Understanding the Cardiac Cycle Phases

Sep 11, 2024

Cardiac Cycle Lecture Notes

Overview of the Cardiac Cycle

  • The cardiac cycle involves all mechanical events where blood flows through the heart's chambers.
  • Average duration: 0.8 seconds.

Key Concepts Discussed

  • Differences in atrial vs. ventricular pressure
  • Differences in arterial vs. ventricular pressure
  • Phenomena related to AV valves (Atrioventricular Valves)
  • Phenomena related to semilunar valves (Pulmonary and Aortic Valves)
  • Correlation with EKG components

Phases of the Cardiac Cycle

1. Mid to Late Ventricular Diastole

  • Diastole: relaxation phase of the heart.

  • Blood returns to the heart from:

    • Inferior vena cava
    • Superior vena cava
    • Coronary sinus
    • Pulmonary veins

  • Atrial Pressure > Ventricular Pressure: opens the AV valves (Tricuspid and Bicuspid/Mitral).

  • 70-80% of blood flows passively into ventricles due to gravity.

  • EKG: Atrial depolarization (P wave) occurs as SA node fires, pushing the remaining 20% of blood into ventricles.

  • This phase is also known as the period of ventricular filling.

2. Isovolumetric Contraction

  • Ventricles begin to depolarize and contract, creating increased pressure but no blood is ejected yet.
  • Ventricular Pressure < Arterial Pressure: semilunar valves remain closed.
  • AV Valves: close when ventricular pressure exceeds atrial pressure; creates the first heart sound (S1) or "Lub".
  • End-diastolic volume (EDV) is reached as blood accumulates in the ventricles.

3. Ventricular Ejection

  • Ventricles continue to contract, leading to increased pressure until it exceeds arterial pressure.
  • Ventricular Pressure > Arterial Pressure: semilunar valves open, allowing blood to be ejected.
  • AV Valves: remain closed.
  • This phase is known as mid to late ventricular systole or ventricular ejection.
  • EKG: QRS complex signifies ventricular depolarization and contraction.

4. Iso-volumetric Relaxation

  • Ventricles relax post-ejection, leading to a decrease in pressure.
  • Remaining blood volume post-contraction is known as end-systolic volume (ESV).
  • Arterial Pressure > Ventricular Pressure: semilunar valves close, producing the second heart sound (S2) or "Dub".
  • EKG: Ventricular repolarization appears as the T wave.

Summary of Pressure Relationships

  • Atrial pressure is greater than ventricular pressure during diastole.
  • Ventricular pressure becomes greater than atrial pressure during contraction, leading to valve closures.
  • Arterial pressure is always greater than ventricular pressure until ejection occurs.

Conclusion

  • The cardiac cycle continuously repeats, with each cycle taking approximately 0.8 seconds.
  • Understanding these phases is crucial for grasping cardiac output and overall heart function.