The Cardiac Cycle

Cardiac Cycle Summary

  • Definition: Sequence of electrical and mechanical events in each heartbeat.

  • Phases:

    • Systole: Ventricular contraction.

    • Diastole: Ventricular relaxation and filling.

  • Main Phases of Cardiac Cycle:

    1. Passive Filling

    2. Atrial Contraction

    3. Isovolumic Ventricular Contraction (IVC)

    4. Rapid Ejection

    5. Isovolumic Ventricular Relaxation

Wiggers Diagram

  • Purpose: Visual aid for teaching cardiac physiology.

  • Components:

    • X-axis: Time

    • Y-axis:

    • Blood pressure (aortic, ventricular, atrial)

    • Ventricular volume

    • Electrocardiogram

    • Optional: Arterial flow, Heart sounds

  • Illustrates coordinated changes in cardiac cycle values.

Atrial Contraction

  • Triggered by P wave on ECG.

  • Results in:

    • Contraction of atria

    • Blood flow across AV valves

    • Small pressure increase noted as the "a-wave" in left atrial pressure.

  • Atrial kick contributes significantly to ventricular filling.

  • Heart Sound: Fourth heart sound (S4) may occur due to vibration during atrial kick.

Isovolumic Contraction

  • Initiated by QRS complex leading to:

    • Contraction and rapid increase in intraventricular pressure.

    • Closure of AV valves creates first heart sound (S1).

  • Ventricular volume remains unchanged during this phase.

Ejection Phase

  • Rapid Ejection: Blood ejected into aorta/pulmonary artery after valve opening.

  • Reduced Ejection: Ventricular repolarization and relaxation occur, reducing ejection rate.

Isovolumic Relaxation

  • Starts diastole; valves close, creating second heart sound (S2).

  • Ventricular pressures drop but volume remains constant.

  • The peak left atrial pressure (v-wave) occurs here.

Rapid Filling

  • A-V valves open, blood floods ventricles, relaxing further.

  • May produce a third heart sound (S3) during filling, often benign in children but pathological in adults.

Stroke Volume and Cardiac Output

  • Stroke Volume (SV): Difference between end-diastolic volume (EDV) and end-systolic volume (ESV).

  • Cardiac Output (CO): Product of SV and heart rate (HR). Normal range ~ 5-6 L/min.

Regulation of Cardiac Output

  • Cardiac Index: CO normalized per body surface area (BSA). Normal range 2.6 - 4.2 L/min/m².

  • Heart Rate Impact: Rapid heart rate inversely affects SV due to reduced filling time.

  • Factors Influencing Stroke Volume:

    • Preload: Initial chamber stretch.

    • Afterload: Resistance against ventricular contraction.

    • Inotropy: Force of muscle contraction independent of sarcomere length changes.

  • Definition: Sequence of electrical and mechanical events in each heartbeat.

  • Phases:

    • Systole: Ventricular contraction, where the heart pumps blood out to the body.

    • Diastole: Ventricular relaxation and filling; the heart chambers fill with blood.

  • Main Phases of Cardiac Cycle:

    1. Passive Filling: Blood flows passively from the atria into the ventricles, filling them before the atria contract.

    2. Atrial Contraction: Triggered by the P wave on the ECG, the atria contract to push blood into the ventricles, contributing significantly to ventricular filling.

    3. Isovolumic Ventricular Contraction (IVC): Initiated by the QRS complex, the ventricles contract, increasing intraventricular pressure while the volume remains unchanged as all valves are closed. This phase results in the closure of AV valves and the first heart sound (S1).

    4. Rapid Ejection: Blood is ejected into the aorta and pulmonary artery due to the opening of the semilunar valves as the ventricular pressure exceeds aortic and pulmonary pressures.

    5. Reduced Ejection: This phase occurs after the peak of ejection. Ventricular repolarization and relaxation begin, leading to a gradual decrease in the rate of blood ejection.

    6. Isovolumic Relaxation: This phase marks the beginning of diastole, where the closure of semilunar valves creates the second heart sound (S2). The ventricular pressure decreases while the ventricular volume remains constant.

    7. Rapid Filling: A-V valves open as the pressure in the ventricles drops below that in the atria, allowing blood to flood the ventricles. This may produce a third heart sound (S3), especially in younger individuals, but it can be pathological in adults.