Let's move on to the group 4 antiarrhythmic drugs. These are
the calcium channel blocker drugs. Now I've discussed them
multiple times in our angina lecture, in our heart failure
lecture, and in our hypertension lecture.
Let's talk about these drugs as they act on the AV node. Now
we are going to be focusing only on verapamil and diltiazem
because if you remember the other type of calcium channel
blockers like nifedipine, don't really act on the heart.
These drugs act on the heart. I also want to mention too that
verapamil is more cardiac active than diltiazem.
So you can think of verapamil as being a cardiac drug,
diltiazem being more of a mixture of cardiac and vascular,
and drugs like nifedipine being vascular drugs. Let's move on
to AV nodal arrhythmia. So take a look at that action potential.
How is it different to you? Well, remember that these drugs
are state and use dependant. So if you are using the calcium
current a lot, for example if you have fast heart rate, then
you are going to have more response to these drugs.
The AV conduction velocity is decreased. Do you remember what
that is called? That's called dromotropy. The PR and ERP are
increased. PR means PR interval, the space between the P wave
and the R wave of the QRS complex. And the ERP is the
effective refractory period. Now, nifedipine and amlodipine
are not good antiarrhythmics. The other thing that happens
with nifedipine and amlodipine, which are not antiarrhythmic
drugs, is they cause a reflex sympathetic discharge,
what we call reflex tachycardia. So these drugs should not
ever be thought of as group 4 antiarrhythmics like the other
calcium channel blockers, you have to think of them as just
hypertension drugs and they can actually induce tachycardia
because of their reflex action.