Mechanism of ECG

THE HEART- FUNCTIONS AND PHYSIOLOGY

1. The physiology and functions of the heart are the studies of functions of the heart, which comprises-: the flow of blood, the cardiac cycle, myocardium structure, the electrical conduction system of the heart and their interdependence on each other. 
2. The essential features of physiology and functions of the heart are given below-:

• There are four chambers in the heart- The TWO UPPER CHAMBERS are called the left and right atrium. These two chambers allow the blood to enter the heart. The TWO LOWER CHAMBERS are called the left ventricle and right ventricle. These two chambers help in sending the blood out of the heart.

• The blood from the lungs or other parts of the body comes to the upper chamber and then is pumped to the lower section. Blood from the lower part of the body is pumped back again from where it came from.

• The circulation of blood takes in the form of loops-: The Pulmonary Loop-This loop circulates the blood between the lungs and the heart. The deoxygenated blood is transported to the lungs where oxygen is absorbed, and carbon dioxide is released; The Systemic Loop- This loop circulates between the heart and the rest of the body. The oxygenated blood is transported to the rest of the body except the lungs, and the deoxygenated blood is transported back to the heart.

• The whole cycle is called HEARTBEAT, from blood pumping to the atria and back to ventricles. Heart rate is the speed of heartbeat per minute. The standard heart rate for a normal resting human heart is 60-100 beats per minute(bpm).CARDIAC CYCLE

1. The cardiac cycle is the behaviour of the human heart from the starting of one heartbeat to the starting of the next heartbeat. It comprises systole and diastole of both the atria and ventricles. In each cardiac cycle, two sounds are produced- LUB and DUB, which can be easily heard with the help of a device called STETHOSCOPE.
2. ELECTROCARDIOGRAPHY
3. Electrocardiography is one the fastest and most straightforward techniques used to evaluate the heart. Electrocardiography is used to determine numerous problems related to the heart. Electrocardiography is used to diagnose- abnormal heart rhythm, if the arteries are blocked or narrowed, the previous history of heart attack, chest pain and many more functions.
4.  During electrocardiography, electrodes are placed on the skin, and the heart’s electrical activity is measured in terms of a graph of voltage versus time. 

• Resting Electrocardiography- In this type of electrocardiography, the patient lies on the back with a bare chest and is still. Any movement in the body, coughing or sneezing will hamper the readings.

• Exercise Electrocardiography- In this type of electrocardiography, the readings are measured while the patient is physically active, usually riding an exercise bike.

• Holter Monitor- In this type of electrocardiography, three or four electrodes are attached to the patient’s chest. A belt is tied or hung around the neck, recording the ECG data and transferring it to the computer.

1. The working of an electrocardiograph machine is based on the fact that when a muscle contracts, it generates a small electric current which can be measured with the help of electrodes when attached to the skin. 
2. The electrode measures the current from the body and transmits it to the amplifier present in the electrocardiograph. The amplifier present in the electrocardiograph amplifies the signal and draws it on a sheet of paper in the form of a graph.
3.  INSTRUMENTATION
4.  The electrocardiograph machines consist of a central unit and a set of electrodes. They are- 

• CARDIAC MONITOR- It is a device that directly sticks to the chest and records the electrocardiograph without any wires. 

1.  ELECTRODES- These are the conductive pads patched on the body surface. If two electrodes are patched in two different parts, the electrodes will measure the electrical potential difference between the two parts. Such a pair of electrodes are called LEADS. Generally, ten electrodes are used to record electrocardiographs which work as 12 leads.     

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4. ELECTRODE NAME	ELECTRODE PLACEMENT METHOD
   RA	On the right arm, not placed on thick muscle
   LA	On the same location of RA, but on left arm
   RL	On the lower end of inner calf muscle of right leg
   LL	On the same placed of RL but on left leg

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6. INTERPRETATION
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8. Interpretation of the ECG is the process of observing the pattern of the heart’s electrical activity. The signals obtained by the electrocardiography machine gives the heart rate and heart rhythm. The interpretation is made based on the following points-:
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• RATE -The heart rate range between 60 to 100 beats per minute is considered ‘NORMAL’ in adults. A heart rate with a degree less than 60 in adults is called BRADYCARDIA, and a range above 100 in adults is called TACHYCARDIA. 

• AMPLITUDES AND INTERVALS- An acceptable amplitude range is considered in a specific time interval and time duration. The different waves and their acceptable time duration are stated below-:

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2. FEATURE	DESCRIPTION	DURATION
   P wave	It represents the depolarisation of the atria	＜80 ms
   QRS complex	It represents the fast depolarisation of the left ventricle and right ventricle	80 to 100 ms
   ST segment	It represents the depolarization time of the ventricles
   T wave	It represents the repolarization of the ventricles	160 ms

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4.                                     RHYTHM- RHYTHM- Normal rhythm produces P wave, QSR complex and T wave. Any deviation from normal rhythm causes irregularities in the heartbeat.

• EXPERIMENTAL ADVANTAGES
• The technique of Electrocardiography has many advantages in the field of medical sciences. A few of them are listed below-:

• Electrocardiography helps in warning post-cardiac arrest by measuring the heart rate and rhythm.

• Electrocardiography is painless, quick and safe.

• Electrocardiography provides accurate diagnostic information.

• Electrocardiography helps in detecting the heart conditions of patients before any surgery.

Conclusion

1. Electrocardiography is a beneficial technique in determining the heart conditions of a patient by 
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3. measuring the heart rate and rhythm. The electrodes are placed on the skin, and a graph between voltage and time is obtained, which depicts the electrical conductivity of the heart. The amplitudes and intervals of the readings are matched with the predefined reference value values, and a person’s heart condition is evaluated.