SENSORIZED MICROCATHETER

Smart Microinstrumentation Adressing Fetal Surgery – First Results

A. Sieber

1, 2

, K. Houston

, A. Menciassi

, G. Nauer

and P. Dario

Profactor Research and Solutions GmbH, Seibersdorf, Austria

Scuola Superiore Sant’Anna,Pisa, Italy

ECHEM, Wiener Neustadt, Austria

Keywords: Fetal surgery, Endoscopy, Fetoscopy, Pulmonary Atresia, Bio-impedance, Catheter.

Abstract: Pulmonary Atresia is a malfunction that is diagnosed in about 1 out of 20.000 fetus. The authors propose a

novel surgical intervention where the fetal heart is accessed with an endoscopic intervention through the

umbilical cord. The key for this innovative procedure is a novel micro-catheter that is equipped with sensor

and actuators that allow active navigation inside the heart and also tissue characterisation. The present paper

presents the first prototype.

1 INTRODUCTION

1.1 Fetal Surgery

Birth defects occur in 1/28 of births and are the

leading cause of infant deaths. Costs for treatment

after birth are sometimes higher than surgery costs.

Surgical interventions on the fetus during pregnancy

allow a higher intra-uterine survival rate and an

improved postnatal outcome. Till now for a fetus

with diagnosed congenital malformation abortion,

continuation of the pregnancy until termination with

a Cesarean delivery, change in timing mode or place

of delivery were the only possibilities. Fetal surgery

may now provide a solution in these cases.

Starting from the two main American centres

(Harrison, M. R., 2003) that have been performing

fetal surgery for more than twenty years - University

of California, San Francisco Fetal Treatment Center

and Children’s Hospital of Philadelphia, Center for

Fetal Diagnosis and Treatment - nowadays about a

dozen worldwide centres provide prenatal surgical

intervention and many others carry on research and

experiments for specific fetal surgical applications.

(Raul A. Cortes and Diana L. Farmer, 2004)

Fetal surgery is still intended for a restricted

number of malformations that can not be

successfully or efficaciously treated after birth.

However, since 1981 many life-threatening fetal

pathologies have been treated through in-utero

surgical corrections, approaching prenatal

interventions as a valid alternative to neonatal

therapy or induced abortion.

At the moment open fetal surgery is the standard

procedure. It is similar to standard surgical

interventions, but causes a high level of stress for

both the fetus and the mother. An alternative can be

performing a key-hole surgical intervention on the

fetus with the help of endoscopic microtools. This

procedure is commonly known as fetoscopy and

allows an intervention on the fetus in its natural

environment causing less uterine trauma, less fetal

manipulation but preterm labor, damage to uterine

membranes and manipulation difficulties. (Sydorak,

R. M. , Albanese, C.T., 2003; Danzer, E., Sydorak,

R.M., Harrison M.R., Albanese C.T, 2003; Flake,

A.W., 2003 ; Berris M., Shoham M., 2006;

Papadopulos, N.A., Papadopoulos, M.A., Kovacs,

L., Zeihofer, H.F., Henke, J., Boettcher, P., Biemer,

E., 2005).

These procedures are performed through the use

of small trocars and a combination of

videofetoscopic and sonographic visualization.

Paediatric surgeons are trying to apply standard

minimal invasive instruments, designed by medical

engineers for different kind of surgery, to fetal

surgery applications. These instruments may be

suitable for some interventions, but are far too big

for interventions in an early development stage of

the fetus. Thus one of the main problems fetoscopy

is facing is the lack of suitable micro

instrumentation.

190

Sieber A., Houston K., Menciassi A., Nauer G. and Dario P. (2008).

SENSORIZED MICROCATHETER - Smart Microinstrumentation Adressing Fetal Surgery – First Results.

In Proceedings of the First International Conference on Biomedical Electronics and Devices, pages 190-195

DOI: 10.5220/0001047301900195

 SciTePress

1.2 Pulmonary Atresia

During pregnancy the necessary oxygen is not

supplied through the fetal lungs but by the placenta.

The Foramen Ovale is an opening between the right

and the left atrium that allows blood to pass by the

pulmonary tract. After birth this opening is usually

closed. Pulmonary Atresia (Daubeney, P.E.F.,

Wang, D., Delany, D.J., Keeton, B.R., Anderson,

R.H., Slavik, Z., Flather, M., Webber, S.A., K.,

2005; Litovsky, S., Choy, M., Park, J., Parrish, M.,

Waters, B., Nagashima, M., Praagh, R.V. & Praag,

S.V., 2000) is a malfunction that may appear during

pregnancy: it is an incorrectly developed pulmonary

valve.that is, instead of a valve there is just a

membrane (compare Figure 1A and Figure 1B).

Figure 1A: Healthy heart 1B: Heart with absent pulmonary

valve (http://www.americanheart.org/).

No blood supply to the lungs is possible in this

case which usually causes the death of newborns

when oxygen supply by the placenta is not given

anymore. Furthermore anatomic obstruction to the

right or left ventricular outflow tract may cause

ventricular dysfunction, can divert fetal blood flow

in the uterus and result in cardiac chamber

hyperplasia. Thus severe aortic or pulmonary

stenosis can result in a hypoplastic left or right

ventricle with an inability for the ventricular

chambers to support the systemic or pulmonary

circulation. Theoretically early relief of the fetal

aortic or pulmonary stenosis may prevent such

occurrence and might preserve the right or left

ventricular function. In the case of pulmonary

atresia this can be achieved by a punctuation of the

pulmonary membrane to enable a pulmonary blood

flow and a further correct development of the valve.

(Tworetzky, W., Wilkins-Haug, L., RW. Jennings,

2004; Kohl, T., Witteler, R., Strämper, D.,

Gogarten, W., Asfour, B., Reckers, J., Merschhoff,

G., Marcus, A.E., Weyand, M., Van Aken, H.,

Vogt, J., Scheld, H.H., 2003)

Pulmonary atresia can be diagnosed in the 12-

14th week of gestation. The surgical intervention

should be performed as soon as possible. In the 14th

week the fetus size is about 9-14cm and has a

weight in the range of 60 - 200g. In this

development stage the pulmonary membrane has a

diameter of approximately 1mm.

Pulmonary atresia occurs in about one out of

every 20,000 live births. An early surgical

intervention is the only alternative to abortion and

could allow normal development of the pulmonary

valve and the right ventricle.

Figure 2: Fetoscopic approach to access the fetal right

ventricular through the umbilical cord

2 METHODS

2.1 Fetoscopic Approach

We propose a minimally invasive surgical

procedure in the case of pulmonary atresia which

includes the following steps:

(1) Externalisation of the uterus where the fetus

remains in its own environment

(2) Accessing the right ventricle with a flexible

and steerable microcatheter through the

umbilical cord (need for a microcatheter

(outer diameter <1mm), steering mechanism

(3 degrees of freedom), the catheter needs to

be highly flexible, position feedback systems

need to be available to track the catheters tip)

(Figure 2)

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191

(3) guiding the catheters tip in front of the

pulmonary membrane (need for a steerable

catheter)

(4) recognition of the tissue in front of the

catheter: Due to the small dimensions of the

pulmonary tissue and the surrounding tissue it

is very difficult to distinguish between those

just by vision on an ultrasound picture. Tissue

characterisation and recognition sensors may

then be the solution for a reliable tissue

distinction.

(5) once the pulmonary membrane is detected the

perforation takes place (need for a cutting

tool)

It is clearly visible that the development of

suitable microinstrumentation is the key to this novel

surgical technique. To proof the feasibility of the

approach, we designed a first prototype for such a

smart catheter that is equipped with tissue

characterisation sensors and a steering mechanism.

2.2 Steering Mechanism

To be able to reach the right ventricle through the

umbilical cord (figure 2), the catheter needs to be

equipped with steering capabilities (Ascari, L.,

Stefanini, C., Menciassi, A., Sahoo, S., Rabischong,

P., Dario, P., 2003). The multi-lumen catheter

consists of a very flexible ending and a less flexible

part. In the walls of the catheter 4 thin diameter

lumen are integrated, each one for one steering wire.

Pulling on these 4 wires and releasing at the same

time the wire which is on the opposite side in the

catheter will primarily result in a movement of the

flexible end part of the catheter. Two

microcontroller driven servo drives are used to pull

and release the wires. This microcontroller is then

connected to a personal computer, which, equipped

with a haptic force feedback joystick, allows a

precise control of the catheter. A third degree of

freedom is realized by either manually or servo

supported driving the catheter forward and

backwards.

2.3 Electrical Impedance Sensor

Bio-impedance spectroscopy allows tissue

classification and identification by recording and

analyzing the electrical impedance at different

frequencies (Rigaud, B., Hamzaoui, L., Chauveau,

N., Martinez, E., Morucci, J., 1994; Cao, H.,

Tungjitkusolmun, S., Choy, Y. B., Tsai, J. Z.,

Vorperian, V. R., Webster, J. G., 2002). From the

electrical point of view cell membranes appear as

capacitors. In comparison to low frequency electrical

current where the current path is leading mainly

through extra cellular fluid, high frequency electrical

current is able to penetrate the cells. Different tissues

can be distinguished by comparison of their

characteristic impedance over frequency and phase

over frequency plots. Principle Component Analysis

can then be used to classify a tissue by a recorded

data set.

For impedance spectroscopy two or four

electrodes configuration are state of the art. Four

electrodes impedance measurement allows higher

accuracy, as two electrodes are used to drive in the

electrical current and the other two, which are

normally arranged in between the first two ones, are

used the small sensing electrode.

It must be kept in mind that for tissue

classification it is not necessary to record accurate

impedance data from the electrical point of view. It

is just important that the training data sets are

recorded with the same electrode configuration to

give comparable recordings.

2.4 Spectrophotometric Sensor

To enable a reliable tissue distinction a second

sensor system based on a different principle is

required. Tissues can be distinguished by their

colour, so pulmonary valve tissue appears rather

white in comparison to the surrounding more red

endocardium. Integration of two optical fibres is the

basis for the recording of optical reflectance

spectrum in front of the catheters tip. One fibre is

used for guiding the necessary light from a white

LED to the point of interest. Reflected light is then

received with a second fibre leading to an optical

spectrophotometer working in the visible range.

Unfortunately in normal condition the heart is filled

with blood. Haemoglobin is a strong light absorber,

where furthermore the wavelength dependent light

absorption also is dependent on the oxygenation

status of haemoglobin. Measuring tissue

characteristics with a spectrophotometric method is

therefore not possible in the presence of whole

blood.

2.5 Washing System

To solve the above described problem we integrated

another lumen in the catheter to provide washing

solution with a small amount of physiological saline

solution blood in front of the catheters tip can be

washed away. Thus blood in the measuring zone is

substituted with the washing solution, which enables

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5 7

1: Tip

2: flexible and steerable part of the catheter

3: not steerable but flexible part of the catheter

4: connector/distributor

5: pressure sensor

6: op tical fibers

7: impedance electrode(s)

8: tube

9: precision pump

10: reservoir with physiological solution

11: controller for the precision pump

12: light source and spectrophotometer

13: electrical impedance spectrometer and rf generator

14: Master computer

Figure 3: System setup.

spectrophotometric reflectance measurement

(Sieber, A., 2007). In figure 3 the principle setup

including the peristaltic pump for the physiological

saline solution is shown.

2.6 Tissue Fixation

Impedance and photometric spectrum recording

requires mechanically stable conditions – the tissue

in front of the catheter should not move relatively to

the catheter, which is difficult to realize in a moving

environment like a beating heart. To solve this

problem the washing system described above has a

second functionality: After the blood in front of the

catheter is substituted with a small amount of

physiological saline solution (Figure 4, 1-3), the

washing solution pump can be driven backwards,

thus sucking in washing solution and creating

suction in front of the catheter. Tissue in front of the

catheter is sucked to the tip and a reliable electrical

and mechanical connection is established (Sieber, A.,

2007), and electrical impedance and optical

spectroscopy are performed (Figure 4, 4-5). The

maximum suction pressure used in this setup is -50

mbar.

Figure 4: tissue characterisation scheme.

3 RESULTS

3.1 Catheter Prototype

To prove the feasibility of the concept a catheter

was fabricated with the following specifications

(Figure 5 and 6):

 diameter: 3,5 mm

 steerable tip, 2 DOF, servo actuated

 four electrodes for electrical impedance

spectroscopy and radio frequency cutting

 two 500 μm optical fibers for optical

reflectance spectroscopy

 housing micromachined from PEEK with 5

axis Kern CNC milling centre

 integrated washing / suction channel

Figure 5: Catheter tip design (A) and first prototype (B).

Figure 6: Catheter prototype (A) and control with a

joystick (B).

A joystick is deployed for the control of the bending

of the catheter tip (Figure 6B).

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193

3.2 System Setup

Figure 3 shows the principle setup. The catheter

consists of the PEEK tip (1) with the integrated

electrodes, the washing channel and the optical

fibers, the steerable part (2) and the passive flexible

part (3). It is connected to a distributor (4) where a

pressure sensor (5) is mounted. Another port is

connected to the pump providing the washing

solution from a reservoir (10). The pump is

connected to a microcontroller ATMEL Atmega 32

(11). This microcontroller also serves as a controller

for two servos driving the Bowden cables of the

catheter needed for active bending of the catheter

tips (not displayed). The optical part (12) consists of

a light source (white LED) and a microspectrometer

[Microparts]. The electrodes are connected to a

programmable precision LCR meter [TEGAM]. All

the components 11, 12 and 13 are then connected to

a PC. The software is written under National

Instruments LabView.

3.3 Tissue Distinction

Several electrical impedance spectra of in total 10

dissected bovine pulmonary valves and surrounding

endocardium were recorded (in the presence of

saline solution).

Figure 7: test setup: recording impedance spectra of

pulmonary valve tissue.

Therefore a second slightly larger (5mm

diameter) catheter tip was fabricated from peek

again using the Kern CNC milling centre. During

the recording the negative suction pressure was kept

constant at -25 mbar.

The impedance spectra were recorded from

10kHz to 5 Mhz with a induced signal of 1V peak to

peak. Pulmonary valve and endocardium tissue can

be clearly distinguished by electrical impedance

spectroscopy (see plots shown in Figure 8 and 9).

Figure 8: Impedance vs frequency endocardium: blue;

pulmonary valve: red; saline solution: yellow.

Figure 9: Phase vs frequency: endocardium: blue;

pulmonary valve: red; saline solution: yellow. The

negative peak at approximately 4 Mhz seems to be a result

of resonance.

4 CONCLUSIONS

Pulmonary atresia is a malfunction that occurs in

approximately 1 out of 20000 fetus. It can be

diagnosed in the 15

week of pregnancy. A feasible

approach to correct the malfunction is described,

but it requires sophisticated instrumentation.

The fabrication of the first prototype is a major step

towards the final catheter, which will be the key for

a successful early treatment of pulmonary atresia

thus offering an affected fetus a prospect to a future

without handicaps.

5 FUTURE WORK

Next steps will be catheter insertion tests of the

prototype on the animal model, enlargement of the

impedance spectra database and in parallel the

design of the miniaturized version. We envisage the

substitution of the bowden wires (the actuation

wires for bending of the catheters tip) by smart

actuators such as Ion Polymer Metal Composites –

which will enable a reduction of the overall

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diameter of the catheter to 0,8 mm, or SMA

actuators (Mineta, T., Mitsui, T., Watanabe, Y.,

Kobayashi, S., Haga, Y., Esashi, M., 2002) .

Additionally coils will be integrated, in order to

give a position feedback (Salomon, O., Kosa, G.,

Shoham, M., Stefanini, C., Ascari, L., Dario, P.,

Zaaroor, M., 2006; Aurora NDI).

ACKNOWLEDGEMENTS

The work described in this paper was supported by

the Austrian Research Centers GmbH, by the

Fondazione Cassa di Risparmio di Pisa in the

framework of the ”microSURF” project for the

development of innovative tools and techniques in

fetal surgery, and by the ASSEMIC project, a Marie

Curie Research & Training Network (MRTN-CT-

2003-504826).

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