Question 12 from the first paper of 2006 and Question 9 from the second paper of 2002 discussed and compared the utility of  lignocaine, magnesium and amiodarone in the management VT. This is a very specific subject. A very specific answer is offered, consisting only of what the college asked for. Antiarrhythmic pharmacology is a deep rabbit hole, and one could become lost there. An ideal resource to read quickly before the exam would have to be the 1998 review article by Kovey.

Lignocaine, magnesium and amiodarone for VT

Features

Lignocaine

Magnesium

Amiodarone

Class

Class 1a antiarrhytmic

Divalent cation

Class 3 antiarrhytmic (though it has effects of all 4 classes)

Administration
and dosage

IV
initial bolus 1-1.5mg/kg.

Then, 4 mg/kg for the first hour, then tapering infusion to 1mg/kg for 24 hrs

IV

10-20mmol/L given over 15-60 minutes,

or 5 mmol boluses followed by 20mmol infusion

IV

150-300mg, followed by an infusion of 900mg over 24 hrs

Pharmaco-kinetics

Rapid hepatic metabolism into inactive metabolites.
Half life 1-2hrs

Rapid distribution; some proportion becomes intracellular; the rest is renally excreted.

Rapid distribution, with a vas volume of dsitribution; becomes bound to tissue proteins.
Half-life 15-60 days

Mechanism

Inhibits voltage-gated sodium channels, decreasing the duration of action potentials and decreasing the  duration of repolarisation

Mechanism uncertain; appears to act as an antagonist to the entry of calcium into depolarising cells.
Depresses the activity of excitable tissues.

Beta-blockade
Voltage-gated soidum channel blockade
Potassium channel blockade
Calcium channel blockade
Prolongs the duration of both action potential and refractory period.

Adverse effects

Neurological disturbances eg. paraesthesia, seizures

Muscle weakness, decreased reflexes, hypotension

Prolongation of QT interval, risk of Torsades.
Array of chronic toxicities included pulmonary fibrosis and hypothyroidism or hyperthyroidism

Classification of antiarrhythmic agents

Vaughan-Williams classification (1970)

Class 1 antiarrhythmic drugs all depress phase zero of the action potential by decreasing membrane conductance of Na+. These are split into 3 subclasses, according to what they do to the action potential duration. The classes are 1a, 1b and 1c.

  • Class 1a prolong the action potential duration. (quinidine, procainamide and disopyramide)
  • Class 1b shorten the action potential duration. (lignocaine and mexelitine)
  • Class 1c keep the action potential duration virtually the same (flecainide and propafenone)

Class 2  antiarrhythmic drugs are beta blockers (eg. propanolol, metoprolol)

Class 3 antiarrhythmic drugs are potassium channel blockers (eg. amiodarone and sotalol)

Class 4 antiarrhythmic drugs are calcium channel blockers (eg. verapimil and diltiazem)

Obviously this classification has problems.

  • Where do digoxin and adenosine fit? What about sodium lactate? The classification omits them.
  • The system does nothing to suggest which class is useful for which role
  • Many of the drugs have multiple effects and could just as easily be classified in another group

Pharmacological properties of antiarrhythmic agents

From the abovelinked 1998 review article by Kovey,  the following table was shamelessly stolen, flaunting copyright for all the right reasons (FOAMED).

comparative pharmacological properties of antiarrhythmic drugs

It says "adapted with permission from Siddoway". That book is no longer in print; it appears as a reference in the list below.

References

Nawrath, H., et al. "Class I Antiarrhythmic Drug Effects: What Is the Basis for Subgroups Ia, Ib and Ic." Cardiac Arrhythmias: The Management of Atrial Fibrillation (2013): 39.

Vaughan-Williams, E. M. "Classification of antiarrhythmic drugs." Cardiac arrhythmias 449 (1970).

Rosen, Michael R. "The sicilian gambit-a new approach to the classification of antiarrhythmic drugs based on their actions on arrhythmogenic mechanisms." Circulation 84.4 (1991): 1831-1851.

Kowey, Peter R. "Pharmacological effects of antiarrhythmic drugs: Review and update." Archives of internal medicine 158.4 (1998): 325-332.

Siddoway, L. A., P. J. Podrid, and P. R. Kowey. "Pharmacologic principles of antiarrythmic drugs." (1995): 355-368.