cardiac cycle

Cardiac cycle

Def

It is the sequence of events during a cardiac beat

Various changes that follow during a heart beat, undergo cyclic repetition

One complete cycle of such cardiac events of a beat is the cardiac cycle

Duration

72times/m

One beat takes 0.8s

If the heart beats faster, the time is less than 0.8sec

If the slower it would be little more

Events

Both atria and ventricles undergo systolic and diastolic phase with every beat

There is a an atreal systole and an atreal diastole, ventricular systole and ventricular diastole

Atrial events

These include atrial systole and atrial diastole .Atrial systole occurs for 0.1 s

Atrial systole is followed by 0.7sec of aerial diastole

During atrial systole the atria contract pouring their blood into their respective ventricles

During atrial diastole ,  the atria relax and the blood from superior and inferior vena cava in the right atrium and the pulmonary veins in the left atrium, fill them  up. After this they again contract to perform atrial systole

Ventricular events

As the atrial systole ends, the ventricular systole begins

This continues for 0.3 sec and is followed by ventricular diastole

Isometric ventricular contraction period

As ventricular systole starts, the atrioventricular valves (AV valves) of both ventricles close. This produces the first heart sound. At this time semi lunar (SL) valves are closed

In the beginning of ventricular systole there are closed cavities and ventricular pressure is mounting up without any decrease in its volume

In isometric contraction period, its onset is marked by closure of AV valves and termination by opening of SL valves. This period remains for 0.05 sec

Intraventricular pressure is highest during this period

Rapid and slow ejection period

After 0.05 sec isometric contraction periods, semi lunar valves open and due to ventricular systole, blood is ejected out into respective arteries

This is period , ejection period (0.15 sec)

The intraventricular pressure starts rising

The intrventricular volume step-down and blood is poured out with maximum force

In the late part of ventricular systole  , intravintricular pressure declines and blood is poured with slow speed. This is called slow ejection period (0.1 scc)

Pro- diastolic and isometric relaxation period

The aorta and pulmonary trunk recoil and blood tries to rush back into the respective ventricles

But this is suddenly stopped by the swift closure of semi lunar valves

This produces the second heart sound, heralding the end of ventricular systole

The ventricular between the start of ventricular diastole and the closure fo SL valves (0,04 sec)  is known is know as prodiastilic period

The next to this period is isometric relaxation period (0.08 sec)

This is the period during which both the valves (AV and SL)  are closed but ventricles and atria are in diastolic condition

This is the period when intra-ventricular pressure is minimum and heart is relaxing

Rapid and slow inflow period

The isometric relaxation period ends with the opening of AV valve and because ventricles are under diastolic condition , rapid inflow of blood starts

This is known as rapid inflow period

Third heart sound is heard on echophine at this juncture and it lasts for 0,03 sec

The next phase is slow inflow period (9.2 sec) and is also known as diastalsis

When ventricular diastole is about to end, the atrial systole has started and it overlaps for 0.1 sec

During this phase because of active contraction of atria, filling of ventricles start rapidly

This is called rapid filling period and fourth heart sound is heard at this stage on echo phone

ECG Changes

The events of cardiac cycle can be traced on ECG

During atrial systole P wave is seen

QRS seen in the beginning of ventricular systole

The last T wave can be seen in the beginning of ventricular diastole

Heart sound records (Phonocardiogram/ echo-cardiograph or echocardiograph

The heart sound can be recorded using suitable equipment phonocardiograph

The first heart sound is recorded on the closure of A-V valves

Second heart sound can be recorded at the time to the closure of semi lunar valves

Two small sounds are recorded during the sudden rush of blood into the ventricles