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An "account" of digitalis and atrial fibrillation
Insights
Digitalis slows ventricular rate in atrial fibrillation by increasing the atrioventricular node's refractory period and enhancing concealed conduction. This mechanism ensures a slower, more controlled ventricular response.
Area of Science:
- Cardiology
- Pharmacology
- Electrophysiology
Background:
- Atrial fibrillation causes rapid, irregular ventricular rates due to chaotic atrial impulses.
- The atrioventricular (AV) node-His system normally filters these impulses, but digitalis modifies this function.
- Understanding the electrophysiologic principles of atrial fibrillation and AV node function is key to digitalis's effects.
Observation:
- Digitalis exerts an "opium-like" action on ventricular rate control in atrial fibrillation.
- It acts through two primary pharmacologic mechanisms.
- These mechanisms involve altering the refractory periods of the AV node and atrial myocardium.
Findings:
- Digitalis increases the refractory period of the AV node, reducing conduction.
- It decreases the atrial myocardium's refractory period, increasing the number of impulses reaching the AV node.
- This leads to greater concealed conduction within the AV node, slowing the ventricular rate while maintaining its randomness.
Implications:
- Digitalis uniquely increases AV node refractory period and concealed conduction.
- It remains a primary treatment for sustained atrial fibrillation.
- Further research into these mechanisms could optimize atrial fibrillation management.
Abstract:
This review deals with the mechanisms by which digitalis exerts its "opium-like" action on the ventricular rate in patients with atrial fibrillation. To understand the effect of digitalis on ventricular rate and rhythm, it is essential to learn more about the basic electrophysiologic principles responsible for: atrial fibrillation as such, and the scaling function of the atrioventricular (AV) node-His system. It may be assumed that during atrial fibrillation, the atrial excitatory process results in randomly spaced impulses that reach the AV junction from random directions with random strength. The refractory period of and concealed conduction in the AV node enable the AV conduction system to scale down the shower of atrial impulses to a random ventricular response with a considerably lower rate. Digitalis decreases the ventricular rate through two synergistic pharmacologic actions: digitalis increases the refractory period of the AV node, and digitalis decreases the refractory period of the atrial myocardium through a direct and indirect effect (vagotonic and vagomimetic). A decrease in the refractory period of the atrial myocardium results in more atrial impulses reaching the AV junction in a given unit of time. More atrial impulses result in a greater degree of concealed conduction in the AV node and, thus, in a slower ventricular rate. The ventricular rhythm remains random during digitalis treatment. Digitalis is the only drug that effects an increase in the refractory period of and concealed conduction within the AV node. Two hundred years after its discovery, digitalis remains the drug of choice for the treatment of patients with sustained atrial fibrillation.