A 52-year-old man with a history of hypertension is being evaluated for atrial fibrillation. His ECG shows a prolonged QT interval. Regarding the molecular mechanisms underlying the cardiac action potential duration (APD) and refractoriness, all of the following are true EXCEPT:

Explanation

## Cardiac Action Potential Duration and Refractoriness ### Refractory Periods Defined | Period | Timing | Na+ Channel State | Membrane Potential | Excitability | |--------|--------|-------------------|-------------------|-------------| | **Absolute Refractory** | Phase 0–early Phase 3 | Inactivated | Still depolarized | Cannot fire (no matter stimulus strength) | | **Relative Refractory** | Late Phase 3–early Phase 4 | Partially recovered | Hyperpolarized (below threshold) | Can fire only with **suprathreshold** stimulus | | **Supernormal Period** | Very late Phase 3 | Mostly recovered | Near resting | Can fire with **subthreshold** stimulus (rare) | ### Key Point: **Decreased outward K+ current (IK) PROLONGS APD and INCREASES the absolute refractory period—it does NOT shorten APD.** This is a critical misconception. When K+ efflux is reduced, repolarization is delayed, the action potential lasts longer, and the cell remains refractory for a longer time. ### High-Yield: The relationship between APD and refractoriness: - **Longer APD** → **Longer absolute refractory period** → **Lower arrhythmia risk** (less time for re-entry) - **Shorter APD** → **Shorter absolute refractory period** → **Higher arrhythmia risk** (more time for re-entry) ### Clinical Pearl: Class III antiarrhythmics (e.g., amiodarone, sotalol) **block K+ channels**, which **decreases IK**, **prolongs APD**, and **increases the refractory period**—thereby suppressing re-entrant arrhythmias. The option incorrectly suggests that decreased IK shortens APD, which is backwards. ### Mnemonic: **K+ OUT = Longer APD** — When K+ efflux is blocked, the action potential cannot repolarize quickly, so it lasts longer. ### Pathophysiology of EADs and DADs ```mermaid flowchart TD A[Prolonged Phase 2 Plateau]:::outcome --> B{Membrane potential still positive}:::decision B -->|Yes| C[L-type Ca2+ channels reopen]:::action C --> D[Inward Ca2+ current > Outward K+ current]:::outcome D --> E[Early Afterdepolarization EAD]:::urgent E --> F[Risk of Torsades de Pointes]:::urgent G[Delayed Repolarization]:::outcome --> H[Increased intracellular Ca2+]:::outcome H --> I[Abnormal SR Ca2+ release]:::action I --> J[Delayed Afterdepolarization DAD]:::urgent ``` [cite:Guyton & Hall Textbook of Medical Physiology 14e Ch 10]