How to Read ECG? ECG & EKG Interpretation in 2024

Understanding how to read an ECG is crucial for healthcare professionals, as it helps in diagnosing various heart conditions. This blog post is designed to guide you through the essentials of ECG interpretation, offering a systematic approach to avoid common errors. By following these steps, you can ensure accurate readings and better patient care.

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We’ll start by emphasizing the importance of verifying patient details before beginning ECG interpretation. This includes confirming the patient's name, and date of birth, and ensuring that the ECG's date and time are correct. Additionally, we'll cover the basics of ECG calibration, typically set at 25mm/s and 10mm/1mV, which is vital for accurate readings.

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Finally, the blog will provide a clear explanation of what constitutes a normal adult heart rate, helping you quickly identify whether the heart rate is within the normal range (60-100 bpm) or if there are signs of tachycardia (above 100 bpm) or bradycardia (below 60 bpm). By the end of this post, you'll have a solid foundation in ECG interpretation, empowering you to make informed decisions in your clinical practice.

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What is Heart Rate?

Heart rate is the number of times your heart beats per minute, indicating how hard your heart is working to pump blood throughout your body. It's a key vital sign that can provide insights into your overall heart health and fitness levels.

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How to Read an ECG?

Reading an ECG is a fundamental skill for healthcare professionals, enabling the detection of various heart conditions. This guide breaks down the key steps in ECG interpretation, from verifying patient details to assessing heart rate, rhythm, and axis.

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1. Patient Details

Before interpreting an ECG, it's essential to confirm that the patient's details are correct. Double-check the patient's name, date of birth, and identification numbers to ensure they match the information on the ECG. This step is crucial for accurate diagnosis and treatment, as mislabeling can lead to incorrect clinical decisions.

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2. Situation Details

Next, review the situation details, including the date and time the ECG was performed. It's important to know when the ECG was taken, as this can affect the interpretation, especially in acute settings where time-sensitive cardiac events are being monitored.

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Ensure the ECG calibration is correct usually set at 25mm/s for paper speed and 10mm/1mV for voltage. Proper calibration is vital for accurate measurement and interpretation.

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3. Measuring the Rate on an ECG

To determine the heart rate from an ECG, count the number of QRS complexes within a specific time frame. The simplest method is to count the number of QRS complexes in a 6-second strip and multiply by 10 to get the beats per minute (bpm). A normal adult heart rate ranges from 60-100 bpm. Rates above 100 bpm indicate tachycardia, while rates below 60 bpm suggest bradycardia.

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4. Assessing the Rhythm on an ECG

Assessing the rhythm involves examining whether the heartbeats are regular or irregular. A regular rhythm will have evenly spaced QRS complexes. Irregular rhythms can be indicative of conditions such as atrial fibrillation or other arrhythmias. Look for a consistent pattern in the timing and shape of the P waves, QRS complexes, and T waves.

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5. Assessing the Axis on an ECG

The axis of the heart refers to the general direction of the electrical activity during depolarization. Assessing the axis helps in identifying conditions such as left or right axis deviation, which can be associated with specific cardiac abnormalities like hypertrophy or conduction defects.

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The axis is determined by looking at the QRS complex in leads I, II, and III, as well as in the augmented limb leads (aVL, aVF). A normal axis lies between -30° and +90°.

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What is A Normal Adult Heart Rate?

A normal adult heart rate typically ranges from 60 to 100 beats per minute (bpm). If the heart rate exceeds 100 bpm, it's considered tachycardia, while a heart rate below 60 bpm is classified as bradycardia.

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• Normal (60-100 bpm): A heart rate within this range indicates a typical resting heart rate for a healthy adult.

• Tachycardia (> 100 bpm): A heart rate above 100 bpm, often indicating stress, fever, or underlying cardiac conditions.

• Bradycardia (< 60 bpm): A heart rate below 60 bpm, which can be normal for athletes or indicate an issue with the heart's electrical system.

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Regular Heart Rhythm

A regular heart rhythm, also known as sinus rhythm, is when the heart beats in a consistent, evenly-spaced manner. This rhythm indicates that the electrical impulses controlling the heart are functioning correctly, leading to a steady flow of blood throughout the body.

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• Consistent Timing: Each heartbeat is spaced evenly, with similar intervals between QRS complexes on an ECG.

• P Wave Presence: In a regular rhythm, each P wave is followed by a QRS complex, reflecting proper atrial and ventricular contraction.

• Normal Heart Rate: The rhythm typically falls within the normal heart rate range of 60-100 bpm, ensuring efficient circulation.

• Stable Electrical Activity: Regular rhythm suggests that the heart’s electrical conduction system is operating normally, without irregular impulses or arrhythmias.

• Count the Number of Large Squares: Identify the number of large squares between two consecutive R-R intervals on the ECG.

• Calculate the Heart Rate: Divide 300 by the number of large squares counted. This formula works because the standard paper speed for ECGs is 25 mm/s, and there are 300 large squares in one minute.

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Example:

• If there are 4 large squares between two R-R intervals: 

• 300/4 = 75 beats per minute

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This calculation provides a quick and accurate estimate of the heart rate based on the ECG trace.

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Irregular Heart Rhythm

An irregular heart rhythm indicates that the heartbeats are not evenly spaced, which can suggest a range of cardiac issues or arrhythmias. Properly assessing an irregular rhythm involves calculating the heart rate from a rhythm strip to understand the pattern and frequency of the beats.

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Counting and Calculation

• Count the Number of Complexes: On a rhythm strip, typically lasting 10 seconds, count the total number of QRS complexes.

• Calculate the Heart Rate: Multiply the number of complexes by 6 to estimate the average heart rate over one minute.

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Example:

• If there are 10 complexes on the rhythm strip: Heart Rate=10×6=60 beats per minute\text{Heart Rate} = 10 \times 6 = 60 \text{ beats per minute}Heart Rate=10×6=60 beats per minute

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Heart Rhythm

A patient's heart rhythm can be classified as either regular or irregular. Irregular rhythms are often further categorized into two types: regularly irregular and irregularly irregular. Understanding these patterns is crucial for diagnosing and managing various cardiac conditions.

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Types of Irregular Rhythms

• Regularly Irregular: This type exhibits a recurring pattern of irregularity, such as in atrial flutter with a regular pattern of skipped beats or premature beats.

• Irregularly Irregular: This rhythm is completely disorganized with no discernible pattern, as seen in atrial fibrillation where the intervals between beats vary randomly.

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Assessment Techniques

• Mark Out R-R Intervals: To assess the rhythm, mark several consecutive R-R intervals on a piece of paper. Slide this paper along the rhythm strip to determine if the intervals remain consistent or vary.

• Checking for AV Block: If you suspect an atrioventricular (AV) block, map the atrial rate and the ventricular rhythm separately. Mark the P waves and R waves, and observe if the PR interval changes, if QRS complexes are missing, or if there is complete dissociation between atrial and ventricular activity.

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Cardiac Axis

The cardiac axis refers to the general direction of electrical activity as it spreads through the heart during depolarization. This directional flow is essential for understanding how the heart's electrical impulses are conducted and can provide valuable information about heart health.

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Timing and Direction

• Timing: The cardiac axis typically spans from approximately 11 o’clock to 5 o’clock on an ECG. This indicates a normal range of electrical activity direction in a healthy heart.

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Determining the Cardiac Axis

• Cardiac Axis: To determine the cardiac axis, evaluate the QRS complexes in leads I, II, and III. These leads provide a comprehensive view of the heart’s electrical activity from different angles, allowing you to assess whether the axis falls within the normal range or deviates, which could suggest conditions such as hypertrophy or conduction abnormalities.

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Normal Cardiac Axis

The normal cardiac axis represents the typical direction of electrical activity in the heart, indicating proper heart function and conduction. In a healthy individual, the cardiac axis should fall within a specific range that reflects balanced electrical impulses.

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Key Points and Leads

• Typical Findings: On a standard ECG, a normal cardiac axis is indicated by the most positive deflection in Lead II compared to Leads I and III.

• Lead II: This lead will show the highest positive deflection, reflecting the direction of the heart’s electrical activity towards the left and downward.

• Leads I and III: While these leads will also show positive deflections, they will be less pronounced compared to Lead II.

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Right Axis Deviation

Right axis deviation (RAD) occurs when the heart’s electrical axis shifts to the right, which can be indicative of underlying cardiac conditions. This deviation affects how electrical impulses travel through the heart, and recognizing it is crucial for accurate diagnosis and treatment.

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Key Points and Leads

• Typical Findings: In the case of right axis deviation, Lead III will show the most positive deflection, while Lead I will exhibit a negative deflection.

• Lead III: The prominent positive deflection in Lead III reflects the shift in the heart’s electrical axis towards the right.

• Lead I: A negative deflection in Lead I suggests that the electrical activity is deviating from its normal direction.

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Right axis deviation is often associated with right ventricular hypertrophy or other conditions affecting the right side of the heart. Identifying these ECG patterns helps in diagnosing and managing related cardiac issues.

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Left Axis Deviation

Left axis deviation (LAD) occurs when the heart's electrical axis shifts to the left, which can indicate various cardiac conditions. This deviation alters the usual direction of electrical activity and is essential for diagnosing potential issues with heart function.

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Key Points and Leads

• Typical Findings: In the left axis deviation, Lead I will show the most positive deflection, while Leads II and III will exhibit negative deflections.

• Lead I: The prominent positive deflection in Lead I indicates that the electrical activity is directed more toward the left side of the heart.

• Leads II and III: Both leads will show negative deflections, reflecting the deviation of electrical activity away from these leads.

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Left axis deviation is often associated with heart conduction abnormalities, such as those seen in left ventricular hypertrophy or certain types of heart block. Identifying these patterns on an ECG aids in diagnosing and managing related cardiac conditions.

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P Waves

The P waves on an ECG represent atrial depolarization and are crucial for assessing the rhythm and function of the heart. Analyzing P waves helps determine if the atria are functioning correctly and if the heart’s electrical signals are traveling as expected.

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Questions to Assess P Waves:

1. Are P Waves Present?

Check the ECG to see if P waves are visible. Their presence is essential for evaluating atrial activity.

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2. Does A QRS Complex follow each P Wave?

Ensure that every P wave is followed by a QRS complex, indicating proper atrioventricular (AV) conduction.

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3. Do the P Waves Look Normal?

Examine the duration, direction, and shape of the P waves to ensure they are within normal limits. Abnormalities may indicate atrial issues.

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4. If P Waves Are Absent, Is There Any Atrial Activity?

Look for other signs of atrial activity:

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• Sawtooth Baseline: Indicates flutter waves, suggesting atrial flutter.

• Chaotic Baseline: Indicates fibrillation waves, suggesting atrial fibrillation.

• Flat Line: Indicates no atrial activity, which could be due to an atrial standstill or other issues.

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Hint

If P waves are absent and there is an irregular rhythm, it may suggest a diagnosis of atrial fibrillation, where the atria do not contract effectively, leading to a chaotic baseline on the ECG.

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PR Interval

The PR interval on an ECG represents the time it takes for electrical impulses to travel from the atria through the AV node to the ventricles. It is a crucial measurement for assessing the conduction between the atria and ventricles.

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Number Estimations

• Normal Range: The PR interval should be between 120 to 200 milliseconds (ms), which corresponds to 3 to 5 small squares on the ECG paper. This range indicates a normal delay in AV conduction.

• Measurement: To measure the PR interval, count the number of small squares from the beginning of the P wave to the start of the QRS complex and ensure it falls within this normal range.

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An abnormal PR interval can indicate various conduction disorders, such as AV block, where the interval may be prolonged or shortened.

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Prolonged PR Interval

A prolonged PR interval (>0.2 seconds) indicates a delay in the conduction of electrical impulses from the atria to the ventricles, which can be associated with various degrees of heart block.

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1. First-Degree Heart Block (AV Block)

• Description: This condition is characterized by a consistently prolonged PR interval greater than 200 milliseconds (MS) or 5 small squares on the ECG.

• ECG Findings: The PR interval remains prolonged and fixed for each beat. For example, if it is 6 small squares (300 MS), this would indicate a first-degree heart block.

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First-degree heart block is characterized by a consistently prolonged PR interval exceeding 200 milliseconds (MS) or 5 small squares on the ECG. This condition reflects a delay in the electrical signal as it travels from the atria to the ventricles. 

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The PR interval remains fixed and does not vary between beats, indicating a persistent, albeit fixed, delay in AV node conduction. This block is generally less severe but may signal underlying issues with electrical conduction in the heart.

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2. Second-Degree Heart Block (Type 1)

• Description: Also known as the Mobitz Type 1 or Wenckebach phenomenon, a progressively increasing PR interval characterizes this block until a QRS complex is dropped.

• ECG Findings: The PR interval gradually lengthens until a beat is skipped (i.e., a QRS complex is dropped). For example, if the PR interval starts at 4 small squares and progressively increases to 6 small squares before a QRS complex is dropped, this pattern repeats with each subsequent beat.

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Second-degree heart block Type 1, or Mobitz Type 1 (Wenckebach phenomenon), features progressively lengthening PR intervals until a QRS complex is dropped. Initially, the PR interval is normal, but it gradually increases until a beat is missed. 

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After the dropped beat, the interval shortens, and the pattern starts again. This block reflects a variable delay in AV conduction that eventually causes some beats to be skipped, often leading to a recurring cycle of interval prolongation and missed beats.

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3. Second-Degree Heart Block (Type 2)

• Description: Also known as Mobitz Type 2, this block features a consistent PR interval with occasional drops of QRS complexes.

• ECG Findings: The PR interval remains constant (e.g., 5 small squares) while QRS complexes are intermittently missing. For example, if every 3rd or 4th P wave fails to produce a QRS complex, it indicates a 3:1 or 4:1 block, respectively.

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Second-degree heart block Type 2, or Mobitz Type 2, is characterized by a consistent PR interval with occasional, predictable drops of QRS complexes. Unlike Type 1, the PR interval remains constant, but every 3rd or 4th P wave fails to produce a corresponding QRS complex, indicating a 3:1 or 4:1 block. This type is more serious and less predictable than Type 1 and often requires closer monitoring and potentially more urgent intervention.

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4. Third-Degree Heart Block (Complete Heart Block)

• Description: This severe block occurs when there is a complete failure of electrical communication between the atria and ventricles.

• ECG Findings: P waves and QRS complexes appear independently of each other. For example, P waves may occur regularly every 6 seconds, while QRS complexes occur independently, possibly at a different rate (e.g., a junctional escape rhythm with QRS complexes of less than 0.12 seconds or a broad-complex escape rhythm with QRS complexes longer than 0.12 seconds).

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Third-degree heart block, or complete heart block, occurs when there is no electrical communication between the atria and ventricles. This results in P waves and QRS complexes occurring independently of each other. The atria and ventricles function separately, with the heart's rhythm maintained by an escape pacemaker. Depending on the location of the pacemaker, this can result in narrow-complex or broad-complex escape rhythms.

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Complete heart block is a serious condition that often requires immediate treatment to ensure adequate cardiac function. In third-degree heart block, the heart’s rhythm is maintained by an escape pacemaker from either above or below the bifurcation of the bundle of His. Narrow-complex rhythms originate above the bifurcation, while broad-complex rhythms arise below it.

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Tips For Remembering Types of Heart Block

Understanding where each type of heart block occurs within the heart’s conduction system can help in distinguishing them and remembering their characteristics.

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First-Degree AV Block

• Location: Between the SA node and the AV node.

• Tip: Picture it as a delay in the signal transmission from the atrium to the AV node. It’s like a slow but consistent relay in the atrial region.

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Second-Degree AV Block

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Mobitz I (Wenckebach):

• Location: Within the AV node.

• Tip: Think of it as a progressive delay within the AV node, causing periodic dropped beats due to varying conduction speeds.

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Mobitz II:

• Location: After the AV node, in the bundle of His or Purkinje fibers.

• Tip: Visualize this as a block in the pathways beyond the AV node, where the PR interval remains constant, but some QRS complexes are dropped.

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Third-Degree AV Block

• Location: At or beyond the AV node.

• Tip: Imagine a complete blockage in the conduction pathway after the AV node, where the atria and ventricles function independently with their pacemakers.

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Shortened PR Interval

A shortened PR interval can indicate one of two scenarios:

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1. Closer Origin of the P Wave:

• The P wave originates closer to the AV node, reducing the conduction time. This may be due to individual anatomical differences, such as a smaller atrium or a less fixed SA node position.

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2. Accessory Pathway:

• The atrial impulse may be reaching the ventricles through a faster, abnormal pathway rather than the usual route. This accessory pathway can lead to a shortened PR interval and may be associated with a delta wave, often seen in conditions like Wolff-Parkinson-White (WPW) syndrome.

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QRS Complex

The QRS complex represents the electrical activity of the ventricles as they contract and pump blood. It's crucial for diagnosing various heart conditions. Here’s a breakdown of its components:

• Width
• Height
• Morphology

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Width of the QRS Complex

The width of the QRS complex can be categorized as either narrow or broad, and understanding these variations is crucial for diagnosing heart conditions:

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Narrow QRS Complex (< 0.12 Seconds)

• Description: A narrow QRS complex measures less than 0.12 seconds (or less than 3 small squares on the ECG).

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Key Points:

• Normal Conduction: Occurs when the electrical impulse travels efficiently through the bundle of His and Purkinje fibers, leading to well-coordinated ventricular depolarization.

• Implication: Indicates that the heart’s conduction system is functioning properly, and the impulse is spreading through the ventricles in a synchronized manner.

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Broad QRS Complex (> 0.12 Seconds)

• Description: A broad QRS complex measures more than 0.12 seconds (or more than 3 small squares on the ECG).

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Key Points:

• Abnormal Conduction: Occurs when there is an abnormal depolarization sequence, such as:

• Ventricular Ectopic: The impulse spreads slowly across the myocardium from an ectopic focus in the ventricle, leading to a broad QRS.

• Bundle Branch Block: The impulse travels quickly through one branch but spreads slowly through the other, resulting in a broad QRS complex.

• Implication: Indicates a disruption in the normal conduction pathway, which could be due to conditions such as bundle branch block or ventricular ectopic rhythms.

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Bundle Branch Block

Bundle branch block is characterized by broad QRS complexes and includes two main types: left bundle branch block (LBBB) and right bundle branch block (RBBB). To quickly recognize these blocks on an ECG, you can use the William Marrow mnemonic, which helps identify key features in leads V1 and V6.

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1. Left Bundle Branch Block (LB)

Mnemonic Hint: William

Two Ls in William signify Left Bundle Branch Block.

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ECG Features:

• V1: Deep S wave, which may be notched, resembling a "W".

• V6: Broad, "M"-shaped R wave.

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2. Right Bundle Branch Block (RB)

Mnemonic Hint: MaRRoW

• Two Rs in MaRRoW signify the Right Bundle Branch Block.

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ECG Features:

• V1: RSR' pattern, which looks like an "M".

• V6: Broad S wave, resembling a "W".

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Using these mnemonics helps quickly identify and differentiate between LBBB and RBBB by focusing on the characteristic patterns observed in the ECG leads.

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Height of the QRS Complex

The height of the QRS complex reflects the amplitude of the ventricular electrical activity and can indicate various heart conditions. It is categorized as either small or tall:

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Small QRS Complexes

Description:

• Definition: Amplitude less than 5 mm in the limb leads or less than 10 mm in the chest leads.

• Implications: Small complexes may be seen in conditions like obesity, chronic lung disease, or diffuse heart disease where the electrical signals are less pronounced.

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Tall QRS Complexes

Description:

• Definition: Amplitude greater than normal, often indicating a measurement exceeding 5 mm in the limb leads or 10 mm in the chest leads.

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Implications:

• Ventricular Hypertrophy (LVH): Tall complexes are often associated with conditions like left or right ventricular hypertrophy. This occurs when the heart muscle thickens, leading to increased electrical activity.

• Body Habitus: This can also be seen in tall, slim individuals due to their body frame

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‍Measurement Algorithms: Several algorithms can help assess LVH, including:‍

• Sokolow-Lyon Index: Compares the amplitudes of specific QRS complexes.

• Cornell Index: Considers both QRS amplitude and the duration of the QRS complex.

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Morphology of the QRS Complex

The morphology, or shape, of the QRS complex, provides important diagnostic clues and helps in identifying underlying conditions.

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Key Points:

1. Normal Morphology: The QRS complex should have a consistent shape that appears narrow and regular.

2. Abnormal Morphology: Variations can indicate issues such as:

• Ventricular Hypertrophy: Often associated with tall, notched, or widened complexes.

• Myocardial Infarction: May show changes like abnormal Q waves or altered shapes.

• Electrolyte Imbalances: These can cause unusual patterns in the QRS complex.

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Delta Wave

The delta wave is a distinctive feature seen in the ECG of patients with specific arrhythmias and conduction abnormalities. It is associated with certain types of pre-excitation syndromes.

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Description

• Definition: A delta wave is an initial slurred upstroke in the QRS complex, indicating early ventricular depolarization.

• Appearance: It appears as a slurred, gradual upstroke at the beginning of the QRS complex, creating a characteristic "swooping" appearance.

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Associated Condition

Wolff-Parkinson-White (WPW) Syndrome:

• Pre-excitation: The delta wave is most commonly associated with WPW syndrome, where there is an extra electrical pathway (accessory pathway) between the atria and ventricles.

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Conduction: This pathway allows electrical impulses to bypass the normal AV node conduction delay, causing early depolarization of the ventricles.

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Key Points for Interpretation:

• Identification: Look for a slurred upstroke at the start of the QRS complex in the affected leads.

• Diagnosis: The presence of a delta wave, especially in conjunction with a short PR interval, can suggest WPW syndrome or other pre-excitation syndromes.

• Clinical Implications: Delta waves can lead to abnormal heart rhythms and may require further evaluation and management to prevent potential complications.

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Understanding and recognizing delta waves are crucial for diagnosing pre-excitation syndromes and guiding appropriate treatment.

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Q-Waves

Q-waves are part of the QRS complex and represent the initial negative deflection seen in some ECG leads. They provide important diagnostic information regarding past or ongoing myocardial infarction (heart attack). Here's a brief overview:

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Normal Q-Waves

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Description:

• Definition: Small, initial negative deflection in the QRS complex.

• Location: Normally present in leads I, aVL, V5, and V6.

• Size: Generally small and not wide or deep.

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Abnormal Q-Waves

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Description:

• Definition: Larger, deeper, and wider than normal Q waves.

• Indication: Often indicative of a past myocardial infarction or ongoing heart damage.

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Size:

• Width: Greater than 1 small square (0.04 seconds)

• Depth: Greater than 1/4 of the height of the R wave in the same lead.

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Diagnostic Significance

• Acute Myocardial Infarction: The presence of abnormal Q-waves can suggest a previous heart attack, particularly if observed in leads corresponding to the affected heart area.

• Evolution: Q-waves may take hours to days to develop after an infarction and can persist long-term, even after the heart has healed.

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Recognizing abnormal Q-waves helps in diagnosing and assessing the extent of myocardial damage and guiding treatment strategies.

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R and S Waves

The R and S waves are integral components of the QRS complex, which represents the electrical depolarization of the ventricles. Understanding these waves helps in diagnosing various heart conditions.

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R Wave

Description:

• Definition: The R wave is the first positive deflection in the QRS complex.

• Significance: Represents the depolarization of the main mass of the ventricles.

• Appearance: Typically, the R wave is tall and prominent in the ECG tracing.

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S Wave

‍Description:

• Definition: The S wave follows the R wave and is a negative deflection.

• Significance: This represents the completion of ventricular depolarization as the electrical impulse travels through the ventricles.

• Appearance: Often appears as a downward deflection after the R wave and may be small or absent in some leads.

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Key Points for Interpretation:

R Wave Height:

• Increased Height: This may suggest conditions like ventricular hypertrophy.

• Decreased Height: This can be seen in conditions like myocardial infarction or obesity.

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S Wave Depth:

• Increased Depth: Often associated with conditions like right ventricular hypertrophy or bundle branch block.

• Normal or Shallow: Usually indicates a normal pattern of ventricular depolarization.

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Analyzing the R and S waves helps in assessing the electrical activity of the ventricles, identifying heart conditions, and guiding appropriate treatment.

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J Point and J Point Segment

The J point and the J point segment are important in ECG analysis as they help in assessing various cardiac conditions, especially concerning the ST segment and overall ventricular repolarization.

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J Point

Description:

• Definition: The J point is the point where the QRS complex transitions into the ST segment on the ECG.

• Location: Marks the end of the QRS complex and the beginning of the ST segment.

• Significance: Indicates the junction between ventricular depolarization and repolarization.

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J Point Segment

Description:

• Definition: The segment immediately following the J point, including the ST segment.

• Importance: Used to evaluate changes in the ST segment, which can indicate issues such as ischemia or infarction.

• Normal Appearance: The ST segment should be flat and at the same level as the baseline.

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Key Points for Interpretation:

Elevation or Depression at the J Point:

• ST Segment Elevation: This may indicate acute myocardial infarction or pericarditis.

• ST Segment Depression: This can be a sign of myocardial ischemia or injury.

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Baseline Measurement:

• Comparison: Evaluate the J-point segment against the baseline to detect abnormalities. The normal ST segment should be isoelectric (flat) or slightly above/below the baseline.

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ST Segment

The ST segment on an ECG is the interval between the end of the S wave and the beginning of the T wave. It represents the period when the ventricles are in a state of electrical equilibrium after depolarization and before repolarization begins.

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In a healthy heart, this segment should appear as a flat, isoelectric line, indicating that the heart's electrical activity is stable during this phase. Any deviations from this baseline such as elevation or depression can signal underlying cardiac issues and warrant further investigation to diagnose potential pathology.

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ST-Elevation

ST-elevation is a critical ECG finding indicating potential myocardial infarction. It is considered significant when the ST segment rises more than 1 mm (one small square) above the baseline in at least two contiguous limb leads or more than 2 mm in two or more chest leads.

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This elevation typically signifies that a substantial portion of the heart muscle is not receiving adequate blood supply, often due to an acute, full-thickness myocardial infarction.

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Proper interpretation of ST elevation involves measuring the extent of the elevation and noting its location on the ECG to help identify the affected region of the heart and guide appropriate treatment.

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ST Depression

ST depression is an important indicator of myocardial ischemia, which occurs when the heart muscle is not receiving enough blood. It is considered significant when the ST segment is depressed by 0.5 mm or more below the baseline in at least two contiguous leads.

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This finding suggests that the heart is experiencing reduced blood flow, often due to conditions such as angina or stress. Proper evaluation of ST depression helps in diagnosing the severity of ischemia and determining the appropriate management strategies to address the underlying issue.

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T Waves

T waves on an ECG represent the repolarization of the ventricles, marking the end of the cardiac cycle of electrical activity in the ventricles. They follow the QRS complex and reflect the process by which the ventricles recover and prepare for the next heartbeat.

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Tall T Waves

• Definition: T waves are considered tall if they exceed 5 mm in the limb leads and 10 mm in the chest leads.

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1. Associated Conditions:

• Hyperkalemia: Tall, tented T waves are a classic sign of elevated potassium levels in the blood.

• Hyperacute STEMI: Tall T waves can also be an early indicator of an acute ST-elevation myocardial infarction.

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Understanding the significance of T-wave abnormalities helps in diagnosing and managing various cardiac conditions effectively.

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Inverted T Waves

T wave inversion is an important ECG finding that can indicate a range of conditions depending on its distribution across leads. Normally, T waves are inverted in lead V1, and inversion in lead III can be a normal variant. However, inversion in other leads is often a nonspecific sign and can be associated with several conditions:

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• Ischemia: T wave inversion may indicate myocardial ischemia, suggesting reduced blood flow to the heart muscle.

• Bundle Branch Blocks: Inverted T waves are seen in V4-V6 in the left bundle branch block (LBBB) and V1-V3 in the right bundle branch block (RBBB).

• Pulmonary Embolism: T wave inversion can be a sign of a pulmonary embolism.

• Left Ventricular Hypertrophy: Inversion in the lateral leads may suggest left ventricular hypertrophy.

• Hypertrophic Cardiomyopathy: Widespread T wave inversion can be associated with hypertrophic cardiomyopathy.

• General Illness: T wave inversion is also observed in about 50% of patients in intensive care units (ITU), often related to general illness.

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To interpret T wave inversion accurately, observe its distribution and apply it within the clinical context of the patient's overall condition.

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Biphasic T Waves

Biphasic T waves, characterized by two distinct peaks or deflections, can be a notable ECG finding indicating underlying conditions. These waves have an initial positive deflection followed by a negative one, or vice versa, and can be associated with:

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• Ischemia: Biphasic T waves may suggest myocardial ischemia, reflecting a compromised blood flow to the heart muscle.

• Hypokalemia: They can also indicate hypokalemia (low potassium levels), which can affect the heart’s electrical activity.

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Evaluating biphasic T waves involves considering their presence along with other clinical findings to determine the appropriate diagnosis and management.

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Flattened T Waves

Flattened T waves are an ECG finding that can be indicative of various underlying conditions. When T waves are flattened, they lose their typical peak, appearing more flat and level. This change can be a non-specific sign and may represent:

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• Ischemia: Flattened T waves can suggest myocardial ischemia, where reduced blood flow to the heart muscle affects its electrical activity.

• Electrolyte Imbalance: They can also be associated with electrolyte disturbances, such as hypokalemia or hypomagnesemia, which impact the heart’s electrical conduction.

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Interpreting flattened T waves requires assessing them in conjunction with other clinical data to identify the potential cause and guide appropriate treatment.

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U Waves

U waves are a less common but noteworthy ECG finding, appearing as a small deflection following the T wave. Typically, U waves are more visible in leads V2 or V3 and are defined as being greater than 0.5 mm in amplitude. Their prominence can increase with slower heart rates, such as in bradycardia. U waves can be associated with several conditions:

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• Electrolyte Imbalances: They are often seen in cases of hypokalemia or other electrolyte disturbances.

• Hypothermia: U waves may appear in individuals with low body temperature.

• Antiarrhythmic Therapy: Medications such as digoxin, procainamide, or amiodarone can induce U waves.

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Proper assessment of U waves involves considering their presence alongside other clinical information to diagnose potential underlying issues.

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Documenting your ECG Interpretation

1. Patient Details:

• Name and Date of Birth: Confirm with ECG details.

• Date and Time: Record when the ECG was done.

• Calibration Settings: Ensure settings are standard (25 mm/s, 10 mm/1mV).

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2. Heart Rate:

• Regular Rhythm: Calculate using large squares (300 divided by the number).

• Irregular Rhythm: Count complexes in a 10-second strip and multiply by 6.

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3. Heart Rhythm:

3. Regular vs. Irregular: Identify patterns and any dissociation between atrial and ventricular activities.

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4. Cardiac Axis:

• Normal: From 11 o’clock to 5 o’clock.

• Right Axis Deviation: Lead III positive, Lead I negative.

• Left Axis Deviation: Lead I positive, Leads II and III negative.

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5. P Waves:

• Presence and Pattern: Check if followed by a QRS complex and assess the shape.

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6. PR Interval:

• Normal: 120-200 ms.

• Prolonged or Shortened: Document any deviations.

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7. QRS Complex:

• Width: Narrow (<0.12 seconds) or broad (>0.12 seconds).

• Height: Small or tall; note hypertrophy.

• Morphology: Describe shapes and patterns.

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8. ST Segment:

• Elevation: >1 mm in limb leads or >2 mm in chest leads.

• Depression: ≥0.5 mm indicating possible ischemia.

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9. T Waves:

• Normal: Upright and following QRS.

• Abnormalities: Note if tall, inverted, flattened, or biphasic.

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10. U Waves:

• Presence and Size: Document if >0.5 mm and possible associated conditions.

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11. Additional Notes:

• Patient Symptoms and Clinical Context: Integrate with ECG findings for comprehensive interpretation.

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Summary

To interpret an ECG, start by verifying patient details and calibration settings. Measure the heart rate using large squares for regular rhythms or count complexes on a 10-second strip for irregular rhythms. Assess the heart rhythm for regularity and determine the cardiac axis by analyzing leads I, II, and III.

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Examine P waves for presence and pattern and measure the PR interval. Evaluate the QRS complex for width, height, and morphology. Check the ST segment for elevation or depression, and review T waves for abnormalities. Document any U waves and integrate findings with patient symptoms for accurate diagnosis.