Cardiac Cycle Overview

Overview

This lecture explains the cardiac cycle, linking the electrical (ECG/conduction system) and mechanical (contraction/relaxation) events in the heart and how these produce heart sounds.

Electrical Events of the Cardiac Cycle

• The heart's electrical activity is measured by the ECG (electrocardiogram).
• The SA node initiates depolarization, spreading through the atria and causing the P wave (atrial depolarization).
• The AV node delays the signal, seen as a flatline after the P wave.
• Depolarization through the bundle branches produces the Q wave (septal depolarization).
• Depolarization of the thick ventricular muscle generates the R wave (ventricular depolarization).
• Further spreading through the ventricles causes the S wave.
• The QRS complex represents overall ventricular depolarization.
• Repolarization (resetting) of the ventricles produces the T wave (ventricular repolarization).

Mechanical Phases of the Cardiac Cycle

• The heart alternates between contraction (systole) and relaxation (diastole).
• Ventricular Filling (Diastole): Blood passively enters ventricles; AV valves open, semilunar valves closed.
• Atrial Contraction (Systole for atria): Atria actively push remaining blood into ventricles; AV valves still open, semilunar valves closed.
• Isovolumetric Contraction (Ventricular Systole): Ventricles contract, all valves closed, no blood movement.
• Ventricular Ejection (Ventricular Systole): Ventricles push blood out; AV valves closed, semilunar valves open.
• Isovolumetric Relaxation (Diastole): Ventricles relax, all valves closed, no blood movement.

Valve Status and Heart Sounds

• Valve closure produces heart sounds.
• S1 (first heart sound) occurs when AV valves close (start of isovolumetric contraction).
• S2 (second heart sound) occurs when semilunar valves close (start of isovolumetric relaxation).

ECG and Mechanical Event Correlation

• Mechanical contraction follows electrical depolarization, usually starting halfway through the corresponding wave.
• Atrial contraction follows P wave; ventricular contraction follows QRS complex.
• S1 heard around halfway through QRS; S2 heard after the T wave.

Key Terms & Definitions

• Depolarization — Electrical activation that triggers contraction.
• Repolarization — Electrical resetting of heart muscle.
• SA Node — Primary pacemaker of the heart.
• AV Node — Delays electrical signal before it reaches ventricles.
• QRS Complex — ECG representation of ventricular depolarization.
• Systole — Contraction phase of the heart.
• Diastole — Relaxation phase of the heart.
• Isovolumetric Contraction/Relaxation — Phases with no volume change; all valves closed.
• Preload — Amount ventricles are filled before contraction.
• AV Valves (Tricuspid/Bicuspid) — Separate atria from ventricles.
• Semilunar Valves — Valves at aorta and pulmonary artery exits.

Action Items / Next Steps

• Review ECG tracing basics and the relationship to heart mechanics.
• Study the timing and significance of heart sounds S1 and S2.
• Understand valve status during each cardiac cycle phase.