ADVERTISEMENTS:
In this article we will discuss about:- 1. Meaning of Cardiac Cycle 2. Duration of Cardiac Cycle 3. Phases.
Meaning of Cardiac Cycle:
SA node, the pacemaker has the property of automaticity and rhythmicity. Because of this, it produces action potentials, which spread all along the atrial and ventricular muscle fibers. This in turn brings about depolarization and repolarization. Following this, various changes occur in the heart, which is repeated from beat to beat. These events are known as the events of the cardiac cycle.
The events are (Fig. 3.24):
i. Mechanical changes in the form of contraction (systole) and relaxation (diastole) in atria and ventricles.
ii. Hemodynamic changes that is pressure and volume changes in atria and ventricles.
iii. Acoustic changes that is the production of heart sounds.
Duration of Cardiac Cycle:
Duration of cardiac cycle depends on the heart rate per minute. For example, if the heart rate is 60 per minute, the cardiac cycle duration will be 1 sec. There is an inverse relationship between the duration of cardiac cycle and heart rate. Hence when the heart rate is increased to 120 beats per minute, the cardiac cycle duration will be about 0.5 sec.
ADVERTISEMENTS:
Duration of atrial and ventricular systole and diastole will be as follows: when the heart rate is about 75 times per minute the duration of systole and diastole in different chambers will be (Table 3.5);
Atria:
a. Systole is about 0.1 sec.
b. Diastole is about 0.7 sec.
Ventricular:
a. Systole is about 0.3 sec.
b. Diastole is about 0.5 sec.
Phases of Cardiac Cycle:
ADVERTISEMENTS:
From the above, it is obvious that the systole of atrial chambers will be followed by the systole of ventricular chambers. The ventricular and atrial systole will never coincide. Part of ventricular diastole and atrial diastole occur simultaneously. That is part of atrial diastole will occur when the ventricles are also in diastolic phase.
The further events of the cardiac cycle occurring in the ventricular chambers are discussed on the basis of intraventricular pressure changes. The pressure recording and the volume changes in the ventricular chambers can be made out with the help of cardiac catheterization (Fig. 3.25).
At the end of the atrial systole, the ventricular systole starts. The ventricular chamber is filled with blood. The left intraventricular pressure is about 5-8 mm Hg. As the chamber begins to contract, the intra ventricular pressure begins to rise. Blood in the ventricle tries to go back into the atrium.
ADVERTISEMENTS:
This is prevented by the closure of atrioventricular valves and this is responsible for the production of the 1st heart sound. The semilunar valves, which are present at the beginning of aorta, are yet to open, as the aortic pressure is around 80 mm Hg. Hence the ventricle now contracts isometrically as a closed chamber. Because of this, the intraventricular pressure rises sharply.
So, the phase of ventricular systole during which both the AV and SL valves are in closed state, and which gives raise to sharp increase of intra ventricular pressure is known as isovolumetric contraction/isovolumetric ventricular contraction. The duration of this phase is about 0.05 sec.
The ventricular pressure rapidly rises from about 5 mm Hg to about 80 mm Hg. When the intraventricular pressure rises beyond 80 mm Hg, the SL valves are forced open and this leads the next sub-phase of ventricular systole, which is known as maximum ejection phase.
a. Maximum Ejection Phase:
ADVERTISEMENTS:
i. Duration of this phase is about 0.11 sec.
ii. The ventricular fibers contract isotonically.
iii. The pressure in the chamber increases gradually to about 120 mm Hg.
iv. Approximately, 70% of the stroke volume is pumped out from the ventricle into the aorta during this phase.
ADVERTISEMENTS:
b. Reduced Ejection Phase:
i. Duration is about 0.14 sec.
ii. Some of the ventricular muscle fibers have already started relaxing.
iii. About 30% of the stroke volume is pumped out into the aorta during this phase.
iv. The intraventricular pressure slowly starts decreasing.
c. Protodiastole Phase:
ADVERTISEMENTS:
i. Duration is about 0.04 sec.
ii. This is the time interval from the end of ventricular systole to the closure of the SL valves.
iii. When once the intraventricular pressure falls below the aortic pressure, blood from the aorta tends to flow back into the ventricle.
iv. This is prevented by the sudden closure of aortic valves.
v. The closure of aortic valves is responsible for the production of the 2nd heart sound.
The time interval between the 1st and 2nd heart sounds is known as clinical systole.
ADVERTISEMENTS:
d. Isovolumetric Relaxation Phase:
i. Duration is about 0.08 sec.
ii. AV valves which were closed at the beginning of ventricular systole are still in the closed state and the SL valves have also got closed.
iii. Now the ventricular muscle starts relaxing, and the ventricle relaxes as a closed chamber. Therefore, the pressure in the ventricle falls sharply without any alterations in the volume of blood in the ventricle
iv. The pressure falls rapidly to as low as zero mm Hg.
Right from the moment the closure of AV valves, the blood that is returning to the heart from the venous compartment keeps getting accumulated in the atria. This is responsible for slow raise of pressure in the atrium. The increase of pressure in the atrium continues until the AV vales open which occurs at the end of isovolumetric ventricular relaxation and this will lead to the next phase namely the initial rapid filling phase.
e. Initial Rapid Filling Phase:
i. Duration is about 0.09 sec.
ii. During this phase, there is pressure gradient between the atrium and the ventricle.
iii. Due to this, blood starts flowing from the atrium to ventricle.
iv. The sudden rush of blood from the atrium to ventricle is responsible for the production of the 3rd heart sound.
v. About 70% of ventricular filling occurs during this phase.
vi. Since no active contraction of the muscle is involved for ventricular filling, only a pressure gradient facilitates this, the ventricular filling occurs by a passive process.
Clinical significance:
Conditions like atrial fibrillations wherein atrial muscles stop contracting. Due to the passive process of filling of ventricle, cardiac output does not fall considerably.
Diastasis (slow filling phase):
i. Duration is about 0.19 sec.
ii. As blood flows from the atrium to ventricle, the pressure falls in the atrium.
iii. Rushing of blood from the atrium into the ventricle, blood accumulates in the ventricle and increases the intraventricular pressure.
iv. Due to this, the pressure gradient between the atrium and ventricle gradually decreases. This stops the further blood flow from the atrium to the ventricle during this phase.
Though there is no blood flow, this phase has lot of practical significance, when there is increase or decrease of heart rate; cardiac cycle duration will also change. Normally, the duration of the ventricular systole does not get altered much when compared to the duration of the ventricular diastole.
Even in the ventricular diastole, it is the duration of the diastasis that is affected. When there is an increase of heart rate, the duration of the diastasis is compromised. Therefore, in spite of an increase in the heart rate, ventricular filling remains fairly normal.
f. Final Rapid Filling Phase:
i. Duration is about 0.1 sec.
ii. This corresponds to the phase of atrial systole.
iii. Now the active contraction of atrial muscle pumps blood from atrium to ventricle.
iv. About 25% of ventricular filling occurs during this phase.
v. Blood flow into the ventricle causes the production of 4th heart sound.
Phonocardiogram: Refers to graphical recording of heart sounds.
Heat sounds can be heard using a stethoscope at specified areas on the precardial region. Almost anyone who is trained better, can hear the 1st and 2nd heart sounds and sometimes the 3rd sound also. But the 4th sound can only be graphically recorded. The heart sounds are affected in conditions like stenosis of valves, incompetence of valves, etc.
At times, in very rare cases, the 1st heart sound may split due to asynchronous closure of mitral and tricuspid valves.
The 2nd heart sound is replaced or followed by murmur in aortic incompetence.
Some of the important features of 1st and 2nd heart sounds have been shown in Table 3.6.
