Basic Dysrhythmia Workshop Part I

Cardiac Chambers and Valves

  • Cardiac Chambers:

    • Right Atrium (RA) (1): Receives deoxygenated systemic venous blood from the Superior Vena Cava (SVC) and Inferior Vena Cava (IVC).
    • Right Ventricle (RV) (3): Receives blood from the right atrium and pumps it through the pulmonic valve into the pulmonary artery toward the lungs.
    • Left Atrium (LA) (5): Receives oxygenated blood returning from the lungs via the pulmonary veins.
    • Left Ventricle (LV) (7): Receives oxygenated blood from the left atrium and pumps it under high pressure through the aortic valve into the aorta to supply systemic circulation.

  • Atrioventricular (AV) Valves:

    • Tricuspid Valve (2): Positioned between the Right Atrium and Right Ventricle; prevents backflow into the RA during ventricular contraction.
    • Mitral Valve (Bicuspid Valve): Positioned between the Left Atrium and Left Ventricle; prevents backflow into the LA during ventricular contraction.

  • Semilunar Valves:

    • Pulmonic Valve (4): Located at the exit of the Right Ventricle into the pulmonary trunk/artery.
    • Aortic Valve (6): Located at the exit of the Left Ventricle into the ascending aorta.

  • Supporting Valve Structures:

    • Papillary Muscles: Muscular projections arising from the ventricular walls.
    • Chordae Tendineae: Fibrous, tendon-like cords connecting papillary muscles directly to the AV valve leaflets, preventing valve eversion during systole.

Overview of Cardiac Structures

Cardiac Wall Structure

  • Layers of the Heart Wall (from exterior to interior):
    • Pericardium: The protective sac enclosing the heart, consisting of two sub-layers:
    • Parietal Pericardium: Outer membrane composed of a dense fibrous layer, loose connective tissue, and mesothelium.
    • Visceral Pericardium (Epicardium): Outer tissue surface of the heart wall composed of mesothelium and loose connective tissue.
    • Myocardium: Thick middle muscular layer composed of specialized cardiac muscle fibers responsible for mechanical contraction.
    • Endocardium: Inner lining layer of smooth endothelium and loose connective tissue that lines the cardiac chambers and covers heart valves.

Layers of the Heart Wall

Coronary Blood Flow and Perfusion

  • General Oxygen Consumption and Perfusion Dynamics:

    • The myocardium consumes 70%70\% of the oxygen contained in coronary arterial blood (compared to 15% to 20%15\%\text{ to }20\% consumed by the brain).
    • Coronary artery flow originates at the root of the aorta directly above the aortic valve.
    • Coronary vessels originate on the epicardial surface and penetrate inward into the myocardium and endocardium to perfuse all cardiac muscle cells.
    • Coronary arteries are best perfused during diastole, as ventricular contraction during systole compresses intramyocardial vessels.

  • Right Coronary Artery (RCA):

    • Anatomical Branches: Conus arterial branch, small cardiac vein (running alongside), septal artery branches, marginal arterial branch, A-V nodal arterial branch, and posterior descending artery (at the crux).
    • Perfusion Areas:
    • Right Atrium (RA) and Right Ventricle (RV).
    • Inferior surface of the heart (including the left inferior ventricle).
    • Sinoatrial (SA) node in 55%55\% of the population.
    • Atrioventricular (AV) node in 90%90\% of the population.
    • Internodal tracts.
    • Posterior 1/31/3 of the interventricular septum.

  • Left Main Coronary Artery (LCA):

    • Originates from the aorta and bifurcates into two main vessels:
    • Left Anterior Descending (LAD) Artery:
      • Anatomical branches include diagonal arterial branches and septal artery branches.
      • Perfuses the anterior wall of the Left Ventricle (LV), the interior surface of the heart, the anterior 2/32/3 of the interventricular septum, and both Right and Left bundle branches.
    • Circumflex Artery:
      • Runs along the coronary groove between the Left Atrium and Left Ventricle around the lateral aspect of the heart.
      • Perfuses the Left Atrium (LA), lateral wall of the Left Ventricle (LV), and posterior wall of the Left Ventricle (LV).
      • Perfuses the SA node in 45%45\% of the population and the AV node in 10%10\% of the population.

  • Coronary Venous Drainage:

    • Deoxygenated myocardial blood drains via the great cardiac vein, middle cardiac vein, small cardiac vein, and left posterior coronary vein into the coronary sinus, which empties directly into the Right Atrium.

Anatomy of Coronary Blood Flow

Electrophysiology and Cellular Properties

  • Fundamental Properties of Cardiac Muscle:

    • Automaticity: The inherent capability of cardiac cells to spontaneously initiate an electrical impulse without external nervous system stimulation.
    • Excitability: The capacity of cardiac muscle cells to respond to an electrical stimulus.
    • Conductivity: The ability to transmit an electrical impulse (transmission of electrical energy) from cell to cell across cardiac tissue.
    • Contractility: The ability of myocardial muscle cells to shorten and generate physical tension in response to electrical stimulation.

  • Cellular Electrical States:

    • Polarization (Ready State): Resting potential of the cardiac cell; internal cellular environment maintains a net negative electrical charge relative to the outside. Potassium (K+K^+) ions remain concentrated inside, while sodium (Na+Na^+) ions reside outside.
    • Depolarization (Discharge State): Electrical discharge phase where sodium (Na+Na^+) rapidly influxes into the cell, reversing internal potential from negative to positive and triggering contraction.
    • Repolarization (Recovery State): Recovery phase during which potassium (K+K^+) exits and sodium (Na+Na^+) is actively pumped out to restore resting negative intracellular electrical charges.

Electrical Activity at the Cellular Level

Cardiac Conduction System

  • Pacemaker Structure and Hierarchy:
    • Sinoatrial (SA) Node:
    • Located at the junction of the Right Atrium and Superior Vena Cava.
    • Serves as the primary pacemaker ("battery") of the heart, initiating electrical impulses in most individuals.
    • Inherent firing rate: 60–100 beats/min60\text{--}100\text{ beats/min}.
    • The term sinus rhythm signifies that impulse initiation originates within the SA node.
    • Atrioventricular (AV) Node:
    • Located at the junction of the Right Atrium and ventricle (junction of all four chambers).
    • Delays impulse transmission to ensure complete atrial emptying prior to ventricular contraction.
    • Inherent firing rate: 40–60 beats/min40\text{--}60\text{ beats/min}.
    • Rhythms originating in the AV node are designated as nodal or junctional rhythms.
    • Ventricular Conduction System:
    • Bundle of His: Directly connects the AV node to the bundle branches in the interventricular septum.
    • Bundle Branches: Bifurcates into the Right Bundle Branch (RBB) and Left Bundle Branch (LBB).
    • Purkinje Fibers: Fingerlike projections extending directly into the ventricular muscle tissue, transmitting impulses into every individual ventricular cardiac cell.
    • Inherent ventricular rate: 20–40 beats/min20\text{--}40\text{ beats/min} (Purkinje fibers maintain an inherent rate of 30–40 beats/min30\text{--}40\text{ beats/min}).
    • Pacemaker Speed Hierarchy:
    • SA Node (60–100 beats/min60\text{--}100\text{ beats/min}, fastest) →\rightarrow AV Node (40–60 beats/min40\text{--}60\text{ beats/min}) →\rightarrow Ventricles (20–40 beats/min20\text{--}40\text{ beats/min}, slowest).
    • Latent Automaticity:
    • Most regions within the heart possess the potential to initiate impulses, but only assume active pacemaker roles under abnormal or pathological conditions.

The Electrical System of the Heart

The Cardiac Cycle and Mechanical Events

  • Overview:

    • The cardiac cycle represents the rhythmic repetition of coordinated electrical and mechanical pumping events.
    • Mechanical pumping action occurs when myocardial cells contract in response to prior electrical stimulation.
    • Composed of two distinct physical phases: systole and diastole.

  • Electrical Events:

    • Electrical conduction begins with impulse generation in the SA node, propagation across the atria (P wave: atrial depolarization), impulse delay at the AV node, rapid transmission through the Bundle of His, bundle branches, and Purkinje fibers (QRS complex: ventricular depolarization), followed by recovery (T wave: ventricular repolarization).
    • Electrocardiographic monitoring commonly utilizes lead II and MCL1 to visualize impulse conduction through heart tissue. The positive lead serves as the active viewing point ("does the looking").

Electrical Events of the Cardiac Cycle

  • Mechanical Events:
    • Systole (Contraction Phase):
    • Calcium release exposes active binding sites on actin and myosin, permitting crossbridge overlap that produces cellular shortening and contraction.
    • Frank-Starling Law: The force of ventricular contraction is directly dependent upon the degree of myocardial stretch (actin-myosin overlap length) achieved prior to release.
    • During systole, the aortic and pulmonic valves open, allowing ejection into systemic and pulmonary circulation, while AV valves remain tightly closed.
    • Diastole (Relaxation and Filling Phase):
    • Myocardial actin and myosin crossbridges stretch as blood fills the ventricles.
    • Ventricular filling requires approximately 7–12 mmHg7\text{--}12\,\text{mmHg} of pressure to obtain optimal LV stretch for maximal subsequent contraction.
    • A failing Left Ventricle may require elevated filling pressures to achieve adequate stretch and contractile force.
    • During diastole, the mitral and tricuspid valves open to allow ventricular filling, and coronary arterial perfusion reaches its peak.

Autonomic Nervous System Regulation

  • Sympathetic Nervous System (SNS):

    • Mediates the "fight or flight" stress response.
    • Innervates both the atria and ventricles.
    • Secretes catecholamines: epinephrine and norepinephrine.
    • Cardiovascular Effects:
    • Increases SA node impulse firing rate (chronotropic effect via β1\beta_1 receptors).
    • Increases conduction velocity through the AV node (dromotropic effect).
    • Increases cardiac contractility (inotropic effect via β1\beta_1 receptors).
    • Causes peripheral arteriolar vasoconstriction (mediated via α\alpha receptors).

  • Parasympathetic Nervous System (PNS):

    • Dominates cardiovascular regulation under resting, stable physiological conditions.
    • Innervates only the atria.
    • Reaches the heart via the Vagus Nerve (Cranial Nerve X).
    • Secretes acetylcholine.
    • Cardiovascular Effects:
    • Decreases SA node firing rate (negative chronotropic effect).
    • Decreases AV node conduction speed (negative dromotropic effect).
    • Decreases atrial contractility (negative inotropic effect).
    • Decreases cellular irritability.
    • Promotes generalized vasodilation.

Autonomic Innervation of the Heart