Cardiac Physiology Basics and ECG Analysis

Basics

• Normal heart weight: females (250-300 g); males (300-350 g).
• Wall thickness: right ventricle (0.3-0.5 cm); left ventricle (1.3-1.5 cm).
• 90% of the heart's mass is made up of myocytes (heart muscle cells), but they only make up 25% of the cells.
• Heart function relies on the conduction of electrical impulses.
• The sinoatrial node (SA node) is the pacemaker of the heart, located in the right atrium.
• The SA node discharges at 70–80 times/minute and is innervated by vagal and sympathetic nerve fibers.
• The AV node is a "gatekeeper" that prevents too fast depolarization of the heart.

ECG

• Normal ECG strip: 10 seconds
• To calculate heart rate: multiply ventricular depolarization (QRS) count in 10 seconds by 6
• Interpretation: check if the P wave precedes the QRS complex and analyze the ST waves.

Heart Sounds

• First heart tone ("lub"): produced by the vibration of blood against closed atrial valves, marking the start of systole.
• Second heart tone ("dub"): caused by the closure of the aortic and pulmonary valves at the end of systole.
• S2 can sometimes be split into P2 (pulmonary valve sound) and A2 (aortic valve sound) during inspiration (inhaling).

Congestive Heart Failure

• Characterized by the heart's inability to pump sufficient blood throughout the body.
• Caused by factors that impair heart function: myocardial infarction, hypertension, arrhythmias, valve defects, pulmonary hypertension, and so forth.
• Main cause: volume overload, requiring the heart to pump more blood.
• Symptoms: shortness of breath during exercise or at rest, fatigue, pulmonary edema, peripheral edema, ascites, nocturia (urinating at night), and jugular vein distension.
• Treatment: address underlying causes, weight reduction, fluid management, sodium restriction, and medication (antihypertensives, diuretics, beta blockers, and antiarrhythmics).

General Causes of Heart Failure

• Left side of the heart:

  • Hypertensive heart disease: left ventricle works harder to overcome increased pressure in the aorta, leading to left ventricular hypertrophy.
  • Coronary heart disease.
  • Arrhythmias.

• Right side of the heart:

  • Cor pulmonale: right ventricle hypertrophy or dilation due to impaired lung perfusion.
    • Acute cor pulmonale: pulmonary embolism (PE).
    • Chronic cor pulmonale: chronic obstructive lung disease, lung tissue loss (tuberculosis, cancer surgery).
  • Mechanism: right ventricle works harder to pump blood through the lungs.
  • The heart compensates until it fails.

Shock

• Three most frequent types of shock:
  • Cardiogenic shock: heart failure to pump blood (MI, arrhythmias, PE, heart tamponade).
  • Hypovolemic shock: blood volume loss (bleeding, burns, trauma).
  • Septic shock: systemic infection (mainly gram-negative organisms – endotoxic shock).
• Other types of shock: anaphylactic shock and neurogenic shock following spinal cord injury.

Clinical Symptoms of Shock

• Anxiety, restlessness, disorientation.
• Hypotension.
• Rapid pulse (tachycardia).
• Cool, clammy, mottled skin.
• Decreased urine production (oliguria).
• Rapid breathing (hyperventilation).

Blood Circulation Before and After Birth

• Before birth, oxygenation occurs through the placenta, not the lungs.
• Fetal blood bypasses the lungs through the foramen ovale between the right and left atria.
• Shortly before/after birth, the foramen ovale closes.
• Blood from the pulmonary artery takes a shortcut through the ductus arteriosus to the aorta.

Atrial Septal Defect (ASD)

• A left-right shunt occurs when the septum wall does not form properly.
• Consequences are usually less severe than ventricular septal defects, with no symptoms before age 30.
• Results in right heart hypertrophy eventually.
• Systolic murmur!
• Paradoxical embolus: thrombosis can travel from the right heart to the left heart and cause a stroke if the foramen ovale doesn't close (patent foramen ovale - PFO).

Patent Ductus Arteriosus (PDA)

• In the fetus, blood from the right heart bypasses the lungs through shortcuts (foramen ovale and ductus arteriosus) directly to the aorta.
• After birth, the ductus arteriosus usually closes within 15 hours.
• If it doesn't close, blood flows from the aorta (with higher blood pressure) into the lungs.
• This increases blood flow through the lungs and decreases oxygenated blood in the aorta.
• The heart has to work harder.
• Clinical feature: machine-like sound over the heart.
• Right-to-left shunts causing cyanosis.

Tetralogy of Fallot (ToF)

• RIGHT-TO-LEFT SHUNT!
• A large VSD will lead to right heart hypertrophy and eventually a reversal of the shunt.
• Deoxygenated blood flows from right to left, causing cyanosis (Eisenmenger's syndrome).
• Prognosis is very poor if this occurs.