Ischemic Heart Disease and Conduction Disorders

Ischemic Heart Disease and Conduction Disorders

Overview of the Heart

  • Size and Location
    • Approximately the size of a fist.
    • Located in the mediastinum.
  • Anatomical Structure
    • Base: Upper part of the heart.
    • Apex: The pointed end.

Acute Coronary Syndrome (ACS)

  • Definition: ACS includes myocardial infarction (MI) and/or unstable angina.
  • Cause: Results from ischemia, which leads to cell death in myocardial infarction.

Myocardial Infarction (MI)

  • Mechanism: Prolonged ischemia causes cellular necrosis.

Heart Structure

  • Tissue Layers:
    • Epicardium: Outermost layer.
    • Myocardium: Muscular layer that facilitates contraction.
    • Endocardium: Innermost lining of the heart chambers.
  • Coronary Circulation:
    • Right Coronary Artery: Supplies blood to the right side of the heart.
    • Left Coronary Artery:
    • Left Anterior Descending (LAD): Supplies blood to the front and bottom of the left ventricle and the front of the septum.
  • Exercise Impact: Increases the formation of coronary collateral vessels.

Heart Wall Composition

  • Layers:
    • Pericardium: Protective sac around the heart.
    • Epicardium: Outer layer.
    • Myocardium: Middle muscular layer.
    • Endocardium: Inner layer lining the chambers.

Coronary Arteries Overview

  • Right Coronary Artery: Supplies right side of the heart.
  • Left Main Coronary Artery:
    • Circumflex Artery: Supplies left atrium and part of the left ventricle.
    • Left Anterior Descending (LAD): Supplies the anterior wall of the heart.

Cardiac Chambers and Circulation Overview

  • Four Chambers of the Heart:
    • Right Atrium: Receives deoxygenated blood from superior and inferior vena cava and coronary sinus.
    • Right Ventricle: Pumps deoxygenated blood to the lungs via pulmonary artery.
    • Left Atrium: Receives oxygenated blood from pulmonary veins.
    • Left Ventricle: Pumps oxygenated blood to the body through the aorta.
  • Circuits:
    • Pulmonary Circuit: Low pressure; deoxygenated blood returns to the left atrium.
    • Systemic Circuit: High pressure; oxygenated blood delivered to body tissues.

Blood Pressure Types

  • Systolic Pressure (SBP): Blood pressure during contraction (systole).
  • Diastolic Pressure (DBP): Blood pressure during relaxation (diastole).
    • Example: Normal BP reading of 120/80.

Cardiac Conduction System Overview

  • Components:
    • Sinoatrial Node (SA Node): Primary pacemaker of the heart.
    • Atrioventricular Node (AV Node): Relay point between atria and ventricles.
    • Bundle of His: Pathway for electrical signals from the AV node.
    • Bundle Branches: Calculate signal direction in the ventricles.
    • Purkinje Fibers: Distribute electrical impulse throughout the ventricles.

Cardiac Muscle Action Potential

  • Phases of the Cardiac Action Potential:
    • Phase 0: Rapid depolarization; fast sodium (Na+) channels open; potential rises from -90 mV to +20 mV.
    • Phase 1: Peak; abrupt closure of fast sodium channels.
    • Phase 2: Plateau; calcium (Ca++) channels open and Ca++ enters to enable cardiac muscle contraction.
    • Phase 3: Repolarization; potassium (K+) channels open, allowing K+ to move out of cells.
    • Phase 4: Resting potential; Na+/K+ pump re-establishes the ion concentration gradient.

Refractory Period

  • Absolute Refractory Period:
    • Cells cannot be restimulated.
    • Corresponds to phases 0, 1, 2, and a part of phase 3.
    • Ensures the heart can relax and fill without being reactivated, protecting against arrhythmias.

Dysrhythmias (Arrhythmias)

  • Disruption of Electrical Signals:
    • Supraventricular: Affects SA node and atria.
    • Ventricular: Impacts bundle of His and Purkinje fibers.
  • Types: Tachyarrhythmias (fast heart rate) and bradyarrhythmias (slow heart rate).

ECG Overview

  • Electrocardiogram (ECG) Function: Measures voltage changes on the skin due to cardiac action potential propagation.

  • Ischemic Events Detection:

    • Slowed action potential propagation due to ischemia.
    • Infarction blocks propagation; localized changes observable in the ECG.

  • Normal ECG Components:

    • P Wave: Atrial depolarization.
    • QRS Complex: Ventricular depolarization.
    • T Wave: Ventricular repolarization.

  • 12-Lead ECG: Provides multiple views of heart's electrical activity.

Coronary Artery Disease (CAD)

  • Definition: CAD is a vascular disorder characterized by atherosclerosis and/or blood clots causing occlusion of coronary arteries.
  • Consequences:
    • Myocardial Ischemia: Localized temporary insufficiency of blood supply, primarily due to increased workload leading to stable angina.
    • Acute Coronary Syndromes (ACS): Includes unstable angina and myocardial infarction leading to irreversible tissue damage.

Myocardial Ischemia Details

  • Occurs When: Local O2 demand exceeds delivery due to factors like atherosclerosis.
  • Symptoms: Chest pain due to hypoxia; recoverable if oxygen deficit is less than 20 minutes.

Angina Pectoris

  • Stable Angina: Occurs with middle CAD due to narrowed arteries that limit blood flow, triggered during exertion.
  • Symptoms: Chest pain, crushing sensation, radiating pain to shoulder and arm; may feel like indigestion.
  • Risk Factors: Smoking, diabetes mellitus, hypercholesterolemia, metabolic syndrome.