How Does an ECG Work?

An ECG is the heart's electrical activity through repeated cardiac cycles. It is a graph of the electrical activity in the heart- voltage over time. The impulses are recorded using electrodes on the skin, arms, and legs. These electrodes detect electrical changes, depolarization followed by repolarization, during the heartbeat.

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ECG usually refers to a 12-lead ECG taken while lying down. Apple Watch and a Holter monitor are other devices that record ECG signals using variable electrodes. A conventional 12-lead ECG employs 10 electrodes placed on the patient's limbs and the surface of the chest.

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The heart's electric potential is measured from 12 different angles or leads, capturing the magnitude and direction of the heart's electrical depolarisation within 10 seconds. In this blog, we will explore how the ECG works.

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Physiology and Function of the Heart

We must know the fundamentals of ECG, so let’s review the basics of heart physiology and function.

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• There are two upper chambers, the right and left atria, and two lower chambers, the ventricles. The atria pump blood to the ventricles on the same side, and the right and left ventricles pump blood to the lungs and the body, respectively.

• Blood circulation has two loops: one through the lungs, the pulmonary loop, and the other through the body, the systemic loop.

• It ensures that the blood gets oxygenated in the lungs, and this oxygenated blood provides oxygen to other body parts.

• The cardiac cycle is from blood entering the heart to its emptying. It is reflected as heart rate or beats per minute.

• The heart operates automatically and is self-exciting, a unique feature compared to other body muscles that require nervous stimuli.

• The contractions in the heart occur spontaneously and are sensitive to the nervous system (sympathetic for arousing and parasympathetic for depressing ) and hormonal influences.

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Cardiac conduction system:-

The cardiac conduction system has cells that keep the heart beating. It controls the rhythm and rate by contraction and relaxation. It ensures that blood flows properly through the heart and the rest of the body. The sinoatrial node is the heart’s natural pacemaker. It is located in the right atrium.

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This node generates an electrical signal that causes the atria, the upper chambers, to contract. The signal then moves to the atrioventricular node, which delays it until the atria empty blood into the ventricles. The signal travels down the bundle of His, which divides into right and left branches ending in Purkinje fibres.

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These fibres spread the signal through the ventricles, prompting them to contract and pump blood to the lungs or the rest of the body. The sympathetic nervous system speeds up the heart rate during stress or exercise, while the parasympathetic nervous system slows it down during relaxation. The heart maintains a steady beat.

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Cardiac muscle and electrical activity:-

Cardiac muscle exhibits autorhythmicity, which means it can generate electrical impulses without external stimuli. Pacemaker cells initiate and regulate the heartbeat. There are two primary types of cardiac muscle cells.

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• Myocardial contractile cells: 99% of heart muscle cells are responsible for contractions that pump blood.

• Myocardial conducting cells make up one percent of the conduction system and propagate the electrical impulse in the heart.

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Cardiac muscle cells are short and branched, containing a single central nucleus. They are connected by intercalated discs, which allow contractions. The cardiac muscle cells have striations similar to skeletal muscles and numerous mitochondria to fulfill the high energy demand.

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Cardiac muscle relies on calcium ions for contraction, and the calcium influx contributes to a plateau phase in the action potential, resulting in sustained contraction.

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• When an electrical impulse originates from the sinoatrial node, it spreads across the atria, causing atrial depolarization. It is seen as a P-wave on the ECG.

• The signal slows at the AV node, allowing the atria to empty. It passes through the bundle of His, bundle branches, and Purkinje fibres and causes ventricular depolarization, representing the QRS complex.

• Finally, the ventricles repolarize, forming a T-wave.

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The ECG records these changes in electrical potential and provides a graphical representation of the heart's electrical activity.

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Cardiac parameters of interest:-

1. Heart rate: reflects the frequency of a complete heartbeat from its generation to the beginning of the next beat. It can be determined using ECG and Photoplethysmography (PPG) sensors.

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2. The interbeat interval: (IBI) is the time interval between individual heartbeats. The R-R interval is used for the analysis.

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3. Heart rate variability (HRV): is the natural variation of IBI values from beat to beat. It decreases under conditions of emotional stress. HRV is reduced in individuals who experience frequent and prolonged daily worry and in patients who have post-traumatic stress disorder (PTSD).

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4. Sensors are used for IBI and HRV analysis because they are more sensitive to certain signal characteristics than PPG sensors.

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Measuring Heart Activity:-

Heart activity can be recorded using non-invasive techniques like ECG and PPG.

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Electrocardiogram

Though the amount of electricity is small, it can be picked up by ECG electrodes attached to the skin. ECG records the electrical activity of the heart muscle - depolarization (a negative change in the electric charge), which propagates as pulsating electrical waves towards the skin.

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The ECG setup has four electrodes on the four extremities (right arm, left arm, right leg, and left leg). ECG electrodes require conductive gel to increase the conductivity between the skin and electrodes.

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Photoplethysmography (PPG)

As the blood moves, blood pressure changes even in the outer layers and small vessels. The peripheral blood flow can then be measured using optical sensors attached to the fingertip, earlobe, or capillary tissue. The device has an LED that sends light into the tissue and records how much light is absorbed or reflected to the photodiode (a light-sensitive sensor) as the blood moves into the tissue.

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More light is absorbed as the blood flows towards the tissue, and more light is reflected as it moves away. PPG uses dry sensors that can be attached quickly compared to an ECG, making devices like the Apple Watch easy to use.

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Electrocardiogram

Let us now understand how the ECG works.

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Principle of the electrocardiogram:-

Cardiac action potential and cardiac conduction form the basis of the ECG. The SA node generates the cardiac action potential, which is transmitted through the cardiac conduction pathway. It causes changes in the membrane voltage (potential) across the membrane of cardiomyocytes. This action potential change results in a continuous cardiac cycle.

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The cardiac impulse transmits through the heart, spreads to the surrounding tissue, and finally reaches the skin of our body. This electric impulse from the skin is received by the electrodes of the ECG machine and processed to produce a waveform.

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Procedure for obtaining an electrocardiogram-

1. The patient lies flat on a bed in a comfortable and relaxed position.

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2. Remove all metallic objects and electrical devices.

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3. Attach the electrodes in their respective location .

• The RA electrode is placed on the right forearm.

• The LA electrode is placed on the left forearm.

• The RL electrode is placed on the right lower leg.

• The LL electrode is placed on the left lower leg.

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4. The V1 is placed in the 4th intercostal space, right of the sternum.

• V2 is placed in the 4th intercostal space to the left of the sternum.

• V4 is attached to the fifth intercostal space in the midclavicular line.

• V3 is attached between V2 and V4.

• V5 is attached horizontally to the left of V4 in the left anterior axillary line.

• V6 is attached horizontally, left of V5, in the mid-axilliary line.

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5. Lead wires are connected to the electrodes and the ECG machine system on.

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6. The ECG machine auto-records the electrical impulse activity. ECG is thus recorded, the leads are disconnected from the electrodes, and the electrodes are detached.

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ECG and Einthoven’s Triangle:-

Einthoven’s triangle is a fundamental concept in ECG developed by Sir William Einthoven, a Dutch physician. An ECG monitors the electrical activity of the heart. It is represented in the form of waves called ECG waves or complexes. It involves placing three electrodes on specific body locations to create a triangle.

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The three corners of this triangle are named after the respective electrodes:

• Right arm (RA).

• Left arm (LA).

• Left leg (LL).

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There are three standard placements of electrodes:

• Standard limb leads.

• Augmented limb leads.

• Precordial leads.

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1. Standard limb leads I, II, III

They display a potential difference between two limbs simultaneously. One limb holds a positive electrode, the other negative.

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• Lead I: It measures the potential difference between the left and right arm. The positive electrode is on the left arm, and the negative electrode is on the right. The lead is oriented at 0 degrees, viewing the heart from the left.

I = LA – RA

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• Lead II measures the potential difference between the left leg and the right arm. The positive electrode is on the left leg, and the negative electrode is on the right arm. The lead is oriented at 60 degrees and gives a view from the lower left.

II = LL – RA

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• Lead III: The negative electrode is on the left arm, and the positive electrode is on the left leg; they form an angle of more than 120 degrees

III = LL-LA

2. Augmented limb leads (aVL, aVR, aVF)

Leads aVL, aVR, and aVF are augmented unipolar limb leads. The ECG has six limb leads, three of which are augmented unipolar and three of which are standard bipolar.

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The leads are unipolar as they utilise a single positive electrode, compared to a combination of other electrodes. These leads are named after the locations of their positive electrodes, which are placed on the left arm (aVL), the right arm (aVR), and the left leg (aVF).

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These leads capture the heart's electrical activity in the frontal plane using an axial reference system. Each lead is derived by combining the average of electrical signals received from the two opposite electrode positions.

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3. Precordial leads V1 to V6

The precordial leads capture electrical impulses from different heart regions and provide a horizontal view of the heart's electrical activity. In a normal ECG, there is a progressive increase in R Wave amplitude from V1 to V5 and then a decrease in V6. This pattern is our wave progression, which indicates ventricular depolarisation from the right to the left ventricle.

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Poor R-wave progression indicates the absence of a normal increase in R-wave amplitude across the precordial leads. A R-wave amplitude is < 3 mm in lead V3. A reversal or decrease of amplitude in V2 to V3 or V3 to V4 is associated with specific cardiac conditions. It can even show up in the ECG of healthy individuals due to factors such as incorrect placement.

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4. Instrumentation of ECG:-

The ECG machine is a sophisticated machine that has several components working together. It is the core of instrumentation and has multiple components: electrodes, a signal processor, a monitor, and a printer.

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• A standard 12-lead ECG uses 10 electrodes placed on the patient’s body. These electrodes transmit electric signals from the heart to the skin, and each electrode captures electrical activity from different areas of the heart.

• Lead wires connect electrodes and ECG machines, causing a transmission of impulses.

• The central signal processor increases the signal strength for interpretation and removes any noise that distorts the reading.

• Data processing: The signals are converted into ECG waves that we can identify.

• Display and printing: The signals are sent to the monitor or a tracing sheet.

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5. Obtaining the ECG:-

The following steps are followed for obtaining an ECG:-

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• Patient preparation: The patient is made to lie flat on a bed in a comfortable, relaxed position, removing all metallic objects and electrical devices.

• Electrode placement: The electrodes are attached to the patient’s body in their respective locations.

• Connecting lead wires: The electrodes are attached through lead wires to the ECG machine.

• Switching on the machine: The machine detects electric currents when switched ON.

• Recording the ECG: It records signals to produce an ECG tracing.

• Post procedure, the leads are disconnected from the electrodes, which are removed from the patient’s body.

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6. Artifacts in ECG:-

ECG artifacts are abnormalities not related to the electrical activity of the heart. As a result of artifacts, normal components of the ECG can get distorted, which may be caused by:-

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Internal factors

• These are physiological causes and could be due to the patient's motion-like tremors and shivering. Simple movements such as brushing and combing hair can also produce disturbances in ambulatory ECG monitoring.

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External factors

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1. Electromagnetic interference

• Powerline electrical disturbances.

• Electrocautery machine.

• Electrical devices.

• Radio frequency devices like a mobile phone.

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2. Cable and electrode malfunction

• Proper use of electrode gel.

• Misplaced leads.

• Loose connections.

• Inappropriate settings.

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3. Medical equipment in operating theatres and intensive care units also affects ECG monitoring.

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7. Limitations of ECG:-

• ECG gives information about the heart's electrical activity and not about the heart’s anatomy.

• ECG can give false results due to problems in the electrodes or due to metallic or electrical devices nearby.

• The interpretation of the ECG report is subjective. Thus, conclusions may vary among healthcare professionals.

• ECG displays the electrical activity for a short duration.

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Conclusion

ECGs help diagnose diseases and manage heart health. They show the heart's electrical activity and help detect arrhythmias, heart attacks, and structural abnormalities. They provide doctors with essential information to make decisions about treatment and care.

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Monitoring heart health with an ECG prevents potential issues from becoming more serious by identifying them early and saving many lives. Gauze provides support, care, and consultation if you experience chest pain, shortness of breath, or palpitations.