Cardiac Conduction System and Antiarrhythmic Drugs

Study Notes

The heart's electrical system controls its pumping action
Five elements make up the cardiac conduction system: SA node, AV node, bundle of His, bundle branches, and Purkinje fibers
Normal heart rhythm begins with electrical signals from the SA node, causing atria to contract and pushing blood into ventricles
Cardiac cells are divided into contractile and conducting cells
Contractile cells generate force for heart contractions, while conducting cells initiate electrical impulses
The SA node is the heart's natural pacemaker due to its fastest automaticity
The action potential of pacemaker cells differs from heart muscle cells, particularly in their membrane potentials and ion channels involved
Tachyarrhythmias are deviations from normal heart rhythm, classified based on heart rate as bradyarrhythmias or tachyarrhythmias
Tachyarrhythmias can be caused by abnormal automaticity, triggered activity, or reentry mechanisms
Antiarrhythmic drugs are classified based on their dominant mechanism of action as Class 1, 2, 3, or 4 agents
Class 1 drugs block sodium channels, inhibiting the depolarization process and slowing conduction velocity
Class 2 drugs are beta blockers, preventing the action of catecholamines on the heart and decreasing heart rate and contractility
Class 3 drugs block potassium channels, prolonging the action potential and refractory period
Class 4 drugs block calcium channels, slowing conduction in the SA and AV nodes and reducing heart contractility
Other antiarrhythmic agents, such as Digoxin, Adenosine, and Magnesium Sulfate, do not fit neatly into these classes and have unique mechanisms of action