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Cardiac Cycle Simulation

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Background Information

Introduction

The cardiac cycle is the sequence of mechanical and electrical events during a single heartbeat. When the heart beats 75 times per minute, one cardiac cycle lasts 0.8 seconds. The duration shortens as the heart rate increases and lengthens as the heart rate decreases. Because it is recurring, the end of one cycle immediately precedes and prepares the heart for the start of the next cycle.

Cardiac cycle and circulatory system animation

Heart Structures Review

Before continuing the presentation of the cardiac cycle, it may be beneficial to first review the names and functions of the heart chambers, valves, and major vessels.

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Cardiac Cycle Segments

The cardiac cycle consists of two main periods (parts or divisions): ventricular systole and ventricular diastole. Pacemaker cells in the heart walls regulate the timing and duration of these periods.

◉ Ventricular systole makes up about 1/3 of the cardiac cycle. During this time, the chamber walls contract and eject blood from the heart into large arteries that unite with the pulmonary and systemic circulatory systems.

◉ Ventricular diastole makes up the remaining 2/3 of the cardiac cycle. During this time, the walls of the chambers relax, and blood enters the heart through large veins from the pulmonary and systemic circulatory systems.

Physiologists typically subdivide ventricular systole and diastole periods into several phases (or stages) to better study the events in the heart chambers and major blood vessels. The approach and number of phases vary but generally resemble the scheme below.

Phases and Basic Events

Ventricular Systole:

Ventricular Isovolumetric Contraction

The blood-filled ventricles start contracting during this phase, increasing the pressures in the chambers. When the ventricle pressures rise above the atria pressures, it closes the atrioventricular valves. Because all the heart valves are closed, blood cannot exit the ventricles, so the blood volume in these chambers remains unchanged or “isovolumetric.”

Ventricular Ejection Phase

The ventricular walls continue to contract, causing the blood pressure in these chambers to increase. When the ventricular blood pressures rise above those in the large heart arteries, it forces the semilunar valves to open, allowing blood to flow into the aorta and pulmonary trunk arteries. Initially, the blood flows rapidly out of the ventricles but slows as the phase continues.

Ventricular Diastole:

Ventricular Isovolumetric Relaxation

The ventricles start relaxing, decreasing the blood pressure in the chambers. When the ventricular pressures fall below the pressures in the large arteries, it closes the semilunar valves. Because all the heart valves are closed, blood cannot enter the ventricles, so the blood volume in these chambers remains unchanged or “isovolumetric.”

Passive Ventricular Filling

The pressure on the blood in the ventricular chambers decreases as the ventricular walls continue relaxing. When the ventricular pressures fall below the atrial pressures, it forces the atrioventricular valves to open. Initially, blood in the atria rapidly enters the ventricles. Filling slows as blood from the heart’s large veins flows into the ventricles after passing through the atria.

Atrial Systole

The previous phase nearly fills the ventricles with blood. To complete the filling process, the atria contract to actively inject additional blood into the ventricles just before they contract.

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Cardiac Cycle Starting Points

Because the phases are part of a cyclic process, there is no set (or standardized) starting point for the cardiac cycle. Physiologists often select the atrial systole phase as the starting point of the cardiac cycle because it coincides with the P wave at the beginning of the electrocardiogram. Another commonly used starting point is the ventricular isovolumetric contraction phase. It is selected because it marks the beginning of ventricular systole.

Atrial Systole Start Point

Atrial systole phase as the starting point of the cardiac cycle.

Isovolumetric Contraction Start Point

Isovolumetric contraction phase as the starting point of the cardiac cycle.

Regulation of Blood Flow

Blood flows through the heart from areas of higher pressure to areas of lower pressure, which changes with each cardiac cycle phase.

The pressure increases in the heart chambers when cardiomyocytes (myocardiocytes or heart muscle cells) contract and decreases when the cardiomyocytes relax.

Electrochemical activity (action potentials) in cardiomyocyte membranes controls their rhythmic contraction and relaxation. The pace of cardiomyocyte action potentials is, in turn, regulated by the heart’s conduction system.

Heart valves direct the blood flow by allowing the blood only to enter the proper low-pressure areas.

As the blood moves from areas of higher pressure to lower pressure, it produces corresponding changes in heart chamber blood volumes.

Cardiac cycle blood flow animation

Wiggers Diagram

The Wiggers diagram, named after its developer, Carl Wiggers, is a composite of several graphs related to the cardiac cycle. Physiologists use the information the Wiggers diagram provides to interpret and comprehend the changing events associated with each part of a heartbeat.

Wiggers Diagram

Wiggers diagram

X-Axis Components

The X-axis (horizontal axis) of the Wiggers diagram displays the sequence and durations of the main divisions and subdivisions (phases) of the cardiac cycle.

X-Axis

Wiggers diagram: x-axis

Y-Axis Components

The Y-axis (vertical axis) displays the amplitudes of several heart events associated with each part of the cardiac cycle, including chamber pressures, chamber volumes, electrical activity, and sounds. The recordings are taken from the left side of the heart because the ventricle is thicker and produces more forceful contractions than the right.

Chamber and Arterial Pressures

Wiggers diagram: pressures

Ventricular Volumes

Wiggers diagram: volumes

Electrical Activity

Wiggers diagram: electrocardiogram

Heart Sounds

Wiggers diagram: phonocardiogram

Interactive Display

Use the interactive display below to put the cardiac cycle events in continuous motion. As the sequence of phases sweeps across the screen, notice how the heart’s electrical and mechanical activities are related. Also, observe how the heart’s mechanical activities affect heart pressures, volumes, and sounds.

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Procedures and Assessments

Press the appropriate button for each phase to see it individually displayed along with the associated heart actions.

View the animated heart to compare the phase event graphs and heart actions.

To assess the phase events, use the portion of the Wiggers diagram highlighted by the phase scan. Use the hide/show buttons to select individual graphs.

Isovolumetric Contraction

Procedure: Click the ventricular isovolumetric contraction button to view a scan of this phase and the associated heart actions.

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Assessment: Analyze the phase events by answering the following questions.

Ventricular isovolumetric contraction takes place during early systole.

The QRS complex.

The cardiomyocytes are depolarizing, which initiates contraction.

The ventricular walls start contracting, which applies pressure to the blood in the chambers. The AV valves close when the pressure in the ventricles becomes higher than in the atria.

The AV valves close at the point where the ventricular and atrial pressures intersect.

They remain closed during this phase because the ventricular pressure is less than the pressure in the major arteries.

The volume remains relatively stable because all the valves are closed. The volume remains stable because all the valves are closed. These factors are the reason why the phase is called isovolumetric contraction.

The pressure rises rapidly because the closed heart valves block blood movement out of the chambers.

The source of the heart sounds are vibrations produced by the closing AV valves.

This phase lasts 0.05 secs or 4% of the cycle.


Ventricular Ejection

Procedure: Click the ventricular ejection button to view a scan of this phase and the associated heart actions.

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Assessment: Analyze the phase events by answering the following questions.

Ventricular ejection takes place during mid to late systole.

This phase runs from the QRS complex’s end to the T wave’s mid-portion.

The ventricular walls fully contract.

The peak ventricular pressure is about 120 mmHg.

The pressure decreases when the cardiomyocytes start repolarizing and stop contracting.

The volume initially decreases rapidly and then slows. The rate of ejection slows due to the decrease in ventricular pressure.

The semilunar valves open when ventricular pressure exceeds the pressure in the major arteries.

The aortic pressure rises and falls in response to changes in the ventricular pressure.

The amount of blood ejected is approximately 70 ml.

About 60 ml of blood remains in the ventricles.

This phase lasts about 0.22 secs or 28% of the cycle.


Isovolumetric Relaxation

Procedure: Click the ventricular isovolumetric relaxation button to view a scan of this phase and the associated heart actions.

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Assessment: Analyze the phase events by answering the following questions.

The ventricles are in early diastole.

This phase takes place at the end of the T wave.

More cardiomyocytes repolarize and enter a state of relaxation.

The ventricular pressure decreases rapidly.

The ventricular pressure remains higher than the atrial pressure during this short phase.

The ventricular pressures drop below pressures in the large arteries.

The semilunar valves close at the point where the ventricular and atrial pressures intersect.

The volume remains relatively steady. With closed AV and semilunar valves, blood cannot flow in or out of the ventricles as the ventricles start to relax. These factors are the reason why the phase is called isovolumetric relaxation.

The source of the heart sounds are vibrations produced by the closing semilunar valves.

It lasts about 0.08 secs or 10% of the cycle.


Passive Ventricular Filling

Procedure: Click the passive ventricular filling button to view a scan of this phase and the associated heart actions.

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Assessment: Analyze the phase events by answering the following questions.

The ventricles are in the mid-portion of diastole.

This phase occurs along the flat (isoelectric) line between the end of the T wave and the mid-portion of the following P wave when the cardiomyocytes are at rest.

The cardiomyocyte membranes are in a state of rest between action potentials.

The AV valves open at the start of this phase when the ventricular pressures fall below the atrial pressures.

No. Initially, the volume increases rapidly as accumulated blood in the atria enters the ventricles. The rate slows as blood from the major veins passively flows through the atria into the ventricles.

The pressures stay near 0 mmHg.

The pressure does not increase because the ventricles are fully relaxed and have space for the incoming blood.

The ventricles add about 70% to 80% of their total filling during this phase.

This phase lasts about .35 secs or 44% of the cycle.


Atrial Systole (Active Ventricular Filling)

Procedure: Click the atrial systole button to view a scan of this phase and the associated heart actions.

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Assessment: Analyze the phase events by answering the following questions.

The ventricles are in the late stage of diastole.

This phase takes place from the middle of the P wave to the middle of the QRS complex, the P-R segment.

The atrial cardiomyocyte membranes depolarize to initiate contraction.

This phase adds 10% to 30% (15 ml to 30 m) to the total filling of the ventricles.

The ventricles fill to about 120 ml to 130 ml.

The ventricular pressure slightly increases because they have space for the added blood.

The blood does not flow back into the major veins because their pressures are higher than the atrial pressures.

This phase lasts about 0.1 secs or 14% of the cycle.

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References and Attributions

Anesthesia Key – Cardiac Cycle

Advances in Physiological Education (American Physiological Society) – Constructing the Wiggers diagram using core concepts: a classroom activity

Physiological Reviews (American Physiological Society) – Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

N. I. H. National Library of Medicine – The Cardiac Cycle and the Physiological Basis of Left Ventricular Contraction, Ejection, Relaxation, and Filling

N. I. H. National Library of Medicine – Physiology, Cardiac Cycle

Researchgate – Transmembrane ionic currents underlying cardiac action potential in mammalian hearts

Rice University (OpenStax) – Cardiac Cycle

Science Direct – Isovolumetric Contraction

University of California Cardiovascular Imaging Lab – Cardiac Cycle

University of California Davis (LibreTexts) – Heart Sounds

University of Cape Town Clinical Research Centre – The Cardiac Cycle, Wiggers Diagram

University of British Columbia – Basic Physiology and Approach to Heart Sounds

University of Oslo – From the action potential to the ECG

University of Texas Medical Branch – Cardiac Cycle

University of Utah Medical School – HyperHeart