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Class I antiarrhythmics: Sodium channel blockers
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Antiarrhythmic drugs help control arrhythmias or abnormal heartbeats.
There are four main groups of antiarrhythmic medications: class I, sodium-channel blockers; class II, beta-blockers; class III, potassium-channel blockers; class IV, calcium-channel blockers; and miscellaneous antiarrhythmics, or unclassified antiarrhythmics.
We’ll focus on class I antiarrhythmics which are further broken down into 1a, 1b, and 1c. All three groups work on Na+ channels in the cardiac myocytes, so class I medications are also called Na+ channel blockers.
When a heartbeat doesn’t follow this path, it’s called an arrhythmia, and there are two main causes - abnormal automaticity and abnormal reentry.
As a result, this area of the heart essentially flips roles with the SA node, firing so fast that the pacemaker cells in the SA node don’t get a chance to fire. At that point, the heartbeat is being driven by the ventricles.
Alternatively, there can be an abnormal reentry which often results from scar tissue in a ventricle after a heart attack.
Scar tissue doesn’t conduct electricity, so the signal just goes around and around the scar, and each cycle can cause the ventricles to contract.
Here, the signal might move back up the accessory pathway, since oftentimes it’s bidirectional, meaning the signal can go from atrium to ventricle as well as from ventricle to atrium. This creates a reentry circuit that causes extra contractions that occur in between the signals coming from the SA node.
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