Lecture: The Cardiac Cycle

Definition

• Cardiac Cycle: All mechanical events where blood flows through the different chambers of the heart; takes approximately 0.8 seconds.
• Key aspects to discuss:
  • Atrial vs. Ventricle pressure
  • Arterial vs. Ventricle pressure
  • Status of AV valves (atrioventricular) and SLV (semilunar valves)
  • EKG correlation

Anatomy of Valves

• Semilunar Valves: Pulmonary and aortic

Mid to Late Ventricular Diastole

• Diastole: Relaxation phase
• Blood is returning to the heart from
  • Inferior vena cava
  • Superior vena cava
  • Coronary sinus
  • Pulmonary veins (to the left atrium)
• Event: Ventricular filling
  • Blood accumulates in atria, increasing atrial pressure
  • AV valves (tricuspid and bicuspid/mitral) open as atrial pressure is greater than ventricular pressure
  • 70-80% of blood flows passively into ventricles
• Atrial depolarization towards end of ventricular diastole
  • Additional 20% blood pushed into ventricles
  • EKG: P wave

Isovolumetric Contraction (Early Ventricular Systole)

• Systole: Contraction phase
• Ventricles start contracting
  • Ventricular pressure rises but is less than arterial pressure
  • Semilunar valves (aortic and pulmonary) remain closed
  • AV valves snap shut due to higher ventricular pressure compared to atrial pressure
  • Heart Sound: S1, “Lub”
  • EKG: QRS complex

Ventricular Ejection (Mid to Late Ventricular Systole)

• Ventricular pressure increases (120 mmHg in left, 25 mmHg in right)
  • Semilunar valves open due to pressure being greater than arterial pressure
  • Blood ejected into aorta and pulmonary trunk
  • Valve Status: AV valves closed, Semilunar valves open
  • Arterial vs. Ventricular Pressure: Arterial pressure is less than ventricular pressure
  • EKG: QRS complex

Isovolumetric Relaxation (Early Ventricular Diastole)

• Ventricles start relaxing
  • Aortic and pulmonary pressure greater than ventricular pressure
  • Semilunar valves snap shut
  • Heart Sound: S2, “Dub”
  • Valve Status: All valves closed (brief moment)
  • Arterial vs. Ventricular Pressure: Arterial pressure greater than ventricular pressure
  • EKG: T wave (ventricular repolarization)
• Dicrotic Notch: Brief rise in aortic pressure upon semilunar valve closure

Summary

• Cardiac cycle phases:
  • Mid to late ventricular diastole (ventricular filling)
  • Isovolumetric contraction
  • Ventricular ejection
  • Isovolumetric relaxation
• Cycle time: ~0.8 seconds