Development of a New Catheter-Tip Pressure Transducer

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INTRODUCTION

Anesthesiologists and clinicians requiringaccurate, continuous monitoring of intravas-cular pressures such as those of radial artery,pulmonary artery, and central vein regarddirect measurement to be of fundamentalimportance. A pressure measuring systemincludes a transducer, which convertsmechanical pressure movement into an elec-trical signal. The transducer is usually con-nected to a fluid-filled catheter in whichpressure waveform travels down from ablood vessel to the transducer. Also, thecatheter-tip transducer system is used toavoid the use of a fluid-filled system. In thispaper, we deal with the development of acatheter-tip transducer manometer system.

A catheter-tip transducer is comprised of aminiature transducer attached to the periph-eral end of an intravascular catheter, whichconsequently, can be introduced directly intoa blood vessel. Since this system does not

need fluid-filled catheters, its frequencyresponse is high and can be well used in

high-fidelity pressure measurement.However, the disadvantages of these trans-ducers are that they are expensive and frag-ile with unstable baseline drift. Althoughimprovements in this field of instrumenta-tion have been made, performance remainsunsatisfactory. The problems lie in the struc-ture of the transducer.

To eliminate the above-mentioned prob-lems, we have introduced a new type of dif-ferential transducer that is capable of shift-

ing the baseline by applying appropriatepressure via the external port.

Structure of differential type catheter-tiptransducer

Figure 1 shows the structure of a conven-tional gauge type catheter-tip transducer [1,2]. Applied pressure distorts a diaphragmand a piezo element connected to thediaphragm converts the distortion into anelectrical signal consisting of small voltagesproportional to the pressure. Calibration of

baseline and sensitivity adjustment are car-ried out by exposing the diaphragm to

Tokai J Exp Clin Med., Vol. 24, No. 3, pp.85-92, 1999

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Development of a New Catheter-tip Pressure Transducer

Yoshio KINEFUCHI, Haruo FUKUYAMA, Toshiyasu SUZUKI,Masahiro KANAZAWA and Mamoru TAKIGUCHI

Department of Anesthesiology, Tokai University School of Medicine

(Received May 19, 1999; Accepted June 21, 1999)

Two major disadvantages of the conventional catheter-tip transducer have been incapabilityto calibrate baseline pressure and to adjust sensitivity after insertion. A vast majority of con-ventional catheter-tip transducers are classified as gauge type. Due to their structure they areinherently incapable of recalibrating after insertion. We have overcome this problem by

developing a new differential type catheter-tip transducer equipped with a lumen that con-nects a small chamber at the backside of the transducer to another external port. This lumenis capable of pressure passage. The output of this type depends on the difference between

the two imposed pressures. This passage makes a baseline standard possible, when the endof the lumen is exposed to atmospheric pressure. When pneumatic pressure is imposed tothe end of the lumen using a syringe, for example, the transducer output shifts up and downin accordance to that pressure, enabling baseline pressure recalibration and verifying the

degree of sensitivity after insertion. By obtaining the following data, we confirmed the sta-bility and availability of this transducer: Baseline drift less than 0.04 mmHg/8 hour, fre-quency characteristics flat up to 60 Hz, and common mode rejection ratio more than 46 dB.

Key words : Catheter-tip transducer, Differential transducer, CV catheter, Baseline drift,Frequency characteristics

Yoshio KINEFUCHI, Department of Anesthesiology, Tokai University School of Medicine, Bohseidai, Isehara, Kanagawa 259-1193

Japan


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atmospheric pressure and then to a staticpressure within the physiological range.Therefore, this procedure can not be doneafter introduction of the catheter into ablood vessel.

Figure 2 shows the fundamentals of ourproposed new transducer structure. Thesmall chamber at the backside of the piezoelement connected to a passage of pressurelumen has an opening at the hand side ofthe catheter. As depicted in the figure, weattempted to control the pressure in thechamber at the backside of the piezo ele-ment. The transducer output is proportional

to the difference between the two pressuresapplied to the diaphragm and to the lumenopening. Thus it is referred to as a differen-tial type transducer.

Figure 3 shows the differential type

catheter-tip transducer made from a com-mercially available multi-lumen CV catheterof 20 cm long and 7.5 French in diameter(KS7.5FR-3W, Kawasumi, Japan). The inter-nal diameter of the lumen connected to thechamber is 0.9 mm. A hub is attached to theopening of the lumen to facilitate pneumat-ic pressure application using a syringe oranother specially designed instrument.

86 Y. KINEFUCHI et al.

Fig. 1 Gauge type transducer: An applied pressure distorts adiaphragm and a piezo element mounted on the ceramic stand.

Distortion of the element is converted into an electrical signal.

Fig. 2 Differential type transducer: The small chamber at the backsideof the piezo element is connected to a pressure lumen that hasthe opening at the hand side of the catheter. The output oftransducer is proportional to the difference between twoapplied pressures to the diaphragm and to the lumen.


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METHOD

The differential type transducer functionsin the same way as a conventional gaugetype transducer when the opening of the

lumen is exposed to atmospheric pressure,thus providing a high fidelity baseline level.Furthermore, applying an appropriate pres-sure to the opening enables compensation ofbaseline drift and removal of the DC com-ponent (i.e. the mean pressure) in order toexpand the dynamic range of small pressuresignals.

On the other hand, it is not clear whetheror not the chamber and lumen at the back-side of the piezo element enlarges baselinedrift or reduces frequency characteristics of

the transducer. Detailed inspections concern-ing these functions have been carried outusing the following methods.(1) Baseline drift

The baseline drift was measured underconditions where the differential type trans-ducer was kept in a contained air environ-ment to minimize the effect of airflow. Theopening of the lumen also was released to

the same environment. The transducer out-put was connected to the Servo recorder (U-255MD, Pantos, Japan) via the bridge ampli-fier (AP601G, Nihon Kohden, Japan). Itsmaximum 8-hour baseline deviation was

measured and evaluated by comparing it tothat of a conventional gauge type transducer(CVPS, Kawasumi, Japan).(2) Differential characteristics

The differential type transducer wasfixed, with sealing tape, to a disposabletransducer dome (1295c, Hewlett-Packard,USA) and its output was connected to theServo recorder as mentioned above. In orderto measure the applied pressure to the trans-ducer, a pressure transducer (TNF-R,Ohmeda, USA) was connected directly to the

dome. Pneumatic pressure ranging from 0mmHg to 50 mmHg was applied simultane-ously to both the dome and the opening ofthe lumen. Common mode rejection ratio(CMR) was calculated as maximum outputdeviation/applied pressure ratio. CMR indi-cates whether or not, the performance of theoutput remained unchanged when signals inthe common mode were applied to the two

New catheter-tip pressure transducer 87

Fig. 3 Appearance of the catheter-tip differential transducer: Thephoto shows the catheter-tip transducer which is 20 cm longand 7.5 French in diameter. The wiring indicates the output ofthe transducer. One of the hubs is attached to the opening ofthe lumen connecting to the chamber.


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inputs, thereby allowing differential charac-teristics to be evaluated. The smaller CMRvalue results in more desirable differentialcharacteristics [3].(3) Frequency characteristics

The chamber and lumen at the backsideof the element appeared to be frequency-dependent, hence influencing frequencycharacteristics of the catheter-tip transducer.To assess the degree of influence, frequencycharacteristics of the differential type trans-ducer were measured using the experimentalapparatus shown in Figure 4. The output ofthe programmable oscillator (OSC-2L, Japancircuit design, Japan) controlled by a com-puter (PC9801, NEC, Japan) was connectedto a loudspeaker (P5160, Fostex, Japan) via

a custom-built amplifier. The speaker gener-ates a sinusoidal pressure waveform of 1-60Hz into the dome by oscillating a built-indiaphragm. The catheter was fixed to thedome, with the opening of the lumen kept atatmospheric pressure. For pressure refer-ence, a pressure transducer was connecteddirectly to the dome as mentioned above,and the dome was filled with Ringer lactatesolution. Outputs from the two transducerswere simultaneously read into the computer.

By changing the frequency to progressivelyhigher values, the ratio of the two pressuresignals was calculated at each frequency,and a characteristics chart was constructed.

Similar frequency characteristics of a gaugetype transducer (CVPS, Kawasumi, Japan)and an ordinary CV (central venous)catheter without a transducer (1001,Ohmeda, USA) were also measured for com-

parison. The CV catheter was filled withRinger lactate solution. Normal care wastaken to fill the system slowly and ensure theabsence of air bubbles. Thereafter, the nat-ural frequency (fn) and the damping coeffi-cient () were calculated as follows [4]:

fn=fr/(1-22)1/2

2={1-(1-1/Ar2)1/2}/2

Where fr is the resonant frequency and Ar is the peak amplitude at resonant fre-

quency. Higher natural frequency and lowerdamping coefficient result in better wave-form fidelity and less time lag [5, 6].

RESULTS

(1) Baseline driftFigures 5 (a) and (b) show the baseline

drifts of the differential type and gauge typetransducers, respectively. Maximal deviationsare 0.04 mmHg and 0.1 mmHg, respectively.Both transducers comprise the same piezo

elements made by the same manufacturer,therefore, the differential type structureimproved the baseline drift by one-half com-pared to that of the gauge type structure.


Fig. 4 Block diagram of the measuring system: The left section shows

the pressure source consisting of a loudspeaker and a dome. Acatheter and a transducer as a reference are connected to thedome, and two pressure signals are read into a computer simul-taneously. A frequency characteristics is constructed by calcu-lating a ratio of the two pressure signals at each frequency.


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(2) Differential characteristicsFigure 6 shows the output variation of the

differential type transducer when static 5mmHg increments and decrements of pres-sure ranging from 0 to 50 mmHg wasapplied simultaneously to both the

diaphragm and the opening of the lumen. Alinear correlation was observed between thetransducer output and applied pressure. Anegligible amount of hysteresis was alsoobserved. CMR (common mode rejection

ratio) is indicated by the constant ratio


Fig. 5 Baseline drifts: Maximum deviations are 0.04 mmHg (uppertrace) and 0.1 mmHg (lower trace). The differential type trans-ducer improved the baseline drift by one-half compared to that

of the gauge type.

Fig. 6 Differential characteristics: The output is approximately pro-portional to the added pressure. A narrow hysteresis loop is alsoobserved. In this case, the slope indicates CMR (common moderejection ratio) and is calculated as -46 dB. See text for furtherdetails.


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between them. The maximal output devia-tion was 0.25 mmHg when a pressure of 50mmHg was applied. Therefore, CMR can becalculated as 0.25/50 = 0.005, determined as-46dB (decibels, calculated as 20log 0.005).

(3) Frequency characteristicsFigure 7 shows the frequency characteris-tics of 3 types of catheters; (a) differential,(b) gauge, and (c) normal CV catheter with-out transducer. The ordinate indicates theamplitude ratio between applied (A0) andoutput (A) pressures. No trace differencewas detected between (a) and (b). Their fre-quency characteristics showed little frequen-cy dependency and were flat up to 60 Hz.Trace (c) showed frequency dependency; 7.1Hz of resonant frequency, and 1.4 of reso-

nant amplitude, resulting in 8.4 Hz of nat-ural frequency (fn) and 0.38 of dampingcoefficient (), obtained by using Equation[1] and [2]. By plotting these values into theevaluation chart [7], the highest frequency,i.e. the highest frequency component withwhich the catheter can derive with fidelitywas determined as 2.2 Hz.

DISCUSSION

Employing a differential type transducer

may eliminate problems that occur in theuse of conventional gauge type transducers.

The baseline drift of the differential typetransducer was reduced to one-half com-pared to the gauge type. This result agreedwith expectations since the differential typehad a high level of baseline fidelity.Generally the intravascular transducer worksat body temperature, so baseline drift causedby temperature changes may not affect mea-surement errors. However, transducer cali-bration is performed before vessel insertion,and insertion may result in a temperaturerise of approximately 10 degrees. Thereby,baseline drift of a few mmHg may beinevitable. The differential type transducercan solve this problem by recalibrating afterinsertion, as mentioned above.

As shown, CMR of the differential typetransducer was 1/200, indicating that sensi-tivity difference between the diaphragm andthe lumen was less than 1 %, and that thetransducer can be recalibrated through the

lumen instead of the diaphragm. For exam-ple, when a transducer is suspected of base-line shift and shows a pressure of P1, themagnitude of the baseline shift is deter-

mined as P1-P2 when pressure P2 is appliedthrough the lumen so as to keep the outputat zero. Therefore, transducer output may becorrected by applying pressure P2-P1through the lumen using a device such as a

syringe. Also, when higher sensitivity orexpanded dynamic range is required inorder to measure small pressure changes, theDC component or mean pressure componentshould be temporarily eliminated by addingthe corresponding pressure from the lumen.The sensitivity of the transducer can be con-firmed as follows: when two different pres-sures, (for example when P1 and P2 areapplied in turn through the lumen and thecorresponding transducer outputs) yield thechanged portion ofP, sensitivity can be

calculated as P/(P2-P1). Also, linearity canbe checked easily by changing P1 or P2.

It was feared that the differential typetransducer may have frequency-dependentbehavior. Fortunately, the influence of thelumen was not extreme. As shown, frequen-cy characteristics of the differential typetransducer were flat and proved to be simi-lar to those of conventional gauge typetransducers. Frequency components of a CVpressure waveform are estimated to be up to

7 Hz or less [8, 9]. Frequency characteristicsof both the differential and gauge typetransducers are wide enough to satisfy CVpressure waveform bandwidth requirements.On the other hand, the frequency responsecurve of a normal CV catheter without trans-ducer had a resonant peak of 7.1Hz withinthe above mentioned frequency bandwidth,resulting in pressure waveform distortions. Although no actual distortions occurred,waveform delay time is prolonged, anderrors in timing and the relationships of cer-tain cardiovascular signals may result [5, 7,10]. An example of waveform delay isdemonstrated in Figure 8. The lower tracederived by a normal CV catheter showed notonly waveform distortion but also 76 mil-liseconds of delay time from the upper tracederived by the catheter-tip transducer. Thisled us to conclude that the catheter-tip trans-ducer should be used in the field of high-fidelity manometry.

Why does the lumen at the backside of

the piezo element seem to have no effect onfrequency characteristics? This lumen acts asinertance, i.e. as the impedance elementwhose magnitude is proportional to the fre-



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quency. Therefore, the impedance of thelumen increases with frequency and thehigher the frequency rises, the harder thelumen becomes. Of course, transducer sensi-tivity of zero frequency is sufficient, so con-

sideration of the influence of the lumen onthe frequency characteristics is unnecessary.Several commercially available PA (pul-

monary artery) catheters with the catheter-tiptransducer also have similar problems as the

above-mentioned CV catheters [11]. Further-more, they are quite expensive. The sameconcept introduced in this paper concerningdifferential type transducers might also beapplied to PA catheters. High-fidelity PA

pressure waveforms can perhaps beobtained with ease. Thus, the evaluation ofPA catheters can expect a dramatic increase.However, there still remains the problem ofconsiderable expense.


Fig. 7 Frequency characteristics: Trace (a), (b) and (c) show frequen-cy characteristics of the differential type, gauge type, and nor-mal CV catheter, respectively. Traces (a) and (b) are flat up to60 Hz. Trace (c) has 7.1 Hz of the resonant frequency, and 1.4

of the resonant amplitude. See text for further details.

Fig. 8 Waveform distortion and time lag caused by a normal CV

catheter: The lower trace derived from a normal CV catheterhad 76 milliseconds time lag compared to the upper tracederived from the differential type transducer. Occurrence oftime lag is inevitable for a fluid-filled catheter manometer sys-tem.


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York, McGraw-Hill Book Company, pp. 21-50, 1971.4) Kleinman B: Understanding natural frequency and

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physiological systems. Philadelphia, WB Saunders,pp. 77-91, 1966.

7) Kinefuchi Y, Suzuki T, Takiguchi M, et al :Evaluation of dynamic response of catheter-manometer systems for pulmonary arterial pressure.J Appl Physiol 77: 2023-2028, 1994

8) Patel DJ, Mason DT, Ross J, et al: Harmonic analysisof pressure pulses obtained from the heart and greatvessels of man. Am Heart J 69: 785-794, 1965.

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10) Hurst JW, Schlant RC: Examination of the veins andtheir pulsation, the heart. Edited by Hurst JW. NewYork, McGraw-Hill Book Company, pp.193-201,1978.

11) Kinefuchi Y, Suzuki T, Takeyama K, et al: The lim-its of the frequency characteristics of PACs (pul-monary artery catheters). J J Circulation Control 13:89-95, 1992.


Development of a New Catheter-Tip Pressure Transducer

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