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The Cardiac Cycle

The Cardiac Cycle

The heart beats rhythmically in a sequence called the cardiac cycle—a rapid coordination of contraction (systole) and relaxation (diastole).
The Process
Electrical signals—sent from the sinoatrial (SA) node in the right atrial wall to the atrioventricular (AV) node between the right atrium and right ventricle—cause both atria to simultaneously contract. When the signal reaches the AV node, it pauses for approximately a tenth of a second, allowing the atria to contract and empty blood into the...

Electrophysiology of Normal Cardiac Rhythm

Electrophysiology of Normal Cardiac Rhythm

The normal cardiac rhythm is a synchronized electrical activity that facilitates the regular and coordinated contraction of the heart muscle. This process is essential for efficient blood circulation throughout the body. The fundamental elements involved in establishing and maintaining this rhythm include the unique electrical properties of cardiac muscle cells, the sinoatrial (SA) node's pacemaker function, the specialized conducting system, and the ionic mechanisms underlying each phase of...

Correlation between ECG and Cardiac Cycle

Correlation between ECG and Cardiac Cycle

The electrical signals recorded on an electrocardiogram (ECG) occur before the mechanical processes of contraction and relaxation during the cardiac cycle.
A cardiac action potential originates in the SA node and spreads throughout the atria and the AV node in approximately 0.03 seconds. This results in the P wave in an ECG and triggers atrial contraction. The action potential is then briefly slowed at the AV node, allowing the atria to contract and fill the ventricles with blood before...

Cardiac Cycle

Cardiac Cycle

The cardiac cycle refers to the sequence of events that occur in the heart from the beginning of one heartbeat to the next. It's characterized by alternating periods of contraction (systole) and relaxation (diastole) of the heart muscles.
During the cardiac cycle, blood flow through the heart is regulated entirely by changing pressure gradients. This sequence of events begins with the heart in a state of total relaxation, known as mid-to-late diastole, during which blood passively flows from...

Specialized Characteristics of Cardiac Muscles

Specialized Characteristics of Cardiac Muscles

The primary role of cardiac muscles is to propel blood throughout the cardiovascular system. The cardiac muscle cells, or cardiomyocytes, exhibit specialized characteristics that allow them to perform this function.
Cardiac muscle cells are smaller than skeletal muscles, averaging 10–20 mm in diameter and 50–100 mm in length. However, they have large energy demands for continuous contraction and relaxation. This energy is almost exclusively derived from aerobic metabolism of energy reserves in...

Cardiac Action Potential

Cardiac Action Potential

Cardiac action potentials are essential for proper heart function, enabling the rhythmic contractions needed for adequate blood circulation. Nodal cells and Purkinje fibers, specialized for electrical conduction, generate these action potentials.
The cardiac action potential process involves a series of phases characterized by the movement of ions across the cardiac cell membranes, leading to the depolarization and repolarization of the cardiac myocytes.
Ionic Basis of Cardiac Action Potentials

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Related Experiment Video

Updated: Jul 16, 2026

Patient-specific Modeling of the Heart: Estimation of Ventricular Fiber Orientations

Patient-specific Modeling of the Heart: Estimation of Ventricular Fiber Orientations

Published on: January 8, 2013

Interaction between spiral and paced waves in cardiac tissue.

Konstantin Agladze¹, Matthew W Kay, Valentin Krinsky

¹Pharmacology and Physiology Department, The George Washington University, 2300 Eye Street, Washington, DC 20037. phynas@gwumc.edu

American Journal of Physiology. Heart and Circulatory Physiology

|March 27, 2007

Summary

High-frequency antitachycardia pacing (ATP) can terminate lethal arrhythmias by guiding cardiac rotating waves. This study experimentally confirms ATP-induced wave drift and collision, offering insights into defibrillator mechanisms.

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Optocardiography and Electrophysiology Studies of Ex Vivo Langendorff-perfused Hearts

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Patient-specific Modeling of the Heart: Estimation of Ventricular Fiber Orientations

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Optocardiography and Electrophysiology Studies of Ex Vivo Langendorff-perfused Hearts

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Published on: November 7, 2019

Area of Science:

Cardiology
Biophysics
Electrophysiology

Background:

Implantable cardioverter-defibrillators (ICDs) use antitachycardia pacing (ATP) to prevent lethal arrhythmias.
A proposed ATP mechanism involves paced-induced drift of cardiac rotating waves leading to termination.

Purpose of the Study:

To provide direct experimental evidence for the paced-induced drift mechanism of rotating waves in cardiac tissue.
To investigate the interaction between rotating waves and pacing stimuli in cardiomyocyte monolayers.

Main Methods:

Initiation of rotating waves in neonatal rat cardiomyocyte monolayers using premature stimuli.
Observation of wave propagation patterns using the Ca(2+)-sensitive indicator fluo-4.
High-resolution monitoring of spiral tip-paced wave interactions and numerical simulations.

Main Results:

Stable spiral waves were observed to pin to local heterogeneities in the myocyte layer.
High-frequency pacing successfully terminated spiral activity in most cases.
Experimental data and simulations demonstrated paced-induced drift and wave break leading to instability or boundary collision.

Conclusions:

Cardiac rotating waves can be pinned by tissue heterogeneities.
Overdrive pacing can displace rotating waves.
Paced-induced wave drift and subsequent instability/collision provide a mechanism for terminating arrhythmias via ATP.