Background of ECG

Ecg is called as Electrocardiogram which is used for measuring or representing the electrical signals produced from the heart muscle. The proposal or demonstration regarding the electrical activity of the heart was made long back in the 19th century by marchand and others. In1901 a scientist known as William Enthovien invented the string galvanometer which gave the trustworthy and straight forward method for the measurement of electrical signals from the heart. Later when years have passed its transfered from the laboratory to the clinic. What ever the electrocardiographers has achieved in building this tool, it was started to use world wide and became a important tool for detecting and diagnosing the heart conditions and also used for the treatment for every kind of heart disease[2]. Normally if there is any expansion in the heart muscle or else if there is a cardiac disorders in the heart , ecg gives the information about this .The heart which generates the electric field is described by the vector quantities .The frequency range is used for diagnosing is 0.05Hz to 150Hz[3]. Normally in the heart, the electrical signals travels from one cell to other cell very easily because all of the cells are very compact to each other .Usually in the heart the electricity will be generated when the cells are depolarized(due to the movement of the ions) and there will be no flow of electricity at the time of the rest which means the cells will be in polarized state. There are different cells in the heart which are called as automatic cells, these cells depolarizes quickly and have got a major role in producing the rhythm of the heart. These cells are called as pacemaking cells. The electricity will be generated when the pacemaking cells are in sinoatrial node where it depolarizes very quickly and generate the heart rate[1]. Normally Ecg is performed in different ways i.e depends upon the patients disease .Most of the time the test will be taken at the time of the rest, in case the patient has got some of the disease like coronary artery disease , so the test will be done during exercise.

The test on human's will be done by using the electrodes, so here up to 12 electrodes will be used and it will be attached to the skin with the help of gel. Gels are mainly used for the proper connection between the skin and the electrodes and main thing is that it reduces the skin resistance and will have the pretty good electrical connection. Areas where the electrodes will be placed must be shaved, these electrodes are used to measure the signals produced by the heart and check the rhythm of the heart. The voltage will be taken between pairs of the electrodes.


To design an Eclectrocardiogram machine by making use of 2 elctrodes in order to measure the electrical activity and to check the condition of the heart. Here only 2 electrodes are used which are applied to both the hands of the human beings and the output of this will be seen in oscilloscope.



The main function of the heart is to pump the blood to the whole body. Totally it has got four chambers, they are left atrium, left ventricle, right atrium, right ventricle. In heart there is a flow of both oxygenated and non oxygenated blood .Non carbonated blood flows from lungs to the heart, where it enters in to the atria and contraction of the blood takes place, and further it pushes it to ventricular region and once again the contraction process takes place in the ventricular region and finally it spreads all over the body. carbonated blood flows from heart to lungs which is nothing but the non oxygenated blood. Here the flow of the blood will be in reverse as that of oxygenated blood i.e. first the blood will flow in to the right atrium of the heart where contraction of the blood takes place and kicks the blood in to right ventricle. Once again the contraction process takes place in the ventricles and kicks the blood away from the heart to lungs.[5]

The rate of the heart is operated by a pacemaker and it resides in the right atrium, it is called as sino-atrial node. Now take an example of a gas lighter where a single spark of a gas is enough to ignite the gas stove, in the same way by considering the heart, the single spark of a blood is enough in order to excite all the walls of the heart to contract. First the atria is excited and it is followed by the ventricular.

The Heart rate is increased by sinus node which produces a huge amount of sparks per minute and it promoted by adrenaline. The nervous system is one which controls the adrenaline. The electrical activity of the heart is described here.

The electrical pulses are produced in sinoatrial node, then it moves to every corner of the heart, especially in left and right atrium. As soon as the electrical pulses moves, the contraction in the heart takes place, mainly it takes in the upper chamber of the heart where atriums are present. The electrical pulses next travels in to different part of the heart i.e. atrio ventricular which resides on the top of the ventricles and stays there over a period of time. In this period of time the blood content in the right and left atrium will be emptied and will be transferred to the ventricles[6]. The contraction takes place in the heart and the blood will be flown in to pulmonary artery and aorta. In this way the electrical conductivity takes place.

Applications of Ecg

  1. It is mainly used for Monitoring purpose
  2. It is used to check the rhythm of the heart.
  3. If the patient has got some jerks and feels some pains in the heart, so ecg is used to check whether there is any sign of heart attack.
  4. It is used for observing the activity of the pacemaker.
  5. If the patient had a block or any hole in his heart, so to check that ecg is used.

Different instrumentation for the measurement


Going back to the history, the ecg was founded by willem Einthoven in 1903.The theory which he proposed was measuring the activity of the heart by using different techniques and as the years passed, now-a-days electrodes are used in order to measure the electrical signals produced from the heart. For standard ecg there are 12 leads for ecg [1], normally lead means the measuring the signal voltages between the electrodes. It is divided in to unipolar lead and bipolar lead. Universally for the measurement purpose 3 electrodes are used, where two of them are applied to both left and right arms and third is applied to the ground.

Now let us describe the wave form which is shown above. Considering the positive end of the bipolar electrode, when it senses the current there will be a positive deflection and simultaneously there is upward movement which is shown in the graph. so when it does not senses the current there will be negative deflection and causes the downward movement. If the flow of the current is perpendicular means normally there will be no deflection in the recordings . Here in this graph we can define segments as the distance between the deflections and the waves as the distance between the intervals [1].

In the graph, it is divided in to four waves named as P,Q,R,S,T. Considering the top part of the heart i.e. atria where the depolarization has taken place is shown by P wave. The time needed for depolarization which spreads from the atria to antrioventricular node is measured by the total time period of p wave. The total time period of the p wave lasts for 0.1 seconds[4] .Considering the interval between P-R where it starts at P and finishes at R, shows that time needed for any electrical pulses for the depolarization of the atria and continues to travel to ventricular which is the bottom part of the heart. Normally in adults the time period between P-R lasts for 0.12 to 0.2 seconds which is normally called as sinus rhythm . Here the depolarization of the ventriculars takes place between the QRS time period, generally in normal conditions if there is any absence or insufficient of oxygen is there in the heart means the ST segment will be depressed. Finally the repolarization of the ventricular is shown by the T-wave,


  1. Electrode: The main use of the transducer where it senses the electrical signal produced from the heart, it will be in millivolts electrode is to calculate the activity of electrical signals which is produced by the heart, so in order to get the detail analysis of the body lead is used to get the information from the different location..Here Ag/AgCl electrodes are used which are normally treated as a non polarized electrode.
  2. Ecg Amplifier: Amplifiers are used because to amplify the electrical signals produced from the heart, so usually the heart signal will be in terms of milli volts. This is not enough to get the correct information from the heart, so the signals are amplified.
  3. Optocoupler: Optocouplers are generally known as opto-isolators, this is a small device which are normally used in all electronic circuits to isolate from different circuits. In ecg it is mainly used to isolate the human body from the circuit or the output.
  4. Bandpass filters: The filters are used to remove the noise. The noises produced by the body like muscle noise which are amplified in the amplifier circuit produces a distortion,. so in order to remove these noises filters are used. Normally bandpass filters are formed by lowpass and highpass filters.
  5. Oscilloscope: oscilloscope is used for viewing the output of the ecg, Generally a graph will be represented on the screen of the oscilloscope and can check the normality and rhythm of the heart.

The diagram which is shown above is of the electrodes connected to the body of the human being .Normally electrodes are nothing but the leads and even the leads are also known as the measurement of the electrical potential between the pairs of electrodes, the standard ecg uses 12 lead .These leads are divided in to 3 parts normally known as lead1, lead2, lead3,but these lead consists of electrodes in order to check the heart from the different angles. Amongst the 3 leads mainly lead 1 is used universally which consists of 3 electrodes. In this 3 electrodes 2 of them are applied to the inputs and third one is given to the ground. The 2 electrodes applied to the inputs senses the electrical signal produced from the heart and sends the signals for processing. Mainly these electrodes are attached to the left and right arm with help of the gel and third electrode is used as a ground electrode and which is applied to the left foot.

Here the above diagram shown is normal 3 lead ecg and the triangle which is shown is known as Enthoven's triangle. The tip of the triangle shows that electrical connectivity between the heart and the limbs. Lead 1 shows the potential between the right and the left arm, the second lead shows the potential difference between the left leg and the right arm and the third lead shows the potential difference between the left arm and the left leg.

Enthoven's law states that in the electrocardiogram at any given instant the potential of any wave in lead 2 is equal to the sum of the potentials in lead I and III.[9]

Problem using 3 electrode Ecg.

Most of surgeons in the world make use of 3 electrode ecg. But normally there are many electrodes which are placed on the body to analyse the heart in the different angle which gives the correct picture of the heart.3 electrodes are used to find out the affected area in the restricted part, so here all of the electrodes are not used. But according to the analysation even the 3 electrodes are more for detecting. Third electrode which is applied for the left foot is mainly used for the ground purpose, here this electrode is not necessary because it produces more leakage currents and patient feel more uncomfortable if they have more electrodes on them. And the main thing cost will be more if we use more electrodes and even the complexity will be more in designing the circuit. Skin irritation will be more when there more electrodes[11]. This is not a portable one, so that we can use it at home for the measurement purpose. The ground referenced capacity is large, so that currents from the mains causes the interference[18]. The input capacitance between the body and the ground will be large when we compare with the ground free electrode and other major problem is that signal to noise ratio is poor[19]. So these are all the problems which can be overcome by using the 2 electrodes ecg

Electrode Electrocardiogram

2 Electrode which normally tells that only 2 electrodes are used in the whole system in order to detect the signal from the heart. These 2 electrodes are normally applied for both the arms and records the pulses generated from the heart. The speciality of this design is to remove the ground electrode which is normally used everywhere in hospitals. 2 Electrode design is portable and can be used in the houses for normal check up whilst is useful for few applications that include biotelemetry, portable holter monitors and portable arrhythmia monitors[10].

Biotelemetry: Here telemetry means a technique, normally it can be called as communication technique which collects all the information or the measurements in the remote locations and sends it for the monitoring purpose. This technique is used in the biomedical field for the monitoring the ecg, eeg signals. Biotelemetry uses the wireless communication technique, where the transmitters and receivers are used to transfer the potential difference between the electrodes.

Holter Monitors: Holter Monitor is a portable device which is used for monitoring the electrical activity of the heart. This device uses as series of electrodes to record the signal, but it is also applicable when only 2 electrodes are used. It is very small device where it is placed near the patients belt. In the early days the audio tapes were used to record the signal and due to the technology improvement now-a-days flash memories are used to record the signals. These recorded signals are tested with the help of the computer.

Arrhytmia Monitors: Arrythmia monitors uses the Electrocardiogram testing inorder to check the abnormalities in the rhythm of the heart. Basically Arrhytmia means change or irregularity in the rhythm of the heart. So to check what kind of abnormality or reasons are there in change in the rhythm of the heart, the arrhythmia monitors are used.

Block Diagram of 2 electrode ECG

Biopotential Electrodes

Electrodes in the ecg system are attached to the human body to measure the biopotential signal produced by the heart or muscle cells. In this project i am using 2 electrodes, these are applied to the hands of a person . Basically electrodes are used as a transducers, which normally converts the ionic flow from the body to a electron current through an electrolyte and to a electric potential measurable at the front end of the ecg system[16].There two types of electrodes namely polarisable and non polarisable. When the electrodes are polarized, it normally act as a capacitors and when it is non polarized, it acts like a resistor. Normally for ecg measurement purpose we use non polarized electrode.[14] The electrodes are made from different material like silver, gold etc. But normally silver silver chloride electrodes are used for the measurement purpose.

Silver -silver Electrodes: The electrodes are designed by using a conductive metal, silver and it is bordered with salt and silver chloride. This is applied to body via an electrolyte to which a gel is applied[13] . These electrodes are normally used because it gives the stable junction potentials. Here sodium based electrolytic gel are used because of motion artifacts and provides a good connectivity between the electrodes and skin. When the metal combines with a gel which is applied to the electrode produces a solution between the skin and electrode interface. When the silver contacts this solution it produces Ag+ ions . When the ions are dissolved an electric field will be produced , so that the ionic equilibrium will be set up[14]. A double layer will be formed when the Ag+ and cl- ions are settled at the surface of the electrode. Now a capacitance will be formed when the charges of the double layer are separated by a dielectric material. Here a series resistance will be added because, the electrodes are non polarized and they act like a resistance. But this is not acceptable circuit because when the frequency goes to zero, the impedence will not goes to infinity . So to avoid this a parallel resistance will be added to the capacitor. This is shown in the figure below

Ecg Amplifier

Why we need an Amplifier? Basically, Amplifiers are nothing but the ampliflying the small amount of voltage by some amount. In Ecg system amplifiers are used to amplify the signals which are produced from the heart. Normally the signals produced from the heart will be in the range of millivolts , so this can't be used for the measurement purpose . Here in the above diagram “V” is the electric potential from the electrodes this will be in the range of millivolts, so it is fed to the amplifiers to amplify the signal[14]. There will some problems occurring during the amplification, such as electrical interference, magnetic induction and source impedance unbalance. Discussing about the electrical interference, which shows that the voltage what we get in the amplifiers is not a pure main voltage. There will be some other interference, but one major interference is from the power lines. So here the noise from the power lines, walls and mainly from the ceiling combines with the current in to the patient and flows through the electrode skin impedances. so this shows that there is a small voltage of noise added with the mains 50hz voltage. The main challenge is to get the ecg signal from the noise, as the ecg signal is a differential voltage because it is taken from the two electrodes. So 50hz noise will be there in the both of the electrodes which is applied to the right and left arms. In order to reject this, we need a amplifier normally a instrumentation amplifier in the input side of the ecg machine. Here main motto of using the instrumentation amplifier is to reject the maximum amount of common mode voltage in order to reject the main interference. The model of a instrumentation amplifier is shown below.

The instrumentation amplifier shown is the standard Ecg amplifier where it is having two input buffer amplifier and a single ended output differential amplifier. Here at input side the differential voltage is accepted from the electrodes, the voltage will be in milli volts and the output at the single ended output amplifier will be interms of volts . The two op-amps at the input side are designed in such a way that, it should have a high input impedance which allows to use a high source resistance. The single ended output differential amplifier gives the high gain and rejects the common mode voltage. The main characteristics of the instrumentation amplifiers are low DC offset voltage, High common mode rejection ratio and high input impedance[17].

Electrical Isolation

Electrical isolation is an important thing in the ecg instrument. Here isolation should be made between the patient and the instrument. This isolation doesnot allow any of the leakage currents to pass through the instrument to the patient. If there is any leakage currents especially leaking to the heart , it will be a dangerous to the patient. In order to avoid the leakage currents an electrical isolation is made. Normally isolation is done by electrically and optically, in electrical isolation an transformer is used. As the transformer is a high freuency device, so biopotential signal need to modulate using the high frequency carrier, this will be a tedious one[16]. Another type of isolation is optical isolation, here mainly the optocouplers are used. The mechanism of this isolation is the conversion of electrical signal in the optical signal and again back to the electrical signal. First optical signal is produced by the LED, from the electrical signal produced from the amplifiers. Then this optical signal is modulated and transmitted to the photodetector and this detector demodulates and converts to the electrical signal. In this way the optical isolation is done.

Common mode rejection block

Here this block provides the major role in this design. Mainly the two electrode Ecg improves the patient safety by avoiding the third electrode from the body.Here we will analyse the situation that how can we avoid the third electrode. So normally in order to get the better performance, common mode voltage should be zero . To achieve it most of the ecg designers uses 3rd electrode. Here when we get the common mode voltage, it will be passed through the body and to the ground through the ground electrode inorder to compensate with the common mode error. Now we need to reduce the common mode error without using the ground electrode . Here simple technique is used to avoid it, that is the block below the instrumentation amplifier collects the voltage from the amplifier and reverses the polarity of the signal. Normally we use V-I converter operational ampliers in this block where the signal is converted from voltage to current and fed back to the instrumentation amplifier. So in this way we will do the rejection of common mode voltage without using the 3rd electrode.


The filtering is done after the ecg signal is amplified in the instrumentation amplifier. The amplified signal will be having lot of noise content, so inorder to remove that filters are used. Here mainly the lowpass filter is used inorder to remove the noise from the signal. The low pass filter is basically used to allow the low frequency signals and does not allows the signals which are above the cut-off frquency. Now here normally we use maximumof 150hz for diagonising, so the filter will be designed to the cutoff frequency . The lowpass filter will be used as cascaded RC filters because the frequency of the ecg will be maxium but the output of the opamp will not be able to handle with the frequency signal[15]. So for this cascaded RC filters is used . So further if there is any filteration is required, a notch filter is used inorder to remove the nosie from the signal.



  1. The internet journal of health ISSN:1528-831 2000 Volume 1 Number2
  2. Electrocardiography David M.Mirvis. Ary L.Goldberger
  3. Biomedical Instrumentation by R.Khandpur, Publisher Tata Magraw Hill
  4. Lake C. Chapters 1 and 2. Clinical Monitoring. WB Saunders Company, 1990. ISBN 0-7216-2961-X
  5. Dr Juliette Lee, Royal devon and Exeter hospital-Article issued on 2000-http://www.nda.ox.ac.uk/wfsa/html/u11/u1105_01.htm
  6. http://en.wikipedia.org/wiki/Electrocardiography
  7. How stuff works-http:// health.how stuff works .com/heart.htm
  8. http:// www.heartsite.com/html/electrical activity.html
  9. http://www.perfusion.com.au/CCP/Biomedical electronics/Biomed
  10. ENEL 563 Biomedical Signal Analysis www2.enel.ucalgary.ca
  11. Pflgers] Archiv fr die gesamte Physiologie des Menschen und der Tiere, 1913
  12. Ground Free Ecg Recording With Two Electrodes Nitish V Thakor, Student, Member, IEEE, and Jhon Webster, Senior Member, IEEE
  13. Open ecg-project 3 lead wireless ecg-www.open-ecg-project.org
  14. A simple Electrocardiograph Wynn VT and Miller N physics education
  15. Medical Elecronics, DR . Neil Townsend, Michaelmas Term 2001
  16. Elecrocardiogram Data Acquisition and wireless transmission by Patrick O.Bobbie, Chaudary Zeeshan Arif, Hema chaudary, Sagar pujari.
  17. “Biopotentials and Electrophysiological Measurement” by Nitish V.Thakor
  18. Instrumenation Amplifier, National semiconductor Linear brief1 march 1969.
  19. Comments on “ Ground Free Ecg Recording With Two Electrodes” by B.C.Towe
  20. Ecg Measurement using Capacitive Coupling Electrodes for Man-Machine Emotional Communication by T.Maruyama, M.Makikawa, Member, IEEE

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