12-Lead ECG modulator and demodulator equipment for telephony transmission

12 -LEAD ECG MODULATOR AND DEMODULATOR EQUIPMENT FOR TELEPHONY TRANSMISSION SL. Tora l, B. Lara Az n al; JM. Quero and L.G. Franquelo Departmen t of Electronic Eng ineering, Un i versity of Sev ille Avda. Camino de los Descubr imientos, 41092, Seville, Spain E-ma il: [email protected] Tfno: +34 954487371 Fax: +34 954487373 ABSTRA CT This paper describes a teleassisten cy system that consists of a modulator equipment for the acquisition. storage and FM transmission of the typic al 12 lead ECG signals and a demodulalOf equipment for recovering the original signals in a medical ceme r. Switched telephone network and GSM network can be used to transmir the audio frequency modulated signal carrying E CG Information. This project h as been developed in collaboration with Teleasiscencia Cardiotest. S.L. 1. INTRODUC T IO N Consumers are dema nding more access to health inforI nation and more convenient hea l th care service. Particularly, people living in rural or remote areas la ck the access on urgent and special medical care . In these cases, health insura nce companies must provide a medical assistance within a minimum distance . Besides, consumer satisfact ion in all te lemedicine program evaluations has been very high, Given the choice between convenience of telemedicine and traveling to see the consultant in pe rson. they overwhelmingly choose telemedicine. I n this paper. we propose a modulator and demodula - tor equipment for ECG transmissIon . The link between the patient and the specialist is established using a frequency modulated audiO signal transmitted by the telephone network. Figure I shows the basic now of information. The remote po i nt (a patient, a non specialist doctor) ca n connect with a medical center to transmit the pr ev iously stored ECG signal using the ECG modulalOr. The ECG demodulator of the host center can recover the original ECG signal and then a specialist can diagnose any heart disfunction. ". -a - 10 ---::---:-10 ... - REMOTE POINT MEDICAL CENTER Figure I: Block diagram of ECG system The 12 -lead ECG equipment is actually an expansion module for the basic equ ipment described in {II. In next 722 two seClions. we explain the hardware and software of the ECC modulator and demodulator. Section III shows the final prolOtype and some resu lts, and finally conclusions will be pointed out in section IV 2. 12·L EA D MOD U LATOR The 12 lead modulator eqUipment must optimize several restri ctions: area . power consumption and simulta neous acquisition. I t must be taken into a cco um that ECG signals must be sensed and amplified by a factor of 1000. Electrocardiographic signals ma y be corrupted by various kind of noise (21 . Typical examples are : • Power line Interference. It is the main source of noise, because of the cables attached to the 10 electrodes. • Electrode co nta ct noise. It is a transient interfe rence cause by the loss of contact between the electrode and the skin. • Motion artifacts. It consists of transient baseline c hanges caused by chang es in the electrode-skin impedance because of movements of th e patient . • Other noise sources. There are several other noise sources due to muscle co ntra ction. respiration or radio frequency interferen ces. To record an ECG an electric circuit between the heart and the electrocardiograph must be completed. For this purpose, electrodes are placed on different parts of the body surface. Ten electrodes are connected to the modu lator equipment by means of cables: nine of them are Mexploringelect rodes while the last one is the reference placed on the right leg. Depending on the position of electrodes and the measured vectors, we can di stinguish three groups of ECG signals 131. • Bipolar standard leads. Electrodes placed on the right arm (RA), left arm (LA) and l eft leg (LL) are used to pi ck up the potential variations on these extremiCies. Bipolar leads named dl. dIl and dIl l are obtained by recording respectively the potential differences between LA and RA. LL and RA, and LL and LA. • Unipolar extremity leads. Unipolar extremity leads are obtained by recording the differences between RA. LA a nd LL a nd a point n amed Wilson's cen tral terminal. This point is g iv en by the s um of the potentials from RA. LA and LL. These leads are known as aVR. a VL and aVF. • Unipo lar pre cordial leads. The unip olar pr ecordial ECG is obtained by placing the exploring el ec trode on thl:: classical six locations of the an teri or and left portions of the chest. U nipolar precordial l eads are obtained a~ th e difference between th ese six positions and th e Wllson's central terminal. They are prefixed by the letter ~V~ fo ll owed by numbers 1 thr ough 6. which indicate the correspo ndin g chest sites . The acquisition stage is pelformed by a low power ins trum e ntation amplifier w ith a programmab le gain fact O f. ECG interferences are tr e ated in th e following way: ,Jower line int e rf eren ces are attenuated tha nk s to common mode rejection ratio (CMRR) of the amplifier; besides. a 50 Hz notch fil te r is di gita ll y jmplemented in the mi croprocessor c hip . Input signals are high pass filt ered to elimin ate ba se line c han ges. Finally, electrodes co n sists of patch es with a conductor gel t hat guarantees a good c onta ct wjth the body surface. It is a r eq uir em ent of th e pr oject that th e 12 l eads must be acquired simultaneousl y in ten seconds. That m ea ns that the input signals mu st be multiplexed for the 8-bit analog to digital converter of the micr o-co mr oller. Fi gure 2 shows a bl oc k diagr am of th e modulator equipment. Using the typical 10 elec tr odes, the 1 2l ead ECG signals are acquired in parallel and them multiplexed for th e analog to digital conversion. Sa mpling frequency is 100 Hz . The micro-controller (MC6 8L II from Motorola 141) stores th e ECG signals usin g th e analog to dig it al co nv erter (10 seconds fo r each lead) In the R EG po sition of th e eqU ipment and sends th e modulated ECG sig nals to the speake r in the PLAY position, showing the correspo nding ECG l ea d in a fo ur digits alphanumeric dis pl ay . ~ ~-.r - -, ~ ~ '-- --' "- u Figure 2: Block di agra m of the 12 -lead ECG modul ator 3. DEMODULATOR Figure 3 Is a block diagram of th e demodulator equi pment. The audio signal is ac quired and preamplified. Figure 3: Block diagram of the ECG demodulator Then a frequency-voltage converte r recover the original SignaL After a filter s ta ge. the ECG signal is digitally co nvened using the analog to digital co nv erte r of th e mlcroco ntroller c ir c uit . Finally. th e digital signal is transmitted to a host PC using a se rial RS-232 link or a infrar ed link (lrDA). The equipment can wo rk w ith batteri es or a DC power s up pl y. 4. PROTOTYPE AND R ESULTS The modulator equip ment h as been divided in an analog board and a digital board. Figure 4 is a picture of th e anal og board (t op view). It includes the acquisition of the 12-1 ea d Signals. SMT co mp one nt s allow to achieve size restrictions. There is a potentiometer for eac h Signal to se t th e ze ro valu e. Figure 4: Modulator: analog board Figure 5 shows the digital board. It is based on th e micro-controller circuit that controls the analog multi - plexers. the four digit alphanumeric display and a low battery circuit detector. Figure 5: Modulator : digital board Finally. figure 6 is a pictur e of the demodulator equipment . The board includes a DC power supply (on the right bottom part of the picture) the preamplifier and the frequency to voltage converter (on the le ft bottom) and the serial connector and the IrDA Jed on the top of th e piCture. The micro-controller and the driver for the serial link are placed on the other la yer of the board. Figure 6: Demodulator board An example fthe demodulated signals are shown in fig. ures 7 and 8. ParUcuiariy. fig. 7 shows the bipolar and unipolar extremities leads, and fig. 8 shows unipolar precordJal leads. Vertical axis is the captured digital value transmitted to the PC. 5. CONCLUSIONS A tel easistency system for ECG signals has been developed in collaboration with a medical center. The system co nsists of a portable 12 l ead modulator equipment, used by rhe patient or a non specialist doctor, and a demodulator located in this medical center . The main restrictions are related w ith the modulator because it will be used by the patient. I t must be of reduced size. with the maximum autonomy and user friendly. This equipment can be useful in remote areas without a cardiology specialist and in mandatory mecltcaJ revisions in factories where hundreds of ECC must be done. Simultaneous acquisition of the 12 lead allows a record time of ten seconds for each revision. 6. REFERENCES 111 S.L. ToraI. J.M. Quero. M. Ele na Prez . L.G. Franquelo. SlvfT Bo ard for Ac quisition of El ec trocardiograph Sig - nals and CSM Tr ansmission. Proc. DCIS·2000 .. Montpellier. pp. 805814. Nov. 2000. 121 G.M. Friesen . T.C. Jannet. M.A. Jadallah. S.L. Yates. S.R. Quint and H.T. Nagle. A Comparison of Noise Sensiviey of Nine QRS Decection Algorithms. IEEE Trans . Biomed. Eng .. vol. 37. no. I. pp. 85-98. Aug. 1990. 131 J. W. Hurst and RC. Schlant. The Heart. McGrawHill. Health Proffesions Division. 71h edition. 1990. 141 Motorola In c.. MC68HCll Reference Manual . Mo· torola. Rev. 3. 199 1. 724 " '" ~II "" ' 2SO "" ,- 200 -200 200 'SO • 'SO J I 'SO '00 -'00 '00 SO - SO SO " " " 2 2 .VR oVL ,VF 2SO • 2SO 2SO 200 -200 2bo 'so - 'SO I .I '" , oon, ' ' 00 ' 00 SO - SO SO " " , , , 2 , 2 time ($) rime (5) MIe(s) Figure 7: 12 lead EC G: bipolar and unipolar extremities leads V. V2 V3 2SO - 2SO "" 200 200 200 'so - 'SO 'SO ' 00 -'00 '00 50 f SO SO " , " 2 , , , 2 W V, V6 2SO 2SO 200 200 '50 'so ' SO '00 - "Xl '00 I SO - SO SO " , " , " , time (5) t ime (I) lime ($) Fi gure 8: 12 lead ECC: unipolar precordial leads 71'