Cardiac Conduction System and Electrocardiogram (ECG)

Sinoatrial (SA) Node and Cardiac Conduction

Location: SA node is in the right atrium near the entrance of the superior vena cava.
Function: natural pacemaker of the heart; initiates all heartbeat and determines heart rate.
Propagation: electrical impulses from the SA node spread through both atria, stimulating atrial contraction.

Location: AV node is on the other side of the right atrium near the AV valve.
Function: serves as the electrical gateway to the ventricles; delays the passage of impulses to the ventricles.
Significance of delay: ensures the atria have ejected all blood into the ventricles before the ventricles contract.
Signal transmission: the AV node receives signals from the SA node and passes them to the AV bundle (bundle of His).

AV bundle divides into right and left bundle branches, which conduct impulses toward the apex of the heart.
Signals are then passed onto millions of Purkinje fibers and spread throughout the ventricles.

ECG/EKG is a composite recording of all action potentials produced by the nodal and other cardiac cells.
Each wave or segment on the ECG corresponds to a specific event in the cardiac electrical cycle.
When the SA node fires, atrial depolarization occurs, which is represented by the P wave on the ECG.

Atrial systole begins about one hundred milliseconds after the P wave begins.
- Timing: t_{ ext{atrial systole}} \approx 100\ \mathrm{ms}
PR interval: measures from the start of the P wave to the start of the QRS complex.
- Represents the time between atrial depolarization and ventricular depolarization.
- Reflects conduction through the AV node.
- In formula form: PR\ interval = t{\text{start}(P)} \to t{\text{start}(QRS)}}.
QRS complex: represents ventricular depolarization.
- Q wave: corresponds to depolarization of the interventricular septum.
- R wave: depolarization of the main mass of the ventricles.
- S wave: represents the last phase of ventricular depolarization moving toward the base of the heart.
Atrial repolarization: occurs during the QRS complex but is obscured by the large QRS complex.
ST segment: reflects the plateau phase of ventricular action potentials.
- This is when the ventricles contract and pump blood.
- In other words, the plateau phase corresponds to the time ventricles are in contraction while staying electrically isoelectric.
T wave: represents ventricular repolarization, just before ventricular relaxation (diastole).

The electrical cycle initiates depolarization, leading to contraction (systole) and blood ejection, followed by relaxation (diastole) as repolarization completes.
The sequence of events: SA node firing → atrial depolarization (P wave) → AV node delay → ventricular depolarization (QRS) → ventricular contraction (ST segment) → ventricular repolarization (T wave) → ventricular relaxation → cycle repeats with each heartbeat.

Prominent concept: the conduction system coordinates one-way propagation of impulses to ensure efficient, timed cardiac pumping.
Real-world relevance: ECG interpretation hinges on recognizing the P wave, PR interval, QRS complex, ST segment, and T wave to assess rhythm, conduction delays, and ventricular function.
Foundational link: these electrical events underlie the mechanical events of the cardiac cycle (atrial and ventricular systole and diastole).
Practical implications: abnormalities in any component (e.g., prolonged PR interval, wide QRS, ST segment changes) can indicate conduction defects, ischemia, or chamber dysfunction.

The cycle repeats itself with every heartbeat.
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