Comprehensive Study Guide on the Pathophysiology of Ischemic Heart Disease and Myocardial Infarction and Pharmacology

Definition and Pathophysiology of Ischemic Heart Disease (IHD)

• Definition of Ischemic Heart Disease (IHD): A condition characterized by a mismatch where the volume of blood flowing to the heart fails to meet the metabolic demands required by the cardiac myocytes.
• Primary Cause: For most ischemic conditions, the cause is an obstruction of the coronary arteries, which reduces the total blood supply to the heart.
• Oxygen Supply and Respiration:
  • The reduction in blood flow inherently reduces the oxygen supply.
  • Under normal conditions, tissues undergo aerobic respiration.
  • In the presence of ischemia, there is an immediate shift toward anaerobic respiration.
• Metabolic Consequences:
  • Increased production of lactic acid.
  • Decrease in blood pH.
  • Accumulation of metabolites, which triggers the clinical sensation of angina pectoris.

Clinical Presentation and Symptomatology

• Angina Pectoris: Simply defined as chest pain.
• Qualitative Description: Patients frequently describe the sensation of ischemic heart disease as:
  • Crushing pain.
  • Squeezing pain.
  • Burning chest pain.
• Physical Signs: A classic diagnostic sign is when the patient clenches their fist over their chest.
• Pain Radiation Pattern: Pain typically originates on the left side of the chest and radiates to:
  • The left shoulder.
  • The left arm.
  • The throat.
  • The jaw.
  • The back of the body.
• Associated Sympathetic Responses: Due to the immediate activation of the sympathetic nervous system, patients may experience:
  • Sweating.
  • Tachycardia (fast heart rate).
  • Anxiety.
• Respiratory Distress: Increased sympathetic response raises left ventricular pressure within the pulmonary system, which impairs proper gaseous exchange, leading to shortness of breath (dyspnea).

Classification of Ischemic Heart Disease

• Coronary Artery Disease (CAD):
  • Stable Angina.
  • Variant Angina: Also referred to as "Zarian angina" in specific contexts.
• Acute Coronary Syndrome (ACS):
  • Non-ST elevated acute coronary syndrome (NSTE-ACS): Further divided into:
    • Unstable Angina.
    • Non-ST elevated myocardial infarction (NSTEMI).
  • ST-elevated myocardial infarction (STEMI).

Pathophysiology of Coronary Artery Disease (CAD)

• Stable Angina:
  • Underlying Cause: Atherosclerosis in the coronary arteries restricting blood supply to the cardiac myocytes.
  • Plaque Characteristics: Characterized by an atherosclerotic plaque with a thick, stable fibrous cap. This cap is robust and has a low risk of rupturing.
  • Mechanism: The physical presence of the plaque narrows the arterial lumen. Blood flow cannot increase proportionally to meet increased myocardial oxygen demand.
  • Clinical Triggers: Symptoms are not present at rest. Episodes are precipitated by physical exertion (exercise) or stress, as the heart must pump harder.
  • Vascular Dynamics: The affected arteries are thick and stiff, leaving little room for vasodilation.
  • Endothelial Dysfunction:
    • Vasoconstriction: In a healthy state, physical exertion triggers the sympathetic nervous system. As a compensatory mechanism, endothelin triggers the release of nitric oxide ($NO$) for vasodilation. In stable angina, the damaged endothelium cannot release sufficient $NO$, allowing sympathetic vasoconstriction to predominate.
    • Thrombotic Prevention Failure: Normally, the endothelium releases anticoagulant and antiplatelet factors like prostacyclin to inhibit platelet aggregation. Damaged arteries fail to release prostacyclin, leading to increased platelet aggregation potential.
  • Predictability: The clinical presentation is stable and predictable. Pain is short-lasting, reversible, and shows no changes in frequency or severity for at least two months.
• Variant Angina (Zarian Angina):
  • Mechanism: Unlike other forms of IHD, this is not characterized by atherosclerosis. It is caused by a spontaneous spasm of the coronary artery producing vasoconstriction.
  • Triggers: Stress and emotional disturbances are known precipitating factors.
  • Temporal Pattern: Occurs predominantly at night or when the patient is at rest.
  • Prevalence: Considered relatively rare.

Pathophysiology of Acute Coronary Syndrome (ACS)

• Mechanism of Blockage: Occurs when a pre-existing plaque ruptures, blocking the passageway. This leads to increased vasoconstriction, platelet aggregation, and thrombus formation.
• Unstable Angina:
  • Involves a ruptured plaque where platelets have begun to aggregate.
  • The degree of occlusion is higher than in stable angina.
  • The thrombi are rich in platelets with minimal coagulation factors.
  • Because occlusion is partial (not fully occluded), oxygen supply is limited but present; therefore, necrosis of the myocardium is not likely to occur.
  • Symptoms: More frequent and severe than stable angina; pain may persist even at rest.
• Myocardial Infarction (MI):
  • Defined as a "heart attack" where complete or prolonged occlusion leads to the death of cardiac myocytes (myocyte necrosis).
  • NSTEMI: Involves partial occlusion and results in partial-thickness damage to the ventricles. There is no ST-segment elevation on the ECG.
  • STEMI: Involves complete occlusion and results in full-thickness damage to the ventricles. This significantly affects cardiac conduction, resulting in a raised ST segment.

Electrocardiogram (ECG) Interpretation and Cardiac Biomarkers

• Electrocardiogram (ECG) Components:
  • P Wave: Corresponds to atrial depolarization.
  • QRS Complex: Corresponds to the depolarization of the ventricles.
  • T Wave: Corresponds to the repolarization of the ventricles.
  • ST Segment: Represents the interval between the end of ventricular depolarization and the beginning of repolarization.
  • ST Interval: Represents the time from the end of ventricular depolarization to the end of ventricular repolarization.
• Biomarkers and Clinical Indicators:
  • Cardiac Troponins (Troponin I and Troponin T): Released into systemic circulation upon cardiomyocyte damage. Troponin T is the primary marker used via blood tests to confirm a heart attack.
  • Creatine Kinase (CK): Indicative of muscle-related injury; the myocardium-specific form (Creatinine Kinase myocardial bound) is used to identify cardiac muscle injury.
  • Lactate Dehydrogenase (LDH): Leaks into the systemic circulation and interstitial fluid following cell damage.
• Troponin Summary by Condition:
  • Stable Angina: Normal.
  • Unstable Angina: Normal.
  • NSTEMI: Elevated.
  • STEMI: Elevated.

Progression and Repair of Myocardial Infarction

• Pathological Timeline - Acute Events (< 2 Minutes):
  • Ischemia causes a switch to anaerobic respiration, leading to a drop in pH.
  • Energy Failure: Cellular energy levels decrease, causing the failure of the sodium potassium ATPase ($Na^+ K^+ ATPase$) pump.
  • Pump Dynamics: In a functional pump, sodium is pumped out and potassium is pumped in. In failure, sodium ions ($Na^+$) accumulate inside the myocytes.
  • Cellular Oedema: Water follows the sodium concentration gradient, leading to intracellular accumulation of water and myocardial edema.
• Pathological Timeline - Irreversible Injury (> 20 Minutes):
  • Irreversible injury and necrosis occur.
  • Proteins and enzymes (Troponins I and T, CK, LDH) leak into the interstitial fluid and circulation due to damaged cell membranes.
  • Leakage of intracellular proteins and rise in interstitial fluid manifests as oedema of the myocardium after approximately $4\,h$.
• Repair Mechanism Phases:
  1. Inflammatory Phase (0-4 Days): Heightened response characterized by cardiomyocyte death, neutrophil infiltration, and the release of pro-inflammatory mediators.
  2. Proliferative/Healing Phase (0-3/4 Weeks): Angiogenesis occurs (new blood vessels form). Collagen synthesis and the regeneration of new cardiac myocytes (referred to as "marocytes" in the transcript) take place.
  3. Formation Phase (2/3-4/6 Weeks): Formation of mature scar tissue (fibrosis).
  • Note: Scar tissue weakens the ventricles and does not restore normal function.

Complications of Myocardial Infarction

• Tissue Necrosis:
  • Persistent necrosis leads to the heart's inability to pump, resulting in congestive heart failure.
  • Cardiac Tamponade: Occurs following ventricular wall rupture when proteins and enzymes leak into the interstitial fluid within the myocardium.
  • Papillary Muscle Infarction: Leads to mitral regurgitation.
• Electrical Instability:
  • Damage to conduction pathways or changes in ion permeability (related to the $Na^+ K^+ ATPase$ pump failure) alter electrical responses.
  • Results in irregular heartbeats (arrhythmias).
• Impaired Contractility:
  • Difficulty in contracting or relaxing leading to blood stasis.
  • Thromboembolism: Stasis promotes ventricular thrombus formation. If a thrombus breaks off, the resulting emboli can cause a stroke.
  • Cardiogenic Shock: A medical emergency where vital organs (brain, kidneys) suffer a lack of blood supply due to pump failure, resulting in hypotension and reduced coronary perfusion.
• Pericardial Inflammation:
  • The post-MI inflammatory cascade can cause excessive infiltration of mediators into the outermost layer of the heart (the pericardium).
  • This condition is known as pericarditis.