G2iii / 18A12: Ventricular diastole

18A12: Exam Report

Briefly describe the cardiac events that occur during ventricular diastole.

29% of candidates passed this question.

Many answers lacked structure and contained insufficient information. Better answers defined diastole and described the mechanical events in the 4 phases of diastole. A common error was the ECG events in diastole. The electrical events and coronary blood flow should have been mentioned.

G2iii / 18A12: Describe the cardiovascular events that occur during ventricular diastole

Diastole = the period of the cardiac cycle where the ventricles relax & fill

  • Lusitropy = the ability of cardiac myocytes to relax
  • Events of diastole divided into:
    • Ionic ∆
    • ECG ∆
    • Mechanical ∆

NOTE: Most cardiac work occurs during diastole

1) Ionic

  • Cytosolic [Ca2+] regulates active events of myocytes
  • ↓[Ca2+] causes Ca2+ to dissociate from TnC & inhibits cross-bridging
  • ↓Ca2+ is facilitated by:
    1. SERCA pump → pumps Ca2+ back into SR
    2. Sarcolemma Ca2+-ATPase → moves Ca2+ extracellularly
  • Facilitates myocyte relaxation (lusitropy)

2) ECG

      1. Isovolumetric relaxation: ventricular repol. Is being completed → end of T-wave on ECG
      2. Rapid filling: no electrical activity by cardiac cells ∴isoeletric line on ECG
      3. Slow filling: depol. spreads from SA Node across atria → P wave on ECG
      4. Atrial contraction: atrial depol. complete → end of P wave. Depol spreads from atria → AV Node = PR segment on ECG

3) Mechanical Events

    • There are 4 phases of ventricular diastole
      1. Isovolumetric relaxation
      2. Rapid filling
      3. Reduced filling
      4. Atrial systole

4. Isovolumetric relaxation

  • All ventricles relax
  • Ventricular pressure falls
  • Outflow tract P > ventricular P → blood flow reverses → aortic + pulmonary valves shut
  • Valves shut = S2 = commencement of DIASTOLE
  • All valves are shut → ventricular volumes remain constant
  • Volume in ventricle KA ESV ~50mL

5. Rapid filling

    • Atrial P > ventricular P → AV valves open
    • Ventricle fills with blood → fast because atria were so full prior to AV valve opening
    • Filling is passive
    • Once ventricles are fully relaxed → their pressure begins to rise as they fill

6. Reduced filling

  • Passive ventricular filling nears completion
  • Ventricle is filling with blood & expanding, but becoming less compliant
  • As the ventricular P increases, the rate of blood flow from atria → ventricle will fall

7. Atrial systole

    • Atria contract → drives more blood into ventricle
    • Atrial pressure falls → reserving pressure gradient across AV valves → AV valves shut
    • At the end of this phase, ventricles are filled to their ECV ~120mL with P 3mmHg
Atrial systole3