Understanding ECG: Basics of Cardiac Conduction and Interpretation

The objectives of this session focus on gaining a comprehensive understanding of the heart's anatomy, the electrical conduction system, and the interpretation of electrocardiograms (ECGs). By the end of this session, students should be able to identify different aspects of heart function as represented on an ECG, recognize common rhythms, and understand the normal parameters associated with various cardiac cycles.

Effects of Conduction on the Heart

The heart's dynamic functions depend heavily on its conduction system:

Atrial contraction: Initiated at the Sinoatrial (SA) Node and progresses to the Atrioventricular (AV) Node.
Cardiac pause: A slight pause occurs from the AV Node to the Bundle of His, allowing the heart to fill effectively.
Ventricular contraction: Transmits impulses from the Bundle of His to the left and right bundle branches, facilitating the expulsion of blood from the heart.

It is essential to note that contraction equals the expulsion of blood while relaxation allows for passive filling of blood into the heart.

Important Terms

Familiarity with key terminology is critical in ECG study:

Lead: The visual output on the ECG printout, identified by notations such as I, II, III, etc.
View: Refers to perspectives such as inferior, anterior, lateral, or septal.
Electrode: A gel sticker used to establish contact with the skin.
Cable: Connects the monitor to the patient via stickers.

Limb Leads Placement

In performing an ECG, lead placement is critical for accurate results:

Right Arm Lead (RALA)
Left Arm Lead (LL)
Right Leg Lead (RL)

Lead II is often preferred for monitoring as it tends to provide the most consistent view of the heart’s electrical activity.

Chest (Precordial) Lead Placement

Precordial leads are placed on the chest to give additional insight into the heart's electrical activity, allowing for a comprehensive overview across the heart’s surface.

Basic ECG Components

Understanding the components of an ECG is crucial:

The ECG graph is divided into small blocks, where:

1 mm: Equals 0.04 seconds.
5 small blocks: Equals 0.20 seconds.

The primary components of an ECG and their normal limits include:

P wave: Represents atrial contraction; duration is less than 0.12 seconds (3 small blocks).
PR interval: Indicates a cardiac pause between 0.12 to 0.20 seconds (3-5 small blocks).
QRS complex: Indicates ventricular contraction; duration is less than 0.12 seconds (3 small blocks). It can be narrow or wide.
T wave: Represents ventricular relaxation; duration is approximately 0.16 seconds (4 small blocks).

ECG Components and Limits

One key interval to monitor is the QT interval:

Normal QTc value: QTc is prolonged if it exceeds 440ms in men or 460ms in women. A QTc interval greater than 500 is associated with an increased risk of torsades de pointes.
Short QTc: A QTc is considered abnormally short if it is less than 350ms.
Rule of thumb: A normal QT is generally less than half the preceding RR interval.
J point: The point where the ST segment ends and relates to whether it is elevated or depressed above or below the isometric line.

ECG Interpretation

The interpretation of an ECG can be conducted through a 4-point analysis involving:

Rate: Establishing whether the heart is beating fast or slow.
Rhythm: Determining if the rhythm is regular or irregular.
P waves: Noting if there is a 1:1 correlation with every QRS complex.
QRS Width: Estimating whether the QRS segment is wide or narrow.

Estimation of the heart rate can utilize the following methods:

300 rule: Calculate by dividing 300 by the number of big blocks between two R waves; best for regular rhythms.
6 second strip rule: Count the number of QRS complexes in 6 seconds (30 big blocks) and multiply by 10; suitable for irregular rhythms.

Looking for Additional Indicators

Further assessment includes checking for:

ST segment elevation: Indicates possible ischemic events.
ST segment depression: Could suggest myocardial ischemia or a previous infarction.

Abnormal Rhythms

The recognition of abnormal rhythms is essential in clinical practice:

Tachycardia: Can refer to a rapid heart rate.
Ectopic beats: Can be classified into:

Unifocal: Same origin of ectopic beats.
Multifocal: Originating from different sites.

Atrial Fibrillation: Characterized by the absence of discernible P waves and an irregular rhythm.
Atrial Flutter: No discernible P waves are observed, typically featuring a saw-tooth pattern.
Ventricular Fibrillation (VF): A critical condition needing immediate intervention.

AV (Heart) Blocks

Understanding heart blocks is crucial. Different degrees of AV block manifest distinct ECG patterns:

1st Degree AV Block: Associative prolongation of the PR interval with a 1:1 P wave for every QRS complex.
2nd Degree Type 1 AV Block: A progressively prolonged PR interval that leads to a dropped beat before the sequence repeats. P waves outnumber QRS complexes.
2nd Degree Type 2 AV Block: This type has normal PR intervals, but dropped beats occur sporadically after certain P waves.
3rd Degree AV Block: Signifies a complete dissociation between the P waves and QRS complexes, with some P waves possibly hidden within QRS complexes.

Visual Representation of AV Blocks

AV blocks can be remembered using a relatable story:

1st Degree: Like a husband arriving home late consistently at the same time.
2nd Degree Type 1: A husband who progressively comes home later until he fails to show.
2nd Degree Type 2: One who comes home on time until suddenly missing.
3rd Degree: Represents a divorce, indicating a complete separation of rhythm and association.

Counseling

Understanding ECGs is imperative for healthcare professionals to ensure proper patient care and management in cardiovascular conditions. Further education about interpreting ECG patterns is beneficial for enhancing clinical diagnosis and patient outcomes.

Resources for Further Learning

Students are encouraged to explore additional materials such as "How to Read an ECG" by Geeky Medics, and "QT Interval" from the LITFL ECG Library for a deeper understanding of electrocardiography.

Comprehensive Overview of ECG and Cardiac Conduction

Comprehensive Overview of ECG and Cardiac Conduction