Cardiac Action Potential

A 72-year-old woman with a history of atrial fibrillation is on digoxin therapy. She presents with nausea, visual disturbances (yellow-green halos), and a heart rate of 35 bpm. An ECG shows a prolonged PR interval and a characteristic 'sagging' ST segment. Which phase of the cardiac action potential is most directly affected by digoxin toxicity in the AV node?

A 52-year-old man with a history of recurrent syncope presents to the cardiology clinic. ECG shows a prolonged QT interval (QTc 520 ms). To confirm the diagnosis of Long QT syndrome and identify the underlying ion channel defect, which investigation is the most appropriate next step?

Which ion channel is responsible for the most common cause of prolongation of the action potential duration (APD) in cardiac myocytes?

A 72-year-old woman from Mumbai with a history of myocardial infarction 3 months ago is admitted with acute chest pain. Her ECG shows ST elevation in leads II, III, and aVF (inferior STEMI). Cardiac catheterization reveals complete occlusion of the right coronary artery. During the acute phase of myocardial necrosis, the ischaemic zone shows a marked reduction in intracellular ATP and accumulation of intracellular Ca²⁺. Which phase of the cardiac action potential is most severely disrupted in this ischaemic tissue, and what is the primary ionic mechanism?

Which ionic mechanism best distinguishes the plateau phase (Phase 2) of the ventricular action potential from the rapid repolarization phase (Phase 3)?