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Prepared By: S. Ehtesham Al Hanif (Hridoy) [0510035

BIO (BIO-MEDICAL) ENGINEERING ECG

ME 471- BIO-ENGINEERING / BIO-MEDICAL

TOPICS: ECG

Prepared By,

S. EHTESHAM AL HANIF (HRIDOY)

STUDENT ID: 0510035

E-MAIL: [email protected]

MOBILE: 88-01670839383

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What is an electrocardiogram (ECG)?

The electrocardiogram (ECG) is a noninvasive test that is used to reflect underlying heart conditions bymeasuring the electrical activity of the heart in detail.

Interpretation of these details allows diagnosis of a wide range of heart conditions. These conditions can vary from minor to life threatening. The term electrocardiogram was introduced by Willem Einthoven in 1893 at a meeting of the Dutch Medical

Society.

In 1924, Einthoven received the Nobel Prize for his life's work in developing the ECG.How is an ECG performed?

ECG leads (electrical sensing devices) are attached to the body while the patient lies flat on a bed or table. Leads are attached to each extremity (4 totals) and to 6 pre-defined positions on the front of the chest. A small amount of gel is applied to the skin, which allows the electrical impulses of the heart to be more easily

transmitted to the ECG leads.

The leads are attached by small suction cups, Velcro straps, or by small adhesive patches attached loosely to theskin.

The test takes about 5 minutes and is painless.What can be detected on the ECG? The underlying rate and rhythm mechanism of the heart. The orientation of the heart (how it is placed) in the chest cavity. Evidence of increased thickness (hypertrophy) of the heart muscle. Evidence of damage to the various parts of the heart muscle. Evidence of acutely impaired blood flow to the heart muscle. Patterns of abnormal electric activity that may predispose the patient to abnormal cardiac rhythm disturbances.

When is an ECG performed?

As part of a routine physical examination or screening evaluation. As part of a cardiac exercise stress test. As part of the evaluation of symptoms of chest pain, shortness of breath, dizziness or fainting, or palpitations.

Basic Anatomy of the Heart:

The heart is a 4-chambered muscle whose function is to pump blood throughout the body. The heart is really 2 "half hearts," the right heart and the left heart, which beat simultaneously. Each of these 2 sides has 2 chambers: a smaller upper chamber called the atrium (together, the 2 are called

atria), and a larger lower chamber called the ventricle.

Thus, the 4 chambers of the heart are called the right atrium, right ventricle, left atrium, and left ventricle.

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Right atrium receives venous blood from the body and pumps it to the right ventricle. Right ventricle pumps blood through the lungs (add o2) Blood then flows into the left atrium Contraction of LA-blood moves to LV-contraction of LV-blood pumps to general circulation and returns to RA

through venous

Heart rhythm is controlled by electrical signal initiated spontaneously by special muscle cells located in righatrium known as sinoatrial (SA) node or the pacemaker

SA node fires at regular intervals about 72 times per minute Electrical signal then passes from the SA node into the atrioventricular (AV) node, causing contraction of LV andRV

Schematic of an action potential moving down the wall of the heart. Some of the ion current, indicated by the circles, passes through the torso, indicated by the resistor. The potential on the chest wall is due to current flow through the resistance of the torso.

Nerves of the heart:

The heart's function is so important to the body that it has its own electrical system to keep it runningindependently of the rest of the body's nervous system.

Even in cases of severe brain damage, the heart often beats normally. An extensive network of nerves runs throughout all 4 chambers of the heart. Electrical impulses course through these nerves to trigger the chambers to contract with perfectly synchronized

timing.

Heart Function and the ECG

The ECG records the electrical activity that results when the heart muscle cells in the atria and ventriclescontract.

Atrial contractions (both right and left) show up as the P wave. Ventricular contractions (both right and left) show as a series of 3 waves, Q-R-S, known as the QRS complex. The third and last common wave in an ECG is the T wave. This reflects the electrical activity produced when the ventricles are recharging for the next contraction

(repolarizing).

Interestingly, the letters P, Q, R, S, and T are not abbreviations for any actual words but were chosen many yearsago for their position in the middle of the alphabet.

The electrical activity results in P, QRS, and T waves that have a myriad of sizes and shapes.

When viewed from multiple anatomic-electric perspectives, these waves can show a wide range oabnormalities of both the electrical conduction system and the muscle tissue of the heart's 4 pumping

chambers.

ECG waves

Fig. The P wave, QRS complex,

and the T and U waves.

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N.B.: In some individuals, a small peak occurs at the end or after the T wave and is the U wave. Its origin hasnever been fully established, but it is believed to be a repolarization potential.

In standard ECG recording there are five electrodes connected to the patient: right arm (RA), left arm (LA), lefleg (LL), right leg (RL), and chest (C).

These electrodes are connected to the inputs of a differential buffer amplifier through a lead selector switch. Each heartbeat produces a set of P-QRS-T waves. This set of waves, in turn, is recorded and analyzed from each of 12 points of view. Six of these points of view are the locations of the 6 pads placed across your chest. These are called V1, V2, V3,

V4, V5, and V6 (pronounced Vee One, Vee Two, and so on).

The other points of view represent combinations of the pads placed on the arms and legs. These are called I, IIIII , aVR, aVL, and aVF.

The interpretation of the waves produced by each of these 12 views provides valuable information about thefunctioning of your heart.

ECG Waves and Interval

What is P Wave? During normal atrial depolarization, the main electrical vector is directed from the SA node towards the

AV node, and spreads from the RA to LA.

This turns into P Wave on the ECG. What reveals?

1. Relationship between P waves and QRT complexes helps distinguish various cardiac arrhythmias.2. Shape and duration of the P waves may indicate atrial enlargement

What is PR Interval? Interval between P waves and QRS complex. It is usually 120 to 200 ms.

What reveals?1. PR interval > 200 ms: First degree heart block2. Shape and duration of the P waves may indicate atrial enlargement

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What is QRS Complex? The QRS complex is a structure on the ECG that corresponds to the depolarization of the ventricles. QRS complex is larger than the P wave.

What reveals?1. Normal QRS complex is 60 to 100 ms in duration represented by three small squares or less, but any

abnormality of conduction takes longer, and causes widened QRS complexes.

What is ST segment? The ST segment connects the QRS complex and the T wave and has a duration of 80 to 120 ms

What reveals?1. Flat, downsloping, or depressed ST segments may indicate coronary ischemia.

What T wave? T wave represents the repolarization (or recovery) of the ventricles. Interval from QRS complex to the apex of the T wave is referred to as the absolute refractory period. Last half of the T wave is referred to as the relative refractory period (or vulnerable period).

What reveals?1. Inverted (or negative) T waves can be a sign of coronary ischemia, left ventricular hypertrophy.2. Tall symmetrical T waves may indicate hyperkalemia.3. Flat T waves may indicate coronary ischemia or hyperkalemia.

What is QT segment? The QT interval is measured from the beginning of the QRS complex to the end of the T wave. Normal values for the QT interval are between 0.30 and 0.44 (0.45 for women) seconds

What reveals?1. QT interval varies based on the heart rate, and various correction factors have been developed to

correct the QT interval for the heart rate.

2.

QT interval represents the time needed for the the ventricles to depolarize and repolarize.

What is U wave? The U wave is not always seen. It is typically small, and, by definition, follows the T wave. U waves are thought to represent repolarization of the papilary muscless.

What reveals?1. Prominent U waves are most often seen in hypokalemia.2. An inverted U wave may represent myocardial ischemia or left ventricular volume overload.

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ECG Lead Connection

SIX LEADS CONNECTION ON THE CHEST

Electrocardiographic View of The Heart

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ECG Lead

It refers to the wire that connects an electrode to the electrocardiograph. A 12 lead electrocardiograph usually uses 10 wires/electrodes A lead records the electrical signals of the heart from a particular combination of recording electrodes which are

placed at specific points on the patient's body.

When a depolarization wavefront (or mean electrical vector) moves toward a positive electrode, it creates apositive deflection on the ECG in the corresponding lead.

When a depolarization wavefront moves away from a positive electrode, it creates a negative deflection on ECGin the corresponding lead.

When a depolarization wavefront (or mean electrical vector) moves perpendicular to a positive electrode, itcreates an equiphasic (or isoelectric) complex on ECG.

ECG Electrode

A major problem of obtaining an accurate ECG involves the metal electrodes. Electrically, the body can he treated as a bag of salt water, and in salt water current flows in the form of moving

ions.

In wires and the metal from which electrodes are made, however, current flows in the form of moving electrons At the interface between the body and a metal electrode, ion flow must be converted to electron flow through a

chemical reaction.

If ordinary metals are used for electrodes, polarization results from this chemical reaction. At one or both electrodes, gas bubbles form due to electrolysis, and the resulting electrode-to-solution interface

is electrically unstable.

This instability produces electrical noise and drift which may be much larger than the ECG signal. Theseproblems may be avoided by using silver-silver chloride electrodes). These electrodes are easily made by

electrodepositing a silver chloride coating on pure silver electrodes.

Silver-silver chloride electrode is the natural choice for the typical patient monitoring At the electrode-to-solution interface, complex layers of positive and negative charge form. This electrical double layer should not be disturbed by patient movement, which might cause artifacts

(undesirable voltage changes).

Hence the metal electrode is recessed from the skin, and space between the electrode and the skin is filled witha conductive paste. The plastic electrode case is attached to the skin by a pressure-sensitive adhesive, similar to

that used on masking tape.

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Block diagram of an ECG machine

In standard ECG recording there are five electrodes connected to the patient: right arm (RA), left arm (LA), leftleg (LL), right leg (RL), and chest (C).

The potentials picked up by the patient electrodes are taken to the lead selector switch. By means of capacitive coupling, the signal is connected symmetrically to a differential preamplifier.

Preamplifier gives a stabilizing effect.

The amplified output signal is picked up single-ended and is given to the power amplifier. The power amplifier is generally of the push-pull differential type. The output of the power amplifier is single-

ended and is fed to the pen motor, which deflects the writing arm on the paper.

The auxiliary circuits provide a 1 mV calibration signal and automatic blocking of the amplifier during a change inthe position of the lead switch. It may include a speed control circuit for the chart drive motor.

ECG record details

Electrocardiograms are almost invariably recorded on graph paper wiith horizontal and vertical lines at 1 mmintervals with a thicker line at 5 mm intervals. Time measurements and heart rate measurements are made horizontally on the electrocardiogram. For routine work, the paper recording speed is 25 mm/s. Amplitude measurements are made vertically in

millivolts. The sensitivity of an electrocardiograph is typically set at 10 mm/mV.

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Patient Monitoring

After amplification, the ECG signal must be displayed. When a routine diagnostic ECG is taken, a permanent record is required for analysis and a pen recorder is

usually used.

In some recorders ink fed through a small capillary tube writes on ordinary paper. Other recorders have a heated stylus that melts off a thin, white wax coating that is on black paper, thus

producing a black trace on a white background.

A more convenient approach is to use the ECG oscilloscope to continually monitor a patient. It shows one to two cycles of the ECG, which is enough for a doctor or nurse to assess the patient's condition. Modern monitors use a microcomputer to store the ECG information and use it to continually refresh the trace. The trace slowly moves across the screen and never fades. In many intensive care units (ICUs) several patient

are monitored by the use of multiple traces on a single large oscilloscope.

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ECG Telemetric System

Use Telephone for transferring ECG data Doctor or Nurse can monitor from distance place

Instalation in hospitals, allows long-term ECG monitoring of patients on bed or while moving.

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On-line ECG data wireless transfer, real-time analysis, alarms, patient state detection, automatic data and evenrecord, history, patient data trends.

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