Chapter 19: The Heart – Coronary Artery Disease and Cardiac Physiology

Coronary Artery Disease

Myocardial Ischemia and Infarction

Coronary artery disease (CAD) is a condition where the arteries supplying blood to the heart muscle become narrowed or blocked. This can lead to reduced oxygen delivery to the heart tissue.

• Myocardial Ischemia: Partial obstruction of coronary blood flow, resulting in angina pectoris (chest pain).

• Myocardial Infarction (MI): Complete obstruction of coronary blood flow, causing death of heart tissue (heart attack).

Example: A patient experiencing chest pain during exercise may be suffering from myocardial ischemia. If the artery becomes fully blocked, a myocardial infarction can occur.

Arteriosclerosis and Atherosclerosis

These are two related but distinct conditions affecting arterial walls:

• Arteriosclerosis: Hardening or thickening of arterial walls, which can lead to CAD or strokes.

• Atherosclerosis: Formation of lipid deposits (plaques) in the arterial tunica media, often associated with endothelial damage and smooth muscle cell hyperplasia. These plaques restrict blood flow.

Example: Atherosclerosis in the coronary arteries is a major cause of heart attacks.

Treatment Options for Blocked Arteries

• Vessel Replacement: Removing and replacing the damaged vessel with a segment from the leg (e.g., coronary artery bypass grafting). This is complex and risky.

• Balloon Angioplasty: A balloon is inflated inside the vessel to compress the plaque and restore blood flow. Plaques may regrow after the procedure.

• Stent Placement: Inserting a small mesh tube (stent) to keep the vessel open and maintain blood flow.

Additional info: Stents are often coated with drugs to reduce the risk of re-narrowing (restenosis).

Left Ventricular Hypertrophy

When arteries are blocked, the heart must work harder to pump blood, leading to increased blood pressure and enlargement of cardiac muscle cells (hypertrophy). Over time, this can result in heart failure because cardiac muscle cells have limited ability to regenerate.

• Hypertrophy: Increase in cell size, not cell number.

• Heart Failure: The heart becomes unable to pump blood effectively.

Cardiac Muscle Cell and Electrical Activity

Action Potentials and Contraction

Cardiac muscle cells have unique electrical and contractile properties compared to skeletal muscle:

• Longer Action Potential: Cardiac muscle cells have a prolonged action potential and contraction time.

• Refractory Period: Cardiac muscle cannot be re-stimulated during the refractory period, preventing tetany (sustained contraction).

• Skeletal Muscle: Can undergo wave summation and tetany, which is not possible in cardiac muscle.

Example: If the heart could enter tetany, it would stop pumping, which would be fatal.

Cardiac Cycle

Phases of the Cardiac Cycle

The cardiac cycle is the sequence of events in one heartbeat, beginning with atrial contraction and ending with ventricular relaxation.

• Atrial Systole: Contraction of the atria, pushing blood into the ventricles.

• Ventricular Systole: Contraction of the ventricles, ejecting blood into the arteries.

• Ventricular Diastole: Relaxation of the ventricles, allowing them to fill with blood.

Additional info: The cardiac cycle ensures coordinated pumping of blood through the heart and to the body.

Cardiac Physiology

Cardiac Output and Related Volumes

Cardiac output is a measure of the heart's efficiency in pumping blood.

• Cardiac Output (CO): The volume of blood pumped by each ventricle per minute.

• Formula:

$CO = HR \times SV$

• Heart Rate (HR): Number of heartbeats per minute.

• Stroke Volume (SV): Volume of blood pumped by one ventricle per beat.

• End-Diastolic Volume (EDV): Amount of blood in the ventricles at the end of atrial systole, just before ventricular contraction.

• End-Systolic Volume (ESV): Amount of blood remaining in each ventricle after systole (ejection phase).

Example: If HR = 70 beats/min and SV = 70 mL/beat, then $CO = 70 \times 70 = 4900$ mL/min.

Term	Definition
Cardiac Output (CO)	Volume of blood pumped per minute
Heart Rate (HR)	Beats per minute
Stroke Volume (SV)	Blood pumped per beat
End-Diastolic Volume (EDV)	Blood in ventricles at end of atrial systole
End-Systolic Volume (ESV)	Blood remaining after ventricular contraction

Additional info: Cardiac output is a key indicator of cardiovascular health and is regulated by factors such as autonomic nervous system activity, venous return, and contractility.