A 58-year-old man from Delhi presents to the emergency department with acute onset weakness in both legs and difficulty breathing. He was found unconscious at home 2 hours ago. On examination, he is alert but unable to move his legs; respiratory rate is 8/min. Serum potassium is 7.8 mEq/L (normal 3.5–5.0). An ECG shows peaked T waves and prolonged PR interval. Which of the following best explains the neurophysiological basis of his muscle weakness?

Explanation

## Pathophysiology of Hyperkalemia-Induced Muscle Weakness ### Normal Resting Membrane Potential **Key Point:** The resting membrane potential (RMP) in neurons and muscle cells is approximately −70 mV, maintained primarily by the Na⁺/K⁺-ATPase pump and differential ion permeability. ### Effect of Hyperkalemia on RMP When extracellular K⁺ rises (here 7.8 mEq/L), the K⁺ concentration gradient (K⁺ᵢₙ/K⁺ₒᵤₜ) decreases. Using the Nernst equation: $$E_K = 61 \log \frac{[K^+]_{in}}{[K^+]_{out}}$$ A smaller gradient means the RMP becomes **less negative** (depolarized toward the K⁺ equilibrium potential). The cell depolarizes from −70 mV toward approximately −40 to −50 mV. ### Why Muscle Becomes Weak (Paradoxical) **High-Yield:** At mild-to-moderate hyperkalemia, depolarization brings the membrane potential closer to threshold, making the cell *more* excitable initially (causing the peaked T waves and early arrhythmias). However, as depolarization persists: 1. **Sodium channel inactivation gates close** — voltage-gated Na⁺ channels have two gates: activation (opens at threshold) and inactivation (closes during repolarization). When the cell is held in a depolarized state, inactivation gates remain closed. 2. **Loss of excitability** — even though the membrane is closer to threshold, Na⁺ channels cannot open because they are inactivated. The cell becomes **inexcitable** despite depolarization. 3. **Unopposed K⁺ efflux** — the sustained depolarization also drives K⁺ out of the cell, but the inability to generate action potentials prevents muscle contraction. **Clinical Pearl:** This explains why the patient has **flaccid paralysis** with hyperkalemia — not because the cell is hyperpolarized (which would prevent threshold), but because depolarization-induced sodium channel inactivation prevents action potential generation. ### ECG Correlates - **Peaked T waves** → early depolarization effect - **Prolonged PR interval** → conduction slowing due to partial depolarization of atrial and nodal tissue - **Wide QRS** → ventricular conduction delay - **Sine wave** (in severe hyperkalemia) → imminent cardiac arrest from complete inexcitability ### Management **Key Point:** Treatment aims to shift K⁺ intracellularly (insulin + glucose, β₂-agonists) or remove K⁺ (diuretics, cation exchangers), thereby restoring the K⁺ gradient and RMP toward normal. ## Summary Table | Condition | RMP | Excitability | Mechanism | |-----------|-----|--------------|----------| | Normal | −70 mV | Normal | Balanced Na⁺/K⁺ gradients | | Hyperkalemia (mild) | −50 to −60 mV | ↑ (initially) | Closer to threshold | | Hyperkalemia (severe) | −40 to −50 mV | ↓↓ (paradoxical) | Na⁺ channel inactivation | | Hypokalemia | −80 to −90 mV | ↓ | Hyperpolarized, far from threshold | ```mermaid flowchart TD A[Hyperkalemia<br/>K+ out increases]:::outcome --> B[K+ gradient decreases]:::outcome B --> C[RMP becomes less negative<br/>Depolarization]:::outcome C --> D{Mild vs Severe?}:::decision D -->|Mild| E[Cell closer to threshold<br/>↑ Excitability]:::action D -->|Severe| F[Sustained depolarization]:::action F --> G[Na+ channel inactivation gates close]:::outcome G --> H[Loss of excitability<br/>Flaccid paralysis]:::urgent E --> I[Peaked T waves, arrhythmias]:::outcome H --> J[Respiratory failure if severe]:::urgent ```