Cardiac Anatomy, Physiology, and Coronary Artery Disease

• Left Coronary Artery: This artery divides into the left anterior descending (LAD) and the left circumflex artery. These branches supply the left atrium, the left ventricle, and the intraventricular septum.
• Right Coronary Artery: This artery supplies the right atrium, the right ventricle, and a portion of the posterior wall of the left ventricle. In approximately 90% of individuals, the right coronary artery provides the blood supply to the AV node and the bundle of His. Consequently, an obstruction in this artery can lead to severe cardiac conduction defects.

Autonomic and Reflex Regulation

The heart does not beat in isolation; its rate and strength are constantly adjusted by the autonomic nervous system (ANS) to meet the body's metabolic demands.

The Autonomic Nervous System

The ANS is divided into two opposing branches:

1. Sympathetic Nervous System: Often called the "fight or flight" system, stimulation increases heart rate, the speed of impulse conduction through the AV node, and the force of myocardial contraction. It also stimulates alpha-1 adrenergic receptors in vascular smooth muscle, causing vasoconstriction and increasing blood pressure.
2. Parasympathetic Nervous System: Mediated by the vagus nerve, this system acts as a "brake," decreasing the heart rate by slowing impulses from the SA node and through the AV node.

Reflex Regulation: Baroreceptors and Chemoreceptors

The body utilizes specialized sensors to maintain homeostasis:

• Baroreceptors: Located in the aortic arch and the carotid sinus, these receptors detect changes in arterial stretch (pressure). If blood pressure is too high (e.g., due to volume overload), baroreceptors trigger the vasomotor center in the brain to inhibit the sympathetic system and enhance the parasympathetic system, resulting in vasodilation and a decreased heart rate.
• Chemoreceptors: Located in the aortic/carotid bodies and the medulla, these respond to chemical changes in the blood. They trigger an increase in blood pressure and respiratory rate in response to:
  • Hypercapnia (increased $CO_2$)
  • Acidosis (decreased $pH$)
  • Hypoxemia (decreased $O_2$)

Cardiac Function and Auscultation

To understand heart sounds, one must distinguish between the two phases of the cardiac cycle: systole (contraction/ejection) and diastole (relaxation/filling).

Heart Sounds

Clinicians use auscultation to listen for specific sounds that indicate valve function and ventricular health:

• S1 ("lub"): The first heart sound marks the beginning of systole. It is caused by the closure of the tricuspid and mitral valves.
• S2 ("dub"): The second heart sound marks the beginning of diastole. It is caused by the closure of the aortic and pulmonic valves.
• S3 (Ventricular Gallop): An abnormal sound heard early in diastole. It may indicate volume overload (too much fluid), hypertension, or left ventricular dysfunction.
• S4 (Atrial Gallop): An abnormal sound heard in late diastole. It is caused by the atria contracting against a resistant, often hypertrophied, ventricle.
• Murmurs: These are turbulent sounds heard between S1 and S2, typically indicating abnormal blood flow patterns caused by valve disorders.

Coronary Artery Disease (CAD) and Atherosclerosis

Coronary Artery Disease is a progressive condition characterized by the narrowing or occlusion of the coronary arteries. This process is driven by atherosclerosis, the abnormal accumulation of lipids (fats) and fibrous tissue within the arterial walls.

The Stages of Atherosclerosis

Atherosclerosis does not occur overnight; it develops over many years through several distinct stages:

1. Endothelial Injury: The process begins when the inner lining of the vessel (endothelium) is damaged by factors such as smoking, high cholesterol, hypertension, diabetes, or infection.
2. Fatty Streak: This is the earliest lesion, characterized by lipid-filled smooth muscle cells that appear as a yellow tinge within the artery. These can be seen as early as age 18.
3. Fibrous Plaque: As lipoproteins (carriers of cholesterol) transport lipids into the arterial intima, collagen covers the fatty streak, forming a grayish or whitish plaque. This narrows the vessel lumen, reducing blood flow to distal tissues.
4. Complicated Lesion: This is the most dangerous stage. Inflammation or plaque instability leads to ulceration or rupture, causing platelets to accumulate and form a thrombus (clot). This can lead to total occlusion of the artery.

Exam Tip: Ischemia vs. Infarction Remember that ischemia occurs when blood flow is reduced (often when a vessel is $\geq 50\%$ blocked), causing oxygen deprivation. If ischemia is prolonged and not reversed, it can lead to myocardial infarction (MI). Heart cells typically become hypoxic within 10 seconds of total occlusion and can remain viable for approximately 20 minutes before irreversible damage begins.

Clinical Manifestations: Angina

When the demand for myocardial oxygen exceeds the supply provided by the coronary arteries, ischemia occurs, manifesting clinically as angina (chest pain).

• Stable Angina: Usually precipitated by physical exertion.
• Unstable Angina: A more serious condition characterized by new-onset angina that occurs at rest or with minimal exertion, and which increases in frequency, duration, or severity.

Commonly, angina is felt as a substernal sensation that may radiate to the neck, jaw, shoulders, or arms. Patients may also report indigestion or a burning sensation in the epigastric region.