Question 21

College Answer

One possible way to answer this question is to offer a definition of the diastolic
period then to split the events up for description into mechanical events, ECG
events and electrical/ionic events.
Few candidates defined the diastolic period, and whilst many talked about
opening and closing of valves, there was generally a poor understanding of the
sequence of events whereby the left ventricle comes to be filled with blood. The
better answers included a description of the ionic events that occurred at the
various stages of diastole. Many answers lacked any reference to the ECG
events in diastole.
The major weakness in answers was again the failure to include sufficient
information to achieve a pass mark. This was probably as a result of the lack of a
systematic approach when answering a question of this nature.
Syllabus: C1b, 2d,e and C1c, 2e,f
Recommended sources: Textbook of Medical Physiology, Guyton & Hall, Chp 9 – 11
and Review of Medical Physiology, Ganong, Chp 31

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.