5 · Fluid, Electrolyte & Acid–Base Disorders

The bread-and-butter of renal exams. Anchor everything to a few reference numbers and the EKG/clinical signs.

Acid–base — "24 / 7.40 / 40"

Disorder	pH	Primary change	Causes
Respiratory acidosis	↓	↑ pCO₂ (>45)	Hypoventilation — opioids, COPD, neuromuscular
Respiratory alkalosis	↑	↓ pCO₂ (<35)	Hyperventilation — anxiety, PE, fever, salicylates (early)
Metabolic acidosis	↓	↓ HCO₃ (<22)	See anion gap below
Metabolic alkalosis	↑	↑ HCO₃ (>26)	Vomiting, antacids, diuretics

Anion gap = Na − (Cl + HCO₃), normal 10–16. High gap = MUDPILES (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). Normal gap = bicarb loss (diarrhea, RTA).

Potassium

	Definition	EKG	Treatment
Hyperkalemia	>5.5 mEq/L	Peaked T waves, wide QRS	Calcium gluconate (stabilize membrane) → insulin+glucose, bicarb, albuterol (shift) → kayexalate/dialysis (remove)
Hypokalemia	<3.5 mEq/L	Flattened T, U waves	Replete K; replace magnesium; avoid dextrose fluids

Sodium & volume

Hyponatremia (<135): muscle cramps, seizures. Classify by volume — hypervolemic (CHF, cirrhosis, nephrotic), euvolemic (SIADH, hypothyroid), hypovolemic (Na loss). Correct slowly (≤8–10 mEq/L per 24 h) to avoid osmotic demyelination.
Dehydration: tachycardia, orthostatic hypotension, delayed cap refill; ↑Hct/albumin, ↓urine Na, ↑urine osm. Treat with oral rehydration (mild) → NS boluses 20 mL/kg (severe).
Hypervolemia: weight gain, edema, dyspnea (CHF/CKD/cirrhosis); treat cause + furosemide, Na/fluid restriction; monitor daily weights.

SIADH

Excess ADH → euvolemic hyponatremia, concentrated urine (osm >300, urine Na >20), no edema/HTN.
Causes: small-cell lung cancer, CNS (tumor/trauma), pneumonia, drugs (SSRIs, carbamazepine, cyclophosphamide, ecstasy).
Treat: fluid restriction; hypertonic saline only for severe/symptomatic; demeclocycline or vaptans for chronic.

Calcium & magnesium

	Signs	EKG	Treatment
Hypocalcemia (<8.5)	Cramps, tetany, +Chvostek/Trousseau	Prolonged QT	Calcium gluconate; correct for albumin
Hypercalcemia (>10.5)	"Stones, bones, groans, moans", polyuria	Shortened QT	IV fluids first, then bisphosphonates/calcitonin
Hypomagnesemia	Tremor, nystagmus, refractory ↓K/↓Ca	Prolonged QT	Replete Mg (needed to fix K)

Renal tubular acidosis (normal-anion-gap acidosis)

Type	Defect	Serum K⁺	Urine pH	Clue
I (distal)	↓ H⁺ secretion	Low	>5.5	Sjögren, kidney stones
II (proximal)	↓ HCO₃ reabsorption	Low	<5.5	Multiple myeloma, Fanconi
IV	Hypoaldosteronism	High	<5.5	Diabetes, NSAIDs, sickle cell

Metabolic alkalosis & the 3-step ABG

Chloride-responsive (urine Cl <20): vomiting/NG suction, diuretics, post-hypercapnia → give saline.
Chloride-resistant (urine Cl >20): hyperaldosteronism, Cushing, severe K/Mg deficiency, Bartter.

⭐ ABG in 3 steps: (1) pH → which side of 7.40 is the primary process; (2) anion gap = Na − (Cl + HCO₃) — if ≥ ~20 there's a metabolic acidosis regardless of pH; (3) excess/delta gap reveals a hidden second disorder (e.g., a vomiting CKD patient with both metabolic acidosis and alkalosis).