Hypoxic Damage to the Heart and Ischemic Heart Disease

Definition and Core Mechanisms of Ischemic Heart Disease

  • Definition of Ischemic Heart Disease (IHD): Also referred to as Coronary Artery Disease (CAD) or Coronary Heart Disease (CHD). It is a group of pathophysiologically related disorders caused by myocardial ischemia.

  • The Supply-Demand Imbalance: Myocardial ischemia occurs when there is an imbalance between:

    • Cardiac Blood Supply (Perfusion): The volume of oxygenated blood reaching the heart muscle.

    • Cardiac Demand: The metabolic requirement of the heart for oxygenated blood.

Causes of Hypoxic Damage in the Heart

  • Mechanisms of Decreased Supply:

    • Obstruction to Coronary Blood Flow: This is most commonly due to atherosclerotic lesions (90%90\% of cases), but can also be caused by vasospasm or vasculitis.

    • Diminished Blood Volume: Resulting from conditions such as hypotension or shock.

    • Diminished Oxygenation: Caused by underlying conditions like pneumonia or Congestive Heart Failure (CHF).

    • Diminished Oxygen-Carrying Capacity: Resulting from anemia or carbon monoxide poisoning.

  • Mechanisms of Increased Demand:

    1. Thyrotoxicosis: Excessive thyroid hormone levels increasing metabolic rate.

    2. Tachycardia: Increased heart rate reducing the time available for diastolic filling/perfusion and increasing work.

    3. Marked Myocardial Hypertrophy: Often caused by Aortic Stenosis or sustained hypertension, where the thickened muscle mass outstrips its blood supply.

Epidemiological Statistics and Trends

  • Malaysia (2024 Statistics):

    • Principal Cause of Death: Ischaemic heart diseases accounted for 15.1%15.1\% of deaths in 2023, ranking second only to Pneumonia (15.2%15.2\%).

    • Age Trends: The average age of Malaysians suffering a heart attack is 58.5 years old58.5\,\text{years old}. Approximately one in four heart attack patients are younger than 50 years old50\,\text{years old}.

    • Sex Distribution: Males comprise nearly four in five (78.8%78.8\%) of hospital admissions for heart attacks.

    • Ethnic and Age Groups: IHD is a leading killer across Malay, Chinese, Indian, and other Bumiputera groups. It is the principal cause of death for the 41−59 years41-59\,\text{years} age group (20.6%20.6\%).

  • United Kingdom (2023 Statistics):

    • Rank: CHD is one of the leading causes of death and is the leading cause of premature death (before the age of 75 years75\,\text{years}).

    • Mortality Rate: Responsible for approximately 66,00066,000 deaths annually—averaging 180180 people per day or one death every 8 minutes8\,\text{minutes}.

    • Demographics: One in eight men and one in fourteen women die from CHD. It kills more than twice as many women in the UK as breast cancer.

    • Hospital Admissions: Someone is admitted to a UK hospital due to a heart attack every 5 minutes5\,\text{minutes}.

  • Global Burden (GBD 2021 Estimates):

    • Men: CHD is the number one killer (5.0 million5.0\,\text{million} deaths).

    • Women: CHD is the number one killer (4.0 million4.0\,\text{million} deaths).

    • Total: CHD accounts for 9.0 million9.0\,\text{million} deaths globally. Aside from the peak pandemic years of 2020-2021, CHD remains the single biggest killer worldwide.

Pathophysiology of Myocardial Ischemia and Infarction

  1. Obstruction: Usually initiated by atheroma obstructing blood flow, leading to decreased supply (ischemia).

  2. Hypoxia: Reduced oxygen leads to a shift from aerobic to anaerobic respiration.

  3. Acute Cellular Changes: Includes cell swelling, activation of cellular enzymes, and membrane/nuclear damage.

  4. Reperfusion Window:

    • If reperfusion occurs within 20−40 mins20-40\,\text{mins}, the damage is generally reversible.

    • If no reperfusion occurs, the damage becomes irreversible, leading to cell death (necrosis).

  5. Pain Mechanism: Adenosine is released during ischemic stress, which activates pain receptors, causing the patient to present with chest pain.

  6. Resolution: Dead tissue is eventually replaced by fibrous tissue (scarring).

Classification and Characteristics of Angina Pectoris

  • Definition: Angina pectoris (from "angere" meaning strangling and "pectus" meaning chest) is characterized by substernal or precordial chest discomfort caused by transient myocardial ischemia (15 seconds15\,\text{seconds} to 20 minutes20\,\text{minutes}) without myocyte necrosis.

  • Typical or Stable Angina:

    • Nature: The most common form; involves squeezing, gripping, heavy, or burning sensations radiating to the left arm or jaw.

    • Pathophysiology: Caused by fixed stenosis (≥70%\ge 70\% diameter stenosis of a major epicardial artery or ≥50%\ge 50\% in the left main artery).

    • Triggers: Physical activity, emotional stress, or exposure to cold.

    • Relief: Symptoms are relieved by rest (reducing demand) or vasodilators (like GTN spray) within 5 min5\,\text{min}.

  • Vasospastic (Variant/Prinzmetal’s) Angina:

    • Presentation: Episodes occur at rest, often between midnight and early morning when vagal tone is higher.

    • Pathophysiology: Caused by coronary artery spasms. Potential mechanisms include vascular smooth muscle hyper-reactivity, magnesium deficiency, cocaine use, or smoking.

    • Response: Promptly responds to Nitroglycerins.

  • Microvascular Angina (Cardiac Syndrome X):

    • Presentation: Chest pain similar to stable angina but can also occur at rest.

    • Diagnostic Paradox: Coronary angiograms appear normal (no significant epicardial atherosclerosis or spasm).

    • Mechanism: Dysfunction in the smallest blood vessels (microvasculature) that cannot dilate properly under stress or go into spasm.

  • Unstable Angina (Crescendo Angina):

    • Nature: Part of Acute Coronary Syndrome (ACS). It is a medical emergency.

    • Clinical Features: Increasingly frequent, prolonged (> 20\,\text{min}), and severe chest pain occurring at rest or with minimal exertion.

    • Pathophysiology: Plaque rupture with superimposed thrombosis; often > 90\% of the vascular area is obstructed. It is a warning sign of impending Myocardial Infarction (preinfarction angina).

Acute Coronary Syndrome and Myocardial Infarction (MI)

  • Definition of MI: A localized area of ischaemic necrosis within the myocardium due to occlusion of coronary arterial supply.

  • Classification by ECG and Markers:

    • STEMI (ST-Elevation Myocardial Infarction): Complete occlusion of the coronary artery, resulting in transmural infarction (full thickness). Cardiac enzymes are elevated.

    • NSTEMI (Non-ST-Elevation Myocardial Infarction): Partial occlusion, resulting in subendocardial infarction. Cardiac enzymes are elevated.

    • Unstable Angina (UA): Partial occlusion/thrombus, but no elevation in cardiac biomarkers and no ST elevation.

  • Causes of MI:

    • Majority (90%90\%): Acute plaque rupture and superimposed thrombosis.

    • Other (10%10\%): Vasospasm, vasculitis, aortic stenosis, or shock.

Anatomical Localization of Myocardial Infarction

  • Left Anterior Descending Artery (LAD) [50%50\% of cases]: Supplies the anterior wall of the left ventricle and anterior 2/32/3 of the interventricular septum. Infarct area: Septal (V1−V2V1-V2) and Anterior (V3−V4V3-V4).

  • Right Coronary Artery (RCA) [30%30\% of cases]: Supplies the right ventricle/atrium, inferior and posterior wall of the left ventricle, posterior 1/31/3 of the interventricular septum, and the SA/AV nodes. Infarct area: Inferior (Leads II,III,aVFII, III, aVF).

  • Left Circumflex Artery (LCX) [20%20\% of cases]: Supplies the lateral wall of the left ventricle. Infarct area: Lateral (Leads I,aVL,V5,V6I, aVL, V5, V6).

  • Main Left Coronary Artery (LCA): Can cause Anterolateral or extensive anterior infarction (Leads I,aVL,V1−V6I, aVL, V1-V6).

Clinical Presentation and Diagnosis of Acute Coronary Syndromes

  • Symptoms: Prolonged cardiac pain (chest, throat, arms, epigastrium, back), anxiety, and a distinct "fear of impending death."

  • Physical Signs of Sympathetic Activation:

    • Pallor and sweating (diaphoresis).

    • Nausea and vomiting.

    • Tachycardia.

  • Signs of Functional Impairment:

    • Breathlessness/Lung crepitations (suggesting pulmonary edema).

    • Hypotension and narrow pulse pressure.

    • Raised Jugular Venous Pressure (JVP).

    • Oliguria and cold peripheries.

    • Low-grade fever.

  • Special Note: Diabetic patients may experience "silent" myocardial infarctions (lacking typical pain).

  • Diagnosis Triad:

    1. Clinical features/History.

    2. ECG changes (ST elevation vs. ST depression/T-wave inversion).

    3. Serum Cardiac Biomarkers: Measuring proteins that leak from injured myocytes.

Complications of Myocardial Infarction

  • Contractility Issues: Hypotension, coronary perfusion decrease, cardiogenic shock, and congestive heart failure.

  • Tissue Necrosis/Structural Rupture:

    • Papillary Muscle Infarction: Leading to Mitral Regurgitation.

    • Ventricular Wall Rupture: Leading to Cardiac Tamponade (fluid in the pericardial sac).

    • Ventricular Thrombus: Leading to embolism and stroke.

  • Electrical Instability: Arrhythmias.

  • Pericardial Inflammation:

    • Pericarditis: Early inflammation.

    • Dressler Syndrome: A later autoimmune-mediated pericarditis.