Automated Computer Aided ECG system

Automated Computer Aided ECG system D.B. Wijesinghe, W.O.K.I.S. Wijesinghe, M.M.D.T.K Wijewardane, P.M. Wijewardena Department of Computer Science and Engineering, University of Moratuwa, Sri Lanka.

Abstract The Electrocardiogram (ECG) is an essential diagnostic tool that measure and record the electrical activity of the heart. A wide range of heart conditions can be detected when interpreting the recorded ECG signals. These qualities make the ECG a perfect instrument for patient monitoring and supervision. The commonly used ECG-machine used for diagnosis and supervision at the present is expensive and stationary. The aim of this project is to develop a cheap and reliable ECG system to make the costeffective. Wireless patient monitoring has become a more established technology and a natural step in this progress is to develop a reliable ECG system that contributes to the cable reduction in medical and physiotherapy environments. The main focus of this project is to create a reliable wireless ECG sensor system at low cost. A small embedded ECG sensor system prototype has been developed. Using Bluetooth technology the ECG sensor system can connect to an ECG monitoring network. With the use of a microprocessor the analogue signal is digitally converted at a specific sample rate that based on the resolution of the ECG-signals.

Introduction

Methodology

Heart diseases are no.1 leading causes in hospital deaths in Sri Lanka (probably in the world). To diagnose heart problems, status of the heart should be identified.

Sensors

The sinoatrial node, which is the natural pacemaker of the heart, produces electrical impulses which spread through the heart muscle to make the heart contract. These impulses can be detected by the ECG machine. An ECG (electrocardiogram) machine basically records the electrical activity of the heart, from which status of the heart can be obtained like rate and regularity of heartbeats, the size and position of the chambers, the presence of any damage to the heart.

There are 2 main types of sensors: Dry sensors and Wet liquid sensors. Here, wet liquid sensors are used as the noise due to touch is considerably lower than dry sensors. Silver/ silver chloride electrode is used to measure the voltages.

Although ECG machine is such useful, the problem is they are too expensive to use in rural hospitals. So the goal of this project is to solve that problem, to build a cheap and simple ECG system which is adequate to use in basic cardio diagnosis.

1|Page

QRS Detection Peek values of the received signals are detected using Pam Tompkins algorithm.

Low pass filter Low-pass filter is an electronic filter that passes lowfrequency signals but attenuates (reduces the amplitude of) signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. ADC The ADC carries out the process of the analog to digital conversion. It is a member of the family of action status devices which have two control line, action input lines and the status output line. ADC converter is a single chip integrated circuit having a single input connection for the analog signal and multiple pins for digital output.

Buffer and Differential Amplifier

USART

The buffer amplifier conditions the selected input signal to a suitable level for application to the A/D converter.

The USART supports a wide range of software programmable baud rates and data formats and operates in either Synchronous or Asynchronous (UART) mode. It implements automatic parity generation and several error detection schemes. The USART is capable of detecting a wakeup pattern to selectively enable the receiver and implements flow control logic for hardware handshaking.

Driven by the multiplexer, the buffer amplifier, which is usually an instrumentation amplifier, provides impedance buffering, signal gain and common mode rejection. It has high input impedance, 100 MΩs or more to reduce the effects of any signal distortion caused by the multiplexer. The high input impedance also minimizes errors due to the finite on-resistance of the multiplexer channel switches. To improve system sensitivity, the amplifier boosts the input signal. If it is required to have analog signals of differing ranges, connected to the multiplexer input, then a programmable gain amplifier would be preferable where the gain would be set in accordance with the multiplexer selection address. The use of programmable gain amplifiers removes the necessity to standardize on the analog input ranges. Poor signal received from the buffer is improved by the differential amplifier up to a measurable state.

RS232 The RS232 signals are represented by voltage levels with respect to a system common (power / logic ground). The "idle" state has the signal level negative with respect to common, and the "active" state has the signal level positive with respect to common. RS232 has numerous handshaking lines, and also specifies a communications protocol. Control Logic The control logic provides the necessary interface between the ECG analysis system and the elements of the acquisition unit while providing the necessary 2|Page

timing control. It is to ensure that the correct analog signal is selected, sampled at the correct time, initiated the ADC process and signals to the microprocessors on completion of conversion. Problem description During the development progress several problems has occurred most of them easy to handle and what could be described as expected problems. In this chapter of the report some of the more challenging problems will be described and how they were solved. Noise generation: The wanted electric potential that the ECG device is measuring is relative small in comparison to the noise that both the body and electrical wires absorbs form the surroundings. Mostly 50 Hz noise is absorbed from the power cables and all electrical equipment that is power by them. The ECG signals are specified to have a resolution 0150 Hz therefore a low-pass filter that eliminates the 50Hz noise cannot be used. The DC-offset is an

additional problem in developing bioelectrical amplifiers. Another noise generation is due to the stray effect of the alternating current on the patient or because of the alternating current fields due to loops in the patient cable. Other causes of interference are loose contacts on the patient cable as well as dirty electrodes. When the machine or the patient is not properly grounded, power line interference may even completely obscure the ECG waveform. Most efficient way to reduce all unwanted noise is to use a Right Leg Drive loop to eliminate the noise absorbed by the wires and body by inverting the noise and feeding it back to the body via the right leg.

Shifting on the base line: The baseline shift can be eliminated by ensuring that the patient lies relaxed and the electrodes are properly attached. Baseline wander is usually observed immediately after application of the electrodes. It is due to a relatively slow establishment of electrochemical equilibrium at the electrode skin interface. This can be minimized by selecting the proper electrode material, which will reach equilibrium quickly with a good electrode jelly.

Muscle Tremor: Irregular trembling of the ECG trace, without wondering of the baseline occurs when the patient is not relaxed or is cold. Muscle tremor signals are specially bothersome when a patient moves or the muscles are stretched. Therefore, for long term monitoring, the electrodes are applied on the chest and not on the limbs.

Results A 2-channel ECG sensor system with 0-150 Hz resolution has been developed. The DC restoration loop in the system conforms to the restriction of an ECG monitoring system. The analogue to digital 3|Page

conversion and sample rate is controlled by the Microcontroller; the sample rate of 300 Hz conforms to Nyqvist theorem that states that the sample rate must be at least twice the resolution of the signal. To obtain a basic image without filtering, a Matlab program has been used. Following is the raw signal obtained without the base line.

After low pass filter:

After low pass filter spectrum:

After high pass filter: Raw signal with base line:

After high pass filter spectrum: Raw signal spectrum:

Nd

4|Page

References After peek detector:

[1]Handbook of Biomedical Instrumentation (Second edition) – R.S. Khandpur [2]Design and Simulation of portable telemedicine system for high risk cardiac patients – V. Thulasi Bai and Srivatsa S.K.

Discussion: This project has been more of a research project than a digital electronic assignment. Much knowledge in bio medical engineering has to be obtained during the project. A lot of the time has been used for researching what is possible and finding a solution that will fit the application more than just constructing and testing. And the lab facilities (access to an oscilloscope) were limited. Although the desired output (a complete ECG system) has not been gained in the project, the basic idea has been developed into the practical level. The weak electrical signal generated by the sinoatrial node, which is the natural pacemaker of the heart has been improved to obtain a diagram and noise reduction and frequency filtering can be done using Matlab.

[3]ECG QRS Detection – Valtino X. Afonso [4]TRG Wireless ECG sensor for medical healthcare application – Nor Syahidutul Nadiah Ismail, Sharifah H.S. Ariffin and Mohd Rozaini Abd Rahim, Norsheila Fisal [5]The ECG made easy – John R. Hampton [6]Uniform Sampling of ECG Waveform of MIT – BIH Normal Sinus Rhythm Database at Desired Intervals –Jitu Sharma, Vikash Kumar, Shahanaz Ayub, J.P. Saini [7]A Real time QRS Detection algorithm- Jiapu Pan and Willis J. Tompkins [8]Study and analysis of ECG signal using Matlab & Labview as Effective tools – M.K. Islam, A.N.M.M. Haque, G. Tangim, T. Ahammad and M.R.H. Khondokar

5|Page