An ECG from an 8-year-old male with neurosensory deafness and a family history of sudden cardiac arrest demonstrates QT-interval prolongation. Which of the following is this patient most at risk of developing?

First degree atrioventricular block

An 8-year-old girl is brought to the physician by her parents for the evaluation of an episode of unconsciousness while at the playground that morning. She was unconscious for about 15 seconds and did not shake, bite her tongue, or lose bowel or bladder control. Her grandfather died suddenly at the age of 29 of an unknown heart condition; her parents are both healthy. An ECG shows sinus rhythm and a QT interval corrected for heart rate (QTc) of 470 milliseconds. Laboratory studies are within normal limits. Which of the following is the most likely additional finding in this patient?

Subvalvular ventricular outflow obstruction murmur

Skin folds between the mastoid process and acromion

While explaining the effects of hypokalemia and hyperkalemia on the cardiac rhythm, a cardiologist explains that the electrophysiology of cardiac tissue is unique. He mentions that potassium ions play an important role in the electrophysiology of the heart, and the resting membrane potential of the cardiac myocytes is close to the equilibrium potential of K+ ions. This is because of the high resting potassium conductance of the ventricular myocytes, which is regulated by specific potassium channels. These are open at rest and are closed when there is depolarization. Which of the following potassium channels is the cardiologist talking about?

Inward rectifier IK1 potassium channels

Inward rectifier IKACh potassium channels

Fast delayed rectifier IKr potassium channels

Slow delayed rectifier IKs potassium channels

Transient outward current Ito potassium channels

A 40-year-old woman comes to the physician for a 6-month history of recurrent episodes of chest pain, racing pulse, dizziness, and difficulty breathing. The episodes last up to several minutes. She also reports urinary urgency and two episodes of loss of consciousness followed by spontaneous recovery. There is no personal or family history of serious illness. She does not smoke or drink alcohol. Vitals signs are within normal limits. Cardiopulmonary examination shows no abnormalities. Holter monitoring is performed. ECG recordings during episodes of tachycardia show a QRS duration of 100 ms, regular RR-interval, and absent P waves. Which of the following is the most likely underlying cause of this patient's condition?

AV node with slow and fast pathway

Pre-excitation of the ventricles

Mutations in genes that code for myocyte ion channels

Macroreentrant rhythm in the right atria through cavotricuspid isthmus

Fibrosis of the sinoatrial node and surrounding myocardium

Basic electrophysiology for USMLE Step 1: High-Yield Internal Medicine Lessons

Ventricular Action Potential - The Heart's Spark Plug

Phase 0 (Depolarization): Fast voltage-gated Na⁺ channels open for rapid Na⁺ influx.
Phase 1 (Initial Repolarization): Na⁺ channels inactivate. Transient K⁺ channels open for K⁺ efflux.
Phase 2 (Plateau): Ca²⁺ influx via L-type channels is electrically balanced by K⁺ efflux.
Phase 3 (Repolarization): Ca²⁺ channels close. Dominant K⁺ efflux through delayed rectifier K⁺ channels.
Phase 4 (Resting Potential): Maintained at ~-90mV by the Na⁺/K⁺-ATPase pump.

⭐ The Phase 2 plateau creates a long effective refractory period (ERP), preventing summation of contractions and protecting against tetanus.

Pacemaker Potential - The Heart's Drummer

Cardiac Pacemaker Potential Action Potential Diagram

Automaticity of SA/AV nodes is due to an unstable resting potential.
Phase 4: Slow spontaneous diastolic depolarization via $I_f$ ("funny current") channels, allowing slow Na⁺ influx.
Phase 0: Upstroke is from Ca²⁺ influx through L-type calcium channels, triggered at -40 mV threshold.
Phase 3: Repolarization occurs via K⁺ efflux.

⭐ The slope of phase 4 depolarization determines the heart rate. Sympathetic stimulation steepens the slope (faster HR); parasympathetic stimulation flattens it (slower HR).

Conduction Pathway - Electrical Superhighway

Heart Electrical Conduction System with ECG

Origin: Sinoatrial (SA) node, the heart's primary pacemaker.
Pathway: Signal travels from SA node → atria → Atrioventricular (AV) node → Bundle of His → Right & Left Bundle Branches → Purkinje fibers.
Result: Coordinated contraction of atria, then ventricles.

⭐ The AV node physiologically delays conduction, allowing ventricles to fill. This creates the PR interval on an ECG.

ECG & AP Correlation - Reading the Sparks

Cardiac Action Potential and ECG Correlation

ECG waves are summations of multiple action potentials (APs) from different myocardial cells.
The duration of the AP directly correlates with the QT interval.

⭐ Clinical Pearl: The QT interval represents the time for both ventricular depolarization and repolarization. A long QT interval signifies a prolonged action potential, increasing the risk for Torsades de Pointes.

Refractory Periods - The Heart's Downtime

Absolute Refractory Period (ARP): No new AP is possible, regardless of stimulus strength.
- Corresponds to inactivated Na+ channels (Phases 0-2, early Phase 3).
Effective Refractory Period (ERP): Includes ARP; a stimulus may cause a localized, non-propagated depolarization. The primary target for many antiarrhythmics.
Relative Refractory Period (RRP): A stronger-than-normal stimulus can generate a propagated AP as Na+ channels recover.

$Cardiac Action Potential and Refractory Periods$

⭐ Class IA, IC, and III antiarrhythmic drugs increase the ERP, a key mechanism for suppressing tachyarrhythmias by interrupting re-entrant circuits.

Phase 0 represents rapid Na+ influx in non-pacemaker cells versus Ca2+ influx in pacemaker cells.

Phase 3 repolarization is primarily driven by K+ efflux through delayed rectifier channels.

The SA node's automaticity is due to the "funny current" (If), causing spontaneous diastolic depolarization.

Absolute Refractory Period (ARP) is determined by inactivated Na+ channels, preventing summation.

The AV node functions as a critical gatekeeper, characterized by decremental (slow) conduction.

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