Electrolyte Imbalances
3.5-5.0
Normal K+
mEq/L most dangerous when abnormal
≤10-12
Na+ Correction Rate
mEq/L per 24 hours maximum
8.5-10.5
Normal Ca2+
mg/dL
Potassium
the electrolyte most likely to kill and most likely to be testedHypokalemia (K+ <3.5)
- Flattened T waves, U waves appear
- ST depression
- Muscle weakness, cramping
- Increased digoxin toxicity risk
- Replace IV (max 10 mEq/hr peripheral, 20 mEq/hr central)
Hyperkalemia (K+ >5.0)
- Peaked T waves (first sign)
- Flattened P waves, widened QRS
- Sine wave → arrest
- Calcium gluconate for cardiac protection
- Insulin + glucose to shift K+ intracellularly
Sodium
Hyponatremia (Na+ <135)
- Headache, confusion, seizures
- Cerebral edema (water shifts into brain cells)
- Correct slowly (≤10-12 mEq/L per 24 hours)
- Hypertonic saline (3%) for symptomatic/severe
Hypernatremia (Na+ >145)
- Thirst, restlessness, lethargy
- Cellular dehydration (water pulled from cells)
- Free water replacement (D5W or oral water)
- Also correct slowly to prevent cerebral edema
Calcium, Magnesium, and Phosphorus
Hypocalcemia Chvostek sign (facial twitch with tap), Trousseau sign (carpopedal spasm with BP cuff), prolonged QT, tetany, seizures. Treatment: IV calcium gluconate (peripheral) or calcium chloride (central)
Hypercalcemia "Stones, bones, moans, groans" (kidney stones, bone pain, abdominal pain, neuropsych changes), shortened QT. Treatment: NS hydration, loop diuretics, calcitonin, bisphosphonates
Hypomagnesemia Makes hypokalemia and hypocalcemia refractory to treatment, torsades de pointes, tremors, hyperreflexia. Treatment: IV magnesium sulfate, MUST correct before K+ will respond
Hypomagnesemia must be corrected BEFORE hypokalemia will respond to replacement. Magnesium is the gatekeeper that allows potassium to stay inside cells.
Key Values to Memorize
K+ 3.5-5.0 mEq/L
Na+ 135-145 mEq/L
Ca2+ 8.5-10.5 mg/dL
Mg2+ 1.5-2.5 mEq/L
PO4 2.5-4.5 mg/dL
Na+ correction ≤10-12/24h
electrolyte questions are scattered across the entire CCRN
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Practice CCRN QuestionsExam Traps
Correct magnesium BEFORE potassium will respond
Hypomagnesemia causes renal potassium wasting. No matter how much potassium you replace, levels will not come up if magnesium is low. Always check and correct magnesium first when potassium replacement seems ineffective.
Sodium correction must not exceed 10-12 mEq/L in 24 hours
Correcting chronic hyponatremia too rapidly causes osmotic demyelination syndrome (formerly central pontine myelinolysis). Brain cells that adapted to low sodium cannot adjust fast enough when sodium rises quickly, leading to permanent neurological damage.
Peaked T waves are the first ECG sign of hyperkalemia
The progression of hyperkalemia on ECG goes from peaked T waves to flattened P waves to widened QRS to sine wave to cardiac arrest. Peaked T waves are the early warning. By the time you see a sine wave, arrest is imminent.
Calcium and phosphorus are inversely related
When calcium goes up, phosphorus goes down and vice versa. In chronic kidney disease, phosphorus retention drives calcium down, leading to secondary hyperparathyroidism. The exam expects you to predict the reciprocal change.
Refractory Hypokalemia
Giving potassium but levels will not come up? Check magnesium. Low Mg makes hypokalemia refractory to replacement.
Sodium Correction Speed
Hyponatremia correction too fast = osmotic demyelination. Maximum 10-12 mEq/L in 24 hours. Slow and steady.
Potassium is the most dramatic electrolyte. Too high or too low and the heart starts misbehaving. It controls the electrical rhythm of the heart. Even small deviations from normal can cause dangerous arrhythmias.
Magnesium is the conductor that keeps potassium in line. Without the conductor, the lead vocalist cannot perform properly. That is why low magnesium makes potassium replacement futile. Fix the conductor first.
Sodium controls the fluid balance (the stage size). Change it too fast and the stage collapses (osmotic demyelination). The brain needs time to adjust to changes in the stage, so corrections must be gradual.
These two always move in opposite directions, like a seesaw. When one goes up, the other comes down. They share the same stage and only one can be in the spotlight at a time.
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