Electrocardiograms Investigating Heart Disease Biology Essay


Electrocardiogram is used to investigate heart disease through electrical signal recordings and is very much helpful in diagnoses of different heart ailments. It gives us recordings in graph type tracings, shapes like waves and through which we can come to know about the heart ailments.The electrocardiogram was firstly introduced by Willem Einthoven in year 1893 in a conference of Dutch Medical Society and won Nobel prize for his work to develop ECG in year 1924.The 12-lead ECG was introduced in year 1942 to investigate heart activities through 12 different point of views due to which we can easily get the full picture of heart which makes our investigation more easy and simple.[1]

The ECG shows the recording of the heart activities when electrical signals pass through heart due to which the muscle cells of heart located in atria show P waves due to contraction and also ventricles show Q,R,S waves due to contraction and which is known in medical sciences and QRS complexes due to which we can see P wave.[1]

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The T wave which we can see in ECG recordings is a wave when electrical signals pass through the heart and recharge the ventricals for another contraction. The R wave which we can see in recordings is a wave which shows first upward deflection after the P wave and as it is the most easy and critical waveform to find out on the ECG recordings used to give ventricular depolarisation and can be enlarged with ventricular hypertrophy but can be reduced by different processes like as obesity.

There is a relationship between cardiac phase and R-wave which depends upon nonlinearly with heart rate. When the rate of the heart is slow systole occupies 1/3 and diastole 2/3 of the heart cycle. And whenever Heart rate increase systole also increase in the same ratio and diastole decrease to 50% of any heart cycle.There is a reconstruction phase which is optimal at one heart rate and can be motion artefacts at different heart rate in patient.When we analyze these waves P,QRS,T, through different angles we come to understand that there are many abnormalities due to electrical conduction and the muscle tissues of the four pumping chambers of the heart. [1]

Picture Reference:[2]

Difficulties Amplifying Medical Signals and the methods to overcome:

(1) Signal acquisition, including filtering process

(2) Data transformation and collecting data for new processing.

(3) Waveform recognition, a process which identifies the onset and offset of the diagnostic waves.

(4) Feature extraction used to measure amplitudes and different intervals.

(5) Diagnostic classification can be heuristic.

The Processing of ECG signals from a digital ECG gives us initial sampling of the signals from electrodes which are placed on the body surface. The digital ECG remove or suppress low-frequency noise which is a result of baseline wander and movement, and also respiration and higher-frequency noise which is a result from muscle artifact or power-line and also radiation affected electromagnetic interference which results that the ECG signal at the surface of the human body must have to be filtered and amplified by the ECG. The Digital filters can also be made in order to have linear phase characteristics due to which some of distortion produced from classic analogue filters can be neglected. Measurement error can easily affect the accuracy of ECG diagnostic statements.


(1) Estimation of the QRS rate.

(2) The QRS rhythm is regular or irregular.

(3) Determination of QRS complexes to find out normal and constant morphology.

(4) P waves present and of normal and constant morphology.

(5) Relationship between P waves and QRS complexes.

1:-Estimation of the QRS rate.

Estimate the QRS rate can be finding out by counting the number of large 1cm square between two adjacent QRS complexes and then divide the result in 300. If we have 12 QRS complexes in 10 second strip the ventricular rate must be 72/min (12*6)

Normal QRS rate is 60-100/min

Slow QRS rate < 60/min

Fast QRS rate > 100/min

Normal heart rate is between 60 and 100, but it can be adjusted between limits of 50 to 90/min. A sinus tachycardia is over 90 and bradycardia is less than 50. A heart rate of 85 in an athlete can show substantial tachycardia, if the resting rate is 32/minute.

QRS rhythm

Sinus arrhythmia and heart rate variability

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Slight degree of chaotic variation in heart rate, called sinus arrhythmia

Atrial Extrasystoles

The ectopic beat arises at same time after the sinus beat arises.

Supraventricular tachyarrhythmias (SVT)

Irregular SVT

Regular SVT

The atrial rate is 300/min and a 2:1 block which results at rate of 150/min.

QRS Complexes

Normal QRS width is 0.12s/3 and Prolonged QRS width is 0.12s/3. QRS complexes and their morphology is constant. If ectopics are present. QRS can arise from the atria, AV junction and ventricles. If the ectopic QRS complex is narrow then ectopic focus can easily be find above the ventricles in condition of being wide the ectopic focus can be found above the ventricles. If impulses are premature before the very next sinus beat they known as premature contractions. If impulses are late after the next sinus beat they are known as termed escape beats.

Configurations for QRS complexes:

Right Ventricular Hypertrophy (RVH)

QR in V1 & V2 .

R wave in V1 & V2 , with or without ST & T changes

P waves

Normal atrial activation is over in about 0.10s, starting in the right atrium. A good place to look at P waves is in II, where the P shouldn't be more than 2.5mm tall, and 0.11 seconds in duration.

Right Atrial Enlargement (RAE)

The P wave is taller than two small squares (>0.08 sec) in infants and small children and more than three small squares (> 0.12 sec) in older children and adults. P waves are best seen in the inferior (I, II & aVF) and the right chest leads (V & V).

Left Atrial Enlargement (LAE)

The P waves are wide, more than two small squares (> 0.08 sec) in infants and small children and more than three small squares (> 0.12 sec) in older children and adults. These P waves are best seen in the inferior (I, II & aVF) and the left chest leads (V & V).

In V1, another good place to look, depolarisation of the right atrium results in an initial positive deflection, followed by a vector away from V1 into the left atrium, causing a negative deflection. The normal P wave in V1 is thus biphasic. It's easy to work out the corresponding abnormalities with left or right atrial enlargement:

A qR in V1 suggests right atrial enlargement, often due to tricuspid regurgitation! (Observed by Sodi-Pallares).

If the overall QRS amplitude in V1 is under a third of the overall QRS amplitude in V2, there is probably RA enlargement! (Tranchesi).

A P wave originating in the left atrium often has a `dome and dart' configuration.

In sinus rhythm the P waves should be identical in shape and upright in lead II.A change in P wave morphology implies a different pacemaker focus for the impulse Retrograde activation through the A V junction (junctional or ventricular arrhythmias) usually results in the P waves being inverted in lead II. This is because atrial depolarisation occurs in the opposite direction to normal. Sometimes it may be difficult to establish whether P waves are present because they are partly or totally obscured by the QRS complexes or T waves, e.g. in sinus tachycardia P waves may merge with the pre-ceding T waves. In SA block and sinus arrest, P waves will be absent. In atrial fibrillation no P waves can be identified, justa fluctuating baseline. In atrial flutter, P waves are replaced by regular saw tooth flutter waves, rate approximately 300/min.

Relationship between P waves and QRS complexes

The PR interval extends from the start of the P wave to the very start of the QRS complex (that is, to the start of the very first r or q wave). Each P wave is followed by a QRS complex and each QRS complex is preceded by a P wave. Examine the PR interval. The normal PR interval is 0.12-0.20s (3-5 small squares).

SA node block

This is a diagnosis of deduction, as no electrical activity is seen. An impulse that was expected to arise in the SA node is delayed in its exit from the node, or blocked completely. A second degree SA block can be `diagnosed' if the heart rate suddenly doubles in response to, say, administration of atropine. If the SA node is blocked, a subsidiary pacemaker will (we hope) take over, in the atrium, AV node, or ventricle!

AV nodal blocks

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There are three "degrees" of AV nodal block:

First degree block:

simply slowed conduction. This is manifest by a prolonged PR interval;

Second degree block:

Conduction intermittently fails completely. This may be in a constant ratio (more ominous, Type II second degree block), or progressive

Third degree block:

There is complete dissociation of atria and ventricles.

Clearly a bad thing, requiring temporary or even permanent pacing.

The ECG system compromises of five stages:

(1) The first stage is a transducer AgCl electrode, which convert ECG into electrical voltage.  The voltage is in the range of 1 mV ~ 5 mV.

(2) The second stage is an instrumentation amplifier (Analog Device, AD624), which has a very high CMRR (90dB) and high gain (1000), with power supply +9V and -9V.

(3) We use an opto-coupler (NEC PS2506) to isolate the In-Amp and output. 

(4) After the opto-coupler is a bandpass filter of 0.04 Hz to 150 Hz filter.  Its implemented by cascading a low-pass filter and a high pass filter.

 5) Oscilloscope

Figure 1  Function blocks of the ECG system

ECG Signal

 The basic structure of the heart is shown on Figure 2.  Measuring at different region of the heart will retrieve different biopotential.  And, so that it will generate different ECG waveforms.  The ECG generated by each cardiac cycle is summarized on Table 1.


Figure 2  Basic structure of the heart. RA is the right atrium, RV is the right ventricle; LA is the left atrium, and LV is the left ventricle.



Duration at 75 bpm (0.8 second cycle)

Atrial diastole

Ventricular diastole

AV valves opened.

Semilunar valves close.

Ventricular filling.

0.4 seconds

Atrial systole

Ventricular diastole

AV valves open.

Semilunar valves closed. Ventricular filling.

0.1 seconds

Atrial diastole

Ventricular systole

AV valves closed.

Semilunar valves open.

Blood pumped into aorta and pulmonary artery.

0.3 seconds

Table 1  Duration and characteristics of each major event in the cardiac cycle.   

 The ECG is converted into electrical voltage by electrodes.  A typical surface electrode used for ECG recording is made of Ag/AgCl, as shown on Figure 3.  The disposable electrodes are attached to the patients skin and can be easily removed.

Figure 3 A disposable surface electrode.

 The cardiac mechanism of ECG is shown on Figure 4.  In the top figure, the electrocardiogram (ECG) initiates the cardiac cycle.  The cardiac sounds are also shown.  The bottom figure shows that ejection occurs when the pressure in the left ventricle exceeds that in the arteries.

Once the electrodes convert the ECG into electrical voltage, these voltage can be fed into an instrumentation amplifier, and then be processed.

Figure 4  The ECG cardiac cycle.

We measure the ECG by connecting two electrodes on the right and left chest respectively, as shown on Figure 5.  The body should be connected to ground of the circuits, so that we connect the leg to the ground.  If the body is not grounded, no signal would be obtained.

Figure 5 Simplified ECG recording system


3. Circuits of the ECG system

The ECG circuit diagram is shown on Figure 6. 

Figure 6 The ECG circuit diagram

 3.1  Instrumentation Amplifier

We choose Analog Device AD624 instrumentation amplifier to amplify the ECG voltage from electrodes, which is in the range of several mV.

The AD624 is set up with gain of 1000, and is supplied by +9 V and -9V battery power.  Some important features of the AD624 are listed on Table 2.

Supply Voltage

+-9V Battery power

Programmable Gain

1, 100, 200, 500, 1000, 2500


130dB (Gain=500 to 1000)

Gain Bandwidth Product

25 MHz

Input offset

25 μV, max

Table 2 Important features of AD624 Instrumentation Amplifier

A very high CMRR is very essential for Instrumentation Amplifier.  The small ac signal voltage (less than 5 mV) detected by the sensor on the electrodes will be accompanied by a large ac common-mode component (up to 1.5 V) and a large variable dc component (300 mV).  The common-mode rejection specified by the AAMI (Association for the Advancement of Medical Instrumentation) is 89 dB minimum for standard ECG and 60 dB minimum for ambulatory recorders.  The CMRR of AD624 with gain of 1000 is shown on Figure 7.  The equation of the CMRR:

CMRR = differential gain / common mode gain = Adm/Acm

Figure 7 Measured CMRR data of AD624 In-Amp

A 741-Opamp is connected to the reference node (node 6) of AD624 to offset the output DC voltage to 3V .The 741 is connected as a source follower.  The output DC voltage of the AD624 is adjusted to 3V, which is an optimal value after many try and errors.

 3.2 Opto-Coupler

 After the In-Amp, a NEC PS2506 opto-coupler is cascaded to isolate the In-Amp from the rest of circuits, as shown on Figure 6. 

 3.3 Bandpass filter

In general, components of the signal of interest will reside in the 0.67 to 40-Hz bandwidth for standard ECGs and up to 300 Hz to 1 kHz for pacemaker detection.  We take the suggestion by the book of John Webster to have the bandpass filter the frequency range of 0.04 Hz ~ 150 Hz.  The filter is implemented by cascading a low-pass filter and a high-pass filter.  The data of low-pass and high-pass filter are implemented by simple RC components, as shown on Table 3.  The measured transfer function of the bandpass filter is on Figure 8.




Time Constant t

3dB Frequency

Low-Pass Filter

4 MW

1 mF

4.0 sec

0.04 Hz

High-Pass Filter

10 KW

0.1 mF

0.001 sec

159 Hz

Table 3 Filter data


Figurer 8  The measured transfer function of the bandpass filter

 4. Measured ECG signals

The ECG circuit is shown on Figure 9.  We use the Oscilloscope to probe the ECG signal.

The three measured ECG signals, on Figure 10, are respectively from the Instrumentation Amp, opto-coupler, and filter.  The circuits function very well so that the three signals are almost identical to each other.

 Measuring different region of the heart will obtain different ECG signals.  The ECG signal shown on Figure 9 is one of the standard ECG signals.


V at B























We set

1mv = 10mm

p-p 1000K Res

Voltage divider

m2/ 1kg give a 1mv signal

BPF = 1.5 to 40

Outputs on Each Stage:

Multisim Reproduction Data:

The output data collected from each stage while we were building circuit directly related multisim reproduction data. At each stage, we find out that voltage from given frequency same as we got in multisim reproduction data.

The lab work of ECG 12 lead experiment is quiet interesting because we learnt so many thing of equipment response and specially by applying the leads on our own bodies we got the better response by checking graphical traces which showed us almost all the test which we needed to have in our lab result. Specially it is found that ECG can measure all the required details of heart either the person is having heart ailments and can easily measured through the graphical tracing disturbance on the graph.

ECG is one the most common used diagnostic technique and it is very necessary to understand the ECG specifications in detail so that the person can easily work in the environment of ECG working. This lab work gives us detailed overview of how to use the modern ECG recorder and how to get the better result from this advaced technology which is pretty much helpful for human society in world of dignosis.