CB [38] NGU cardiac excitaion introduction to ECG 2024-2025

Introduction

This section introduces the fundamental concepts of cardiac excitation as presented by Dr. Sandra Younan, emphasizing the importance of understanding the heart's electrical system in maintaining effective circulation.

Electrical and Conducting System of the Heart

The heart's electrical wave initiates from the Sinoatrial Node (SA Node), which acts as the primary pacemaker conducting impulses that regulate heart rhythm. The wave subsequently spreads throughout the cardiac conducting system:

• Atrioventricular Node (AV Node): This node plays a critical role in conducting impulses from the atria to the ventricles, ensuring proper timing between atrial and ventricular contractions.

• Bundle of His: This pathway leads from the AV Node and bifurcates into the Right and Left Bundle Branches, facilitating the conduction of impulses to the ventricles.

• Purkinje Fibers: These fibers distribute the electrical impulse throughout the ventricular walls, ensuring coordinated contraction. Identifying pacemaker cells is essential, as they are responsible for the heart's automatic pacing.

Pacemaker Action Potential

The pacemaker action potential is a unique type of electrical activity found in the Sinoatrial Node (SAN) and Atrioventricular Node (AVN). The slope of this action potential is crucial, as it dictates the heart's automaticity (the ability to fire spontaneously) and rhythmicity (the regularity of the firing rate). The flow of cations, specifically sodium and calcium ions, is responsible for generating this action potential. The effects of autonomic nerve stimulation and associated neurotransmitters can significantly alter the slope of the pacemaker potential, illustrating how the autonomic nervous system regulates heart rate (as discussed in CB [006]).

Conduction of Cardiac Action Potential in Cardiac Muscle Cells

Intercalated discs within cardiac muscle sarcolemma are integral components comprising gap junctions and desmosomes. Gap junctions allow the direct flow of depolarizing current between adjacent cardiac muscle cells, resulting in a rapid transmission of the action potential. This swift conduction is essential for synchronized contraction across the entire heart muscle.

Conduction Speed Through the Heart

There is an impulse conduction delay at the AV Node, typically ranging from 0.02 to 0.05 m/s, due to a lower density of gap junctions. This AV delay enables the atria to contract fully and push blood into the ventricles prior to ventricular contraction, optimizing heart efficiency.

Direction of Spread of Excitation Wave

The electrical wave travels through the ventricular walls in a specific manner:

1. From inside to outside of the wall: This ensures that the inner ventricular muscles contract first, gradually spreading the contraction wave outward.

2. From right ventricle to left ventricle: This directional spread helps in the coordinated pumping action.

3. From the apex to the base of the heart: This route provides effective expulsion of blood during ventricular contraction.

4. In the septum: The wave spreads from the left side to the right side, which is crucial for maintaining correct electrical and mechanical timing between the ventricles.

Recording Cardiac Action Potential

The electrical activities of the heart are transmitted through body fluids and can be recorded using body surface electrodes as an Electrocardiogram (ECG). The ECG traces the sequence of atrial and ventricular depolarization and repolarization, which are vital for diagnosing various cardiac conditions. Both limb and chest leads are employed to capture a comprehensive view of heart activity.

Placement of Limb and Chest Leads

• Limb Leads: These include both bipolar and unipolar leads that record ECG signals from the vertical plane of the body.

• Chest Leads: These leads record from a transverse plane (antero-posterior), allowing for detailed monitoring of cardiac electrical activity from the heart's anterior to posterior surfaces.

Bipolar Limb Leads and Einthoven’s Triangle

Bipolar limb leads form an equilateral triangle, known as Einthoven’s triangle, with the heart at its center, facilitating potential difference measurement:

• Lead I: Right arm (-ve) & left arm (+ve)

• Lead II: Right arm (-ve) & left leg (+ve)

• Lead III: Left arm (-ve) & left leg (+ve)

Augmented Limb Leads

The unipolar augmented limb leads are focused within each limb, measuring potential differences with:

• aVR: Positive electrode on the right arm.

• aVL: Positive electrode on the left arm.

• aVF: Positive electrode on the left foot.

Placement of ECG Chest Leads

Accurate positioning of chest leads is critical for reliable ECG readings:

• V1: Located at the 4th intercostal space at the right sternal border.

• V2: Placed at the 4th intercostal space at the left sternal border.

• V3: Positioned between leads V2 and V4.

• V4: Found at the 5th left intercostal space in the midclavicular line.

• V5: Horizontal to V4 in the anterior axillary line.

• V6: Placed horizontally to V4 and V5 in the midaxillary line.

Observing Depolarization and Repolarization of Cardiac Ventricle

Under resting conditions, no potential difference is observed. Upon stimulation, depolarization occurs, resulting in an upward deflection on the ECG recording. If the initial site of depolarization is also the first to repolarize, this will lead to an opposite deflection being recorded as the current moves away from the lead.

Shape of Recorded Wave

The electrode position significantly influences the shape of the recorded wave, affecting the overall interpretation of the ECG.

Normal ECG Waves, Intervals, & Segments

Essential components of a normal ECG include:

• PR segment: Reflects conduction through the AV Node.

• PR interval: Duration from the beginning of atrial depolarization to the beginning of ventricular depolarization.

• ST segment: Represents the period between ventricular depolarization and repolarization.

• QT interval: Duration of ventricular action potential.

Normal ECG of a Single Heart Beat

This illustrates key events including:

• Depolarization of the ventricular septum, occurring from left to right.

• Depolarization wave spreading from the apex of the heart upwards towards the base.

• Repolarization occurring from epicardium to endocardium. Each small square on the ECG grid represents 0.04 seconds in duration, while 10 vertical small squares equal 1 mV.

Hexaxial Reference of the Heart

Utilizing the Einthoven triangle allows prediction of the expected QRS complex in limb leads, aiding in the assessment of cardiac conditions.

Recording from Standard Bipolar Limb Leads

This section provides a visual representation of recordings made using bipolar limb leads, demonstrating standard practices in ECG measurement.

Hexaxial Reference of the Heart (Enhanced)

This section further reviews the placement of unipolar leads (aVR, aVL, aVF) in relation to the hexaxial reference for accurate assessment of QRS complex morphology.

Recording from Augmented Limb Leads

Here we discuss the setups and details necessary for effectively recording augmented limb leads, ensuring comprehensive cardiac assessment.

Overview of Chest Leads

A detailed overview of the positioning of all chest leads and their corresponding views is essential for accurate cardiac monitoring.

Recording from Chest Leads

• V1 and V2: Primarily view the right ventricle.

• V3 and V4: Capture the septal region.

• V5 and V6: Observe the lateral left ventricle.

• Consideration of depolarization patterns is necessary, especially noting negative Q waves in V5 and V6 versus positive R waves in V1, reflecting normal R wave progression from V1 through V6.

Electrical Axis of the Heart

The mean ventricular wave of depolarization is referred to as the cardiac axis, with a normal axis range for QRS located between -30° and +90°.

Axis Deviation

• Right axis deviation: Defined as an axis greater than +90°.

• Left axis deviation: Indicates an axis less than -30°. Utilizing limb leads and quadrant methods aids in the analysis of axis deviations, and understanding the potential causes for these deviations is vital in cardiology practice.

Determining Cardiac Axis

This section illustrates various cases of normal and deviated cardiac axes through practical examples in limb leads.

Determining Cardiac Axis (Continued)

Continued analysis here emphasizes the contrasting features of normal and deviated cardiac axes, enhancing diagnostic skills through practical examples.

Disorders of Conduction: AV Block

• 1st Degree AV Block: Characterized by a delay of signals from atria to ventricles, indicated by a prolonged PR interval.

• 2nd Degree AV Block: A marked delay exists where some QRS complexes are non-conducted. This is further categorized into two types:

  • Mobitz type I (Wenckebach): Features progressive PR prolongation prior to a dropped QRS.

  • Mobitz type II: Demonstrates a more severe and less predictable pattern.

• 3rd Degree (Complete Heart Block): No impulses reach the ventricles, leading to dissociation between P waves and QRS complexes.

Algorithm for Diagnosis of Heart Block

This chart differentiates between various types of heart block based on measurements of PR intervals and R-R intervals, outlining diagnostic pathways.

Bundle Branch Block

Close examination of QRS duration and characteristics is crucial for identifying right and left bundle branch blocks, with notable changes observed in V1 and V6 leads indicative of each type.

Ectopic Rhythm

• Supraventricular extrasystole: Characterized by premature beats originating above the ventricular level.

• Ventricular extrasystole: Represents early ventricular beats often lacking a preceding P wave. An analysis of potential causes is essential in both normal and pathological hearts.

Abnormal Cardiac Rhythms

This section provides an overview of various cardiac rhythm disorders including:

• Atrial fibrillation: An irregular and often rapid heart rate that can lead to blood clots.

• Atrial flutter: A rapid but regular rhythm often characterized by a “sawtooth” pattern.

• Multifocal atrial tachycardia: A condition marked by rapid heartbeats originating from multiple ectopic foci.

• Ventricular fibrillation: A life-threatening arrhythmia leading to ineffective quivering of the heart.

References

A comprehensive list of notable texts and articles on cardiovascular physiology, ECG interpretation, and clinical applications, providing essential background for further study.

Closing Remark

A sincere thank you note to readers for their engagement with the material presented.

Final Statement

Encouragement for unbounded thinking, emphasizing continuous learning and exploration in the field of cardiology.