THE CARDIAC CYCLE

THE CARDIAC CYCLE

VENTRICULAR DIASTOLE

• ISOVOLUMIC RELAXATION: The very beginning of diastole, right after the aortic valve closes, during which both valves are closed.
  • The ventricular muscle is relaxing as ventricular pressure rapidly decreases.
  • Volume remains constant.
• THREE PHASES OF VENTRICULAR FILLING:
  • RAPID VENTRICULAR FILLING:
    • The Mitral Valve opens, when ventricular pressure falls below atrial pressure.
    • Blood rushes into ventricle, very quickly initially.
    • THIRD HEART SOUND (S3): It is turbulent blood flowing past the ventricular wall during early diastole. It is indicative of pathology.
  • SLOW VENTRICULAR FILLING: The later period of diastole. The majority of blood has already entered the ventricles.
  • TOP-OFF PHASE: The blood contributed to ventricles during atrial systole.
• Diastolic Events Associated with the Atria:

• V-Wave: Small increase in atrial pressure associated with the fact that the mitral valve is closed at the very beginning of diastole.
• Y-Descent: Descent of the V-Wave. Decrease in atrial pressure occurring when the mitral valve opens, right after ventricular isovolumic relaxation.
• A-Wave: Small rise in atrial pressure, occurring right before systole, associated with Atrial Systole and cntrxn of atrial muscle.
• FOURTH HEART SOUND (S4): Vibration of mitral valve leaflets during atrial systole, i.e. during the top-off phase of ventricular filling. This occurs concurrent with the A-Wave and is indicative of pathology.
• Atrial Fibrillation: There is an age difference in the seriousness of this. Again, atrial fibrillation isn't a concern with young people but it is with old people.
  • YOUNG: Atrial systole contributes about 20mL to stroke volume
  • OLD: Atrial systole contributes about 40mL to stroke volume.
• An Increased heart rate makes the atrial contribution to stroke volume more significant. Shorter time for ventricular filling ------> The top-off phase contributes more relative volume to ventricles.

VENTRICULAR SYSTOLE: QRS-Complex occurs and ventricles start contracting.

• FIRST HEART SOUND (S1): The Mitral Valve Closes, as ventricular pressure exceeds atrial pressure.
• ISOVOLUMIC CONTRACTION: Period of contraction during which both valves are closed
  • Pressure is increasing.
  • Volume is constant.
• Systolic Events Associated with the Atria:
  • C-WAVE: Small increase in atrial pressure. Occurs during isovolumic contraction, as the ventricle pushes the mitral valve a little upward toward the atrium.
  • X-DESCENT: The decrease in the C-Wave, due to the change of shape of the ventricle from prolate spheroid (football-like) to spheroid. This makes the mitral valve move down and the atrial pressure return to normal.
• Aortic Valve Opens, as ventricular pressure exceeds aortic pressure.
  • Ventricle must achieve systolic arterial pressure in order to open the Aortic valve, so it reaches pressures around 120 mm Hg.
• Ventricular Ejection: 70% of blood is ejected in the first third of systole.
• SECOND HEART SOUND (S2): The aortic valve closes, as ventricular pressure falls below aortic pressure.
  • DICROTIC (AORTIC) NOTCH: When the Aortic Valve closes, there is a temporary retrograde flow of blood against the Aortic valve cusps. This causes an acute decrease in Aortic pressure at the very beginning of diastole.
  • Two Things act in concert to make the Aortic valve close:
    • The left ventricle relaxes so left ventricular pressure decreases.
    • The retrograde blood flow against the leaflets actually aids in the closure of the valve.

HEART SOUNDS: Left Side -vs- Right Side:

• FIRST HEART SOUND (S1):
  • The mitral valve (left side) closes before the tricuspid valve (right side), because the depolarization begins on the left side of the septum.
  • On the other hand, the Aortic Valve (left side) opens a little after the Pulmonary Valve (right side), because there is so much higher volume in the left side, hence more pressure has to build up before valve will open.

SPLIT SECOND HEART SOUND: During inspiration, You should be able to hear the pulmonic and aortic valves close separately during the second heart sound (i.e. a "split" sound).

• Pulmonic Stenosis: In this case the pulmonic valve is not opening well ------> Wide Splitting during inspiration.
• Aortic Stenosis: Causes paradoxical splitting -- i.e. splitting occurs during expiration instead of during inspiration.