Cardiac cycle | PQRST rhythm | Phases of cardiac cycle | Heart and heart beats

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Cardiac cycle | PQRST rhythm | Phases of cardiac cycle | Heart and heart beats

CARDIAC CYCLE



INTRODUCTION

The heart can be thought of as a pump that consists of two independent pumps in series: a right heart that pumps blood through the lungs and a left heart that pumps blood through the peripheral organs. The heart contracts and relaxes rhythmically and thus acts like a pump. The term systole (also called contractile phase) and diastole (also called relaxation phase) usually refer to the ventricular events but may be prefixed by ‘atrial’ to refer to the atrial contraction and relaxation, respectively.

The electrocardiogram is the device which records the electrical events of heart that precede and initiate the corresponding mechanical events as:

> P wave: It is followed by the atrial contraction,

> QRS waves: These are caused by depolarization of the ventricles which initiates contraction of the ventricles and

> T wave: It occurs slightly before the end of the ventricular contraction.


Thus a cardiac cycle includes both electrical and mechanical events that occur from the beginning of one heart beat to the beginning of the next heart beat.


Cardiac cycle | PQRST rhythm | Phases of cardiac cycle | Heart and heart beats

 

PHASES OF CARDIAC CYCLE

At a normal heart rate each cardiac cycle has a duration of-

      75 beats/min is = 0.8 sec

During each cardiac cycle both atria of the heart contract (called atrial systole) and relax (called atrial diastole), and both ventricles of the heart contract (called ventricular systole) and relax (called ventricular diastole).

Hence, each cardiac cycle can be considered to be consist of simultaneously occurring atrial and ventricular cycles with following phases:


Atrial cycle

1. Atrial systole (or atrial contraction phase) is 0.1 s and

2. Atrial diastole (or atrial relaxation phase) is 0.7 s.


Ventricular cycle

Ventricular systole (0.3 s) consisting of:

1. Isovolumic (or isometric) contraction phase (which is 0.05 s) and

2. Phase of ventricular ejection can be further divided into rapid ejection phase (0.1 s) and slow ejection phase (0.15 s).


Ventricular diastole (0.5 s) consisting of:

1. Proto-diastole is 0.04 s,

2. Isovolumic (or isometric) relaxation phase is 0.06 s,

3. Rapid passive filling phase is 0.11 s,

4. Reduced filling phase (or diastasis) is 0.19 s and

5. Last rapid filling phase which coincides with atrial systole is 0.1 s.

 

ATRIAL CYCLE

ATRIAL SYSTOLE

Atrial systole or the atrial contraction phase lasts for 0.1 sand coincides with the last rapid filling phase of ventricular diastole.

Before the beginning of the atrial systole, the ventricles are relaxing, the atrio-ventricular (AV) valves are open and the blood is flowing from the great veins into the atria and from the atria into the ventricles. Therefore, the atria and ventricles are forming a continuous cavity.

When the atrial contraction begins, about 75% of the blood has already flown into the ventricles. Therefore an atrial contraction usually creates an additional 25% filling of the ventricles.


The contraction of atria causes:

> The intra-atrial pressure increases by 4–6 mm Hg in the right atrium and 7–8 mm Hg in the left atrium. The rise in pressure in the right atrium is reflected into the veins and is recorded as a-wave from the jugular vein.

> The ventricular pressure slightly increases due to pumping of blood in the ventricles.

Narrowing of origin of great veins (inferior vena cava and superior vena cava opening in right atrium) and pulmonary veins opening in left atrium decreases venous return to the heart. Some regurgitation of the blood occurs into the great veins as there are no valves present between them  and the atria.


ATRIAL DIASTOLE

Then after the atrial systole ends, the atrial diastole occurs (0.7 s).

This period actually coincides with the ventricular systole and most of the ventricular diastole too.

During the atrial diastole, the atrial muscles relax and gradual filling of the atria occurs due to continuous venous return and the pressure gradually increases in the atria to drop down to almost zero with the opening of AV valves Then the pressure again rises and follows the ventricular pressure during the rest of atrial diastole.


VENTRICULAR CYCLE

VENTRICULAR SYSTOLE

The ventricles become activated by the impulse travelling along the conduction system after the atrial contraction phase is over, and they begin to contract. The ventricular systole lasts 0.3 seconds and is divided into following phases:


1. Phase of isovolumic (isometric) contraction

As the ventricular contraction begins, the ventricular pressure exceeds the atrial pressure very rapidly causing closure of AV valves (this event is responsible for the production of first heart sound).

Since the AV valves have closed and semilunar valves have not opened yet, so the ventricles contract as a closed chamber and the pressure inside the ventricles rises rapidly to a high level.

As here the ventricles contract but the volume of blood in the ventricles does not change so this phase is called isovolumic contraction phase.

Bulging of AV valves into the atria occurs during this phase, producing a small but sharp rise in the intra-atrial pressure called c-wave, due to sharp rise in the ventricular pressure.

This phase continues for 0.05 s, until the pressure in the left  and right ventricles exceeds the pressure in the aorta(80 mm Hg) and pulmonary artery (10 mm Hg) and the aortic and pulmonary valves opens.


2. Phase of ventricular ejection

The ventricular ejection phase starts with the opening of semilunar valves and lasts for about 0.25 s. This phase can further be sub-divided into two phases, these are:

I. Rapid ejection phase: Once the semilunar valves open, the blood is rapidly ejected out for about 0.1 s. About two-thirds of the stroke volume is usually ejected in this rapid ejection phase. Pressure in this case rises to 120 mm Hg in the left ventricle and to 25 mm Hg in the right ventricle. The right ventricular ejection starts before the left one and is  continued even after left ventricular ejection is complete. As both the ventricles ejects almost same volume of blood and the velocity of right ventricular ejection is less than that of the left ventricle.


II. Slow ejection phase:  It refers to the last two-thirds of systole (about 0.15 s), when the rate of ejection begins to slow.

During this period, about a third of the stroke volume is expelled. The intraventricular pressure begins to diminish, reaching a level somewhat lower than that of the aorta, yet momentum maintains the blood flowing forward for a brief time.

The volume fluctuates. At the end of atrial systole, each ventricle has blood volume of about 130 ml. This is called end-diastolic volume.

An average of about 70 ml to 80 ml of blood is ejected out by each ventricle during each systole. This is called stroke volume.

And about 50 mL of blood is left in each ventricle at the end of systole. This is called end-systolic volume.


VENTRICULAR DIASTOLE

1. Protodiastole

As soon as the ventricular systole ends, the ventricles start relaxing and intraventricular pressure falls rapidly. This phase usually lasts for 0.04 s. During this phase, the raised pressure in the distended arteries (aorta and pulmonary artery) immediately pushes the blood back towards ventricles which snaps the semilunar valves to close. Closure of semilunar valves prevents the movement of blood back into the ventricles and produces the second heart sound (S2). It also causes a dicrotic notch (which is a prominent and distinctive feature of the pressure waveform in the central arteries) in the down slope of aortic pressure curve called the incisura.


2. Isovolumic or isometric relaxation phase

This isometric relaxation phase begins with the closure of the semilunar valves and lasts for about 0.06 s.

As the semilunar valves have closed and the AV valves have not opened yet, so the ventricles continues to relax as closed chambers in this phase. This results in rapid fall of pressure inside the ventricles (usually from 80 mm Hg to about 23 mm Hg in the left ventricle).

Since in this particular phase, the ventricular volume remains constant, so this phase is called as the isovolumic or isometric relaxation phase.

This phase ends as soon as the AV valves open, as indicated by the peak of v-wave on the atrial pressure tracing.

 

3. Rapid passive filling phase (0.11 s)

During ventricular systole, the atria are in diastole and venous return continues so that the atrial pressure remains high. But as soon as the AV valves open, the high atrial pressure causes a rapid, initial blood flow into the ventricles. The rapid passive filling phase produces the third heart sound (S3), which is normally not audible in adults but may be heard in children.

As the AV valves open, the atria and ventricles are a common chamber and pressure in both cavities falls as ventricular relaxation continues.


4. Reduced filling and diastasis (0.19 s)

Here in this particular phase, pressure in the atria and ventricles reduces slowly and remains little above zero. This leads to the decrease in the rate of blood flow from the atria to ventricle causing a very slow filling called diastasis.


Note:

It is important to note that about 75% of blood passes from the atria to the ventricles during rapid filling and reduced filling phases of the ventricular diastole.


5. Last rapid filling phase (0.1 s)

The last rapid filling phase of ventricular diastole actually coincides with the atrial systole. As described in the beginning, the atrial systole brings about the last rapid filling phase and pushes the additional 25% of blood in the ventricles. With this phase, the ventricular cycle gets completed.

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