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.

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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Terms of Use

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.

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.

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

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

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

Ventricular Volumes

Electrical Activity

Heart Sounds

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.