Renal
Electrolytes & Acid-Base Disturbances
Renal

Electrolytes & Acid-Base Disturbances

Sodium, potassium, calcium, magnesium + anion gap acidosis MUDPILES.

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◆Hyponatremia (Na <135)

  • •Check serum osm first (rule out pseudo or hypertonic)
  • •Hypotonic + euvolemic: SIADH (CNS, lung CA, drugs — SSRIs, carbamazepine), hypothyroid, glucocorticoid deficiency
  • •Hypotonic + hypovolemic: GI/skin loss, diuretics (urine Na >20 if diuretic; <20 if extrarenal)
  • •Hypotonic + hypervolemic: CHF, cirrhosis, nephrotic
  • •Correct slowly — <8–10 mEq/L per 24 hr to avoid osmotic demyelination (central pontine myelinolysis)
  • •Severe symptomatic (seizure, AMS): 3% hypertonic saline

◆Hypernatremia (Na >145)

  • •Always reflects water deficit (or excess Na)
  • •Diabetes insipidus (central or nephrogenic) → polyuria + polydipsia + dilute urine despite ↑serum osm
  • •Correct slowly to avoid cerebral edema

◆Hyperkalemia

  • •ECG: peaked T → wide QRS → sine wave
  • •Step 1: IV calcium gluconate (membrane stabilization)
  • •Step 2: insulin + glucose, albuterol, bicarb (shift)
  • •Step 3: kayexalate, patiromer, loop diuretic, dialysis (remove)
  • •Stop offenders (spironolactone, ACEi/ARB, NSAIDs, K-sparing)

◆Hypokalemia

  • •Causes: diuretics, vomiting, diarrhea, hyperaldosteronism
  • •ECG: flat T, U waves
  • •Replete K + Mg (low Mg perpetuates low K)

◆Hypercalcemia (stones, bones, abd groans, psychic moans)

  • •Causes: primary HPT (most common outpatient), malignancy (most common inpatient), vitamin D toxicity, sarcoid, milk-alkali, thiazides, immobilization, MEN syndromes
  • •Treatment: IV NS + calcitonin (rapid) + bisphosphonate (durable, e.g., zoledronic acid); steroids for vitamin D-mediated

◆Hypocalcemia

  • •Chvostek (facial twitch with tap) and Trousseau (carpal spasm with BP cuff) signs
  • •Prolonged QT on ECG → tetany, seizure
  • •Causes: hypoparathyroidism (post-thyroidectomy), CKD, vitamin D deficiency, hypomagnesemia, pancreatitis, tumor lysis
  • •IV calcium gluconate for symptomatic; replace Mg first if low

◆Anion gap metabolic acidosis (MUDPILES)

  • •Methanol (formic acid), Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol (oxalate crystals), Salicylates
  • •Methanol → visual changes (retinal damage)
  • •Ethylene glycol → calcium oxalate crystals in urine + ATN
  • •Both treated with fomepizole (alcohol dehydrogenase inhibitor) ± dialysis

◆Non-anion gap metabolic acidosis (HARDASS)

  • •Hyperalimentation, Addison disease, RTA, Diarrhea, Acetazolamide, Spironolactone, Saline

◆Respiratory acid-base

  • •Respiratory acidosis: hypoventilation (COPD, opioids, NMD)
  • •Respiratory alkalosis: hyperventilation (anxiety, PE, salicylates, sepsis)
  • •Salicylates: mixed primary respiratory alkalosis + anion gap acidosis

High-yield pearls

  • ◆Correct hyponatremia too fast → ODS (central pontine myelinolysis); too slow → cerebral edema
  • ◆Always replete Mg before K (low Mg blunts K replacement)
  • ◆Hypocalcemia → check Mg first
  • ◆Methanol vs ethylene glycol: methanol → vision (formic acid attacks retina); ethylene glycol → crystals + AKI
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