Mobile Non Contact Monitoring of Heart and Lung Activity

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MOBILE NONCONTACT MONITORING OFHEART AND LUNG ACTIVITY

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JITHIN C.S.

#S7 ECE

#21

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CONTENTS

INTRODUCTION

A. CAPACITIVE ECG RECORDINGS

B. MAGNETIC INDUCTION MEASUREMENTS1. MONITORING OF THE ISOLATED HEART ACTIVITY

2. SIMULTANEOUS MONITORING OF HEART AND LUNG ACTIVITY

CONCLUSION

REFERENCE

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INTODUCTION Smart and wearable medical devices must developed for diseased

people in their domestic environment . For this research topics wireless communication, textile integration

and body-worn sensor networks are very important physical point of view, possible sensors are based on capacitive,

magnetic or optical measurement techniques we shall focus on the first two physical principles capacitive,

magnetic techniques capacitive ECG recordings monitor the electrical activity of the

heart, magnetic induction monitoring may supervise the mechanical activity of both lung and heart.

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A. CAPACITIVE ECG RECORDINGS

Electrodes like noble metal and Ag–AgCl are used for capacitance measurement.

capacitive measurements of bio-potentials are rather sensitive to noise.

To obtain a proper design, active electrodes with amplification directly at the electrodes and active shielding must be employed .

For that electrode capacitance must be small ,d=0.3,A=4cm*8cm,

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Block diagram of capacitive ECG amplifier with two active electrodes and capacitive driven right leg

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As the input impedance of the operational amplifier is much larger than ( ),it can be neglected at low frequencies, leading to an electrode cut-off frequency of

C1,2 coupling capacitance Desired ECG signal is generated by the polarization of the heart

muscle, and then coupled to the electrode capacities C1and C2 In capacitive ECG measurements, the main source of interference is

due to power lines

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Coupling model for 50-Hz power line

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•The power line interference mainly contributes to the common mode content Vcm on the body surface via the capacitive voltage divider of the power line coupling capacitance Cp and the body-to-ground capacitance Cg.•This common mode content could be reduced by using matched coupling capacitors and also capacitive driven right leg•Any change in body position may lead to a coupling capacities mismatch of the electrodes, resulting in different electrode cut-off frequencies.•A stability analysis of the feedback loop with capacitive coupling is done by using identical coupling capacities C1=C2• Since capacitive feedback approach assumes several idealizations of the interference process ,the interference reduction is not perfect.

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B. MAGNETIC INDUCTION MEASUREMENTS

• The mechanical actions of heart, diaphragm and thorax move blood and air through the body.

• By impedance point of view, this is a movement of well-conducting (blood) and poor conducting (air) matter inside the chest region.

• Inducing eddy currents into the tissue and measuring the re-inducted magnetic field externally, these impedance changes can be monitored.

• This is done using single coil arrangement where excitation and measurement are performed with the same coil.

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B:1) MONITORING OF THE ISOLATED HEART ACTIVITY Impedance changes inside the chest would be due to heart

activity , the single coil measurement device would be placed at the proper height.

Under the following simplified assumptions, however, an analytical approximation for the beating heart can be derived.

the magnetic field penetrating the heart shall be homogenous in direction and in strength equal to the field at the center of the heart

, ,

Any current in the heart volume will only flow in

direction and is driven by the inducted voltage

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Model for the eddy current induction into the heart.

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•The resulting current depends on the material impedance ,leading to eddy current.

•Due to the filling and ejection of blood, the heart radius shallchange between and . The resulting total current change in this ring can be calculated by

•Assume that this current is flowing in a circular ring with a radius of .Then the change of the magnetic field at the origin can be calculated to

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•The resulting impedance change of the coil can be calculated to

•further assume that the field in the full area of the coil is modulated by the factor the impedance changes according to

•Thus, the volume change of the heart changes the eddy-current distribution leading to a re-induced magnetic field coupling back into the excitation coil, as visualized

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B:2) SIMULTANEOUS MONITORING OF HEART AND LUNG ACTIVITY

• Since heart and respiratory activity have significantly different base frequencies, a separation in frequency domain could be theoretically considered

• Preliminary tests shows in simultaneous monitoring of heart and lung activity is cancellation of the respiratory content in the magnetic induction signal.

• Interference can principally be cancelled by the application of parameterized error models and an additional reference signal, air flow sensor.

• Let the real heart activity be denoted by Heart and the real lung volume inside the body denoted by . .

• The relation between the derivative of the actual lung volume and the measured airflow is linear, but neither the exact time delay nor the exact frequency response. Thus, the transmission path is estimated by the causal linear transfer function ,modeled by FIR

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•In order to gain the volume from the airflow signal, the airflow signal is integrated and the effect of is cancelled with the inverse transfer functionThis leads to an estimation of the actual lung volume inside the body

•Frequency shift is a superposition of respiration and heart-related content. The resulting estimation of is fed into an estimation of the nonlinear transfer function from lung to .This estimated nonlinear transfer function is based on a low-orderTaylor-series approach:

•Therefore, the estimated respiration related content of the frequencyshift is calculated to:

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•As the respiratory related content of the frequency shift is much larger than the heart-related content, is sufficient to fit with a least squares approach to .

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Capacitive ECG electrodes integrated into an office chair

Single coil setup for magnetic bioimpedance monitoring integratedinto a textile carrier.Fig.

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CONCLUSION

• The two monitoring techniques presented eliminate the need for electric contact to the body

• Both techniques bear the potential to be integrated into textiles or electronic devices worn on the body

• Both systems are very sensitive to movements, especially relative movements between sensor and body

• The price for these device will be high

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REFERENCE Matthias Steffen, Adrian Aleksandrowicz, and Steffen Leonhardt,

“Mobile Noncontact Monitoring of Heart and Lung Activity”, ieee transactions on biomedical circuit and systems vol. 1, no. 4, pp. 250- 256 december2007

H. Scharfetter, H. K. Lackner, and J. Rosell, “Magnetic induction tomography: Hardware for multi-frequency measurements in biological

tissues,” Ph ysiol. Meas., vol. 22, pp. 131–146, 2001.

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