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
