Cardiac Cycle

Definition: Cardiac cycle refers to the cardiac events that occur from the beginning of one heart beat by spontaneous generation of action potential in SA node to the beginning of the next. When the heart rate is 75/min, the period of one cardiac cycle is 0.8 sec.

Important terms:

  1. Diastole: a period of relaxation during which heart fills with blood
  2. Systole: a period of contraction during which the blood is ejected from the heart.
  3. Isovolumetric: a phase when all valves are closed and ventricle behaves as a closed chamber and volume within ventricle remains constant

Time period of various events of cardiac cycle:

A)     Atrial Cycle: 0.8 sec

1. Systole: 0.1 sec

2. Diastole: 0.7 sec

B)      Ventricular Cycle: 0.8 sec

1. Systole: 0.3 sec

  • Isovolumetric contraction: 0.05 sec
  • Rapid ejection phase: 0.1 sec
  • Reduced ejection phase: 0.15 sec

2. Diastole: 0.5 sec

  • Isovolumetric relaxation: 0.1 sec
  • Rapid filling phase: 0.1 sec
  • Reduced filling phase (Diastasis): 0.2 sec
  • Last Rapid filling phase (Atrial systole): 0.1 sec

Points to understand:

  • Normally, atrial and ventricular systoles never coincide. Ventricular systole occurs during atrial diastole and atrial systole occurs during ventricular diastole.
  • For a period of about 0.4 sec (Isovolumetric ventricular relaxation, Rapid ventricular filling phase and Reduced ventricular filling phase), both the ventricles and atria relax.

Events in Cardiac Cycle:

1. Atrial Systole:

  • Preceded by ECG “P” wave  which begins due to spontaneous generation of action potential in SA node
  • Contributes to last rapid phase of ventricular filling (20% filling) but is not essential for ventricular filling
  • Filling of ventricle by atrial systole gives rise to 4th Heart sound, which is not audible in normal adults
  • “a” wave appears on atrial pressure curve due to increase in atrial pressure (4 to 6 mmHg in right atrium and 7 to 8 mmHg in left atrium)

2. Isovolumetric ventricular contraction:

  • Ventricles are filled with blood: 80% filling occurred before atrial systole and 20% filling occurred during atrial systole
  • Begins after onset of ECG “QRS” complex
  • Immediately after ventricular contraction begins, pressure rises abruptly
  • When Ventricular pressure > Atrial pressure, Atrioventricular (AV) valve closes giving rise to 2nd heart sound (As mitral valve closes before tricuspid valve, 1st heart sound may split)
  • Since AV valves and semilunar valves (aortic and pulmonary valves) are both closed, isovolumetric contraction occurs and there is rapid rise in ventricular pressure.

3. Rapid Ventricular Ejection:

  • When the left ventricular pressure rises above the aortic pressure (~80 mmHg), aortic valve opens and there is rapid ejection (70% ejection) of blood into the aorta.
  • When the right ventricular pressure rises above the pulmonary pressure (~8 mmHg), pulmonary valve opens and there is rapid ejection (70% ejection) of blood into the pulmonary trunk.
  • “c” wave appears on atrial pressure curve due to bulging AV valve on atria due to increasing ventricular pressure
  • Pressure rise in the ventricles is slower because the blood flows into the arteries. The entry of blood into the arteries causes arteries to stretch and pressure increases. During this period, the pressure in the left ventricle and aorta reaches a maximum of 120 mmHg (systolic pressure) and that in right ventricle and pulmonary trunk reaches a maximum of 24 mmHg.
  • Atrial filling begins
  • Onset of ECG “T” wave marks end of both ventricular contraction and rapid ejection.

4. Reduced Ventricular Ejection:

  • Ejection of blood (30% ejection) from the ventricles continues, but is slower
  • Ventricular pressure begins to decrease
  • Aortic and Pulmonary pressure also decreases because of runoff of blood from larger arteries into smaller arteries
  • Atrial filling continues

5. Isovolumetric ventricular relaxation:

  • Repolarization of Ventricles is now complete (marked by end of ECG “T” wave)
  • Ventricular pressure begins to fall
  • When the pressure in respective ventricles < pressure in aorta and pulmonary trunk, the semilunar valves close (closure of aortic valve followed by pulmonary valve) giving rise to 2nd heart sound. Inspiration causes splitting of 2nd heart sound.
  • Dicrotic notch or incisura appears on aortic pressure curve as a “blip” after closure of aortic valve due to short period of backward flow of blood immediately before closure of the valve, followed by sudden cessation of the backflow.
  • Since, AV valves and semilunar valves are both closed; isovolumetric relaxation
  • Ventricular falls rapidly but the elastic walls of the arteries maintain a high pressure in the arteries, even during diastole although there is a fall in arterial pressure.
  • “v” wave appears on atrial pressure curve due to accumulation of blood in atria against closed AV valves
  • When ventricular pressure < atrial pressure, AV valves (Mitral and Tricuspid) open.

6. Rapid Ventricular Filling:

  • Rapid flow of blood, long accumulated in the atria to the ventricles gives rise to the 3rd heart sound, which is normal in children but pathologic in adults.

7. Reduced ventricular filling (Diastasis):

  • Longest phase of cardiac cycle
  • Only a small amount of blood flows into the ventricles at a slower rate from the great veins via atria.
  • During this period, the blood in both atrium and ventricle becomes continuous as if a single cavity.
  • Time required for diastasis and ventricular filling depends on the heart rate. Increase in heart rate decreases the time available for ventricular filling.

8. Last rapid filling phase:

  • Coincides with the atrial systole

Summary:

Phases in Left Ventricle:

  • Isovolumetric contraction: period between mitral valve closure and aortic valve opening; period of highest O2 consumption
  • Systolic ejection: period between aortic valve opening and closing
  • Isovolumetric relaxation: period between aortic valve closing and mitral valve opening
  • Rapid filling: period just after mitral valve opening
  • Reduced filling: period just before mitral valve closure

Heart Sounds:

  • S1: mitral and tricuspid valve closure; loudest at mitral area.
  • S2: aortic and pulmonary valve closure; loudest at left sternal border.
  • S3: in early diastole during rapid ventricular filling phase.
    • Associated with ↑ filling pressures and more common in dilated ventricles (but normal in children and pregnant women).
  • S4: in late ventricular diastole or atrial systole.
    • Associated with ventricular hypertrophy. Left atrium must push against stiff LV wall.

Jugular venous pulse (JVP):

  • a wave: atrial contraction
  • c wave: RV contraction (tricuspid valve bulging into atrium).
  • v wave: ↑ atrial pressure due to filling against closed tricuspid valve.

Flowchart:

5 Comments
  1. October 21, 2011
  2. October 21, 2011
    • October 21, 2011
      • November 19, 2011
  3. November 3, 2011

Leave a Reply

Your email address will not be published. Required fields are marked *