltage ofthe volume-sensing circuit and the change in thechamber fluid volume,by means of three rubbertubes with different diameters. D = outer diameterof the tube;L = linear regression line; r =correlation coefficient.1210 ^42the tonometer and the artery, and (c) the vessel shouldbe firmly supported by underlying structure (bone) toallow the application of the tonometer [1, 12]. In thiswork, the proposed tonometer is devised to measurethe pressure within the radial artery at the wrist.Because the radial artery is superficial and is supportedby a bone (radius), it is obvious that the arterialpressure measurement with the current tonometersatisfies completely with the last two conditions.The force exerted on the tonometer has to adjustaccording to each subject. Previous investigators haverevealed d that different force on the tonometer maylead to different pressure recording, with distortion notonly in the amplitude also in the frequencycomponents [13]. Here, we make use of a bandage toimmobilize the current arterial tonometer. Beforetesting or experiment, we have to properly adjust theforce exerted on the tonometer to achieve a situation inwhich maximum pressure amplitude takes place. Therecorded values of the full pressure waveform withrelative amplitudes can be further processed andcalibrated using the data measured with a commercialsphygmomanometer. Then, a whole blood pressurewaveform useful in clinical applications may beacquired.One calibration method used in the previous studyis that the authors gradually inject saline into thechamber to find out the relationship between the vessel-34- BIOMEDICAL ENGINEERING-APPLICATIONS,BASIS &COMMUNICATIONS0123 4Time [sec](a)5632860 80 100 120 140 160 180 200Descending portion of IBP [mm Hg]0123Time [sec](b)0.00123Time [sec]445566(c)Fig. 6 Clinical recording. (a) Invasive bloodpressure(IBP) from a patinet's femoral arteryusing a catheter with a tip-sensor; (b) Noninvasiveblood pressure(NIBP)and (c)vessel volume from apatent's radial artery using the present tonometer.volume and the voltage output, resulting in anunreliable outcome [11]. Although the method in theresearch for the volume calibration of the tonometer isnot perfect, it has shown certain degree of progress andimprovement. When the measured artery dilates, theexpanded volume actually results in a new distributionto the chamber volume. As a consequence, theimpedance method used in the work is really tomeasure the volume redistribution. The calibrationperformed with this way has demonstrated a linearrelationship between the volume redistribution and theoutput voltage (Fig. 5). With the assumption that theexpanded artery is always entirely in touch with theflexible diaphragm, a reliable assessment of change invessel volume can be achieved if the diameter of theartery of interest is known.An optimal coupling should be considered inmeasuring arterial pressure with either the currenttonometer or the previous ones. In the presentinvestigation, a flexible diaphragm is required to becompliant enough to directly contact the underlying180160140120100806060 80 100 120 140 160 180 200IBP [mm Hg]180160140120100806060 80 100 120 140 160 180 200Ascending portion of IBP [mm Hg]Fig. 7Comparison between the invasive andnoninvasive blood pressures.r = correlationcoefficient.artery. Nevertheless, to approach a least distortion inpresssure transmission from the artery to thetonometer, the tonometer's chamber pressure should bevaried faithfully with the arterial pressure. In fact, it isfrequency components in the time-varying pressure.But, if we would like to develolp a sub-optimalcoupling condition, the chamber pressure of thetonometer may be maintained at a value equal to the-35- 329mean arterial pressure. This sub-optimal couplingapproach has been put into operation in severalprevious investigations [16, 19].One chief advantage of the current tonometer isthat it can be appliedto assessthe change in an arteriallumen, in addition to the arterial blood pressuremeasurement. Once the continuous arterial pressureand volume variations are both acquired with thetonometer at the same time,a so-called apparentcompliance of the arterial wall can be determined asthe ratio of change in vessel volume and change inpressure [11, 24-25]. This will help the physiciandiagnose cardiovascular diseases related to thehardening of the entire arterial system or a specificsegment of an artery.A noteworthy drawback to a sensor-arraytonometer is the use of the discrete elements. Tosatisfy with the clinical applications, the size of thearray must be miniaturized in order that all sensorelements can be accurately placed over an underlyingartery. It always takes time to position the sensors inactual usage. Furthermore, no proper compensationprocess in a sensor-array tonometer is performed forthe motion artifact, affecting the forces converted tothe sensors. This problem always causes the sensor-array tonometer to be too sensitive to externalstimulations. On the other hand, the opening of theflexible diaphragm of the present tonometer is largeenough to cover the contact area with an artery,ensuring that the change in the arterial volume can besuccessfully transmitted into the tonometer's chamber.6. CONCLUSIONWe have developed a bi-functional arterialtonometer in the study to measure the arterial pressureas well as the arterial volume change. The arterialtonometer chiefly contained a fluid chamber, acompliant diphragm, a pressure sensor and fourassessedby the impedance method. The pressurecalibration using a mercury sphygmomanometershowed that a reasonably linear relationship existedbetween the chamber pressure and the voltage outputof the pressure-sensing circuit. The volume calibrationby using vessel-equivalent cylinders with knowndiameters displayed that there was a linear relationshipof the change in vessel volume to the voltage output ofthe volume-sensing circuit. Clinical testing resultsdemonstrated that the noninvasive pressuremeasuremenuch with the proposed tonometer wasessentially consistent with the invasive pressuermeasurement with a catheter-tipped pressuretransducer. In brief, the arterial applanation tonometerdeveloped may be used to dependably record fullVol. 16 No.6 December 2004arterial blood pressure waveform and change in vesselvolume, and further to determine the dynamiccompliance of a superficial arterial wall.ACKNOWLEDGEMENTThis research was in part supported by NationalScience Council, Taiwan, The Republic of China,under the grants of NSC 92-2218-E-214-008 and NSC92-2218-E-214-013.REFERERCES1. Drzewiecki G, Melbin J and Noordergraaf A:Arterial tonometry: review and analysis. JBiomechanics1993; 16: 141-152.2. Ma JG and Xu DZ: A miniaturizedapplanationtonometer. IEEE Trans Biomed Eng 1999; 46: 947-950.3. Stein PD and Blick EF: Arterial tonometry for theatraumatic measurementof arterial blood pressure. JAppl Physiol 1970; 30: 593-596.4. Tungjitkusolmun S: Chapter 8-- Heart andcirculation. In: Webster JG, Ed.Bioinstrumentation.John Wiley & Sons Inc, New Jersey, USA, 2004;262-302.5. Pressman GL and Newgard PM: A transducer forthe continousexternal measurementof arterial bloodressure. IEEE Trans Bio-Med Electron 1963; 10:73-81.6. Iliev I: Improvements of noninvasivelong-termblood pressure recording. J Clin Eng 1996; 392-397.7. Drzewiecki G: Chap 73-- Noninvasiveassessmentof arterial blood pressureand mechanics. In:BronzinoJD, Ed.-in-chief, The biomedicalengineeringhandbook. CRC Press Inc, Florida,USA, 1995: 1196-1211.8. Drzewiecki G, Hood R and Apple H: Theory ofoscillometric maximum andthe systolic anddiastolicdetectionratios.Ann Biomed Eng 1994;22: 88-96.9. Baker PD, Westenskopw DR and Kuck K:Theoreticalanalysisof non-invasiveoscillometricmaximum amplitude algorithm forestimating meanblood pressure. Med Bio Eng Comput 1997; 271-278.10. Lin C-T, Liu S-H, Wang J-J and Wen Z-C:Reduction of interferencein oscillometric arterialblood pressuremeasurementusing fuzzy logic.IEEE TransBiomed Eng2003; 50: 432-441.11. Drzewiecki, G, Solanki B, Wang J-J, and Li J K-J:Noninvasivedeterminationof arterialpressure andvolume usingtonometry. Proc ELECTRO '96 1996;pp: 61-63.-36- BIOMEDICALAPPLICATIONS,12 K an Verdonc P Developmen anmodellin o arteria applanatio tonometry a review Techno Healt Car 2002 10 65-7613.Eckerl JS Arteria tonometry In Webste JGEd.-in-Chief o medica device aninstrumentation Joh Wile & Son Inc NeYork USA 1988 2770-277614 PS Cavallin C Scorzon D Longhin C PinR an Gana A Arteria tonometry principle anclinica application i hypertension Hig BlooPres 1996 5 241-25015 V Th mechanic o th occlusiv cuf ana flexibl diaphrag tonometer Thesis RutgerUniversity Ne Jersey USA 199416 J-J Li S-H Li C- an Hsie J-HModelin th arteria unloade situatio ioscillometri bloo pressur waveformeasuremen usin fuzz logi control J Me BiEn17 RP Gavriel N Lewkowic S an Palt YA rapi noninvasiv bloo pressur measuremenmetho fo discret valu an ful wavefordetermination J App Physio 1996 80 307-31418 M an Yamakosh K A portablinstrumen fo non-invasiv monitorin o beat-bybea cardiovascula haemoynami parameterbase o th volume-compensatio an electricaladmittanc method Me Bio En Compu 2000383319 K-I Shimaz H an Togawmeasuremen o instantaneou arteria bloopressur i th huma finge b th vasculaunloadin technique IEE Tran Biome En1980 150-15520 M Kawad T Shishid T MatsumotN J J an Sunagaw K Estimatio oarteria mechanica propertie fro aorti antonometri arteria pressur waveforms MethodInfMe 1997 36:250-25321 D an Ki SC Measuremen o le arteriacomplianc o norma subject an diabetic usinimpedanc plethysmography An N Y Aca Sc1999;873:112-2022 P an Yuch CB Noninvasiv measuremento centra arteria pressur an distensibilit barteria applanatio tonometr wit a generalizetransfe function implication fo nursing HearLun 437-4423 R an Bhattacharyy GK StatisticsPrinciple an Methods 4t Ed. Joh Wile & Sons.Inc Ne York USA24 S Masanor N Akik K Toshi O anHiroyosh T Accurac o a continuou bloopressur monito base o arteria tonometryHypertensio25 R an Webste JG Noninvasivmeasuremen o complianc o huma le arteriesIEE Tran Biome En 62-67-37