6.3 Electrical conduction system of the heart

The heart's electrical conduction system is like a complex orchestra, with each component playing a crucial role in coordinating the heartbeat. From the SA node's initial impulse to the Purkinje fibers' rapid signal transmission, this system ensures efficient blood pumping throughout the body.

Understanding the heart's electrical system is key to grasping cardiovascular function. It explains how the heart maintains its rhythm, responds to stress, and what can go wrong. This knowledge forms the foundation for diagnosing and treating various heart conditions.

Cardiac Conduction System Components

Nodes and Their Functions

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• The sinoatrial (SA) node acts as the primary pacemaker of the heart
  • Located in the right atrium
  • Spontaneously generates electrical impulses to initiate the heartbeat
• The atrioventricular (AV) node delays the electrical signal between the atria and ventricles
  • Located between the atria and ventricles
  • Allows time for atrial contraction to complete before ventricular contraction begins

Specialized Conduction Pathways

• The bundle of His transmits the electrical signal from the AV node to the ventricles
  • A specialized conduction pathway
  • Connects the AV node to the left and right bundle branches
• The left and right bundle branches conduct the electrical signal to their respective ventricles
  • Transmit the signal from the bundle of His to the ventricles
  • Ensure coordinated ventricular contraction
• Purkinje fibers rapidly transmit the electrical signal throughout the ventricles
  • Specialized conduction fibers
  • Ensure coordinated ventricular contraction from the apex to the base of the heart

Electrical Activation Sequence in the Heart

Atrial Depolarization and Contraction

• The SA node generates an electrical impulse that spreads through the atrial muscle fibers
  • Causes atrial depolarization and contraction
  • Impulse reaches the AV node after atrial contraction is complete
• The electrical signal is delayed at the AV node
  • Allows time for atrial contraction to complete before ventricular contraction begins
  • Ensures proper filling of the ventricles before contraction

Ventricular Depolarization and Contraction

• The signal travels through the bundle of His, left and right bundle branches, and Purkinje fibers
  • Bundle of His divides into left and right bundle branches
  • Purkinje fibers rapidly conduct the signal throughout the ventricles
• Ventricular depolarization and contraction occur from the apex to the base of the heart
  • Coordinated contraction ensures efficient pumping of blood
  • Ventricular repolarization occurs after contraction, allowing relaxation for the next cycle

Autonomic Nervous System Influence on Heart Rate

Sympathetic Nervous System Effects

• The sympathetic nervous system increases heart rate
  • Releases norepinephrine, which binds to beta-1 receptors on the SA node and cardiac muscle fibers
  • Activated during exercise or stress to meet increased physiological demands
• Sympathetic stimulation prepares the body for "fight or flight" response
  • Increases heart rate and contractility to deliver more oxygen to tissues
  • Helps maintain blood pressure during physical activity or stressful situations

Parasympathetic Nervous System Effects

• The parasympathetic nervous system decreases heart rate
  • Vagus nerve releases acetylcholine, which binds to muscarinic receptors on the SA and AV nodes
  • Dominant during rest and digestion to conserve energy
• Parasympathetic stimulation promotes relaxation and recovery
  • Slows heart rate to reduce myocardial oxygen demand
  • Helps maintain resting heart rate and prevents excessive tachycardia

Autonomic Balance and Heart Rate Regulation

• The balance between sympathetic and parasympathetic activity determines the resting heart rate
  • Allows the heart to respond to changes in physiological demands
  • Maintains homeostasis by adjusting heart rate as needed
• Autonomic imbalance can lead to abnormal heart rates and rhythms
  • Excessive sympathetic tone may cause tachycardia or arrhythmias
  • Reduced parasympathetic tone may result in higher resting heart rates and reduced variability

Common Arrhythmias and Their Effects

Sinus Node Arrhythmias

• Sinus bradycardia is a slow heart rate (typically < 60 bpm) originating from the SA node
  • May be caused by increased parasympathetic tone or certain medications (beta-blockers)
  • Can lead to symptoms such as fatigue, dizziness, or syncope in severe cases
• Sinus tachycardia is a fast heart rate (typically > 100 bpm) originating from the SA node
  • May be caused by increased sympathetic tone, exercise, or certain medical conditions (fever, anemia)
  • Usually a normal physiological response to increased metabolic demands

Atrial and Ventricular Arrhythmias

• Atrial fibrillation is a rapid, irregular atrial rhythm caused by disorganized electrical activity
  • Leads to ineffective atrial contraction and increased risk of blood clots (stroke)
  • May cause symptoms such as palpitations, shortness of breath, or chest discomfort
• Ventricular tachycardia is a rapid heart rate originating from the ventricles
  • May be caused by structural heart disease or electrolyte imbalances
  • Can lead to hemodynamic instability and reduced cardiac output
• Ventricular fibrillation is a life-threatening arrhythmia with chaotic, rapid electrical activity in the ventricles
  • Results in ineffective ventricular contraction and circulatory collapse
  • Requires immediate defibrillation to restore normal rhythm and prevent sudden cardiac death

Conduction Disorders

• Heart block refers to a delay or interruption in the conduction of electrical signals through the AV node or bundle branches
  • May be caused by fibrosis, ischemia, or congenital abnormalities
  • Can lead to bradycardia, reduced cardiac output, and symptoms such as fatigue or syncope
• Bundle branch blocks occur when electrical conduction is delayed or blocked in either the left or right bundle branch
  • May be associated with underlying heart disease or conduction system degeneration
  • Can cause abnormal ventricular activation and an altered QRS complex on the ECG