Dr. David Lyness
THE VAUGHAN-WILLIAMS CLASSIFICATION AND CARDIAC ACTION POTENTIAL
Mg acts as a physiologic Calcium Channel inhibitor by slowing slow
L-Type Calcium channel during PHASE 2. This reduces further Ca release by the Sarcoplasmic Reticulum which leads to reduced automaticity, contractility and conductivity through cardiac tissue, including the AVN.
I- Sodium Channel Blockers
II- Beta Blockers
III- K channel blockers
IV - Calcium Channel Blockers
Think of Prince William - SOon to Be King of the Castle
I II III IV
Rapid utilisation of drugs is often necessary to control arrhythmias and prevent
progression of negative sequelae. There are FIVE main classes of drugs that achieve this.
Worth a mention
Whilst amiodarone is a class 3 potassium blocker - this is late at around 8h post dose and primary mechanism of rapid control is Na, K and Ca blockade.
Magnesium prolongs phase two of the cardiac action potential by acting as a Calcium Channel Blocker - so really has Class IV effects - but is class V.
I - Ventricular Arrhythmias, PAF, WPW
II - Prevent occurrence of tachyarrythmias, decrease MI mortality
III - WPW, VT's and AF (amiodarone has a few class actions)
IV - Prevents re-occurance of SVT's, reduces ventricular rate in AF
V - SV arrythymias, good in heart failure with AF. Not for ventricular arrhythmias.
MgSo4 is used in Torsades De Pointes.