Cardiac cycle (The Guyton and Hall physiology)

Sequence of events from the beginning of one systole to the beginning of next consecutive systole.
One heart beat consists of one systole and one diastole.
Each cardiac cycle is initiated by the cardiac impulse which originates from the SA node.
During each cardiac cycle, certain events occur in the heart and these include pressure changes, volume changes, production of heart sounds, closure and opening of heart valves and electrical changes in the heart.

1. Cardiac Cycle Maryam Fida (o-1827) Source: The  Guyton  and Hall Physiology

2. Cardiac Cycle Sequence of events from the beginning of one systole to the beginning of next consecutive systole. One heart beat consists of one systole and one diastole. Each cardiac cycle is initiated by the cardiac impulse which originates from the SA node. During each cardiac cycle, certain events occur in the heart and these include pressure changes, volume changes, production of heart sounds, closure and opening of heart valves and electrical changes in the heart.

3. Cardiac Cycle Left ventricle Right ventricle Left atrium Right atrium Pulmunory Artery Aorta

4. Left Ventricles Cardiac Cycle Phases Isovolumetric Contraction Phase Maximal Ejection Phase Reduced Ejection Phase Protodiastole Isovolumetric Relaxation Phase Rapid Inflow/ Filling Phase Slow Filling Phase / Diastasis Atrial Systole

5. 1.Isovolumetric Contraction Phase The left ventricle is full of blood just at the beginning of ventricle systole. The pressure is 1-3 mm of Hg. And the mitral valve is open. This phase starts with the closure of mitral valve. The ventricular muscle is contracting powerfully but isometrically. ( friction occurs) Rapid increase in the ventricular pressure. No change in the blood volume i.e. remains at 120 ml When ventricular pressure reaches just above 80 mm of Hg , there is opening of Aortic valve. So this phase starts with the closure of mitral valve and ends with the opening of aortic valve. Duration is 0.06 seconds .

6. 2.Maximal Ejection Phase With the opening of aortic valve, blood ejection starts at maximal rate into aorta. Muscle is contracting powerfully but isotonically .( friction absent ) Ventricular pressure rises to maximum 120 mm of Hg. 2/3 rd of Stroke volume(46 ml out of 70 ml) is ejected in this phase. Duration is 0.11 seconds .

7. 3.Reduced Ejection Phase The ventricular contraction becomes less powerful. Pressure falls from maximum. Remaining 1/3 rd (24 ml out of 70 ml ) of stroke volume is ejected out. This phase ends when the ventricular pressure becomes equal or slightly less than the aortic pressure. Duration is 0.14 seconds .

8. ( 0.06+0.11+0.14=0.31) The above three phases constitute the ventricular systole. This adds up to 0.31 seconds . Ventricular systole

9. Atrial pressure

10. 4.Protodiastole This is a short phase. Some blood flow continues from left ventricle into aorta because of momentum, although ventricle has stopped contracting. In this phase, the momentum has overcome by further fall in ventricular pressure because of relaxation. There is some backflow of blood in the first part of aorta and aortic valve closes and with the closure of this valve, the phase ends. Duration is 0.04 sec.

11. 5.Isovolumetric Relaxation Phase Starts with closure of aortic valve. Ventricular muscle is undergoing relaxation Rapid fall in the ventricular pressure. No change in blood volume as both valves are closed When the left ventricular pressure becomes lower than the left atrial pressure, there is opening of mitral valve. And then this phase ends. Duration is 0.06 sec

12. 6.Rapid Inflow / Rapid Filling Phase Starts with the opening of mitral valve. Blood flows rapidly from left atrium into left ventricle. Ventricle is undergoing relaxation so no rise in pressure. There is 2/3 rd filling of ventricle. Rapid filling phase remains for 0.11 sec.

13. 7.Slow Filling Phase / Diastasis Stasis during the diastole In this phase, it appears that no blood is flowing from left atrium into left ventricle. Most of the blood which was in left atrium has already flown into left ventricle As mitral valve is open, small amount of blood returns into left atrium from pulmonary veins and when this blood enters into the atrium it directly goes into left ventricle. So both left atrium and left ventricle acts as a single chamber so it appears no blood is flowing into left ventricle. Only slight filling occurs in this phase. Nearly no change in pressure or volume Duration is 0.20 sec

14. The duration of whole ventricular diastole is 0.54 sec .

15. 8.Atrial Systole Left atrium contracts to push the blood from its cavity into left ventricle. With atrial systole, ventricular systole is completed. Atria contracts towards the end of the ventricular diastole. Remaining 20-25% of the ventricular filling occurs. Duration is 0.11 sec

16. Total cardiac cycle duration is 0.8 sec when heart rate is about 72-75 beats /min. When heart rate increases, duration of cardiac cycle decreases and this effects diastole more as compared to systole. At a very rapid heart rate, diastole becomes too short. At heart rate of 185 beats/min, the duration of cardiac cycle is 0.33 sec and out of this 0.18 sec is systole and diastole is 0.15 sec . so at a very rapid heart rate cardiac output falls because of decreased duration of diastole and so incompetent filling. The cardiac output generally increases up to 150 beats/ min. No increase in cardiac output from 150-180. Beyond 180 beats/min cardiac output falls.

17. 9.Right Ventricular Cardiac Cycle Same phases of the cardiac cycle with difference of pressure values. At the start of the right ventricular systole, pressure in the right ventricle is 0-1 mm of Hg. ISOMETRIC CONTRACTION PHASE: When pressure rises just above 8 mm of hg there is opening of the pulmonary valve. MAXIMAL EJECTION PHASE: Pressure rises maximally to 25 mm of Hg .

18. ATRIAL PRESSURE CHANGES Atria also undergoes systole and diastole. Duration of atrial systole is 0.11 sec atrial diastole is 0.7 sec. so much prolonged atrial diastole which helps in filling. During each cardiac cycle, three waves are recorded from atria i.e. a wave, c wave and v wave. A wave C wave V wave

19. ATRIAL PRESSURE CHANGES A wave: Due to rise in the atrial pressure during systole . C wave: Occurs at the beginning of ventricular systole . During isometric contraction, there is rapid rise in ventricular pressure, there is bulging of the AV valve into atria and thus increasing atrial pressure and this gives the ascend of c wave. The top of the c wave coincides with the opening of semilunar valves, ventricular contracts more powerfully , contraction of myocardial fibers so av valve is pulled back to ventricales and this causes decreased pressure in atria leading to descent of c wave. V wave : due to gradual rise in atrial pressure , when blood accumulates in the atria because of closed av valves. The top of v wave coincides with the opening if av valves. The blood flows from atrium to ventricles rapidly causing descent of v wave.

20. ATRIAL PRESSURE CHANGES In the right atrium, the pressure may rise upto 4 -5 mm of hg And in left atrium the pressure may rise upto 6-8 m of hg. Pulsations recorded from jugular vein is called JVP. JVP is due to backward transmission of pressure changes of right atrium to neck veins and so in JVP we see same waves.

21. PRESSURE CHANGES IN AORTA Maximum pressure is during left ventricular systole. i.e. 120 mm of hg. Minimum pressure is during left ventricular diastole i.e. 80 mm of hg. In aortic pressure curve there is incisura . A sharp rise in the aortic pressure followed by a gradual fall. Inciusra is recorded at the time of closure of aortic valve.

22. PRESSURE CHANGES IN PULMUNORY ARTERY Maximum pressure is 25 mm of hg during right ventricular systole. Minimum pressure is 8 mm of hg during right ventricular diastole.

23. SEQUENCE OF SYSTOLE IN THE FOUR CHAMBERS OF HEART: Right atrial systole precedes left atrial systole. Left ventricular systole begins slightly earlier than the right ventricuylar systole, but blood ejection into the pulmunory artery begins earlier because of low pressure in pulmunory artery than in aorta.