1. Cardiac Cycle

2. Cardiac cycle - introduction
Activity of the heart
Electrical events
Initiation of the depolarisation
Conduction of depolarisation
repolarisation
Mechanical events
Contraction of muscle
Changes in pressure in cardiac chambers
Blood flow
Opening and closure of valves
Mechanical events are consequent to electrical events

3. Cardiac cycle
Cardiac events that occur from the beginning of one cardiac impulse to the beginning of the next are called the
‘cardiac cycle’
Includes electrical and mechanical events

4. Cardiac cycle - introduction
Electrical events
depolarization and origin of cardiac impulse at the SA node
conduction of the impulse to all parts of the atria through atrial muscle (there may be some special pathways that conduct impulse fast ie. have a higher conduction velocity – internodal pathways). The impulse also spreads to the AV node along the atrial muscle
the impulse will not pass from atrial muscle to ventricles directly as they are not in contact (fibrous tissue separates them)

5. Cardiac cycle - introduction
Electrical events
4. AV node has a low conduction velocity and thus the impulse takes a little time to travel across it. It eventually travels across it and reaches the upper end of the bundle of His
5. bundle of His has a high conduction velocity. It conducts the impulse through its branches to the Purkinje fibre network just under the endocardium of the ventricles. Because of this fast conduction all parts of the ventricles are depolarized at almost the same time. But the interventricular septum depolarizes just before the lateral walls of the ventricles

6. Cardiac cycle - introduction
Electrical events
repolarisation of the heart occurs in the same sequence as depolarisation
when the repolarisation is complete the myocardium remains in the resting state (at resting membrane potential) until the SA node generates an action potential again
When the next action potential is generated same events are repeated in the same sequence –events are cyclical

7. Electrical events Initiation of cardiac depolarisation
Spread of cardiac depolarisation
Repolarisation
Period of electrical inactivity 2 1 1 3 4
duration of one cardiac cycle

8. Cardiac cycle - introduction
Contraction
Atria contract first as they are depolarised first
There is a delay at the AV node before the cardiac impulse passes to the ventricles
Therefore when the ventricles start contraction the atria have finished the contraction
Ventricular contraction starts immediately after the atria have finished their contraction
After ventricles finish their contraction both atria and ventricles remain relaxed until the next action potential from the SA node depolarises the heart
When the next depolarisation occurs same events are repeated in the same sequence –events are cyclical

9. Cardiac cycle - introduction
Contraction
Period of contraction of a cardiac chamber – systole
Period of relaxation of a cardiac chamber – diastole
If words systole and diastole are used by themselves they usually mean ventricular events
When referring to atrial events it is customary to use “atrial systole” and “atrial diastole”

10. Cardiac cycle The cyclical events – Atria Ventricles Cycles of events
Atria and ventricles never contract at the same time
But there are times when both atria and ventricles are relaxed at the same time
Atria AS AD
Ventricles VD VS VD
Cycles of events 1 2 3

11. Cardiac cycle Duration of events - Atria
The total duration of a cardiac cycle depends on the frequency of impulse generation by the SA node
If the frequency is 75/min the length of each cardiac cycle is:
Atria AS AD
= 0.8 sec if rate is 75/min
Ventricles VD VS VD 60
= 0.8 sec 75

12. Cardiac cycle Duration of systole and diastole – Atria
In a cardiac cycle of 0.8 sec –
atrial systole – 0.1 sec
atrial diastole – 0.7 sec
ventricular systole – 0.3 sec
ventricular diastole – 0.5 sec
Atria
0.1
0.7
Ventricles
0.1
0.3
0.4

13. Cardiac cycle Duration of systole and diastole –
When the cardiac cycle shortens or lengthens both systole and diastole will shorten or lengthen.
However, the change in systole is less and the change is diastole is more
Atria
0.1
0.7
Ventricles
0.1
0.3
0.4
Cycle duration
0.8
0.4
1.2
Ventricular systole
0.3
0.2
0.4
Ventricular diastole
0.5
0.2
0.8

15. Cardiac cycle

16. Cardiac cycle Ventricular systole – 0.3 sec
Isometric contraction – 0.05 sec
Isotonic contraction – 2.5 sec
Rapid ejection phase
Reduced ejection phase
Ventricular diastole – 0.5 sec
Protodiastolic period – 0.04 sec
Isometric relaxation – 0.08 sec
Ventricular filling – 0.38 sec
Rapid filling phase
Reduced filling phase

17. Cardiac cycle
When a cardiac chamber is in systole the valve (if any) before the chamber is closed and the valve after the chamber is open
During diastole of a chamber the valve before is open and the valve after is closed
AV valves and semilunar valves are never both open at the same time
However, they are both closed during the isometric contraction and isometric relaxation periods

18. Cardiac cycle Volumes related to the cardiac cycle
End diastolic volume – the volume of blood in the ventricle at the end of ventricular diastole. (or the volume at the start of systole) This is the volume that is available for pumping by the ventricle
End systolic volume – the volume of blood left in the ventricle at the end of systole. The ventricle was not able to pump this volume of blood

19. Cardiac cycle Volumes related to the cardiac cycle
Stroke volume – the amount of blood pumped out by the ventricle during one contraction
SV = EDV – ESV
Approximate values for the volumes (adult)
EDV – 150 ml
ESV – 70 ml
SV – 80 ml

20. Cardiac cycle
Pressures in the cardiac chambers during the cardiac cycle – (mmHg, for illustration only)
Right atrium
Right ventricle
Pulmonary artery
Left atrium
Left ventricle
Aorta
Systole
Diastole 3 25 25 10 5 2
120
120 80

21. Events associated with the cardiac cycle
Peripheral arterial pulse -
Ejection of blood into the aorta raises the aortic pressure suddenly
This increase in pressure expands the aortic wall
The pressure wave in the aorta is transmitted very rapidly along the aortic wall and then along walls of all the bigger arteries
This is felt as the arterial pulse
The pressure wave is transmitted faster than the flow of blood
Timing – soon after the onset of ventricular systole, soon after the closure of the AV valve

22. Events associated with the cardiac cycle
Jugular venous pulse -
The right internal jugular vein rises almost vertically up from the right atrium. There are no valves in between Thus it can be used as a ‘manometer’ to detect right atrial pressures
Right Internal
Jugular
SVC
Brachiocephalic
Right
subclavian

23. Events associated with the cardiac cycle
Jugular venous pulse
The pressure wave in the jugular vein resembles the pattern of pressure change in the right atrium
‘a’ – coincides with atrial contraction
‘c’ – coincides with onset of ventricular contraction and carotid plse
‘v’ – coincides with late ventricular systole (atrial filling)
Waves a v c
x descent
y descent

24. Events associated with the cardiac cycle
Apex beat
The base of the heart is fixed by the apex is relatively mobile. During shortening of the myocardium the apex moves but not the base
This movement causes the apex to move anteriorly and to the right.
The shift of the apex is associated with an impulse felt on the precordium which coincides with the onset of ventricular systole, arterial pulse and the ‘c’ wave of the JVP
- The apex beat
The normal position of the apex beat – 5th LICS, MCL

25. Events associated with the cardiac cycle
Heart sounds
Events associated with the closure of valves – closure itself, sudden changes in blood flow etc cause sounds which can be heard over the precordium
Closure of the AV valves – first heart sound; coincides with the carotid pulse, apex beat and occurs just after the ‘a’ wave in the JVP
Closure of the semilunar valves – second heart sound
A third and a fourth heart sound may be heard under some conditions
Third – during rapid ventricular filling
Fourth – during atrial systole