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Telemetric Monitoring of Foetal ECG

B.Hermans and R. Puers

KULeuven, Department ESAT-MICAS, Belgium

L. Lewi, J. Jani and J. DeprestKULeuven, Dept Obstetrics-Gynaecology, Belgium

ABSTRACTIn this paper we propose an implantable telemetric system that meets the clinical needs for

accurate foetal ECG monitoring during labour for all pregnancies where monitoring is

indicated, as well as during pregnancy for a selected number of foetuses undergoing foetal

surgery.

INTRODUCTIONAs a consequence of recent developments, one can state that the foetus has become a patient,

accessible for diagnosis and therapy. Foetuses may become critically ill during their in uterolife, and may benefit from timely delivery or foetal therapy. Until now, continuous in utero

foetal monitoring has not been possible, since it requires permanent and direct access to the

foetus. Therefore, information from the unborn patient can only be retrieved by indirect and

often inaccurate means (ultrasound, doppler and cardiotocography). If these findings are

suggestive of foetal distress, a foetal blood sample must be taken for confirmation. Foetal

blood sampling is an invasive procedure with its associated risks and offers only momentary

information on the foetal status. The availability of a device that allows for continuous and

accurate monitoring of foetal well-being will reduce the inappropriate preterm delivery or

therapy of foetus, with good foetal well-being and avoid foetal death or brain damage, where

foetal distress has gone unrecognised.

Figure 1: Foetal Endoscopy

(Foetoscopy)This device will be an indispensable tool for foetus undergoing foetal surgery using a recently

developed technique ofoetoscopy (foetal - endoscopy). The rationale is to take advantage of

the already established access to the foetus during the operation, and at that time to attach

an autonomous monitoring system to it, through an adapted trocart. See Figure 1 for

surgical foetoscopic technique.

The same system can also be used for monitoring during labour for all pregnancies

where foetal monitoring is required. Whereas foetal ECG monitoring is currentlyavailable only non-telemetricly, which implies that the woman is immobilized to the

trocart

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recording device during labour, the telemetric intrapartum sensor may be applied on the foetal

scalp like the foetal heart rate electrodes, which are currently used. However, mobilization

during labour will be unrestricted, which benefits the progress of labour and increases the

sense of well-being of the woman.

In the present paper two consecutive approaches are highlighted. The first consists of a simple

telemetric device that continuously transmits the foetal information to the outside world. The

analysis is performed at the receiver site.

The second approach is a novel telemetric unit that will also contain on board circuitry to

record a local analysis. This system will be miniaturised in the future to allow easy and safe

access to the foetus.

METHODS

FIRST EXPERIMENTSWe started this research by investigating the possibility of measuring a foetal ECG by

telemetric means. Therefore we used a commercial available sensor

EA-F20 (DSI, Minnesota, figure 2). This 2 lead miniature sensor measures ECG on a

continuous base when it is magnetically activated.

Figure 2: ECG sensor

The signal of the sensor is transmitted and subsequently captured via the maternal abdomen of

the ewe by an antenna. This antenna also provides the necessary electronics to convert the RF

signal into an analog ECG signal. This signal is then further digitalised using a Keithley

Analog-to-Digital PCI/MCIA card. The received digital information is extracted from this

card and stored into the PC using a program developed with Testpoint.

Medical research has shown that the use of ECG to obtain a beat-to-beat analysis (Nijhuis, J.,

et al) can, in combination with ST-analysis, provide the gynaecologist enough information todetect and prevent foetal stress (Sundström, A., et al). Beat-to-beat analysis consists of the

calculation of the R-R interval of the ECG, whereas ST-analysis consists of determining the

T/QRS ratio.

Figure 3 illustrates the above used terms.

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RR-interval

QRS-amplitude

Q

S

R

Q

S

R

PT

T-amplitude

QRS-complexST-interval

contraction

heart chamberschanges during

hypoxia

contraction of

the atria

eart requency

‘

Figure 3: ECG description

To perform the medical analysis, a program in Visual Basic able to extract automatically beat-to-beat and ST analysis, has been written. Figure 4 shows a typical screenshot generated

by this program. The upper trace shows the raw received ECG signal. The extraction of the

QRS-complex is calculated by an algorithm based on a level decision criterion. The result of

this algorithm can be found on the second trace. The third trace is a conversion of the second

one into a beat/minute scale. This trace can be used for a medical beat-to-beat evaluation.

Mean, maximum and minimum heart rate is also automatically calculated. The next trace

shows the variation in heart rate and is also useful for medical interpretation. The ST analysis

can be found on the last trace. With the display of these traces the gynaecologist has sufficient

information to decide on the necessary therapy.

‘

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Figure 4: Analysis program

The telemetric device was implanted subcutaneously in a foetal lamb at day 119 of

gestation (term = 147 days). (Figure 5)

Figure 5: Surgical procedure

After the implantation of the sensor on the foetal lamb, telemetric data was obtained until day

11. On the last day of recording, an elevation in the ST-segment and bradycardia were

present, suggesting foetal hypoxia. On day 12 no foetal ECG signal could be obtained and

indeed the lamb was stillborn 2 days later. Figure 6 shows traces of the first and last day of recording which illustrates clearly the bradycardia and ST changes as described.

Figure 6: ECG recording on first and last day

First day

-0,4

-0,3

-0,2

-0,1

0

0,1

0,2

0,00 0,17 0,34 0,50 0,67 0,84 1,00

time (s)

Last day

-0,6

-0,4

-0,2

0

0,2

0,4

0,00 0,17 0,34 0,50 0,67 0,84 1,00

time (s)

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Table 1 is generated from the written program and demonstrates the daily extracted results

until day 11. The high ST ratio on the first day may be explained by the foetal stress of the

surgical procedure.

First day Day 2 Day 5 Day 6 Day 7 Day 8 Last day(day 11)

ST (T/QRS) 0,36 0,24 0,25 0,23 0,26 0,22 0,47

Baseline HeartRate

212 177 174 185 153 152 108

Table 1: Daily recordings

NOVEL MONITORING SYSTEM

The experiments described above have proven the feasibility of telemetric monitoring

of foetal ECG. Nevertheless this sensor cannot be used in a clinical environment for foetal monitoring. First of all the size of the used sensor is too big for inserting the

sensor through the trocar during fetoscopy. The maximum diameter of a devicecapable of entering the trocar is 0.5 cm; length is not a critical parameter. Another

shortcoming is the absence of any intelligence in the sensor. The used sensor has no

capacity of making any decisions. The monitoring system that we propose will consist

of following parts as shown in figure 7 :

Figure 7: Monitoring System

1 2 3

1 : Sensor 2 : Intelligent Core3 : Bi-directional telemetric unit

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1: Sensor

A 2 lead ECG amplifier has been developed. The biggest challenge is to eliminate the

common mode signal that disturbs the ECG signal, whereas a 3 electrode ECG amplifier

would be used the third electrode will overcome this common mode interference (Nitish V., et

al.). The 2 electrodes may be implanted subcutaneously on the foetus at a distance of 2 – 3

cm.

2: Intelligent core

Instead of transmitting the entire signal, the recorded parameter will be analysed to eliminate

superfluous data. The system will contain algorithms that can automatically detect foetal

health condition. These algorithms will be based on the already described beat-to-beat and ST

analysis. The ultimate goal is to have a system that contains the intelligence to detect any

abnormality and transmit the necessary information.

3: Bi-directional telemetric unit

The device must be able to send and receive data wirelessly with low power consumption.

The possibility to communicate with the device while it is implanted, offers the unique

opportunity to alter the registration parameters or algorithms at the end-users’ request.

Prototype

A first prototype has been assembled. The 2 lead ECG amplifier has been developed using

bootstrapped amplifiers to reduce 50 Hz interference, followed by low and high analog filters

to eliminate signals that are no part of the ECG bandwidth. This creates an operating

frequency range between 0.5 and 150 Hz. The ECG is amplified in such a way that it is

adapted to the full range of the A/D converter. The board also provides a bandpass filter

between 5 and 15 Hz that will extract the QRS complex of the ECG signal (Jiapu P., et al).

A Flash PIC microcontroller PIC12F629 is used. It contains a 10 bit A/D converter. This

microcontroller is programmed to record a sample at a sampling frequency of 300Hz. The

program also provides what is necessary to treat the samples in a certain way that they can be

sent to a RS-232 converter. By doing so, data can be directly recorded on a PC. The boardalso has a transceiver TR3000 (RFM) with the antenna integrated on the PCB. Through this,

ECG can also be transmitted wirelessly. The PCB is powered with +3V and –3V obtained

through silver oxide button cells.

Figure 8 gives the schematic overview of the board. Figure 9 shows a picture of the developed

circuit.

Figure 8: Schematic overview

Figure 9: Developed circuit (9 cm x 4 cm)

ECG amp fil ters & amp

bandpass

filter battery

A/D

RS-232

transceiver

433Mhz

µ c o n t r o l l e r

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ACKNOWLEDGEMENTSThe authors which to acknowledge the support for this program “Verkennende Internationale

Samenwerking” (stimulation of research projects within the fifth framework program),

sponsored by the Flemish Regional Government; VIS/00/009, telemetric and non-telemetric

microsensors for perinatalogy.

LITERATURE CITEDDSI. Data Sciences International. www.transomamedical.com. Minnesota

Nijhuis, J., et al. Foetale bewaking. Elsevier/Bunge. 1998

Sundström A., et al.’ Foetale bewaking met STAN’. Neoventa Medical . 2000

Nitish V. Thakor and John G. Webster. ‘Ground-free ECG Recording with Two Electrodes’.

Trans. Biomed. Eng . Vol 12:pp 699-704. IEEE 1980.

Jiapu P. and Willis J. Tompkins.. ‘A Real-Time QRS Detection Algorithm’. Biomed. E ng . 3:

Vol pp 230-236. IEEE 1985 Trans.

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