7.1 Cardiac anatomy and electrophysiology

The heart's intricate structure and electrical system work together to pump blood efficiently. From the myocardium's powerful contractions to the pericardium's protective embrace, each layer plays a crucial role in cardiac function.

The cardiac conduction system orchestrates the heart's rhythm, starting with the SA node's pacemaker cells. This electrical network ensures coordinated contractions, allowing for proper blood flow through the chambers and valves during each heartbeat.

Cardiac Wall Layers

Myocardium

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• Thick, muscular middle layer of the heart wall composed of cardiac muscle tissue
• Responsible for the contraction and pumping action of the heart
• Consists of specialized cardiac muscle cells called cardiomyocytes
• Cardiomyocytes are striated, branched, and interconnected by intercalated discs which allow for synchronized contraction
• Myocardium is thickest in the left ventricle (pumps blood to the entire body) and thinnest in the atria (pumps blood to the ventricles)

Pericardium

• Double-layered membranous sac that surrounds and protects the heart
• Outer layer is the fibrous pericardium which is tough and inelastic, provides protection and anchors the heart to surrounding structures
• Inner layer is the serous pericardium which consists of the parietal pericardium (lines the fibrous pericardium) and the visceral pericardium or epicardium (covers the outer surface of the heart)
• Between the parietal and visceral layers is the pericardial cavity filled with pericardial fluid which reduces friction during heart contractions

Endocardium

• Thin, smooth inner lining of the heart chambers and valves
• Composed of endothelial cells continuous with the endothelium of blood vessels
• Provides a non-thrombogenic surface to prevent blood clotting inside the heart
• Covers the heart valves (tricuspid, pulmonary, mitral, aortic) and is thicker here to withstand the pressure gradients
• Plays a role in regulating myocardial function through the release of substances like nitric oxide and endothelin

Cardiac Conduction System

Sinoatrial (SA) Node

• Known as the "natural pacemaker" of the heart, located in the upper wall of the right atrium
• Consists of specialized cardiac muscle cells that spontaneously depolarize, generating electrical impulses
• Fastest intrinsic rate of depolarization (60-100 times/min), setting the pace for the heart
• Receives autonomic nervous system input to modulate heart rate (sympathetic increases, parasympathetic decreases)

Atrioventricular (AV) Node

• Located in the interatrial septum near the tricuspid valve, receives impulses from the SA node
• Conducts impulses from atria to ventricles with a delay (about 0.1s), allowing time for atrial contraction and ventricular filling
• Acts as a secondary pacemaker if the SA node fails, with an intrinsic rate of 40-60 beats/min
• Only pathway for impulses to travel from atria to ventricles (AV bundle/Bundle of His arise here)

Purkinje Fibers

• Specialized conduction cells that rapidly conduct impulses through the ventricles
• Arise from the left and right bundle branches which are continuations of the AV bundle
• Allow for coordinated, nearly simultaneous depolarization and contraction of ventricular muscle
• Conduct impulses much faster than typical cardiac muscle cells (about 4 m/s vs 0.5 m/s)

Action Potential

• Brief reversal of membrane potential that occurs in excitable cells like cardiac muscle cells
• Cardiac action potentials differ from neurons - longer duration (200-400ms) and plateau phase
• 5 phases:
  1. Rapid depolarization (Na+ influx)
  2. Initial repolarization (K+ efflux)
  3. Plateau (Ca2+ influx and K+ efflux balance)
  4. Repolarization (K+ efflux)
  5. Resting membrane potential
• Absolute refractory period during phases 1-3 prevents tetanic contraction, allows time for ventricular filling

Cardiac Function

Cardiac Cycle

• Sequence of electrical and mechanical events that occur during one heartbeat
• Divided into systole (contraction and ejection) and diastole (relaxation and filling) for both atria and ventricles
• One cycle:
  1. Atrial systole
  2. Atrial diastole and ventricular systole
  3. Ventricular diastole
• Coordinated by the cardiac conduction system and regulated by autonomic nerves and hormones
• Pressure changes open and close heart valves to ensure one-way blood flow

Systole and Diastole

• Systole: period of contraction and ejection of blood from the chambers
  • Atrial systole: contraction of atria, "atrial kick" contributes final 20-30% of ventricular filling
  • Ventricular systole: contraction of ventricles, ejects blood into aorta and pulmonary arteries
• Diastole: period of relaxation and filling of the chambers
  • Atrial diastole: relaxation of atria, passive filling from venous return
  • Ventricular diastole: relaxation of ventricles, passive filling from atria then atrial kick at end
• Diastole is longer than systole, especially at lower heart rates, to allow for adequate filling

Electrocardiogram (ECG)

• Recording of the electrical activity of the heart over time using electrodes placed on the skin
• Displays the sum of the action potentials of cardiac muscle cells, conducted through body fluids
• Components:
  • P wave: atrial depolarization
  • QRS complex: ventricular depolarization
    • Q wave: septal depolarization
    • R wave: apex depolarization
    • S wave: basal depolarization
  • T wave: ventricular repolarization
  • U wave (not always seen): papillary muscle repolarization
• Important intervals:
  • PR interval: time from atrial depolarization to ventricular depolarization, indicates AV node delay
  • QT interval: time from ventricular depolarization to repolarization, indicates ventricular action potential duration
• Useful diagnostic tool to detect cardiac abnormalities in rate, rhythm, conduction, and structure