A 52-year-old man from Delhi presents to the emergency department with sudden-onset weakness of both legs and inability to move his arms. His wife reports he was found unconscious 2 hours ago. On examination, he is alert but completely paralyzed except for eye movements. Blood pressure is 140/90 mmHg, heart rate 88/min, and respiratory rate 18/min. Serum potassium is 7.2 mEq/L (normal 3.5–5.0), and arterial blood gas shows pH 7.28. An ECG shows peaked T waves and prolonged PR interval. Which ion channel dysfunction best explains the loss of excitability in his skeletal muscles?

Explanation

## Pathophysiology of Hyperkalemic Paralysis ### The Resting Membrane Potential in Hyperkalemia **Key Point:** The resting membrane potential is determined by the Nernst equation and depends critically on the K⁺ gradient across the cell membrane. In hyperkalemia, the extracellular K⁺ rises, reducing the ratio [K⁺]in/[K⁺]out, which **depolarizes** the resting potential from −90 mV toward −70 to −60 mV. $$E_K = 61 \log \frac{[K^+]_{in}}{[K^+]_{out}}$$ ### Why Muscle Becomes Inexcitable 1. **Initial depolarization** (K⁺ = 6–7 mEq/L): The resting potential moves closer to threshold, and some sodium channels open → brief **hyperexcitability** (fasciculations, cramping). 2. **Sustained depolarization** (K⁺ > 7 mEq/L): The membrane potential stabilizes at a depolarized level (−60 to −50 mV). At this voltage, voltage-gated sodium channels undergo **inactivation** — they transition from the closed (resting) state directly to the inactivated state, bypassing the open state. 3. **Loss of excitability**: With sodium channels inactivated, the muscle cannot generate an action potential in response to neural stimulation, resulting in **flaccid paralysis**. ### Clinical Correlation **Clinical Pearl:** The peaked T waves and prolonged PR interval on ECG reflect the same depolarization in cardiac myocytes; the cardiac conduction system is also affected, explaining the arrhythmia risk in severe hyperkalemia. **High-Yield:** Hyperkalemic paralysis is a medical emergency. Treatment includes: - Calcium gluconate (stabilizes the cardiac membrane) - Insulin + dextrose (shifts K⁺ intracellularly) - Beta-2 agonists (albuterol) - Diuretics and potassium-binding resins ### Why Inactivation, Not Blockade? Sodium channel inactivation is a voltage-dependent process where the inactivation gate closes while the activation gate is already closed. This is distinct from pharmacological blockade (e.g., by local anesthetics), which prevents channel opening. In hyperkalemia, the channels are physically present but electrically unavailable because the membrane is held in the depolarized range where inactivation dominates. [cite:Guyton & Hall 14e Ch 5]