In: Anatomy and Physiology
Verapamil is a drug that blocks Voltage-dependent Calcium channels. Would Verapamil be useful in treating tachycardia or bradycardia and why (must explain mechanism of action)?
To explain this answer lets begin with an insight into the phases of the cardiac action potential. The normal resting membrane potential(RMP) of ventricles is about -85 - -95 mV. K+ ions determine this. The SA node(pacemaker) of the heart has the ability to undergo spontaneous depolarization thereby generating an action potential; automaticity. This is transmitted to the muscles of the heart which helps it to maintain its rhythm. The action potential of the ventricles has 5 phases.
Phase 4 : The RMP. The ventricles are in diastole. An action potential is generated in the SA node.
Phase 0: Rapid depolarization phase.Here the Na+ channels open. And Na+ ions moves into the cell.
Phase 1: Na+ channels start getting inactivated, and the K+ chanels open.
Phase 2: The plateau phase wherin there is an inward movement of Ca+ through L type calcium channels and at the same time the K+ channels remain open and K+ keeps moving out.
Phase 3: The Ca+ channels close while the K+ channels are still open ensuring repolarization till the RMP is regained.
Verapamil is a class IV anti-arrhythmic drug which is a calcium channel blocker.
They act by blocking the L type voltage-gated calcium channels which brings Ca+ ions into the heart cells in plateau phase(phase2) and thereby decrease the rate of phase 4 depolarization of the SA and AV nodes thereby decreasing its automaticity and conduction through AV node which is helpful in treating tachycardias.
Hence, Verapamil is given in cases of PSVT(paroxysmal supraventricular tachycardia)as prophylaxis, SVT and atrial flutters and fibrillation as well.