Ischemic Heart Disease and Conduction Disorders

Introduction

  • Topic: Ischemic Heart Disease and Conduction Disorders
  • Author: F.A. Davis Company
  • Focus: Understanding heart structures, functions, diseases, and associated electrical and physiological events.

Anatomy and Physiology of the Heart

  • Basic Structure of the Heart

    • Size: Approximately the size of a fist
    • Location: Mediastinum
    • Parts:
    • Base: Upper part
    • Apex: Pointed end

  • Heart Wall Layers

    • Epicardium: Outer layer of the heart
    • Myocardium: Muscular middle layer responsible for contractions
    • Endocardium: Inner lining of the heart chambers

  • Chambers of the Heart

    • Right Atrium: Receives deoxygenated blood
    • Right Ventricle: Pumps blood to the lungs for oxygenation
    • Left Atrium: Receives oxygenated blood from pulmonary veins
    • Left Ventricle: Pumps oxygen-rich blood to systemic circulation

  • Circulation Overview

    • Pulmonary Circuit:
    • Low pressure system
    • Travels from Right Ventricle (RV) to Left Atrium (LA) for oxygenation in lungs
    • Systemic Circuit:
    • High pressure system
    • Travels from Left Ventricle (LV) to Right Atrium (RA) to deliver oxygen to body tissues

Coronary Circulation

  • Coronary Arteries: Supply blood to the heart muscle
    • Right Coronary Artery
    • Left Coronary Artery:
    • LAD: Left Anterior Descending artery
    • Exercise increases coronary collateral circulation

Cardiac Electrical Conduction System

  • Key Components:
    • Sinoatrial Node (SA Node): Pacemaker of the heart
    • Atrioventricular Node (AV Node): Delays conduction to allow atria to contract before ventricles
    • Bundle of His, Bundle Branches, Purkinje Fibers: Conduct electrical impulses throughout ventricles

Cardiac Cycle and Action Potential

  • Cardiac Muscle Action Potential Phases:

    • Phase 0: Fast sodium channels open; membrane potential rises from -90 mV to +20 mV
    • Phase 1: Peak; abrupt closure of fast sodium channels
    • Phase 2: Plateau phase; slow sodium-calcium channels open
    • Phase 3: Repolarization; potassium (K+) channels open
    • Phase 4: Resting potential; maintained by sodium-potassium pump

  • Refractory Period:

    • Absolute Refractory Period: No re-stimulation possible; prevents fatal arrhythmias
    • Protects heart during relaxation phase and allows time to fill with blood

Electrocardiogram (ECG) Interpretation

  • ECG Overview:
    • X-axis: Time
    • Y-axis: Millivolts
  • Key Waveforms:
    • P Wave: Atrial depolarization
    • QRS Complex: Ventricular depolarization
    • T Wave: Ventricular repolarization
    • 12-Lead ECG: Provides multiple views of heart's electrical activity
  • Measurement Details:
    • Standard ECG measurements include PR interval, QT interval, ST segment, and more.

Acute Coronary Syndrome (ACS)

  • Definition: A range of conditions associated with sudden, reduced blood flow to the heart

  • Types of ACS:

    • Unstable Angina
    • Myocardial Infarction (MI): Results from prolonged ischemia leading to cell death

  • Angina Pectoris:

    • Definition: Chest pain due to myocardial ischemia
    • Types:
    • Unstable Angina: New episode or increased severity, not relieved by rest, medical emergency
    • Chronic Stable Angina: Consistent pain, usually relieved by rest or nitroglycerin
    • Signs and Symptoms:
    • Crushing pain on the left side of the chest, Levine’s sign, pain radiating to shoulders, jaw, and arm

Unstable Angina

  • Incidence: Increasing in the US

  • Causes:

    • Myocardial ischemia due to blood clot (coronary thrombosis), atherosclerotic plaque, vasospasm, microvascular obstruction

  • Risk Factors: Arteriosclerosis and related conditions

  • Pathophysiology:

    • Oxygen demand exceeds supply, leads to lactic acid formation; different causes include vascular spasms and anemia

  • Diagnosis:

    • ECG, cardiac enzymes, high sensitivity cardiac troponin

  • Treatment:

    • Oxygen administration, nitrates, aspirin, and various other interventions to relieve symptoms and prevent MI progression

Surgical and Endovascular Interventions

  • Percutaneous Coronary Intervention (PCI)

    • Angioplasty: Catheter with balloon used to flatten plaque
    • Stenting: Keeps artery open through a drug-eluting stent

  • Coronary Artery Bypass Graft (CABG):

    • Creates alternate routes around occlusions using veins, traditionally with a cardiopulmonary bypass machine, now can be performed “off pump”

Other Angina Forms

  • Prinzmetal’s Angina:

    • Can occur at rest, not typically associated with typical CAD risk factors except smoking and inflammatory states
    • Treatment includes nitrates and calcium channel blockers

  • Chronic Stable Angina:

    • Recurring episodes of transient myocardial ischemia; treated with nitroglycerin, nicorandil, ranolazine, ivabradine

Myocardial Infarction (MI)

  • Causes: Prolonged ischemia leading to irreversible damage due to atherosclerosis, thrombus, or increased myocardial demand

  • Classification of MI:

    • Type 1: Spontaneous due to plaque or thrombus
    • Type 2: Oxygen deprivation, not caused by atherothrombotic occlusion
    • Type 3: Death before measurement of cardiac troponins
    • Type 4: MI due to stent insertion
    • Type 5: MI due to CABG

  • Signs and Symptoms of MI:

    • Crushing chest pain, radiating pain, sweating, nausea, anxiety, possibility of silent MI (no symptoms)

MI Diagnostics

  • ECG Indicators:

    • STEMI: ST segment elevation (transmural MI)
    • NSTEMI: Non-ST segment elevation (subendocardial MI)

  • Blood Tests:

    • CPK-MB: Rises within 4 hours, subsides in 3-4 days
    • cTnI: Cardiac protein that indicates myocardial necrosis, high sensitivity assays are used

  • Other Diagnostic Tests: Echocardiogram, radionuclide myocardial perfusion

MI Treatment

  • Medications:

    • Anti-platelet medications, beta-blockers, calcium channel antagonists, nitrates, anticoagulants, thrombolytics, morphine for pain relief

  • Complications of MI:

    • Dysrhythmias, papillary muscle rupture, thromboembolism, ventricular aneurysm, heart failure, shock

Cardiac Inflammation and Infection

  • Infectious Endocarditis:
    • Commonly bacterial; can damage heart valves and cause serious complications. Risk factors include prosthetic devices and IV drug use.
  • Myocarditis: Type of inflammatory cardiomyopathy, often caused by infections or autoimmune conditions, treatment includes managing workload on the heart.
  • Pericarditis: Inflammation of the pericardium presenting similarly to MI, may lead to cardiac tamponade or effusion

Conclusion

This comprehensive overview encompasses key aspects of ischemic heart disease and conduction disorders. Proper understanding involves knowledge of heart structure, electrical conduction, common diseases, diagnostic approaches, and treatment options. Each detail contributes crucially to diagnosing and managing conditions related to ischemic heart disease effectively.