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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 of...

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Arrhythmias are irregular heart rhythms occurring when the heart's electrical impulses become abnormal. These disturbances can lead to various symptoms, depending on their severity and the underlying cause. Some common factors contributing to arrhythmias include hypoxia, ischemia, electrolyte imbalances, excessive catecholamine exposure, drug toxicity, and muscle overstretching. Arrhythmias can be classified into two main types based on the rate and site of origin of abnormal heart rhythms.

Conduction System of the Heart

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Autorhythmicity is a term that refers to the heart's inherent ability to generate electrical signals and instigate muscle contractions. This self-regulating conduction system within the heart consists of two key components: the pacemaker cells and specialized conducting cells.
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Disturbances in Heart Rhythm

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Recording and Analysis of Circadian Rhythms in Running-wheel Activity in Rodents

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Circadian variation in human ventricular refractoriness

T Q Kong¹, J J Goldberger, M Parker

¹Department of Internal Medicine, Northwestern University Medical School, Chicago, Ill., USA.

|September 15, 1995

Summary

Sudden cardiac death risk is highest in the morning. This study found that ventricular refractory periods, crucial for heart rhythm, show a daily variation, peaking in the early morning. Beta-blockers normalize these periods.

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Area of Science:

Cardiology
Circadian Biology
Electrophysiology

Background:

Sudden cardiac death incidence peaks in the morning.
Circadian variation in myocardial ischemia is a potential factor.
The role of circadian variation in ventricular refractoriness remains unclear.

Purpose of the Study:

To investigate the existence of a circadian variation in ventricular refractoriness.
To determine if this variation correlates with the increased morning incidence of sudden cardiac death.

Main Methods:

Ventricular effective refractory periods were measured hourly over 24 hours in nine subjects with conduction system disease.
Autonomic nervous system function was assessed.
Measurements were repeated during beta-blockade in a subset of subjects.

Main Results:

A significant circadian variation in ventricular refractoriness was observed.
Shortest refractory periods occurred within 2 hours of waking.
Beta-blockade abolished this circadian variation.
Time of day, not catecholamine levels, was the independent predictor of refractory periods.

Conclusions:

A significant circadian variation in ventricular refractory periods exists.
Shortest refractory periods and maximal shortening occur in the early morning.
Beta-adrenergic tone fluctuations largely drive this phenomenon.