The electrical and mechanical events that occur from the beginning of one heartbeat at the beginning of the next heartbeat is called the cardiac cycle.
The first electrical event in the cardiac cycle is the initiation of a spontaneous action potential in the sinus node (SA node) which is located in the superior lateral wall of the right atrium near the opening of superior vena cava.

There is a delay of.01 second for the current to pass from the atria to the ventricles and it allows the atria to contract completion and pump all the blood to ventricles before ventricle contraction begins.

Phases of cardiac cycle :

There are two phases of a cardiac cycle called Systole and Diastole which are divided into further Seven Subphases.
Diastole: this is a period of relaxation
Systole: this is a period of contraction.

Duration of cardiac cycle :

The total duration of a cardiac cycle is equivalent to the time taken by one systole and one diastole that is the time duration taken by one beat.
Duration of the cardiac cycle can easily be calculated from heart rate in normal person the heart rate is 72 beats per minute so time duration by one cardiac cycle is 1/72 beats /minute that is about .0139 minutes per beat or 0.833 seconds per beat.

There are a total of seven phases of cardiac cycle these are:

1. Atrial systole
2. Isovolumetric ventricular contraction
3. Rapid ventricular ejection
4. Reduced ventricular ejection
5. Isovolumetric ventricular relaxation
6. Rapid ventricular filling
7. Reduced ventricular filling (diastasis )

Cardiac Cycle and ECG

In each event, ECG will be used and an event marker and each event are characterized by a physiological heart sound which is produced by opening and closing of heart valves.
Let’s discuss each event separately.

1. Atrial systole:

In this phase, atrial contraction occurs it is due to electrical activation of atria and on ECG it is represented by P wave.
Blood flows from the great veins into the atria and about 80 percent of the blood flows directly to the atria without atrial contraction, atrial contraction acts as a primer pump and pushed the remaining 20 percent of the blood to the ventricles. It is because of this reason when atria fail it difference is unlikely to be noticed unless the person exercises then clinical signs like shortness of breath occasionally develops.
Fourth heart sound is produced in phase and it’s not audible in the normal person and is only audible in ventricular hypertrophy. 

2. Isovolumetric ventricular contraction:

During this phase, electrical activation of the ventricles occurs ventricles begin to contract and the following event occurs

  • ventricular  pressure rise and it becomes greater than the arterial pressure
  • AV valves are closed it is because of the pressure difference
  •  Semilunar aortic and pulmonary valves do not open so there is no emptying and no volume change.

On ECG it is represented by QRS complex.
The closure of the valve results in first heart sound, the mitral valve closes before the tricuspid valve, it is because of this the first heart sound may be split.

4. Reduced ventricle ejection:

In this phase ejection of the blood from the ventricles continues but it is slower it results in a decrease of ventricular pressure.
During this phase blood runoff from large arteries to small arteries so, aortic pressure also decreases.

Period of ejection:
During this phase, ventricular pressure rises to its maximum value that is above 80mm Hg in left ventricles and slightly above 8mmHg in the right ventricles. These pressure push aortic and pulmonary valves open and blood empties into the aorta and pulmonary trunk. It is further divided into two phases
A.  Period of rapid ejection 
It is the first 1/3 phase-in period of rapid ventricular ejection during this 70 % emptying occur.

B. Period of slow ejection

It is the last 2/3 phase in period of rapid ventricular ejection during this phase 30 % emptying occur.
During the period of rapid ventricular ejection, ventricular volume decreases because most of the stroke volume is ejected during this phase.

Atrial filling begins and the onset of T wave on ECG which represents repolarization of the ventricles, and it marks the end of both ventricular contraction and rapid ventricular ejection.

5. Period of isovolumetric ventricular relaxation :

At the end of T waves which shows that repolarization of the ventricles is complete.
Pressure drops which results in aortic valve closure, followed by the closure of the pulmonary valves.
Second heart sound is produced in this phase which is due to the closure of the semilunar valve. Splitting of second heart sound is due to inspiration which delays the closure of the pulmonary valves.
During most of the isovolumetric ventricular relaxation phase, AV valves remain closed.
Ventricles are now relaxed and because of this ventricular pressure decreases. As all the valves are closed in this phase so ventricular volume is constant that is why this phase is called the isovolumetric phase.

6. Period of Rapid ventricular filling:

During this phase, the following changes occur.

  • Ventricles pressure lessens and when it gets less than atrial pressure the mitral valve opens.
  • With the opening of the mitral valve, ventricular filling from the atrium begins.
  • As blood continuously flows into smaller arteries aortic pressure continues to decrease.
  • As blood flow rapidly from the atria into the ventricles it causes third heart sound which is normal in children but in adult, it is associated with the disease.

7. Period of reduced ventricular filling:

It is the longest phase of the cardiac cycle.
During this phase, ventricular filling continues but it is at a slower rate.
This phase is called diastasis.
The time required for diastasis and ventricular filling depends upon heart rate. For example increase in heart rate cause decreased time available for ventricular refilling, decreased end diastolic volume and decreased stroke volume.

Know more: https://opentextbc.ca/anatomyandphysiology/chapter/19-3-cardiac-cycle/
https://www.medicosite.com/cardiac-cycle/

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