Cardiac Electrophysiology Quiz

Cardiac Electrophysiology

• Cardiac Action Potential:
  • Propagation: Action potentials spread throughout the heart, enabling coordinated contraction.
  • Excitability: The ability of heart cells to respond to a stimulus and generate an action potential.
  • Refractory Period: The period following an action potential where the cell is unresponsive to further stimulation.

The SA Node (Sinoatrial Node)

• Pacemaker of the Heart: The SA Node initiates the electrical impulse that triggers each heartbeat.
• Spontaneous Discharge: The SA Node can generate action potentials independently, setting the heart's rhythm.
• Sympathetic Nervous System: Increases the rate of discharge from the SA Node, speeding up the heart rate.
• Parasympathetic Nervous System: Decreases the rate of discharge from the SA Node, slowing down the heart rate.

Action Potential of the SA Node

• Prepotential (Unstable Resting Membrane Potential): A gradual depolarization leading to the action potential.
• Prepotential Mechanisms:
  • Decreased Potassium Efflux: Potassium ions leak out of the cell more slowly during the prepotential, contributing to depolarization.
  • Calcium Influx: Calcium ions enter the cell through T-type calcium channels, further depolarizing the membrane.
  • Funny Current: A mixed sodium and potassium influx through funny channels, also playing a role in prepotential development.
• Sympathetic Stimulation: Makes the prepotential steeper, increasing the rate of discharge.
• Parasympathetic Stimulation: Makes the prepotential more horizontal, decreasing the rate of discharge.

The AV Node (Atrioventricular Node)

• Location: Situated above the fibrous ring, connecting the atria to the ventricles via the bundle of His.
• Slowest Conduction Velocity: The AV Node conducts impulses slowly (0.02 to 0.05 m/sec) due to gap junctions, contributing to a delay in conduction.
• Conduction Delay (0.1 sec): This delay protects the ventricles from rapid atrial rhythms (e.g., atrial fibrillation) by preventing excessively fast ventricular contractions.

Bundle of His (AV Bundle)

• Location: Crosses the fibrous ring, carrying the electrical impulse from the AV Node to the ventricles.

The Bundle Branches

• Location: Branch off from the bundle of His, conveying the impulse to the left and right ventricles.

Purkinje Fibers

• Fastest Conduction Velocity: Purkinje fibers conduct impulses very rapidly (4-5 m/sec), ensuring efficient and synchronized ventricular contraction.

Spread of Cardiac Excitation

• Interventricular Septum: The impulse travels through the interventricular septum, spreading from left to right.

The Electrocardiogram (ECG)

• Electrical Activity of the Heart: The ECG records the electrical activity generated by the heart.
• Summation of Action Potentials: The ECG represents the sum of all action potentials occurring in the heart at any given time.
• Electrodes: Ten electrodes are used for ECG recording.
  • Nine active electrodes placed on the chest and limbs.
  • One inactive electrode on the right leg.
• Electrocardiograph: This device is a modified voltmeter used to record the ECG.
• ECG lead types:
  • Unipolar lead: Uses one active electrode and an inactive electrode, resulting in nine unipolar leads.
  • Bipolar lead: Uses two active electrodes and an inactive electrode, resulting in three bipolar leads.
• Standard ECG: Consists of 12 leads:
  • 3 bipolar limb leads.
  • 3 augmented limb leads.
  • 6 chest leads.