Cardiac Cycle

Master answer

Diastole in the cardiac cycle corresponds with cardiac myocyte relaxation, and includes periods of isovolumetric relaxation and ventricular filling. It lasts approximately 430ms.

Isovolumetric relaxation

• As aortic pressure overcomes ventricular pressure, and the inertia of forward flow of blood out of the LVOT, the aortic valve closes
• The ventricular myocytes relax, while both the mitral (or tricuspid) and aortic (or pulmonary valves) remain closed
• The volume of blood within the ventricle remains constant (LV End-Systolic Volume ~50mls) , however ventricular pressure decreases from 120mmHg to close to 0mmHg (or 20 to 0mmHg in R ventricle

Ventricular filling

• Passive phase
  • As ventricular pressure decreases below the atrial pressure, the mitral (or tricuspid) valve opens allowing forward flow of blood from the atria to the ventricles leading to a progressive increase in ventricular volume and pressure.
  • The first two-thirds of the ventricular filling is due to passive filling
• Active phase
  • 20% of ventricular filling is provided by the atrial kick during the final quarter of ventricular filling resulting in a slight increase in ventricular pressure
  • Ventricular pressure overcomes atrial pressure and the mitral (or tricuspid) valves close

Electrochemical events

• The cardiac action potential is tied to contraction via excitation-contraction mechanism.
• Diastole corresponds to phase IV where voltage gated L and T-type Ca channels close, ceasing Ca influx, and ongoing K efflux repolarizes the myocyte.
• ECG
  • The T wave corresponds with ventricular repolarization and occurs immediately prior to ventricular diastole.
  • The QRS complex corresponds with ventricular depolarization and occurs immediately prior to ventricular systole
  • Diastole corresponds with the terminal part of the T wave to immediately after the QRS complex
• Electrical current through cardiac cycles
  • During depolarization, current runs from the base of the heart to the apex (corresponding to flow down bundle of His) the just before the end of depolarization, the direction of flow reverses (corresponding to flow down Purkinje fibres)
  • During repolarization, the order reverses, from Purkinje system → Bundle of His

Coronary artery perfusion

• Due to the high ventricular pressures impeding blood flow during systole in a starling-resistor model, left ventricular perfusion (or right ventricular perfusion during pressure-overloaded states) occurs predominantly during diastole.
• As right ventricular pressures are significantly lower than aortic pressures even during systole, perfusion continues (in a normal right ventricle)

Sakurai 2016

Answer by Sakurai