Anatomy and Function of the Heart

The Pathology of the Heart

Normal Heart Structure and Function

• Normal weight: 250-300g for females, 300-350g for males
• Left ventricular (LV) thickness: 1.3-1.5cm
• Right ventricular (RV) thickness: 0.3-0.5cm
• Blood supply: Right and Left coronary arteries, which behave as end-arteries functionally
• The heart has three layers: endocardium, myocardium, and epicardium
• The main functional unit is the myocardium, composed of cardiac myocytes
• Ventricular myocytes are the most specialized
• The contractile unit of the myocytes is the sarcomere, composed of myosin, actin, troponin, and tropomyosin

Cardiac Muscle Striated Appearance

• Due to series of sarcomeres
• Myosin pulls actin towards the center of the cell, creating shortening of the functional unit, which leads to contraction
• Increase in degree of shortening leads to increase in contraction
• Moderate dilation of chambers leads to increase in shortening
• Further dilation causes reduction in overlap of myosin and actin, leading to reduced contraction seen in heart failure

Atrial Myocytes

• Smaller than ventricular myocytes
• Less well-arranged
• Atrial cell cytoplasm contains specific atrial granules, which store atrial natriuretic peptide, with regulatory functions such as vasodilation, natriuresis, and diuresis

Intercalated Discs

• Connect cardiac myocytes, containing specialized intracellular junctions
• Function in mechanical and electrical coupling
• Example: gap junction

Valves

• Ensure unidirectional flow of blood
• Composed of Tricuspid, Pulmonary, Mitral, and Aortic valves
• Tricuspid and Mitral valves have leaflets, while Pulmonary and Aortic valves have cusps
• Valves are composed of fibrosa, spongiosa, ventricularis, or atrialis, and endothelial cells
• Valves are repaired by interstitial cells
• Other structures related to valves include chordae tendinae and papillary muscles
• Valves can suffer from prolapse, calcification, and fibrosis

Conduction System

• SA (Sino-Atrial) pacemaker node: located between right atrial appendages and superior vena cava
• AV (Atrio-Ventricular) node: located in the right atrium, along the atrial septum
• Bundle of His: located in the right atrium, extending to the summit of the ventricular septum
• Right and Left Bundle branches: extend to different ventricles
• SA node beats at a rate of 60-100/minute
• Beat is moderated by the AV node and regulated by the autonomic nervous system

Coronary Arteries

• Originate from ostia beneath the aortic valves
• Left coronary arteries: Left Anterior Descending (LAD), Left Circumflex (LCx)
• Right Coronary Artery (RCA)
• Right Coronary Artery is dominant in most cases, supplying the posterior 1/3 of the cardiac septum

Aging and the Heart

• Heart undergoes atrophy with age
• Accumulation of age pigment, lipofuscin, which is brown in color
• Brown atrophy is seen in cachexia and cancer

Congestive Cardiac Failure

• Heart is unable to maintain an output sufficient for the metabolic requirements of the tissues and organs
• Can appear as the end stage of many forms of chronic heart disease
• Causes:
  • Loss of cardiac myocytes
  • Impaired contractility
  • Mechanical cardiac overload
  • Impaired filling of the chambers

Left-Sided Heart Failure

• Causes:
  • Ischaemic heart disease
  • Systemic hypertension
  • Rheumatic heart disease
  • Myocardial diseases

Organ Changes

• Lungs:
  • Increased hydrostatic pressure
  • Pulmonary oedema and congestion
  • Heart failure cells
  • Brown induration of the lungs
• Kidneys:
  • Stimulation of the renin-angiotensin system
  • Salt and water retention
  • Acute tubular necrosis
  • Pre-renal azotaemia
• Brain:
  • Cerebral hypoperfusion
  • Hypoxic symptoms (e.g. irritability, stupor, and coma)

Right-Sided Heart Failure

• Causes:
  • Cor pulmonale
  • Tricuspid and Pulmonary valve lesions
  • Congenital heart disease associated with left-to-right shunt
  • Left heart failure
• Organ Changes:
  • Liver:
    • Congestive hepatomegaly
    • Central haemorrhagic necrosis
    • Nutmeg appearance
    • Cardiac sclerosis
    • Cardiac cirrhosis
  • Spleen:
    • Congestive splenomegaly
    • Siderofibrotic nodules (Ghandy-Gamna bodies)
  • Subcutaneous tissue:
    • Pitting pedal oedema
    • Anasarca
  • Body cavities:
    • Pleural, pericardial, and peritoneal effusion

Hypertensive Heart Disease

• Definition: Response of the heart to increased demands induced by systemic or pulmonary hypertension
• Systemic hypertensive heart disease:
  • LVH (usually concentric) in the absence of other vascular pathology
  • History of hypertension
• Causes of Hypertension:
  • Essential hypertension (90-95%)
  • Secondary hypertension (5-10%)
  • Renal diseases
  • Endocrine diseases
  • Vascular diseases
  • Neurogenic diseases
• Pathogenesis:
  • Increased pressure overload → myocardial hypertrophy
  • Cardiomegaly and LV thickness

Rheumatic Heart Disease

• Definition: Not directly caused by the organism, but by antibodies to streptococci that cross-react with tissues of the heart, kidneys, etc.
• Commoner in blacks, and in warmer climates
• Develops in 50-75% of childhood cases
• 35% of adults
• Organ Changes:
  • Pericardium:
    • Fibrinous or sero-fibrinous exudate
    • Bread and butter pericarditis
  • Myocardium:
    • Aschoff bodies (foci of fibrinoid necrosis surrounded by lymphocytes, macrophages, and plasma cells)
    • Death may occur in acute RF from CCF
  • Valves:
    • Acute valvulitis: hyperaemia, oedema, and thickening of the valves
    • Chronic valvulitis: progressive scarring and deformity of the valve leaflets
    • McCallum's plaque (fibrous plaques on the subendocardium of the left atrium)
• Extra-cardiac lesions:
  • Subcutaneous nodules
  • Polyarthritis (migratory)
  • Pleural and pericardial effusions
  • CNS: Sydenham's chorea (characterized by rapid, irregular, and purposeless movements)

Infective Endocarditis

• Definition: Invasion of the heart valve or the mural endocardium by infective agents

• Two forms:

  • Acute (fatal, fulminant infection)
  • Subacute (smouldering, indolent illness)

• Epidemiology and pathogenesis:

  • Congenital heart disease
  • Valvular defects
  • Rheumatic heart disease
  • Prosthetic valve
  • Immunosuppression
  • Diabetes mellitus
  • Chronic alcoholics
  • Intravenous drug abuse
  • Commoner in males above 50 years

• Bacteriology:

  • Streptococcus viridans (50%)
  • Staphylococcus aureus (20%)
  • Streptococcus pneumonia
  • HACEK group (Haemophilus, Actinobacillus, Cardiobacterium, Eikenella, Kingella)

• Pathology:

  • Vegetations on the heart valve
  • Fibrinous, granular, or ulcerative lesions
  • Heart failure
  • Embolization
  • Immune complex disease
  • Osteomyelitis### Sudden Death

• Sudden death occurs within an hour of symptom onset

• Associated with severe coronary atherosclerosis (>75% lumina narrowing)

• Complications include thrombus or plaque hemorrhage or rupture

• Mechanism is usually an arrhythmia

• Aortic dissection can also cause sudden collapse

Complications of MI

• Ischaemic cardiomyopathy: severe atherosclerosis of all major arteries, inadequate vascular supply, myocyte loss, fibrosis, decreased compliance, compensatory hyperplasia, and hypertrophy
• Common complications: cardiac arrhythmias (75-95%), sudden death (20%), cardiogenic shock (10-15%), left ventricular aneurysm, and mural thrombosis (15-40%)
• Uncommon complications: myocardial rupture, papillary muscle rupture, re-infarction, infarct extension, post-infarction pericarditis, and Dressler's syndrome

Factors Determining Infarct Size

• Size of the myocardial bed at risk and its metabolic/oxygen needs
• Duration of obstruction
• Coronary artery anatomy, dominance, and collaterals
• Site of coronary artery obstruction
• Severity of obstruction
• Rate at which obstruction developed

Laboratory Diagnosis of MI

• None of the available tests are specific or sensitive
• Tests must be correlated with clinical symptoms, ECG, and angiography
• Timing is crucial

Creatinine Kinase

• Total creatinine is from both skeletal and cardiac origin; non-specific
• MB, MM, and BB fractions
• MM: cardiac and skeletal, with isoenzymes
• MB: specific for cardiac, 2% is skeletal
• BB: found in brain, bowel, and bladder, not measured routinely
• CK-MB is measured every 2-4 hours for 9-12 hours after patient is first seen

Troponins

• Troponins I and T are structural components of cardiac muscle and released into blood with myocardial injury
• Very specific, more than CK-MB
• Begins to rise within 3-12 hours, remains elevated for 5-9 days for troponin I and up to 2 weeks for troponin T
• Difficult to diagnose re-infarction because of the prolonged elevation
• Troponin T is not very specific, as it can appear with skeletal myopathies or renal failure

Myoglobins

• Myoglobin is a protein found in skeletal and cardiac muscle
• Elevated before CK-MB, but not specific
• Lactate dehydrogenase (LDH) begins to rise in 12-24 hours, peaks in 2-3 days, and dissipates in 5-14 days
• LDH isoenzymes are more specific

Prevention of MI

• Control of hypertension and diabetes
• Drug and dietary treatment of hypercholesterolemia
• Dietary reduction of plasma cholesterol and LDL
• Reduction of fat intake
• Avoidance of obesity
• Moderation of salt intake
• Cessation of smoking
• Exercise

Cardiomyopathies

• Dilated cardiomyopathy: characterised by gradual development of heart failure associated with 4-chamber hypertrophy and dilatation of the heart
• Hypertrophic cardiomyopathy: characterised by heavy hypercontracting heart, asymmetrical septal hypertrophy, and AD in 50% of cases
• Restrictive cardiomyopathy: characterised by endomyocardial fibrosis, Loeffler's endocarditis, and endocardial fibroelastosis

Myocarditis

• Aetiology: infections, immune-mediated reactions, and toxins
• Morphology: acute phase, enlarged, flabby heart, cut surface shows mottling of ventricular wall, microhaemorrhages
• Histology depends on the causative agent

Cardiac Tumours

• Primary tumours are rare, metastatic tumours are common (5% of patients dying of cancer)
• Primary tumours in descending order of frequency: myxoma, fibroma, lipoma, papillary fibroelastoma, rhabdomyoma, and angiosarcoma
• Myxoma: most common primary tumour in adults, 90% occur in the atria, single tumour, 1-10cm in diameter, sessile or pedunculated, gelatinous mass

Congenital Heart Disease

• Definition: structural abnormalities of the heart or great vessels present at birth
• Incidence: 6-8 per 1000 live births
• Aetiology: mostly unknown, associated with chromosomal abnormalities, environmental factors, and maternal infection

Classification of CHD

• Acyanotic with shunt (Lt to Rt shunt): PDA, VSD, ASD
• Cyanotic (Rt to Lt shunt): tetralogy of Fallot, late cyanotic VSD (Eisenmenger complex), transposition of great vessels, total anomalous pulmonary venous connection, persistent truncus arteriosus, tricuspid atresia
• Acyanotic (no shunt): coarctation of the aorta, aortic stenosis and atresia, pulmonary stenosis and atresia