Cardiac Conduction System

Components of the Cardiac Conduction System

• Sinoatrial Node (SA Node):

  • Located in the right atrium near the entrance of the superior vena cava.
  • Acts as the natural pacemaker of the heart.
  • Initiates all heartbeats and determines heart rate.
  • Electrical impulses spread throughout both atria, stimulating contraction.

• Atrioventricular Node (AV Node):

  • Located on the other side of the right atrium, near the AV valve.
  • Functions as the electrical gateway to the ventricles.
  • Delays electrical impulses to the ventricles, allowing atria to eject blood into the ventricles before they contract.
  • Receives signals from the SA node and passes them to the atrioventricular bundle.

• Atrioventricular Bundle (AV Bundle or Bundle of His):

  • Divides into right and left bundle branches.
  • Conducts impulses toward the apex of the heart.
  • Signals then passed to Purkinje fibers, spreading throughout the ventricles.

Electrocardiogram (ECG or EKG)

• A composite recording of all the action potentials produced by heart nodes and cells.
• Each wave or segment corresponds to events in the cardiac electrical cycle.

Components of the ECG

• P Wave:

  • Occurs when SA node fires and electrical signals spread throughout atria causing depolarization.
  • Atrial contraction (atrial systole) begins ~100 milliseconds after P wave starts.

• PR Interval:

  • Time from start of P wave to start of QRS complex.
  • Represents time between atrial depolarization and ventricular depolarization.
  • Reflects conduction through the AV node.

• QRS Complex:

  • Represents ventricular depolarization.
  • Q Wave: Depolarization of the interventricular septum.
  • R Wave: Depolarization of the main mass of the ventricles.
  • S Wave: Last phase of ventricular depolarization toward the base of the heart.
  • Atrial repolarization occurs during this time but is obscured by the large QRS complex.

• ST Segment:

  • Reflects the plateau of action potentials in the ventricles.
  • This is when the ventricles contract and pump blood.

• T Wave:

  • Represents ventricular repolarization before ventricular relaxation (ventricular diastole).

Summary

• The cardiac cycle repeats with each heartbeat, involving a coordinated sequence of electrical and mechanical events.

Ventricular depolarization is a critical phase of the cardiac cycle, primarily responsible for the contraction of the ventricles, the heart's main pumping chambers. Here's a breakdown of what happens during this process:

1. Electrical Impulse Initiation: The process begins with an electrical impulse generated by the sinoatrial (SA) node, the heart's natural pacemaker. This impulse travels through the atria, causing them to contract and push blood into the ventricles.

2. Impulse Conduction: The impulse then reaches the atrioventricular (AV) node, where it is briefly delayed to allow the ventricles to fill with blood. From the AV node, the impulse travels down the bundle of His, which divides into the right and left bundle branches, and then into the Purkinje fibers.

3. Depolarization of Ventricular Muscle Cells: As the impulse travels through the Purkinje fibers, it causes the ventricular muscle cells to depolarize. Depolarization is a change in the electrical charge across the cell membrane, making the inside of the cell more positive.

4. Ventricular Contraction: The depolarization of the ventricular muscle cells triggers their contraction. This contraction increases the pressure within the ventricles, forcing blood out of the heart: into the pulmonary artery from the right ventricle and into the aorta from the left ventricle.

5. ECG Representation: On an electrocardiogram (ECG), ventricular depolarization is represented by the QRS complex. This is a series of waves that indicate the rapid depolarization of the ventricles.

Overall, ventricular depolarization is essential for the effective pumping of blood throughout the body, ensuring that oxygen and nutrients are delivered to tissues and organs.

Ventricular repolarization is the process that follows ventricular depolarization and is essential for the heart to prepare for the next contraction. Here's what happens during ventricular repolarization:

1. Restoration of Electrical Charge: During repolarization, the electrical charge of the ventricular muscle cells is restored to its resting state. This involves the movement of ions across the cell membrane, primarily the outflow of potassium ions and the cessation of sodium and calcium inflow. This returns the inside of the cell to a more negative charge compared to the outside.

2. Relaxation of Ventricles: As the electrical charge is restored, the ventricles relax. This relaxation is crucial because it allows the ventricles to fill with blood from the atria in preparation for the next heartbeat.

3. Diastole Phase: Ventricular repolarization corresponds to the beginning of diastole, the phase of the cardiac cycle when the heart muscle relaxes and the chambers fill with blood. This is a critical period for the heart to receive oxygenated blood and nutrients through coronary circulation.

4. ECG Representation: On an electrocardiogram (ECG), ventricular repolarization is represented by the T wave. The T wave indicates the period during which the ventricles are returning to their resting electrical state.

Ventricular repolarization is essential for maintaining the heart's rhythm and ensuring that the heart is ready for the next cycle of depolarization and contraction. Any abnormalities in this process can lead to arrhythmias or other cardiac issues.