Electrocardiogram

An electrocardiogram (ECG or EKG) is a diagnostic test that measures the heart's electrical activity over time through electrodes placed on the skin. This non-invasive procedure helps identify heart abnormalities and track overall heart health.

1.0Electrocardiogram Definition

• Electrocardiogram is the technique by which electrical activities of the heart are studied. The spread of excitation through the myocardium produces local electrical potential. 
• This low-intensity current flows through the body, which acts as a volume conductor. This current can be picked up from the surface of the body by using suitable electrodes and recorded in the form of an electrocardiogram. 
• This technique was discovered by Dutch physiologist Einthoven Willem, who is considered the father of electrocardiogram (ECG).

2.0Electrocardiograph

• An electrocardiograph is the instrument (machine) by which electrical activities of the heart are recorded.
• An electrocardiogram (ECG or EKG, derived from the Dutch term "electrocardiogram") represents the heart's electrical activity before mechanical contractions occur. 
• It captures the combined electrical signals from all cardiac muscle fibers as recorded from the body's surface.

3.0ECG/EKG Test

Both terms mean the same thing: an electrocardiogram. EKG comes from the German word, which uses “k” instead of “c” in both parts of the word. However, it’s different from an echocardiogram, which is an ultrasound that creates images of your beating heart.

4.0Uses of ECG

An electrocardiogram helps determine and diagnose the following: 

1. Heart rate 

2. Heart rhythm 

3. Abnormal electrical conduction 

4. Poor blood flow to heart muscle (ischemia) 

5. Heart attack 

6. Coronary artery disease 

7. Hypertrophy of heart chambers. 

5.0ECG Leads

Recording an ECG

• An ECG is recorded by placing a series of electrodes on the body's surface. 
• These electrodes, called ECG leads, are connected to an ECG machine. 
• Typically, electrodes are fixed on the limbs: the right arm, left arm, and left leg. 
• The heart is considered to be at the center of an imaginary equilateral triangle formed by connecting these three points, known as the Einthoven triangle.

Einthoven Triangle and Einthoven Law

• The Einthoven triangle is an equilateral triangle model used to explain the standard limb leads in an ECG recording. 
• The heart is presumed to lie at the center of this triangle, and the electrical potentials generated by the heart appear simultaneously at the points where the limbs connect—the left arm, right arm, and left leg. 

Types of ECG Leads

ECG is recorded using 12 leads, which are classified into two main categories:

1. Bipolar Leads

2. Unipolar Leads

1. Bipolar Limb Leads

• Bipolar limb leads, also known as standard limb leads, are obtained by connecting two limbs.
• Both electrodes act as active recording electrodes, with one being positive and the other negative. There are three types of standard limb leads:

Limb Lead I

Limb Lead II

Limb Lead III

Bipolar Leads

Lead I

• Connection: Right arm to left arm.
• Polarity: Right arm to the negative terminal, left arm to the positive terminal.

Lead II

• Connection: Right arm to left leg.
• Polarity: Right arm to the negative terminal, left leg to the positive terminal.

Lead III

• Connection: Left arm to left leg.
• Polarity: Left arm to the negative terminal, left leg to the positive terminal.

2. Unipolar Leads

In unipolar leads, one Electrode acts as the active (positive) Electrode, and the other serves as the indifferent Electrode, which is a composite negative reference.

Types of Unipolar Leads

1. Unipolar Limb Leads

2. Unipolar Chest Leads

Unipolar Limb Leads

• Active Electrode: Connected to one of the limbs.
• Indifferent Electrode: Formed by connecting the other two limbs through a resistance.

Types of Unipolar Limb Leads

1. aVR Lead

• Active Electrode: Right arm.
• Indifferent Electrode: Formed by connecting the left arm and left leg.

2. aVL Lead

• Active Electrode: Left arm.
•  Indifferent Electrode: Formed by connecting the right arm and left leg.

3. aVF Lead

•    Active Electrode: Left leg (foot).
•    Indifferent Electrode: Formed by connecting the right arm and left arm.

Unipolar Chest Leads

Unipolar chest leads, also known as "V" leads or precordial chest leads, involve placing an indifferent electrode and active electrodes on the chest to capture electrical activity from specific heart regions.

Indifferent Electrode: This is created by connecting the left arm, left leg, and right arm through a resistance of 5000 ohms.

Active Electrode: Placed at six specific points on the chest, known as chest electrodes, labeled V1, V2, V3, V4, V5, and V6. The "V" indicates vector, representing the direction of current flow.

Positioning of Chest Leads:

• V1: Over the 4th intercostal space, near the rightternal margin.
• V2: Over the 4th intercostal space, near the leftternal margin.
• V3: Midway between V2 and V4.
• V4: Over the left 5th intercostal space on the midclavicular line.
• V5: Over the left 5th intercostal space on the anterior axillary line.
• V6: Over the left 5th intercostal space on the midaxillary line.

6.0ECG with Interpretation 

• A normal electrocardiogram (ECG) includes various components such as waves, complexes, intervals, and segments.
• The typical waves recorded by limb lead II include the P, Q, R, S, and T waves. 
• The nomenclature for these waves, introduced by Einthoven, starts with the middle of the alphabet (P) rather than the beginning (A). 
• The major complexes in an ECG are as follows:

1. P Wave: Represents atrial depolarization.

2. QRS Complex: Reflects the initial ventricular depolarization.

3. T Wave: Indicates the final phase of ventricular repolarization.

4. QRS Complex: Represents the complete ventricular complex.

These components collectively provide insights into the heart's electrical activity.

 P Wave:

• The P wave is the initial positive deflection on an ECG, also called the atrial complex. It represents the depolarization of the atrial musculature, which begins at the sinoatrial (SA) node and spreads throughout the atria. This electrical activity leads to atrial contraction. 
• Atrial repolarization is not distinctly visible on the ECG because it is obscured by the larger QRS complex associated with ventricular depolarization. 
• Duration: The normal duration of the P wave is approximately 0.1 seconds.
• Amplitude: The typical amplitude of the P wave ranges from 0.1 to 0.12 millivolts (mV).

QRS Complex:

• The QRS complex, also known as the initial ventricular complex, represents the electrical depolarization of the ventricles, which triggers their contraction. It consists of three main waves:
• Q Wave: A small negative deflection that reflects the depolarization of the basal portion of the interventricular septum.
• R Wave: A tall positive deflection that follows the Q wave, indicating depolarization of the apical portion of the interventricular septum and the apical portion of the ventricular muscle.
• S Wave: A small negative deflection following the R wave, representing the depolarization of the basal portion of the ventricular muscle near the atrioventricular ring.
• Duration: The normal duration of the QRS complex ranges from 0.08 to 0.10 seconds.
• Amplitude:

Q Wave: Typically 0.1 to 0.2 mV.

R Wave: Generally around 1 mV.

S Wave: About 0.4 mV.

T Wave:

• The T wave is the final component of the ventricular complex and typically appears as a positive deflection on an ECG. 
• It represents the repolarization of the ventricular muscle. 
• Duration: The normal duration of the T wave is approximately 0.2 seconds.
• Amplitude: The normal amplitude of the T wave is about 0.3 mV.

U Wave:

• The U wave is not always visible on an ECG and is generally considered less significant.
• It is believed to be associated with the repolarization of the papillary muscles.

Intervals and Segments of an ECG

P-R Interval

• The P-R interval is the time span between the start of the P wave and the beginning of the Q wave. 
• It represents the duration it takes for electrical impulses to travel from the atria, through the AV node, and into the ventricles. 
• This interval reflects both the depolarization of the atria and the conduction of impulses through the AV node.
• P Wave: Represents atrial depolarization.
• Isoelectric Period: A brief period of zero voltage following the P wave, representing the time required for the depolarization to pass through the AV node.
• Duration - The normal P-R interval duration is approximately 0.18 seconds, with a typical range of 0.12 to 0.20 seconds. An interval longer than 0.20 seconds indicates a delay in the conduction of impulses from the SA node to the ventricles, often due to a delay in the AV node, commonly referred to as AV nodal delay.

Q-T Interval

• The Q-T interval is the period between the start of the Q wave and the end of the T wave on an ECG. 
• It reflects the total duration of ventricular depolarization and repolarization, representing the electrical activity within the ventricles. 
• The normal duration of the Q-T interval typically ranges from 0.4 to 0.42 seconds.

S-T Segment

• The S-T segment refers to the interval between the end of the S wave and the beginning of the T wave on an ECG. 
• This segment is isoelectric, meaning it represents a period where the heart's electrical activity is stable.

J Point

• The J point is the precise location where the S-T segment begins. It marks the transition between the QRS complex and the S-T segment.
• Duration of the S-T Segment-  The normal duration of the S-T segment is approximately 0.08 seconds.

R-R Interval

• The 'R-R' interval is the time span between two consecutive 'R' waves on an ECG.
• Significance: This interval represents the duration of one cardiac cycle.
• Normal Duration: The typical 'R-R' interval duration is approximately 0.8 seconds.
• Importance of Measuring the R-R Interval: Evaluating the 'R-R' interval is crucial for calculating:

1. Heart rate

2. Heart rate variability

7.0Abnormal of ECG

• An abnormal ECG can be caused by a variety of factors, including:
• Coronary Artery Disease: Blockages in the coronary arteries affecting blood flow.
• Heart Failure: Chronic condition affecting the heart's ability to pump blood.
• Myocarditis: Inflammation of the heart muscle.
• Pericarditis: Inflammation of the lining around the heart.
• Pulmonary Embolism: Blood clots in the lungs affecting heart function.
• Drug Effects: Certain medications and recreational drugs can alter ECG patterns.