The cardiac cycle describes the sequence of events in the heart from the beginning of one heartbeat to the next. It consists of both electrical and mechanical events in the atria and ventricles over a total duration of 0.8 seconds for each cycle. The cycle includes atrial systole, atrial diastole, ventricular systole (consisting of isovolumic contraction and ejection phases), and ventricular diastole (consisting of relaxation, rapid filling, reduced filling, and last rapid filling phases). Precise timing and coordination of the opening and closing of the atrioventricular and semilunar valves are crucial for efficient pumping of blood throughout the cardiac cycle.

1. An Introduction to Cardiology
Nikhil Vaishnav, Clinical Coordinator
Department of Paramedical & Health Science

2. Cardiac cycle
CHAPTER-7

3. Introduction
Image source : Google
 The cardiac cycle is a sequence of contraction and relaxation of
the atria and ventricles. The cardiac cycle pumps blood throughout the body.
 It begins at the beginning of one heartbeat and ends at the beginning of
another.
 Cardiac cycle consists of one heart beat or one cycle of contraction and
relaxation of the cardiac muscle.
 Duration of each cardiac cycle is 0.8 second if heart rate is 75 beats per
minute 60/75= 0.8 .

4. Image source : Google
Cardiac cycle consists of both electrical and mechanical events . These
events occur from the beginning of one heart beat to the beginning of
the next.
 During each cardiac cycle both atria contact (atrial systole ) and
relax(atrial diastole) ,and both ventricles contract( ventricular systole )
and relax(ventricular diastole) .

5. Cardiac cycle

6. Cardiac cycle

7. Phases of cardiac cycle
Atrial systole & diastole
Atrial cycle : total 0.8
sec duration .
1. Atrial systole(0.1 sec)
2. Atrial diastole (0.7 sec)

8.  Ventricular cycle : total 0.8 sec duration
a. Ventricular systole ( 0.3 second)
i. Isovolumic contraction phase (0.05 sec)
ii. Ventricular ejection phase which is divided into rapid ejection(0.1s) and
slow ejection phase(0.15 sec)
b. Ventricular diastole (0.5 second )
i. Protodiastole(0.04 sec)
ii. Isovolumic relaxation phase (0.06 sec)
iii. Rapid passive filing phase(0.11 sec)
iv. Reduced filling phase (0.19 sec)
v. Last rapid filling phase (0.1 sec)

10. Atrial systole
 Atrial systole (atrial contraction) phase lasts for 0.1 second.
 It is the beginning phase of cardiac cycle.
 There is electrical depolarization of the atria which is manifested by P wave
of ECG.
 During atrial systole both AV valve open , semilunar valve closed.
 Before atrial contraction most of the ventricular filling (75 %) occurs. Blood
flows from the great veins into the atria and then blood flows from the atria
into the ventricles.

11.  Atrial contraction causes additional 25 % filling of the ventricles.
 Fourth heart sound (S4) is noted sometimes during atrial contraction. It is
due to the ventricular wall vibration during atrial contraction.
 Due to atrial contraction ,intra-atrial pressure in the right atrium increased
by 4-6 mm Hg . There is an increased intra-arterial pressure in left atrium by 7-
8 mm Hg.
 The increased pressure in the right atrium is recorded as a wave from the
jugular vein.

13. Atrial systole

15. Atrial diastole
 Once the atrial systole is over, there is atrial diastole period. It lasts for 0.7
second.
 Atrial diastole happens simultaneously with ventricular systole.
 During this period, atrial muscle relaxed.
 There is gradual filling of the atria due to continuous venous return .

18. Ventricular systole
 Once the atrial contraction is over, electrical impulses reach to the ventricles.
 So ventricles get excited and start contracting.
 This phase lasts for 0.3 second. It has 2 phases: Isovolumic contraction and
ventricular ejection phase.

19. Isovolumic contraction
 This phase begins with ventricular depolarisation which is represented by
appearance of QRS complex of the ECG.
 It lasts for 0.05 second.
 When Intraventricular pressure exceeds the atrial pressure, both AV valves
closed .
Closure of the both atrioventricular valves is responsible for first heart sound
(S1).

20.  In this phase both AV valves have closed and semilunar valves have not
opened. Ventricular pressure rises rapidly .
 Since all valves are closed during this phase, there is no change in the
volume of the ventricles (no ejection occurs). Therefore this phase is known as
Isovolumic or Isovolumetric phase.
 There is bulging of atrioventricular valves into the atria . It produces a small
and sharp rise in the intra-arterial pressure called as “c wave “.
 This phase lasts until the pressure in the both ventricles exceeds the
pressure in the aorta and pulmonary artery

23. Ventricular ejection phase
 Ventricular ejection phase lasts for 0.25 second.
 It starts with the opening of semilunar valves.
It is divided into 2 phases: rapid ejection phase and slow ejection phase .

24. Rapid ejection phase
 During this phase , blood is ejected rapidly into the aorta and pulmonary
artery from both ventricles.
It occurs when the Intraventricular pressures exceed the pressures within
the aorta and pulmonary artery.
 It lasts for 0.1 sec.
 Both Semilunar valves open and AV valves remain closed.
Two thirds of the stroke volume is ejected in rapid ejection phase.
 70 % of blood emptying during rapid ejection phase.

26. Slow ejection phase
 It lasts for 0.15 sec.
 During slow ejection phase one third of the stroke volume is ejected.
Both semilunar valves open and AV valves remain closed .
 30 % of blood emptying during slow ejection phase.

29. Ventricular diastole
i. Protodiastole(0.04 sec)
ii. Isovolumic relaxation phase (0.06 sec)
iii. Rapid passive filing phase(0.11 sec)
iv. Reduced filling phase (0.19 sec)
v. Last rapid filling phase (0.1 sec

30. Protodiastole
 The ventricles start relaxing as the ventricular systole over .
It is the first phase of ventricular relaxation.
This phase lasts for 0.04 second
Intraventricular pressure drops more rapidly.
The elevated pressure in aorta and pulmonary artery pushes the blood back
towards the ventricles. It leads to semilunar valves closed .
Closure of the semilunar valves produces the second heart sound (S2)
It also causes incisura , a dicrotic notch in the down slope of aortic pressure
curve .

31. Isovolumic relaxation phase
 This phase starts with the semilunar valve closure and here all valves closed.
 It lasts for 0.06 second.
 In this phase ventricles are going to relax as closed chambers. It causes rapid
fall of Intraventricular pressure (In left ventricle from 80 mm Hg to 2-3 mm Hg).
 Since all valves are closed, there is no change in the volume of ventricles, so
this phase is called Isovolumic or Isovolumetric relaxation phase.
When the AV valves open, Isovolumic relaxation phase ends.
At the end of this phase, left atrial pressure(LAP) raised due to venous return
from the lungs . The peak LAP is termed as “v wave”.

32. Isovolumicrelaxation

33. Rapid passive filling phase
 It lasts for 0.11 second .
 The atrial pressure is high due to continuous venous return during atrial
relaxation.
 The AV valve open due to high atrial pressure .
 There is rapid and passive ventricular filling caused by opening of AV valves .
 A third heart sound(S3) is produced during rapid filling phase . It is normal in
children . Third heart sound is pathological in adults and caused by ventricular
dilation .
 There is a rapid fall in LAP when mitral valve opens .

34. Rapid filling phase

35. Reduced filling phase
 It lasts for 0.19 second.
 During this phase, pressure in the atria and ventricles decreases slowly.
 The rate of blood flow from atria to ventricle is decreased . It causes a very
slow filling called diastasis.
 During rapid filling and reduced filling phase, 75 % of blood passes from the
atria to the ventricles.

36. Last rapid filling phase
 It lasts for 0.1 second.
 The last rapid filling phase of ventricular diastole coincides with the atrial
systole.
 The atrial systole brings about the last rapid filling phase and pushes the
additional 25 % of the blood in the ventricles .
 With this phase, the ventricular cycle is completed .

37. Wiggers diagram

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