Hello guys, this is a much important topic especially in Emergency Medicine. And before going through this post, if you may, brush up your concepts of cardiac action potential.
A quick recap: Imagine a non-pacemaker AP with a flat phase 4, phase 0 upstroke, then a phase 1 downward notch, then phase 2 plateau phase, phase 3 downstroke slow at first, rapid later. Now the channels.
Phase 0- Na+ channels in the open state, it is inactivated in all other phases.
Phase 1- Transient-outward K+ channels
Phase 2- L type Ca2+ channels and Slow K+ channels (IKs)
Phase 3- Delayed rectifier K+ channels; Slow K+ channels(IKs) to Rapid K+ channels(IKr) and finally ultrarapid K+ channels(IKur).
Phase 4- Inward rectifier K+ channels(IKi)
First, Classification:
We have the Vaughan-Williams classification, the Sicilian gambit which is the most accepted albeit with some significant limitations which will be discussed later.
Class I:
These are the Na+ channel blockers and "membrane stabilizers". So, they reduce slope of phase 0 and hence the peak of action potential. And they all prolong effective refractory period(ERP). Because of subtle differences in its members, they are further classified as three subclasses.
IA:
It has moderate efficacy, i.e., it moderately reduces the slope of phase 0. And now look at the letter A, it is pointing upwards. That is coz it increases APD (Action Potential Duration) and ERP(Effective refractory period) both since it blocks IKr channels which are a part of delayed rectifier K+ channels involved in repolarization phase 3, so they prolong both QRS(ventricular depolarization) and QT intervals(Due to increased APD).
Note: This class of drugs have a cumulative effect. They block Na+ channels in the open state during phase 0 and then dissociates from them slowly and incompletely during the diastolic period after QRS complex so that in next beat, some Na+ channels are already blocked from the previous beat. So the QRS prolongation will rise with each beat. And this effect will be exaggerated at higher rates since diastolic period will shorten and more no of Na+ channels will be stuck with drugs.
Hence, in a way, it attacks more strongly if the rate is uncontrollably higher.
Eg. Quinidine, Procainamide, Disopyramide
IB:
It has low efficacy, it weakly reduces the slope of phase 0. Unlike the above class, it decreases APD. On the ECG, it slightly shortens QT interval and have little effect on QRS complex although both are considered therapeutically irrelevant.
Now why does it shorten APD? In the quick AP recap above I lied a bit, in phase 2 plateau phase the depolarizing Ca2+ channels are helped by residual(still open) depolarizing Na+ channels which are blocked by these drugs, so the repolarizing K+ channels dominate earlier and shorten phase 2.
IA vs IB:
IA is like a friend who attaches to you quickly and then doesn't like to leave you. Wheareas, IB is like a friend who takes his good time to attach but then leaves you quickly.
So based on this, unlike IA, IB blocks both open and inactivated Na+ channels, but they do it so slowly that they miss most of the open Na+ channels in phase 0 (the reason behind them producing little changes in QRS complex) and their real effect starts after phase 0 when they block the inactivated Na+ channels and prolong ERP. They detach relatively quickly so they show less cumulative blocking effect, but at higher rates when the diastolic repolarization phase is so short that even these fast-detaching fellas fail to detach and remain stuck on producing cumulative blocking effect beat after beat.
Another question, why are IB drugs not effective in tackling down Atrial arrhythmias?
2 reasons:
1.Unlike ventricular myocyte AP, atrial myocyte AP has a very short plateau phase and APD and as stated above phase 2 is where IB drugs exert their major effect.
2. IB drugs have negligible effect on normal cardiac cells, they mainly show their effect on ischaemic cells. And atrial myocytes by virtue of their less thickness, less demand and adequate blood supply rarely become ischaemic.
Eg., Lidocaine, Phenytoin, Mexiletine
IC:
It is very strong, it significantly reduces the slope of phase 0. But coz its C, it doesn't Care about APD and ERP, so no effect. On the ECG, it prolongs QRS complex significantly and shows cumulative blocking effect in a very similar way to IA drugs.
Eg., Flecainide, Propafenone, Moricizine
Class II: These are Beta-blockers. They prolong phase 4 of AP, which reduces the automaticity and hence controls rate as well as conduction. On ECG, they prolong PR interval.
Class III: These are K+ channel blockers. They prolong phase 3 of AP, so it delays repolarization and prolongs APD and ERP.
Eg., Amiodarone, Dronedarone, Dofetilide, Sotalol, Ibutilide
Class IV: The Ca2+ channel blockers or more specifically the L-type Ca2+ channel blockers. In SA node and AV node, it prolongs both phase 0 and 4, so controls the rate. In myocardial cells, it prolongs phase 2 of AP, so it impedes conduction. On ECG, they prolong PR interval.
Eg., Verapamil, Diltiazem
Class V: Variable Mechanism; including Magnesium Sulfate, Adenosine, Digoxin, Atropine.
The major drawback of this classification is that some drugs like Amiodarone have overlapping features of other classes.
Mnemonic by iKan :) -
Remember, VeraPamil has P in the name so PR interval is Prolonged.
(Cain) from Flecainide sounds like Quain, Q is for QRS interval prolongation.
That's all!
My next post will be on what, why and how of indications of anti-arrhythmics. Stay tuned! :)
-VM
