ECG Basics and Cardiac Conduction System

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Questions and Answers

What does a positive deflection on an ECG indicate?

Electrical activity moves towards the positive electrode (correct)

Electrical activity moves away from the positive electrode

Electrical activity is absent

Electrical activity is moving towards the negative electrode

What characterizes the PR Interval on an ECG?

Indicates ventricular depolarization

Is isoelectric indicating no net electrical movement (correct)

Shows the direction of the Q Wave

Represents the rapid conduction through the AV node

Which condition represents a complete block of conduction through the AV node?

Prolonged PR Interval

2nd Degree Heart Block

1st Degree Heart Block

3rd Degree Heart Block (correct)

In the cardiac conduction system, which node is responsible for generating electrical activity?

SA Node Signup and view all the answers

What is indicated by a flat line on an ECG?

Electrical activity moving perpendicularly to electrodes Signup and view all the answers

What does the Q Wave represent in ECG interpretation?

Septal depolarization Signup and view all the answers

Which part of the heart acts as a slow conductor of electrical potentials?

AV Node Signup and view all the answers

Which lead is described as using a positive electrode placed at the apex of the heart?

Lead 2 Signup and view all the answers

What does a prolonged PR Interval suggest?

Blockage in the cardiac conduction system Signup and view all the answers

What does the mean vector of atrial depolarization point towards?

The AV node Signup and view all the answers

What characteristic of the left ventricle contributes to generating a larger vector for depolarization compared to the right ventricle?

It has a thicker myocardium. Signup and view all the answers

Which component of the ECG represents the conduction period where the electrical potential moves slowly through the AV node?

PR interval Signup and view all the answers

How do augmented unipolar limb leads provide a view of the heart's electrical activity?

Through placement of two negative electrodes and one positive electrode. Signup and view all the answers

What deflection does the mean QRS vector produce on the ECG due to the thicker left ventricle?

Large positive deflection Signup and view all the answers

What crucial cardiac abnormalities can be identified by leads II and III in the event of a myocardial infarction?

Inferior MI Signup and view all the answers

Which leads provide a horizontal view of the heart's electrical activity?

Precordial chest leads Signup and view all the answers

Which lead looks at the heart from the right and slightly superior angle?

AVR Signup and view all the answers

In the context of the QRS complex, what does the S wave signify?

Depolarization at the base of the ventricles Signup and view all the answers

What anatomical feature primarily determines the resultant vector of ventricular depolarization?

Size and thickness of the left ventricle Signup and view all the answers

What is the primary focus of leads V5 and V6 in an ECG?

Electrical activity of the left lateral wall Signup and view all the answers

Study Notes

ECG Basics

An ECG is a graphic representation of the heart's electrical activity.
Electrodes detect electrical activity, displaying it as positive deflections, negative deflections, or flat lines.
Positive Deflection: Electrical activity moves towards the positive electrode.
Negative Deflection: Electrical activity moves away from the positive electrode.
Flat Line: Indicates very slow or perpendicular electrical activity relative to the electrode axis.
Lead II: Uses a positive electrode at the heart's apex and a negative electrode at the base. This lead is used for the explanations provided.

Cardiac Conduction System

SA Node: Initiates atrial depolarization, moving toward the AV node.
The mean vector of atrial depolarization points towards the AV node.
P Wave: Represents atrial depolarization; it's typically positive in Lead II due to the electrical activity moving toward the positive electrode.
AV Node: A slow electrical conductor, causing a ~0.1-second delay before the impulse reaches the bundle of His.
PR Interval: The time taken for the impulse to travel from the SA node to the ventricles, appearing isoelectric (flat) as the impulse is slowed in the AV node.
Prolonged PR Interval: Suggests a conduction block.
- 1st Degree Heart Block: Delays AV nodal conduction.
- 2nd Degree Heart Block: AV node occasionally fails to conduct impulses.
- 3rd Degree Heart Block: Complete AV nodal block.

Ventricular Depolarization

Bundle of His: Carries action potentials to the ventricles.
Interventricular Septum: Left bundle branch depolarizes the septum, resulting in a negative deflection in Lead II (activity moving away from the positive electrode).
Q Wave: Represents septal depolarization.

Ventricular Depolarization: The QRS Complex

Ventricular Depolarization: The impulse spreads through the right and left bundle branches to the Purkinje fibers, causing ventricular depolarization.
Right Ventricle: Generates a smaller depolarization vector due to its smaller size.
Left Ventricle: Generates a larger depolarization vector due to its thicker myocardium.
Resultant Vector: The combined vector of right and left ventricular depolarization is downward and slightly leftward. This produces a positive deflection in Lead II (activity moving towards the positive electrode).
Normal QRS Complex: Represents ventricular depolarization.

Mean QRS Vector

The mean QRS vector is the combined left and right ventricular depolarization vectors.
Its direction is downward and slightly leftward due to the larger left ventricular mass.
This produces a large positive deflection on the ECG.

ECG Wave Components

P wave: SA node to AV node depolarization.
PR interval: AV node conduction time.
Q wave: Septal depolarization.
R wave: Ventricular depolarization at the apex.
S wave: Ventricular depolarization at the base.
ST segment: Time between ventricular depolarization and repolarization.
T wave: Ventricular repolarization; typically a positive deflection as negative charges move away from the positive electrode.

ECG Leads

A standard ECG has 12 leads.
Classified into bipolar limb leads, augmented unipolar limb leads, and precordial (chest) leads.
Bipolar limb leads: Formed by two electrodes (one positive, one negative).
Augmented unipolar limb leads: Derived from Einthoven's triangle (using Wilson's central terminal and Goldberger's modifications).
Bipolar and augmented unipolar limb leads: Provide a frontal view of the heart.
Chest leads: Provide a horizontal (transverse) view of the heart.

Bipolar Limb Leads

Three bipolar limb leads: I, II, and III.
Lead I: Negative electrode on right arm, positive electrode on left arm.
Lead II: Negative electrode on right arm, positive electrode on left leg.
Lead III: Negative electrode on left arm, positive electrode on left leg.

Einthoven's Triangle and Bipolar Limb Leads

Einthoven's Triangle: Formed by connecting electrodes on right arm, left arm, and left leg.
Bipolar Limb Leads: Measure electrical potential differences between two electrodes.
Lead I: Provides a left-side view of the heart.
Lead II: Shows the heart's electrical activity from the inferior view (bottom).
**Lead III:**Also showing the electrical activity from the inferior view (bottom).
Einthoven's Law: Lead I + Lead III = Lead II.
Importance: Leads provide different views, aiding in abnormality detection.
Myocardial Infarction (MI): Leads II and III are important for evaluating inferior MIs, while Lead I is used for left lateral MIs.

Augmented Unipolar Limb Leads

Augmented Unipolar Limb Leads: Electrodes on limbs, use negative charges at 2 points, and a positive electrode at the third.
AVR: Negative electrodes on left arm & leg, positive on right arm - Rightward & slightly upward view.
AVL: Negative on right arm & leg, positive on left arm - Left-lateral view.
AVF: Negative on right & left arm, positive on left leg - Inferior view.

Chest Leads

Placement: V1-V6 (specific intercostal spaces and positions)
Unipolar Leads: Only positive electrode used, picking up electrical activity from chest wall.
V1 and V2: Measure electrical activity from the interventricular septum.
V3 and V4: Measure activity from the anterior ventricular wall.
V5 and V6: Measure left lateral ventricular wall activity.
Posterior MI: ST segment depression and T-wave inversion in V1 and V2 may indicate a posterior MI. Posterior leads (V7-V9) confirm this.

ECG Leads: An Overview

Bipolar limb leads: Frontal view, showing electrical activity from left, inferior, and bottom directions.
Augmented unipolar limb leads: Frontal view, showing right, left, and bottom directions.
Chest leads: Horizontal view, showing activity from the septum, anterior wall, and left lateral wall.
Combined information from all leads gives a comprehensive view of the heart's electrical activity.

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Description

This quiz covers the fundamental concepts of ECG, including the interpretation of electrical activity of the heart through electrodes. It also explores the cardiac conduction system, detailing the roles of the SA node and the P Wave in heart function.