Cardiology: Aortic Stenosis and Heart Failure

Cardiovascular System

• The cardiovascular system comprises the heart, blood vessels, and blood.
• The heart pumps blood throughout the body.
• Blood vessels transport blood to different organs.
• Blood carries important substances between organs.

Blood Components

• Blood consists of formed elements (cells and fragments) suspended in plasma.
• Formed elements include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets.
• Plasma carries blood cells, proteins, nutrients, metabolic wastes, and other molecules.
• Hematocrit is the percentage of blood volume occupied by erythrocytes.

Blood Vessels

• Blood vessels include arteries, arterioles, capillaries, venules, and veins.
• Arteries carry blood away from the heart.
• Veins carry blood toward the heart.

Pressure, Flow, and Resistance

• Pressure is the force exerted by blood; measured in millimeters of mercury (mmHg).
• Flow is the volume of blood moved per unit time (milliliters/minute).
• Resistance describes the difficulty for blood to flow. Resistance is friction that impedes flow.
• Flow rate is proportional to the pressure difference and inversely proportional to resistance.

Resistance Factors

• Blood viscosity—affected by water volume and erythrocyte numbers.
• Blood vessel radius—larger radii decrease resistance

Heart Structure

• The heart has four chambers: two atria and two ventricles.
• The heart has valves that control blood flow.

Cardiac Cycle

• Systole describes the period of ventricular contraction and ejection.
• Diastole describes the period of ventricular relaxation and filling.
• The cardiac cycle has four phases: ventricular filling, isovolumetric contraction, ventricular ejection, and isovolumetric relaxation.

Cardiac Output

• Cardiac output is the volume of blood pumped out of each ventricle per unit time.
• Calculated as heart rate multiplied by stroke volume (CO = HR x SV).
• Normal cardiac output is ~5 liters per minute.

Regulation of Heart Rate

• The SA node sets the heart rate (100 bpm).
• Sympathetic stimulation increases heart rate.
• Parasympathetic stimulation (vagus nerves) decreases heart rate.

Regulation of Stroke Volume

• Stroke volume is the volume of blood ejected from each ventricle per contraction.
• End-diastolic volume (EDV) is the volume of blood in the ventricles before contraction.
• End-systolic volume (ESV) is the volume of blood left in the ventricles after contraction.
• SV = EDV - ESV; typical SV is 70 mL/beat in a resting person.
• Three factors regulate stroke volume: end-diastolic volume, sympathetic nervous system input, and afterload.

The Frank-Starling Mechanism

• The more blood in the ventricle before contraction (EDV), the stronger the contraction.
• Increased venous return leads to increased cardiac output.

Sympathetic Regulation of Stroke Volume

• Norepinephrine increases ventricular contractility at any given EDV.
• Plasma epinephrine also increases myocardial contractility.
• Sympathetic stimulation increases the rate of contraction and relaxation.

Ejection Fraction

• Ejection fraction (EF) is the ratio of SV to EDV, expressed as a percentage.
• Represents the strength of contraction at a given EDV.

Afterload

• Afterload represents the cardiac workload to eject blood.
• Increased afterload reduces stroke volume.

Blood Vessels

• Arteries are elastic tubes, acting as pressure reservoirs.
• Arterioles control blood flow distribution.
• Capillaries are sites for nutrient exchange, fluid moves out by filtration.
• Venules are highly compliant (capacitance vessels) that act as blood reservoirs.

Venous Pressure and Return

• Venous pressure is ~15 mmHg.
• Mechanisms facilitating venous return include the respiratory pump and muscular pump.

Blood Pressure Regulation

• Mean arterial pressure (MAP) is the average pressure driving blood into tissues.
• MAP = Diastolic pressure + 1/3 Pulse pressure.
• Short-term MAP regulation by baroreceptors.
• Long-term MAP regulation by kidneys.

Baroreceptor Reflexes

• Baroreceptors in the carotid sinus and aortic arch detect arterial pressure changes.
• The medullary cardiovascular center integrates baroreceptor input to regulate sympathetic and parasympathetic outflow.
• Other baroreceptors in veins and heart provide feed-forward component to arterial pressure control.

Other Cardiovascular Reflexes and Responses

• Blood pressure is affected by factors including oxygen and carbon dioxide levels, changes in blood flow to the brain, pain, emotions, eating, mood, and stress.

Hypertension

• Hypertension is chronically elevated systemic arterial pressure (above 140/90 mmHg).
• Primary Hypertension- of uncertain cause.
• Secondary Hypertension- identified causes.

Shock

• Shock signifies reduced blood flow to organs and tissues, causing damage.
• Three types include hypovolemic, low-resistance, and cardiogenic shock.

Local Control of Arteriolar Diameter

• Active hyperemia: increased blood flow due to increased tissue metabolic demands.
• Flow autoregulation: maintaining blood flow despite pressure changes.
• Reactive hyperemia: temporary increase in blood flow after occlusion.

Endothelial Cells and Paracrine Regulation

• Endothelial cells secrete compounds affecting arteriolar tone by affecting vascular smooth muscle.
• Nitric oxide (NO) is a crucial vasodilator secreted by endothelial cells.

Blood Supply to the Heart

• Coronary circulation supplies blood to the myocardium.

Cardiovascular Changes During Exercise

• Systemic blood flow to active muscles increases.
• Heart rate and cardiac output increase significantly.
• Overall peripheral resistance tends to decrease.

ECG

• Electrocardiogram (ECG) is a tool used to track the electrical events of the heart.
• ECG waves correspond to different phases of the cardiac cycle.

Atherosclerosis and Heart Failure

• Atherosclerosis causes damage to the heart vessels, reducing blood flow.
• Heart failure occurs when the heart cannot pump adequately.

Coagulation Disorders

• Hemophilia and Von Willebrand's disease are caused by deficiencies in clotting factors.

Drugs to Treat Hypertension and Heart Failure

• Diuretics, beta-blockers, calcium channel antagonists, ACE inhibitors, ARBs, and mineralocorticoid receptor antagonists.

Summary

• This section provides a summary of the cardiovascular system and its associated diseases.