Fluid & Electrolyte Imbalances

Sodium (Na+) — Normal: 136 – 145 mEq/L

Aspect	Hyponatremia (< 136)	Hypernatremia (> 145)
Causes	SIADH, water intoxication, diuretics, vomiting, heart failure	Dehydration, diabetes insipidus, excessive Na intake, Cushing's
Signs/Symptoms	Confusion, headache, nausea, seizures, muscle cramps, cerebral edema	Extreme thirst, dry mucous membranes, agitation, restlessness, elevated temperature
Interventions	Fluid restriction (dilutional), hypertonic saline (severe), monitor neuro status. Correct slowly (< 12 mEq/L per 24 hrs to prevent osmotic demyelination)	Hypotonic IV fluids (0.45% NS), increase water intake, monitor I&O. Correct slowly to prevent cerebral edema

Key Point

"Where sodium goes, water follows." Sodium is the primary regulator of extracellular fluid volume.

Potassium (K+) — Normal: 3.5 – 5.0 mEq/L

Aspect	Hypokalemia (< 3.5)	Hyperkalemia (> 5.0)
Causes	Diuretics (loop, thiazide), vomiting, NG suction, alkalosis, insulin administration	Renal failure, ACE inhibitors, K-sparing diuretics, tissue destruction (burns, crush injuries), acidosis
Signs/Symptoms	Muscle weakness, fatigue, leg cramps, diminished reflexes, constipation/ileus, shallow respirations	Muscle twitching, paresthesias, weakness progressing to paralysis, abdominal cramping, diarrhea
ECG Changes	Flattened T waves, ST depression, U waves, prolonged QT	Peaked/tall T waves, widened QRS, flattened P waves, sine wave → V-fib/asystole
Interventions	PO/IV potassium replacement (never IV push), increase dietary K (bananas, oranges, potatoes)	Calcium gluconate (cardiac protection), insulin + D50 (shifts K into cells), kayexalate, dialysis

Key Point

Potassium and digoxin have a critical relationship. Hypokalemia increases the risk of digoxin toxicity.

Calcium (Ca2+) — Normal: 9.0 – 10.5 mg/dL

Aspect	Hypocalcemia (< 9.0)	Hypercalcemia (> 10.5)
Causes	Hypoparathyroidism, vitamin D deficiency, renal failure, pancreatitis, blood transfusions (citrate)	Hyperparathyroidism, malignancy (bone mets), prolonged immobilization, thiazide diuretics, excessive vitamin D
Signs/Symptoms	Numbness/tingling, muscle spasms, tetany, positive Chvostek's sign (facial twitching), positive Trousseau's sign (carpopedal spasm), seizures, prolonged QT	Muscle weakness, diminished reflexes, constipation, kidney stones, confusion, polyuria, shortened QT, cardiac arrest
Interventions	IV calcium gluconate (slowly with cardiac monitoring), oral calcium + vitamin D, seizure precautions	NS hydration, loop diuretics (furosemide), calcitonin, bisphosphonates, dialysis

Key Point

Calcium and phosphorus have an inverse relationship. When one goes up, the other goes down.

Magnesium (Mg2+) — Normal: 1.3 – 2.1 mEq/L

Aspect	Hypomagnesemia (< 1.3)	Hypermagnesemia (> 2.1)
Causes	Alcoholism, malnutrition, diuretics, diarrhea, proton pump inhibitors	Renal failure, excessive Mg intake (antacids, laxatives), Mg sulfate overdose
Signs/Symptoms	Tremors, hyperactive reflexes, seizures, dysrhythmias (torsades de pointes), positive Chvostek's/Trousseau's (similar to hypocalcemia)	Lethargy, hypotension, bradycardia, diminished/absent deep tendon reflexes, respiratory depression, cardiac arrest
Interventions	IV magnesium sulfate (with cardiac monitoring), oral supplements, increase dietary Mg (green vegetables, nuts, whole grains)	IV calcium gluconate (antagonist), loop diuretics, IV fluids, dialysis. Stop all Mg-containing products

Key Point

When administering Mg sulfate (e.g., for preeclampsia): Monitor DTRs, respiratory rate (≥ 12/min), and urine output (≥ 30 mL/hr). Keep calcium gluconate at bedside as antidote.

Phosphorus (PO4) — Normal: 3.0 – 4.5 mg/dL

Aspect	Hypophosphatemia (< 3.0)	Hyperphosphatemia (> 4.5)
Causes	Refeeding syndrome, alcoholism, antacids (aluminum/magnesium), DKA treatment, respiratory alkalosis	Renal failure, hypoparathyroidism, chemotherapy (tumor lysis syndrome), excessive intake
Signs/Symptoms	Muscle weakness, confusion, seizures, respiratory failure, decreased cardiac output	Tetany, muscle cramps (from reciprocal drop in calcium), calcification of soft tissues, pruritus
Interventions	Oral/IV phosphorus replacement, high-phosphorus diet (dairy, meat, nuts)	Phosphate binders (calcium acetate, sevelamer) with meals, restrict dietary phosphorus, dialysis

Quick Memory Aids

• Hypo = excitable (for Ca, Mg): muscles twitch, spasm, and seize
• Hyper = sedated/weak (for Ca, Mg): muscles are weak, reflexes diminished
• Potassium is the opposite: hypo = weakness, hyper = initially twitchy then weak/paralysis
• Calcium and phosphorus are inverse — correct one and the other may shift
• Always check magnesium if potassium is low and not responding to replacement