Snapshot A 24-year-old woman presents with tachycardia. She reports that this happens every few years without any identifiable causes. Her pulse is 200/min. Her other vital signs are stable. An electrocardiogram shows a patient with narrow complex QRS, tachycardia to 204/min, and a regular rhythm. She is given a medication to slow down the atrioventricular node and attempt to end the narrow complex tachycardia. (Adenosine) Introduction Adenosine mechanism of action ↑ K+ out of the cells, which hyperpolarizes the cell and causes ↓ AV nodal conduction background action potential is initiated by the sinoatrial (SA) node login to view 2 more bullets Calcium channel blockers mechanism of action directly blocks L-type voltage-gated Ca2+ channels (dihydropyridine receptors) background action potential causes a rush of Ca2+ into the cell Ca2+ enters via L-type voltage-gated Ca2+ channels this triggers the release of more Ca2+ from the sarcoplasmic reticulum login to view 2 more bullets Digoxin mechanism of action inhibits extracellular K+ binding site of the Na+-K+ATPase on myocyte cell membrane login to view 3 more bullets background contractility is determined by intracellular Ca2+ login to view 1 more bullet β-blockers mechanism of action indirectly blocks L-type voltage-gated Ca2+ channels (dihydropyridine receptors) via ↓ cAMP background positive inotropy by ↑ sympathetic nervous system is mediated by β1 receptors at the SA and AV nodes