Overview
This lecture explains the components of the cardiac conduction system, their functions, and the interpretation of heart electrical activity using an electrocardiogram (ECG).
Cardiac Conduction System Components
- The SA (sinoatrial) node in the right atrium acts as the heart's natural pacemaker and sets the heart rate.
- SA node initiates electrical impulses that spread across both atria, causing atrial contraction.
- The AV (atrioventricular) node, located near the AV valve, delays impulses to the ventricles to allow complete atrial emptying.
- The AV node transfers signals to the atrioventricular bundle (bundle of His).
- The AV bundle divides into right and left branches, conducting signals toward the apex of the heart.
- Purkinje fibers distribute impulses throughout the ventricular myocardium for coordinated ventricular contraction.
Electrocardiogram (ECG/EKG)
- An ECG records the total electrical activity of the heart from all cardiac cells and nodes.
- The P wave shows atrial depolarization caused by SA node firing.
- Atrial systole begins about 100 ms after the P wave starts.
- The P-Q segment marks the impulse travel time from SA node to AV node.
- The QRS complex represents ventricular depolarization, including:
- Q wave: interventricular septum depolarization,
- R wave: main ventricle mass depolarization,
- S wave: last phase at the base of the heart.
- Atrial repolarization occurs during QRS but is hidden by its large signal.
- The S-T segment represents the plateau phase when ventricles contract and pump blood.
- The T wave indicates ventricular repolarization before relaxation (ventricular diastole).
- The pattern repeats with every heartbeat.
Key Terms & Definitions
- SA node — Natural pacemaker initiating heartbeat and setting heart rate.
- AV node — Electrical gateway delaying impulses to ventricles for proper blood flow.
- AV bundle (bundle of His) — Conducts signals from AV node to bundle branches.
- Purkinje fibers — Fibers distributing impulses through ventricular muscle.
- ECG/EKG — A record of the heart’s electrical events.
- Depolarization — Electrical activation leading to muscle contraction.
- Repolarization — Return to resting state after contraction.
Action Items / Next Steps
- Review the components and functions of the cardiac conduction system.
- Study ECG waveforms and understand what each segment represents.