Renal Physiology – Electrolyte Disorders

Hypokalemia

Definition (serum K⁺):

• Normal: 3.5–5 mEq/L

• Mild: 3.0–3.5

• Moderate: 2.5–3.0

• Severe: <2.5

Causes:

• ↓ Intake

• ↑ Excretion: diarrhea, diuretics, hyperaldosteronism, hypomagnesemia

• Shifts into cells: alkalosis, insulin, β2-agonists

Signs/Symptoms:

• Nonspecific: weakness, cramps

• ECG: flattened/inverted T-waves, U-waves

• Chronic hypokalemia tolerated better than acute

Treatment:

• Oral or IV K⁺ supplementation

• Oral preferred (safer, slower rise)

• IV guidelines:

  • Max: 10 mEq/hr (0.5 mEq/kg/hr)

  • Limit: 250 mEq/day

  • Central line preferred (peripheral causes burning)

  • Continuous ECG monitoring

• Rule of thumb: 10 mEq KCl → ↑ serum K⁺ by ~0.1 mEq/L (slow redistribution)

• Correct hypomagnesemia concurrently

Perioperative Considerations:

• No universal cutoff for canceling surgery

• Risk ↑ when K⁺ <3.5, especially in cardiac surgery (AFib, atrial flutter)

• 1 mEq/L drop in serum K⁺ = ~200–400 mEq total body deficit

Calcium & Phosphate (brief renal overview)

Calcium:

• Reabsorbed in proximal tubule (via Ca²⁺ ATPase & Na⁺/Ca²⁺ counter-transport)

• Regulated by PTH → ↑ renal Ca²⁺ reabsorption, ↑ bone resorption

• Follows Na⁺ and water

• Alkalosis → ↑ Ca²⁺ reabsorption

Phosphate:

• Normal: 2.5–4.5 mg/dL (0.81–1.45 mmol/L)

• Reabsorbed in proximal tubule (~0.1 mmol/min max)

• PTH: ↓ renal phosphate reabsorption → ↑ phosphate excretion

Hyperphosphatemia Causes:

• Kidney disease, hypoparathyroidism, acidosis, DKA, phosphate enemas

• Symptoms similar to hypocalcemia

• Treatment: phosphate binders

Hypophosphatemia Causes:

• Alcoholism, burns, starvation, diuretics, alkalosis, aluminum antacids

• Symptoms: weakness → respiratory failure, heart failure

• Treatment: oral or IV phosphate (0.5 mmol/kg KPhos over 6 hrs)

Magnesium

Normal: 1.8–2.6 mg/dL

• 50% intracellular, 50% bone, tiny fraction extracellular (half protein-bound)

Hypomagnesemia:

• Causes: GI losses (diarrhea), poor intake, diuretics

• Symptoms: anorexia, nausea, weakness, tremors, fasciculations, seizures

• Often with hypokalemia & hypocalcemia

• Treatment: MgSO₄ 2 g IV (15 min, can be faster if urgent)

Hypermagnesemia:

• Causes: renal failure, ingestion (antacids), medical therapy

• Symptoms by level:

  • 4–6 mEq/dL (4.8–7.2 mg/dL): ↓ reflexes

  • 10 mEq/dL (12 mg/dL): hypotension, paralysis, respiratory depression, cardiac arrest

• Treatment:

  • IV calcium (cardiac protection)

  • Diuretics (excretion)

  • Dialysis if severe

Clear takeaway: 

Always correct Mg²⁺ with K⁺/Ca²⁺ disturbances, watch cardiac effects, and consider chronic vs. acute context.