Cardiac Cycle

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

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

Cardiac cycle - introduction

Electrical events

1. depolarization and origin of cardiac impulse at the SA node

2. 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

3. the impulse will not pass from atrial muscle to ventricles directly as they are not in contact (fibrous tissue separates them)

Cardiac cycle - introduction

Electrical events4. 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

Cardiac cycle - introduction

Electrical events6. repolarisation of the heart occurs in the same

sequence as depolarisation

7. 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

Electrical events

1. Initiation of cardiac depolarisation

2. Spread of cardiac depolarisation

3. Repolarisation

4. Period of electrical inactivity

1 12

3

4

duration of one cardiac cycle

Cardiac cycle - introduction

Contraction 1. Atria contract first as they are depolarised first2. There is a delay at the AV node before the cardiac

impulse passes to the ventricles3. Therefore when the ventricles start contraction the

atria have finished the contraction 4. Ventricular contraction starts immediately after the

atria have finished their contraction 5. After ventricles finish their contraction both atria and

ventricles remain relaxed until the next action potential from the SA node depolarises the heartWhen the next depolarisation occurs same events are repeated in the same sequence –events are cyclical

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”

Cardiac cycle

The cyclical events –

Atria and ventricles never contract at the same timeBut there are times when both atria and ventricles are relaxed at the

same time

ASAtria

Ventricles

AD

VD VS VD

Cycles of events

1 2 3

Cardiac cycle

Duration of events -

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:

ASAtria

Ventricles

AD

VD VS VD

7560 = 0.8 sec

= 0.8 secif rate is 75/min

Cardiac cycle

Duration of systole and diastole –

In a cardiac cycle of 0.8 sec –atrial systole – 0.1 secatrial diastole – 0.7 sec

ventricular systole – 0.3 secventricular diastole – 0.5 sec

0.1Atria

Ventricles

0.7

0.1 0.3 0.4

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

0.1Atria

Ventricles0.7

0.1 0.3 0.4

Cycle duration

Ventricular systole

Ventricular diastole

0.8

0.3

0.5 0.2

0.2

0.4 1.2

0.4

0.8

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

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

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

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

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

Cardiac cycle

Systole Diastole

3

120

120

0

10

2

25

0

25

5

0

80

Peripheral arterial pulse -

Ejection of blood into the aorta raises the aortic pressure suddenlyThis increase in pressure expands the aortic wallThe pressure wave in the aorta is transmitted very rapidly along the aortic wall and then along walls of all the bigger arteriesThis is felt as the arterial pulseThe pressure wave is transmitted faster than the flow of bloodTiming – soon after the onset of ventricular systole, soon after the closure of the AV valve

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 betweenThus it can be used as a ‘manometer’ to detect right atrial pressures

Events associated with the cardiac cycle

Right Internal Jugular

SVC

Brachiocephalic

Right subclavian

Jugular venous pulseThe 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)

Events associated with the cardiac cycle

ac

v

x descent y descent

Waves

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

Events associated with the cardiac cycle

Heart soundsEvents associated with the closure of valves – closure itself, sudden changes in blood flow etc cause sounds which can be heard over the precordiumClosure of the AV valves – first heart sound; coincides with the carotid pulse, apex beat and occurs just after the ‘a’ wave in the JVPClosure of the semilunar valves – second heart sound

A third and a fourth heart sound may be heard under some conditionsThird – during rapid ventricular filling Fourth – during atrial systole

Events associated with the cardiac cycle