Question 12

College Answer

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.

Discussion

Definition of diastole

• Diastole is the period of chamber relaxation and cardiac filling which corresponds to
  • The period between the end of the T wave and the end of the PR interval
  • The period during which the mitral valve/tricuspid valves are open.

Events during diastole:

• Isovolumetric relaxation
  • This period begi
  • The ventricles relax without any change in volume
  • The pressure drops until the tricuspid and mitral valves open
  • The beginning of this period corresponds to the peak of the T-wave, and the middle (steep portion) of Phase 3 (repolarisation) of the cardiac myocyte action potential
  • The end of this period corresponds to the end of the T wave on the surface ECG, and the end of Phase 3
• Early rapid diastolic filling
  • During this period the relaxing ventricles have pressure lower than atrial pressure, and they fill rapidly
  • 80% of the ventricular end-diastolic volume is achieved during this phase
  • Coronary blood flow is maximal during this phase
• Late slow diastolic filling
  • Ventricular and atrial pressures equilibrate and the atria act as passive conduits for ventricular filling
  • The end of this phase corresponds to the end of the P-wave on the surface ECG
• Atrial systole
  • The atria contract (right first, then left shortly after)
  • This increases the pressure in the ventricles up to the end-diastolic pressure, and adds about 20ml of extra volume to the end-diastolic volume
  • These events start at the end of the P-wave on the surface ECG, and finish during the PR interval.
  • The end of this phase corresponds to the peak of the R wave, or the Phase 0 (rapid sodium influx) of the ventricular myocyte action potential

Wiggers, Carl J. "Studies on the consecutive phases of the cardiac cycle: I. The duration of the consecutive phases of the cardiac cycle and the criteria for their precise determination." American Journal of Physiology-Legacy Content 56.3 (1921): 415-438.

Braunwald, Eugene, ALFRED P. FISHMAN, and AndrÉ Cournand. "Time relationship of dynamic events in the cardiac chambers, pulmonary artery and aorta in man." Circulation research 4.1 (1956): 100-107.

Luisada, Aldo A., and Donald M. MacCanon. "The phases of the cardiac cycle." American heart journal 83.5 (1972): 705-711.