Cardiac Anatomy, Physiology, and Electrophysiology Study Guide

Cardiac Anatomy and Valves

Atrioventricular (AV) Valves

The heart contains two main atrioventricular (AV) valves that regulate blood flow between the atria and ventricles.

• Tricuspid Valve: Located between the right atrium and right ventricle; has three cusps.

• Bicuspid (Mitral) Valve: Located between the left atrium and left ventricle; has two cusps.

• Function: Prevents backflow of blood from the ventricles into the atria during ventricular contraction.

Semilunar Valves

The semilunar valves are found at the exits of the ventricles and prevent backflow into the heart.

• Aortic Valve: Located between the left ventricle and the aorta; prevents backflow into the left ventricle.

• Pulmonary Valve: Located between the right ventricle and the pulmonary trunk; prevents backflow into the right ventricle.

• Structure: Each has three crescent-shaped cusps.

Cardiac Circulation

Types of Circulation

The heart is central to three major circulatory pathways:

• Circulation through the heart: Blood flows from atria to ventricles and out to the body or lungs.

• Pulmonary Circulation: Carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium.

• Systemic Circulation: Delivers oxygenated blood from the left ventricle to the body and returns deoxygenated blood to the right atrium.

Coronary Circulation

The myocardium requires its own blood supply because it is too thick for nutrients to diffuse from the heart chambers.

• Right and Left Coronary Arteries: Branch from the base of the aorta.

• Major Branches:

  • Right coronary artery: marginal branch, posterior interventricular branch

  • Left coronary artery: circumflex branch, anterior interventricular branch

Anastomosis in Coronary Circulation

Anastomosis refers to the connection between blood vessels, allowing for alternative routes of blood flow. These are important in the coronary circulation to provide collateral circulation in case of blockage.

Coronary Veins and Drainage

• Most coronary veins drain into the coronary sinus.

• The coronary sinus empties into the right atrium.

Cardiac Muscle Cells and Conducting System

Contractile vs. Conducting Cells

Cardiac muscle contains two main cell types:

• Contractile Cells: Responsible for the contraction and pumping action of the heart.

• Conducting Cells: Specialized for initiating and transmitting electrical impulses; capable of spontaneous depolarization.

Conducting cells are found in the heart's intrinsic conduction system and can generate action potentials without external stimuli.

Intrinsic Conduction System Structures

The heart's conduction system coordinates the heartbeat.

• SA node (Sinoatrial node): Located in the right atrium; the heart's normal pacemaker.

• AV node (Atrioventricular node): Located at the junction of atria and ventricles; delays impulse.

• AV bundle (Bundle of His): Transmits impulses from AV node to bundle branches.

• Bundle branches: Carry impulses through the interventricular septum.

• Purkinje fibers: Distribute impulses throughout ventricular myocardium.

Ectopic pacemaker can develop if other cells outside the SA node begin to generate impulses, often due to damage or disease.

Depolarization and Repolarization in Conducting Cells

• Depolarization: Caused by influx of sodium ions ($Na^+$) and calcium ions ($Ca^{2+}$).

• Repolarization: Caused by efflux of potassium ions ($K^+$).

• Role of Calcium Ions: $Ca^{2+}$ ions prolong the action potential and contribute to the refractory period.

Cardiac Muscle Contraction and Refractory Period

Comparison with Skeletal Muscle

• Cardiac muscle has a longer refractory period than skeletal muscle.

• This prevents tetanus and ensures rhythmic contractions.

Role of Calcium Ions

• $Ca^{2+}$ ions enter during the plateau phase of the cardiac action potential, prolonging contraction and refractory period.

• Long refractory period is necessary to prevent sustained contraction and allow the heart to fill with blood between beats.

Electrocardiogram (ECG)

Obtaining and Interpreting an ECG

• ECG (Electrocardiogram): A recording of the electrical activity of the heart obtained by placing electrodes on the skin.

• Detects arrhythmias, conduction abnormalities, and myocardial damage.

ECG Waves and Intervals

ECG Component	Electrical Status	Contractile Status
P wave	Atrial depolarization	Atrial systole (contraction)
QRS complex	Ventricular depolarization	Ventricular systole (contraction); atrial repolarization occurs but is masked
T wave	Ventricular repolarization	Ventricular diastole (relaxation)

ECG Intervals and Segments

• PR (PQ) Interval: Time from onset of atrial depolarization to onset of ventricular depolarization; indicates AV node function.

• ST Segment: Time between ventricular depolarization and repolarization; elevation or depression may indicate myocardial damage.

• Heart Damage: Can alter the appearance of the ECG, such as abnormal Q waves, ST segment changes, or arrhythmias.

Key Equations

• Action Potential Equation (simplified):

$V_m = g_{Na}(E_{Na} - V_m) + g_{K}(E_{K} - V_m) + g_{Ca}(E_{Ca} - V_m)$

Additional info: The above equation represents the membrane potential ($V_m$) as a function of ion conductances and equilibrium potentials for sodium, potassium, and calcium.