Which phase of the cardiac action potential is most commonly affected by hyperkalemia, leading to characteristic ECG changes?

Explanation

## Hyperkalemia and the Cardiac Action Potential **Key Point:** Hyperkalemia most commonly and earliest affects **Phase 3 (repolarization)** by increasing potassium conductance (gK), leading to faster repolarization — the hallmark of which is the **peaked (tall, narrow, symmetric) T wave** on ECG. ### Mechanism of Hyperkalemia on Phase 3 In hyperkalemia, elevated extracellular K⁺ increases the driving force for K⁺ efflux through repolarizing channels (IKr, IKs), accelerating Phase 3 repolarization: - Faster repolarization → **taller, narrower, peaked T waves** (earliest ECG sign) - Shortened action potential duration → **shortened QT interval** - This is the **most common and earliest** electrophysiological manifestation of hyperkalemia ### Progressive ECG Changes in Hyperkalemia | Serum K⁺ (mEq/L) | ECG Finding | Phase Affected | |------------------|-------------|----------------| | 5.5–6.5 | **Peaked T waves**, shortened QT | Phase 3 (repolarization) ← earliest | | 6.5–7.5 | PR prolongation, QRS widening | Phase 0 (depolarization) slowed | | 7.5–8.5 | Loss of P wave, sine wave pattern | Phase 0 severely impaired | | > 8.5 | Ventricular fibrillation / asystole | Membrane inexcitability | ### Why Phase 3 is the Most Commonly Affected Phase **Clinical Pearl (Harrison's Principles of Internal Medicine):** The earliest and most characteristic ECG change in hyperkalemia is the peaked T wave, which directly reflects accelerated Phase 3 repolarization due to increased K⁺ conductance. This occurs at relatively mild elevations (K⁺ > 5.5 mEq/L) and is the most commonly tested and clinically recognized finding. Phase 4 depolarization (resting membrane potential shift) is a secondary consequence and is more relevant to understanding automaticity changes, but it is **not** the phase most commonly cited as the primary ECG-producing disturbance in standard physiology and clinical texts. **High-Yield:** - **Phase 3 affected → Peaked T waves** (most common, earliest) - **Phase 0 affected → QRS widening** (later, more severe hyperkalemia) - **Phase 4 affected → Altered automaticity** (secondary phenomenon) **Mnemonic:** **"K+ Peaks the T"** — Hyperkalemia → ↑K⁺ conductance → faster Phase 3 repolarization → Peaked T waves. *Reference: Guyton & Hall Textbook of Medical Physiology, 14th ed.; Harrison's Principles of Internal Medicine, 21st ed.*