Calcium Channel Blockers
Controlling the heavy flow of calcium to close the AV nodal gates and break reentrant storms.
Working myocytes use fast sodium channels for conduction. They strike like lightning. But the nodal gatekeepers—the SA and AV nodes—rely entirely on the slow, heavy flow of calcium.
Sodium dictates the speed of the working muscle. Calcium dictates the speed of the nodes. If you want to control the gates, you have to control the calcium.
The L-Type Channel
There are different ways to block calcium, but in electrophysiology, we focus on the non-dihydropyridine calcium channel blockers: Verapamil and Diltiazem.
These drugs target the L-type calcium channels. In the working myocardium of the ventricles, L-type channels drive Phase 2, maintaining the long plateau that allows the heart muscle to contract.
But in the SA and AV nodes, L-type channels are the main event. They drive Phase 0. They are responsible for the entire upward stroke of the action potential. Shut down the L-type channel in the node, and you shut down its ability to conduct.
Slowing the Gate
By blocking calcium entry, Verapamil and Diltiazem drastically slow conduction velocity through the AV node. The calcium current takes longer to cross the threshold, and once open, fewer channels participate in passing the signal along.
This not only slows conduction but prolongs the refractory period of the AV node. The node takes much longer to reset and prepare for the next beat.
On the surface ECG, this physical slowing translates to a lengthened PR interval. The atrial signal reaches the node and wades through the sluggish calcium-deprived tissue before finally escaping to the ventricles.
State-Dependence
Like the sodium channel blockers in Class I, Verapamil is state-dependent. It does not bind well to a quiet, resting channel. It prefers a channel that is actively cycling between its open and inactivated states.
This property makes it a brilliant weapon against supraventricular tachycardias like AVNRT. A fast, reentrant circuit spinning through the AV node forces the nodal calcium channels to open and close with furious speed.
The tachycardia itself invites the drug in. The faster the circuit spins, the deeper Verapamil binds, until the conduction slows so much that the wavefront hits refractory tissue and the circuit breaks.
The Danger in VT
Calcium does not just control nodal conduction. In the working myocardium of the ventricles, calcium is the literal trigger for muscle contraction.
Blocking calcium in the ventricles severely reduces contractility—a powerful negative inotropic effect. Furthermore, these drugs relax smooth muscle in the arteries, causing profound vasodilation.
If a patient presents with a wide-complex tachycardia (VT) and you mistakenly give Verapamil thinking it is a supraventricular rhythm with aberrancy, you are delivering a devastating blow. The ventricle, already struggling to pump efficiently due to the fast rate, is suddenly stripped of its contractile force while the blood vessels dilate.
The result is often immediate and catastrophic cardiovascular collapse. This is why the unbreakable rule of clinical electrophysiology exists: A wide QRS tachycardia is VT until proven otherwise, and Verapamil is forbidden.
In electrophysiology, there is an exception to every rule. Fascicular VT, historically known as Belhassen VT, is a specific idiopathic ventricular tachycardia that originates within the Purkinje network.
Unlike typical VT, the reentrant circuit of Fascicular VT relies heavily on slow inward calcium currents in abnormal Purkinje tissue. It is exquisitely sensitive to Verapamil. In this singular scenario, Verapamil will safely and dramatically terminate the tachycardia.
However, the burden of proof is absolute. You must be completely certain of the diagnosis—typically a right bundle branch block morphology with left axis deviation in a young, healthy heart—before breaking the cardinal rule of wide-complex tachycardias.
Key Takeaways
- The Target: Nodal tissue (SA and AV) relies entirely on slow inward calcium currents for Phase 0 depolarization.
- The Effect: Verapamil and Diltiazem slow conduction and prolong refractoriness in the AV node, lengthening the PR interval.
- State-Dependence: These drugs bind more effectively to actively cycling channels, making them highly effective against rapid tachycardias like AVNRT.
- The Danger: Verapamil is absolutely contraindicated in unknown wide-complex tachycardia due to its severe negative inotropic and vasodilatory effects.
- The Exception: Fascicular (Belhassen) VT is a rare, specific idiopathic VT that relies on calcium currents and responds beautifully to Verapamil, provided the diagnosis is certain.