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?

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

A 58-year-old man with a history of anterior wall myocardial infarction presents with frequent premature ventricular contractions (PVCs) on continuous cardiac monitoring. The phase marked **A** in the cardiac action potential diagram represents the rapid depolarization phase driven by voltage-gated sodium channel opening. Which of the following antiarrhythmic drugs would be most appropriate for acute suppression of these ventricular ectopics in the immediate post-MI period?