Cardiac Cycle: The heart’s primary function is to circulate blood throughout the body in a cycle. Every day, the human heart beats around 100,000 times. The cardiovascular system involves systemic and pulmonary circulation and is responsible for the transport of various substances in human beings, and is composed of the heart, arteries, veins, and blood capillaries.  The heart’s valves control blood flow, resulting in structured blood propulsion to the next chamber. The cardiac cycle is a series of heart contractions that pressurise distinct chambers of the heart, forcing blood to flow in one direction. Read on more about the cardiac cycle, meaning, duration, and phases for better understanding.

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Cardiac Cycle: Meaning

The cardiac cycle events are the sequence of events that occur during a heartbeat. The (Sinoatrial) SA node possesses the properties of automaticity and rhythmicity. As a result, it causes action potentials that spread throughout the atrial and ventricular muscle fibres. As a result, depolarization and repolarization occur. Following that, the heart undergoes several changes, which are repeated from beat to beat.

The cardiac muscle cells, or cardiomyocytes, that make up the heart muscle are responsible for pumping the blood. Cardiomyocytes, which are separate muscle cells that are striated like skeletal muscle yet pump rhythmically and involuntarily like a smooth muscle.

The heart contracts (systole) during each cardiac cycle, pushing blood out and pumping it around the body; this is followed by a relaxation phase (diastole), during which the heart fills with blood—the atria contract at the same moment, pumping blood into the ventricles through the atrioventricular valves. A monosyllabic “lub” sound is produced as the atrioventricular valves close. The ventricles contract at the same moment after a slight delay, pumping blood through the semilunar valves into the aorta and the artery carrying blood to the lungs (via the pulmonary artery). The semilunar valves close, producing a monosyllabic “dup” sound.

Duration of Cardiac Cycle

The period of a cardiac cycle is determined by the heart rate per minute. If the heart rate is 60 beats per minute, the cardiac cycle will last 1 second. The duration of the cardiac cycle and heart rate have an inverse relation. As a result, if the heart rate is increased to 120 beats per minute, the cardiac cycle will take around 0.5 seconds to complete.

The duration of atrial and ventricular systole and diastole will be as follows: when the heart rate is around 75 beats per minute, the duration of systole and diastole in different chambers will be (Table)given below,

	Events during Cardiac Cycle
	Cardiac Cycle	0.8 sec
a.	Atrial Cycle	Systole – 0.1 sec
		Diastole- 0.7 sec
b.	Ventricular Cycle	Systole – 0.3 sec
		Diastole- 0.5 sec
	Mechanical Events
a.	Atrial
	Systole	Volume Changes Pressure Changes
b.	Ventricular
	Systole	Volume Changes Pressure Changes
	Diastole	Volume Changes
		Pressure Changes

Phases of Cardiac Cycle

1. The ventricular systole begins at the end of the atrial systole.
2. The ventricular chamber is overflowing with blood. The pressure inside the left ventricle is around 5-8 mm Hg. The intraventricular pressure begins to build as the chamber contracts.
3. The blood in the ventricle then tries to return to the atrium.
4. The first cardiac sound is caused by the closing of atrioventricular valves, which prevents blood return to the atrium.
5. Because the aortic pressure is around 80 mm Hg, the semilunar valves at the opening of the aorta are unable to open.
6. As a result, the ventricle now contracts in an isometric manner as if it were a closed chamber.
7. The intraventricular pressure rises dramatically as a result of this.
8. Isovolumetric contraction/isovolumetric ventricular contraction is the phase of the ventricular systole during which both the AV and SL valves are closed, resulting in a sharp increase in intraventricular pressure.
9. This phase lasts around 0.05 seconds.
10. The ventricular pressure rises quickly from around 5 mm Hg to around 80 mm Hg.
11. The SL valves are forced open when the intraventricular pressure exceeds 80 mm Hg, resulting in the following sub-phase of the ventricular systole, known as the maximal ejection phase.

a. Maximum Ejection Phase

1. This phase lasts around 0.11 seconds.
2. Isotonic contractions occur in the ventricular fibers.
3. The pressure inside the chamber gradually rises to around 120 mm Hg.
4. During this phase, approximately 70% of the stroke volume is pushed out of the ventricle and into the aorta.

b. Reduced Ejection Phase

1. The duration is around 0.14 seconds.
2. Some of the fibers in the ventricular muscle have already begun to relax.
3. During this phase, approximately 30% of the stroke volume is pushed into the aorta.
4. The intraventricular pressure begins to drop gradually.

c. Protodiastole Phase

1. The duration is approximately 0.04 seconds.
2. This is the time interval between the conclusion of the ventricular systole and the SL valves closing.
3. Blood from the aorta tends to flow back into the ventricle when the intraventricular pressure falls below the aortic pressure.
4. The rapid closing of aortic valves prevents the backflow of blood.
5. The production of the second heart sound is caused by the closing of aortic valves. Clinical systole is the time gap between the first and second cardiac sounds.

d. Isovolumetric Relaxation Phase

1. The duration is around 0.08 seconds.
2. The AV valves that were closed at the start of the ventricular systole are still closed, and the SL valves have been closed as well.
3. The ventricular muscle now begins to relax, and the ventricle closes up as a closed chamber. As a result, the pressure in the ventricle drops dramatically, with no changes in the volume of blood in the ventricle.
4. The pressure rapidly drops to 0 millimetres of mercury.

The blood returning to the heart from the venous compartment accumulates in the atria from the moment the AV valves close. This is what causes the atrium’s pressure to rise slowly. The pressure in the atrium continues to rise until the AV valves open at the conclusion of isovolumetric ventricular relaxation, signalling the start of the next phase, the initial rapid filling phase.

e. Initial Rapid Filling Phase

1. The duration is around 0.09 seconds.
2. There is a pressure gradient between the atrium and the ventricle during this phase.
3. Blood begins to flow from the atrium to the ventricle as a result of this.
4. The generation of the third heart sound is caused by a quick rush of blood from the atrium to the ventricle.
5. This phase accounts for roughly 70% of ventricular filling.
6. Because no active muscle contraction is required for ventricular filling and just a pressure gradient is required, the ventricular filling is a passive process.

Clinical Significance

Atrial fibrillation is a condition in which the atrial muscles stop contracting. The cardiac output does not decrease significantly as a result of the passive process of ventricular filling.

Diastasis (Slow Filling Phase)

1. The duration is around 0.19 seconds.
2. The pressure in the atrial decreases as blood moves from the atrium to the ventricle.
3. When blood rushes from the atrium into the ventricle, it collects in the ventricle, raising intraventricular pressure.
4. As a result of this, the pressure gradient between the atrium and ventricle eventually reduces. During this phase, blood flow from the atrium to the ventricle is stopped.

Even though there is no blood flow, this phase has a lot of practical value because when the heart rate increases or decreases, the duration of the cardiac cycle changes. In most cases, the duration of ventricular systole does not differ much from the duration of ventricular diastole.

The duration of the diastasis is influenced even during ventricular diastole. The duration of the diastasis is affected as the heart rate increases. As a result, despite a rise in heart rate, ventricular filling stays relatively normal.

f. Final Rapid Filling Phase

1. The duration is approximately 0.1 seconds.
2. This corresponds to the atrial systole phase.
3. The atrial muscle is now actively contracting, pumping blood from the atrium to the ventricle.
4. This phase accounts for about a quarter of ventricular filling.
5. The 4th cardiac sound is produced by blood flow into the ventricle.

Fig: Cardiac cycle

Automatic Rhythmicity of the Heart

1. The automatic rhythmicity of the heart is its ability to contract spontaneously and at a regular rate.
2. The impulse of contraction of the heart originates in the sinoatrial node(SA node) situated in the right atrium close to the point of entry of the vena cava.
3. The SA node consists of a small number of diffusely oriented cardiac fibres, possessing few myofibrils and few nerve endings from the autonomic nervous system.
4. The cells of the SA node are known as the pacemaker of the heart. It initiates the heartbeat, but the rate at which it beats can be varied by stimulation from the autonomic nervous system.
5. The cells of the SA node maintain a differential ionic concentration across their membranes of 90mV. The cells have a permanently high sodium conductance.
6. The sodium ions continually diffuse into the cells producing depolarisation which leads to a propagated action potential. This is called cardiac impulse.
7. As this wave of excitation passes across the muscle fibres of the heart, it causes them to contract. The cardiac impulse spreads directly from the SA node over the two atria at the rate of 1m/sec.
8. It, however, cannot spread along the common cardiac muscle fibers from the atria to the ventricles because in the mammalian heart, the atrial muscle fibers are completely separated from those of the ventricles by the atrioventricular node(AV node).
9. From AV, the node arises, the main trunk of the ‘Bundle of His‘ to carry the impulse to the ventricles.
10. AV bundle provides the only route for the transmission of the wave of excitation from the atria to the ventricles.
11. The path from the site of origin of the cardiac impulse to the ends of the branches of the ‘Bundles of His’ together known as the conductive system of the heart.

Refractory Period

1. Another significant aspect of cardiac muscles is their ability to contract.
2. Once the heart muscle begins to contract, it is unable to respond to any additional stimuli until it relaxes. This is known as the refractory period.
3. Any stimulation, regardless of intensity, fails to excite the heart muscle during this phase, and a second contraction is not possible.
4. This prolonged refractory period allows the heart muscle to relax sufficiently. As a result, the heart muscle never gets tired or tetanized (state of sustained contraction).
5. The absolute refractory period of contraction occurs in muscles, whereas the relative refractory period occurs in the initial phase of relaxation.

Wiggers Diagram

Fig: Graphical Representation of Heartbeat

Dr. Carl J Wiggers, an American-born physiologist, has given many healthcare students a unique tool to study the heart cycle throughout the last 100 years. Wiggers’ diagram depicts the relationship between pressure and volume over time, as well as the heart’s electrical activity.

In the Wiggers diagram, the X-axis is used to plot time, while the Y-axis contains all of the following on a single grid and is used to demonstrate:

1. Aortic pressure
2. Atrial pressure
3. Ventricular pressure
4. Ventricular volume
5. Electrocardiogram (ECG)
6. Phonocardiogram (Heart Sounds)

Aortic Pressure

The aortic pressure graph depicts the variation in aortic pressure throughout the cardiac cycle. The graph begins with a moderate inclination, then a notch, and finally a smaller incline. Before starting afresh, the graph concludes with a slow fall.

Fig: Representation of Aortic Pressure

The pressure relationship between the right ventricle and the pulmonary artery is similar to this graph. The key distinction is that the pressure is much lower. As ejection proceeds, aortic pressure increases to a maximum (systolic pressure) and starts to fall as the left ventricle relaxes. When the ventricular pressure has fallen below the aortic pressure, the aortic valve closes, and ejection ceases.

Atrial Pressure

1. The atrial pressure wave depicts how atrial pressure changes throughout systole and diastole. The letters a, v, and c stand for three different major pressure shifts.
2. The ‘v’ wave near the end of the atrial pressure wave represents the pressure change generated as the atria fill with blood.
3. The opening of the atrioventricular valve causes a modest drop in atrial pressure.
4. The ‘a’ wave, which depicts the contraction of the atria, follows.
5. As the atrioventricular valves close, the ‘a’ wave is followed by a downward slope.
6. As systole begins, the AV valves close, and a brief period of isovolumetric contraction occurs, producing a second low-pressure peak; the ‘c’ wave is then followed by yet another increase.

Fig: Atrial Pressure in Wiggers Diagram

Ventricular Pressure and Volume

1. Two different curves depict the pressure and volume variations that occur in the ventricle.
2. They are, nevertheless, best interpreted in combination. The ventricular pressure curve has two waves: a small wave at the beginning, followed by a return to baseline pressure, and then a much greater wave.
3. Throughout its cycle, the ventricular volume curve, on the other hand, has a combination of sharp and gentle slopes and inclines.

Fig: Ventricular pressure and ventricular volume on Wiggers diagram

Electrocardiogram (ECG or EKG)

The ECG is a graphical representation of the heart’s electrical activity. It consists of a succession of waves representing depolarization and troughs representing repolarization.

1. Between the depolarization of myocardiocytes and the actual contraction of the muscles, there is a lag.
2. As a result, the ECG waves will arrive ahead of the pressure curve waves (which are caused by the actual contraction of the heart muscle).

Heart sounds

1. The heart sounds are represented by the phonocardiogram during the cardiac cycle.
2. These heartbeats, which can be heard during auscultation, are the result of the heart valves closing. The sounds are usually referred to as “lub” and “dub.”
3. The closure of the atrioventricular valves causes the first heart sound, also known as S1 or the “lub” sound.
4. The closure of the semilunar valves causes the second heart sound, also known as the “dub” sound. This happens during the isovolumetric relaxation phase at the start of diastole.
5. Hearing a third heart sound, often known as S3, is not uncommon. A quick surge of blood from the atria into the ventricles usually causes this.

Summary

The heart contracts (systole) during each cardiac cycle, pushing blood out and pumping it around the body; this is followed by a relaxation phase (diastole), during which the heart fills with blood—the atria contract at the same moment, pumping blood into the ventricles through the atrioventricular valves. A monosyllabic “lub” sound is produced as the atrioventricular valves close. The ventricles contract at the same moment after a slight delay, pumping blood through the semilunar valves into the aorta and the artery carrying blood to the lungs (via the pulmonary artery). The semilunar valves close, producing a monosyllabic “dup” sound.

Phases of the cardiac cycle are atrial contraction, isovolumetric contraction, rapid ejection, reduced ejection, isovolumetric relaxation, rapid filling, and reduced filling. Wiggers’ diagram depicts the relationship between pressure and volume over time, as well as the heart’s electrical activity. The left chambers of the heart are used to demonstrate Aortic pressure, Atrial pressure, Ventricular pressure, Ventricular volume, Electrocardiogram (ECG), and Phonocardiogram (heart sounds).

Frequently Asked Questions (FAQs) on Cardiac Cycle

Q.1. What does the cardiac cycle explain?
Ans: The cardiac cycle describes the human heart’s activity from the start of one heartbeat to the start of the next. It is divided into two parts: diastole, when the heart muscle relaxes and fills with blood, and systole, when the heart muscle contracts and pumps blood vigorously.

Q.2. What are the phases of the cardiac cycle?
Ans: The cardiac cycle is split into 7 phases, including atrial contraction, isovolumetric contraction, rapid ejection, reduced ejection, isovolumetric relaxation, rapid filling and, reduced filling.

Q.3. What is the significance of the cardiac cycle?
Ans: The heart’s primary function is to circulate blood throughout the body in a cycle known as the cardiac cycle. The cardiac cycle is the electrical signals that cause the heart muscles to contract and relax that coordinate the filling and emptying of the heart with blood.

Q.4. What is the systole and diastole of the heart?
Ans: The cardiac cycle is divided into two phases: diastole and systole. They happen as the heart beats, moving blood through a system of blood vessels that transports blood to all parts of the body. When the heart contracts to pump blood out, it is called systole, and when it relaxes after contraction, it is called diastole.

Q.5. What is the difference between the information contained in a phonocardiogram and an electrocardiogram?
Ans: ECG is produced by electrical activities of the heart, and PCG is produced by mechanical activities of the heart; the two have different criteria. As a result, ECG segmentation algorithms cannot be directly used for PCG segmentation.

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