Lecture on the Cardiac Cycle

Introduction

• The cardiac cycle involves all mechanical events where blood flows through the heart chambers.
• Average duration: 0.8 seconds.
• Discussion points include:
  • Atrial vs. Ventricular Pressure
  • Arterial vs. Ventricular Pressure
  • Status of AV and Semilunar Valves
  • Corresponding EKG components

Key Phases of the Cardiac Cycle

1. Mid to Late Ventricular Diastole

• Definition: Diastole is relaxation; thus, the ventricles are in the relaxation phase.
• Blood Flow:
  • Blood returns to the heart via inferior/superior vena cava, coronary sinus, and pulmonary veins.
  • Accumulates in the atria, causing atrial pressure > ventricular pressure.
  • AV valves (tricuspid and mitral) open, allowing passive flow (70-80%) into the ventricles.
• Valves:
  • AV valves open, semilunar valves closed.
• EKG: P wave indicates atrial depolarization, pushing remaining 20% of blood into ventricles.
• Phase Name: Period of Ventricular Filling.

2. Isovolumetric Contraction (Early Ventricular Systole)

• Ventricular Activity:
  • Ventricles start depolarizing and contracting.
  • Isovolumetric contraction—ventricular volume is constant as all valves are closed.
• Pressure Relationships:
  • Ventricular pressure rises but is less than arterial pressure.
  • AV valves close (first heart sound, "lub").
  • Arterial pressure > ventricular pressure, keeping semilunar valves shut.
• EKG: QRS complex indicating ventricular depolarization.

3. Mid to Late Ventricular Systole (Ventricular Ejection)

• Ventricular Activity:
  • Ventricular pressure > arterial pressure, opening semilunar valves.
  • Blood ejected into pulmonary trunk and aorta.
• Valves:
  • Semilunar valves open, AV valves remain closed.
• Pressure Relationships:
  • Ventricular pressure > arterial pressure.
  • Atrial pressure remains low.
• EKG: Still represented by QRS complex.

4. Isovolumetric Relaxation (Early Ventricular Diastole)

• Ventricular Activity:
  • Ventricles start relaxing (repolarization).
  • All four heart valves closed momentarily.
  • Blood distributed to systemic, pulmonary, and coronary circuits.
• Valves:
  • Semilunar valves close (second heart sound, "dub").
  • AV valves remain closed.
• Pressure Relationships:
  • Arterial pressure > ventricular pressure (dicrotic notch observed).
  • Ventricular pressure > atrial pressure.
• EKG: T wave indicating ventricular repolarization.

Conclusion

• Cycle repeats approximately every 0.8 seconds.
• Understanding of pressure dynamics and valve operation crucial to cardiac cycle comprehension.