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Related Concept Videos

The Cardiac Cycle01:13

The Cardiac Cycle

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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...
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Dysrhythmias III: Characteristics of Dysrhythmias01:29

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Dysrhythmias, also known as arrhythmias, are irregular heart rhythms that result from abnormal electrical activity in the heart, affecting its ability to circulate blood efficiently. Tachyarrhythmias, a subset of dysrhythmias, are characterized by abnormally fast heart rates exceeding 100 beats per minute. Here are some types of tachyarrhythmias with their distinct ECG features:Sinus Tachycardia:Sinus tachycardia presents a regular heart rhythm with an increased rate of 101-180 beats per...
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Cardiac Cycle01:29

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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...
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Dysrhythmias IV: Characteristics of Bradyarrhythmias01:18

Dysrhythmias IV: Characteristics of Bradyarrhythmias

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Bradyarrhythmias are cardiac rhythm disorders characterized by a slower-than-normal heart rate, typically defined as fewer than 60 beats per minute. Some of which are discussed here:Sinus BradycardiaSinus bradycardia presents a heart rate lower than 60 beats per minute, with a regular rhythm originating from the SA node. The ECG typically shows normal P waves preceding each QRS complex, a normal PR interval (0.12 to 0.20 seconds), and a normal QRS duration (0.06 to 0.10 seconds).First-Degree AV...
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Electrophysiology of Normal Cardiac Rhythm01:19

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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...
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Correlation between ECG and Cardiac Cycle01:25

Correlation between ECG and Cardiac Cycle

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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...
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Related Experiment Video

Updated: Mar 29, 2026

A Model of Long-Term Ventricular Fibrillation in Isolated Rat Hearts
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Mechanisms of sinus node cycle length changes during ventricular fibrillation.

Stephen L Wasmund1, Christina F Pacchia2, Richard L Page3

  • 1Department of Cardiovascular Medicine, University of Wisconsin, H4/534 Clinical Science Center, 600 Highland Avenue, Madison, WI, 53792-3248, USA. swasmund@medicine.wisc.edu.

Clinical Autonomic Research : Official Journal of the Clinical Autonomic Research Society
|November 25, 2015
PubMed
Summary

Sinus node cycle length changes during ventricular fibrillation (VF) are vagally-mediated. Blocking vagal activity with atropine altered these changes, suggesting a role for the vagus nerve in VF.

Keywords:
Sinus node cycle lengthVagal activationVentricular fibrillation

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Area of Science:

  • Cardiology
  • Autonomic Nervous System Physiology

Background:

  • Sinus node cycle length (SNCL) can increase or show no change during ventricular fibrillation (VF), a phenomenon not fully understood.
  • Previous research indicates that up to one-third of patients exhibit these non-shortening SNCL patterns during VF.

Purpose of the Study:

  • To investigate the underlying mechanism of sinus node cycle length (SNCL) alterations during ventricular fibrillation (VF).
  • To test the hypothesis that vagal nerve activity mediates SNCL changes observed during VF.

Main Methods:

  • Ventricular fibrillation (VF) was induced in anesthetized pigs using direct current (DC) stimulation.
  • Sinus node cycle length (SNCL) changes were measured during VF and compared to baseline.
  • Following defibrillation, pigs were randomized to receive atropine, propranolol, a combination, or placebo, with repeat VF induction to assess drug effects on SNCL.

Main Results:

  • Of 19 pigs with measurable SNCL during VF, 10 showed shortening (S-Group) and 9 showed non-shortening (NS-Group).
  • Atropine significantly reduced the absolute SNCL change in both groups and attenuated shortening in the S-Group.
  • Atropine combined with propranolol also significantly reduced absolute SNCL changes; propranolol or placebo alone had no significant effect.

Conclusions:

  • Sinus node cycle length (SNCL) modifications during ventricular fibrillation (VF) are primarily mediated by vagal nerve activity.
  • These findings suggest a significant role for the autonomic nervous system in modulating cardiac electrophysiology during VF.
  • Further research is needed to explore the clinical implications for defibrillation thresholds and device programming.