7. Severe Electrolyte Disorders

Quick Recap

Endocrine & Metabolic System, Protocol 7/9. Covers sodium, calcium, magnesium, and phosphate disorders. Hypercalcemic Crisis and Hyperkalemia have their own dedicated protocols (Renal System) — cross-referenced here rather than duplicated.

1. Hyponatremia — Diagnostic Framework

Distinguish SIADH from true hypovolemia (both driven by ADH, both show low serum osmolality + high urine osmolality — genuinely hard to tell apart on osmolality alone):

• Urine sodium <20 mEq/L suggests volume depletion; >40 mEq/L suggests untreated SIADH
• Serum uric acid: normal/elevated with volume depletion, LOW with SIADH — a useful adjunct discriminator

Before diagnosing SIADH, withdraw pharmacologic agents that enhance ADH action: nicotine, carbamazepine, antidepressants, narcotics, antipsychotics, certain antineoplastics (stimulate ADH release); chlorpropamide, methylxanthines, NSAIDs (potentiate ADH action); oxytocin and desmopressin (direct ADH analogs). Rule out thyroid hormone or cortisol deficiency, both of which can present as euvolemic hyponatremia mimicking SIADH.

Cerebral Salt Wasting (CSW): associated with neurosurgery/CNS trauma, ESPECIALLY subarachnoid hemorrhage (see SAH protocol, Neurology System — that protocol's explicit "never fluid-restrict" rule stems directly from this mechanism). Mechanism: possibly brain natriuretic peptide release and/or loss of renal sympathetic tone -> excessive renal Na+ excretion -> volume depletion (the KEY distinguishing feature from SIADH) -> secondary ADH release. Hyponatremia does not always correct with volume resuscitation alone, possibly due to concomitant ADH release from the damaged brain; fludrocortisone can help ameliorate the sodium decline in refractory cases.

2. Hyponatremia — Treatment Principles and Rate of Correction

Four treatment objectives: prevent further sodium decline; decrease ICP in patients at herniation risk; alleviate symptoms; avoid EXCESSIVE (overly rapid) correction.

Correction rate (the central safety principle — osmotic demyelination syndrome/central pontine myelinolysis is the feared complication of overcorrection):

• Acute (<2 days duration) OR severe neurologic dysfunction present: correct 1-2 mEq/L/h UNTIL symptoms improve, then SLOW to no more than 0.5 mEq/L/h and no more than 8 mEq/L over 24 hours
• Chronic (>=2 days, or duration unknown — treat as chronic if uncertain): correct at no more than 0.5 mEq/L/h and no more than 6-8 mEq/L over 24 hours
• It is the DAILY change, not the hourly rise, that drives osmotic demyelination risk — rapid correction in the first few hours of a 24h period is acceptable IF the total daily change stays within bounds
• If overcorrection occurs, actively RELOWER sodium with D5W or desmopressin (1-2 mcg q6-8h), with continued close monitoring — a genuine, actionable rescue maneuver rather than just "watch and wait"

Adrogue-Madias equation (estimates expected Na+ change per liter of infusate, though it does NOT account for ongoing urinary losses and is "notoriously inaccurate"):

Change in [Na+] per liter = (infusate [Na+] + [K+] − serum [Na+]) / (total body water in kg + 1)

Infusate Na+ values: 3% hypertonic saline 513 mEq/L; 0.9% NS 154 mEq/L; Ringer's lactate 130 mEq/L (+4 K+); 0.45% saline 77 mEq/L; D5W 0 mEq/L.

Given the equation's inaccuracy, there is NO SUBSTITUTE for frequent serum sodium measurement (initially every 2 hours) during active correction — use the equation for a starting estimate only, then let real-time monitoring drive the actual infusion rate.

Fluid choice logic:

• True hypovolemic hyponatremia: 0.9% NS is effective (shuts off the volume-depletion drive for ADH)
• SIADH: 0.9% NS will WORSEN hyponatremia — urine osmolality exceeds the infusate's, causing net further dilution; use free water restriction as first-line, or 3% hypertonic saline cautiously if urgent correction needed
• Solute tablets (NaCl 2-3g BID-TID, or urea 15-30g TID) can increase daily solute load relative to water intake as an adjunct SIADH strategy
• Loop diuretics promote water excretion by blunting maximal urinary concentrating ability
• Conivaptan (vasopressin receptor antagonist): 20mg IV bolus, then 20mg/24h IV for 2-4 days — risks osmotic demyelination with Na+ overcorrection and hypotension; common toxicities diarrhea, hypokalemia

3. Hypernatremia — Framework and Etiologies

Almost always reflects a relative WATER deficit, via three mechanisms: decreased free water intake, increased free water loss, or excessive hypertonic fluid gain.

Decreased water intake is the MOST COMMON etiology in hospitalized patients — not necessarily from impaired thirst, but from inability to ACCESS or REQUEST water (nursing home residents, intubated ICU patients) — hypernatremia can develop even at maximal appropriate ADH response (urine osmolality >800 mOsm/kg) simply because the patient cannot self-hydrate.

Insensible free water loss: ~400-500 mL/day baseline from skin/respiratory tract in ambulatory adults; increases 100-150 mL/day for each degree of body temperature above 37C; highly variable with sweating, burns, mechanical ventilation, ambient humidity — unmatched insensible losses (on top of calculated free water needs) will drive hypernatremia if not proactively replaced.

Diabetes insipidus (central or nephrogenic): the kidney FAILS to appropriately concentrate urine (>800 mOsm/kg) despite hypernatremia — an inappropriately DILUTE urine in the face of hypernatremia is the key diagnostic clue.

4. Hypocalcemia

Hypomagnesemia, IF PRESENT, MUST BE TREATED FIRST for effective calcium correction — severe magnesium depletion impairs PTH release, meaning calcium repletion alone will be ineffective/incomplete until magnesium is corrected. This sequencing rule is easy to miss and explains "refractory" hypocalcemia that doesn't respond to calcium alone.

If hyperphosphatemia is the cause of low calcium (e.g., tumor lysis syndrome, rhabdomyolysis — see those dedicated protocols): LIMIT calcium replacement to cases with SIGNIFICANT cardiac/neurologic symptoms only — overaggressive correction risks METASTATIC CALCIFICATION given the concurrently elevated phosphate (same principle established in the Tumor Lysis Syndrome protocol's "asymptomatic hypocalcemia needs no therapy" rule).

Profound hypocalcemia from a chelating process (e.g., massive citrate exposure): consider EARLY hemodialysis with higher-calcium dialysate to clear the chelating species.

Hypocalcemia in sepsis/shock states: therapy-guiding evidence is genuinely LACKING — calcium administration increases cardiac contractility/BP in this setting, but some animal data suggest calcium may WORSEN reperfusion injury and mortality in this specific context — a real, unresolved tension worth being aware of rather than reflexively correcting calcium in every septic patient.

Dosing: calcium gluconate 1g IV over 2 min (arrhythmia/phlebitis risk, chloride formulation more irritating than gluconate); calcium chloride via CENTRAL vein only (see Hyperkalemia protocol for the elemental calcium comparison between formulations). Asymptomatic hypocalcemia: oral calcium + vitamin D/calcitriol, given BETWEEN meals and away from thyroid hormone/iron to maximize absorption.

5. Hypercalcemia — Diagnostic Framework

Defined as corrected calcium >10.3 mg/dL or ionized calcium >5.2 mg/dL. First evaluate PTH status:

• PTH elevated or "inappropriately normal" (should be maximally suppressed by hypercalcemia physiologically) -> hyperparathyroid state
• PTH suppressed -> consider vitamin D overdose, or excess 1,25-(OH)2 vitamin D production (sarcoidosis, granulomatous disease, lymphoma), or malignancy (PTH-related peptide/PTHrP production, or lytic bone destruction releasing calcium directly into circulation)

Symptoms typically occur >12 mg/dL, more severe with RAPID elevation (rate of rise matters as much as absolute level, a recurring theme across metabolic protocols in this library). Polyuria is common (calcium acts at the loop of Henle) -> volume depletion. GI symptoms: anorexia, constipation, abdominal pain, rarely pancreatitis. Neurologic: weakness, fatigue, confusion, stupor, coma.

(See the dedicated Hypercalcemic Crisis protocol, next in this system, for full acute management including bisphosphonates, calcitonin, and denosumab dosing.)

6. Hypophosphatemia

Mild (1.9-2.5 mg/dL): common, usually from transcellular shifts, requires NO specific treatment beyond correcting the underlying cause.

Severe/symptomatic (<1.0 mg/dL): IV phosphate therapy indicated.

Dosing table:

Formulation choice: use POTASSIUM phosphate if normal renal function and K+ <4 mEq/L; use SODIUM phosphate if renal impairment or K+ >4 mEq/L — avoids inadvertently worsening hyperkalemia in the wrong clinical context.

If hypotension occurs during infusion, SUSPECT HYPOCALCEMIA and discontinue — rapid phosphate administration can precipitate calcium, causing acute symptomatic hypocalcemia.

Reduce dosing in renal insufficiency (33% of usual dose in severe renal failure) — phosphate clearance is renally dependent.

Transition to oral once phosphate >1.5 mg/dL. 24-36 hours of repletion may be needed to replenish intracellular stores, given phosphate's predominantly intracellular distribution.

No demonstrated benefit to aggressive repletion of asymptomatic hypophosphatemia — specifically, phosphate therapy in DKA has shown NO improved outcomes and may even suggest INCREASED morbidity via resultant hypocalcemia (consistent with the DKA protocol's own conservative phosphate stance).

CRRT patients: check phosphorus every 12-24 hours given ongoing continuous clearance — hypophosphatemia is a well-recognized CRRT complication (see CRRT Indications protocol, Renal System).

7. Investigations (General Electrolyte Workup)

Serum electrolytes (Na, K, Cl, HCO3, Ca, Mg, PO4), renal function, glucose, TSH/cortisol (euvolemic hyponatremia workup), urine sodium and osmolality, fractional excretion of sodium (FENa) where relevant, serum uric acid (SIADH vs volume depletion discrimination), PTH/vitamin D panel for calcium disorders, ECG (calcium/magnesium/phosphate can all cause conduction abnormalities).

8. Organ Support

Careful, rate-controlled IV correction per the specific electrolyte and clinical context above; hemodialysis for refractory/chelation-related hypocalcemia or severe electrolyte derangement with concurrent renal failure; standard ICU supportive care.

9. Consultation Matrix

10. Monitoring Framework

Frequent serum sodium during active correction (every 2 hours initially for hyponatremia), calcium/magnesium/phosphate trend during repletion, ECG monitoring for significant derangements, urine sodium/osmolality trend if differentiating SIADH from volume depletion, renal function trend.

11. Complications

Osmotic demyelination syndrome/central pontine myelinolysis (hyponatremia overcorrection — flaccid paralysis, dysarthria, dysphagia, death in its complete form), cerebral edema (hypernatremia overcorrection or too-rapid hyponatremia correction in the wrong direction), metastatic calcification (aggressive calcium repletion with concurrent hyperphosphatemia), hypocalcemia from rapid phosphate administration, hyperkalemia from wrong-formulation phosphate choice in renal impairment. Prevention: strict adherence to correction rate limits, frequent monitoring rather than relying on predictive equations alone, magnesium-before-calcium sequencing, formulation-appropriate phosphate selection. Rescue: D5W/desmopressin for hyponatremia overcorrection, discontinue phosphate infusion for hypotension/suspected hypocalcemia, hemodialysis for refractory derangements.

12. Escalation & De-escalation

Escalate: severe symptomatic electrolyte derangement not responding to standard correction -> hemodialysis, more frequent monitoring, specialty consultation.

De-escalate: electrolyte normalized/stabilizing within safe correction limits -> transition to oral repletion where appropriate, reduce monitoring frequency, address underlying cause definitively.

13. ICU Discharge Criteria

Electrolytes stable within normal or safely-corrected range, no evidence of overcorrection complications, underlying cause identified and addressed, oral repletion established where applicable, appropriate outpatient follow-up for chronic conditions (SIADH, hyperparathyroidism, etc.).

14. Documentation & Medicolegal Checklist

15. Key Guidelines

Verbalis JG, Goldsmith SR, Greenberg A, et al. Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. Am J Med. 2013;126(10 Suppl 1):S1-S42.

16. Controversies

The precise "safe" correction rate limits for hyponatremia (1-2 mEq/L/h initially vs the stricter 24h totals) reflect expert consensus rather than a single definitive RCT, and real practice variation exists in how conservatively clinicians approach borderline-acute cases. The Adrogue-Madias equation's acknowledged inaccuracy (not accounting for ongoing urinary losses) means its clinical utility is limited to rough initial estimation, yet it remains widely taught/used as if more precise. Calcium administration in sepsis/shock-associated hypocalcemia remains a genuine evidence gap, with some concerning animal data on reperfusion injury not yet translated into firm human clinical guidance.

17. References

1. Disorders of Plasma Sodium, Potassium, Calcium, Magnesium, and Phosphorus chapter. Washington Manual of Critical Care, 4th ed, 2025 (Ch. 26).
2. Electrolyte Abnormalities drug dosing reference table. Washington Manual of Critical Care, 4th ed, 2025 (Ch. 95).
3. Chawla R, Sharma A. Fluid and Electrolyte Disorders (Hyponatremia). ICU Protocols: A Step-wise Approach, 2nd ed. Springer; 2020 (Ch. 1-2).
4. Verbalis JG, Goldsmith SR, Greenberg A, et al. Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. Am J Med. 2013;126(10 Suppl 1):S1-S42.
5. Adrogue HJ, Madias NE. Hyponatremia. N Engl J Med. 2000;342(21):1581-1589 (original equation source).

See also: Hypercalcemic Crisis (Endocrine & Metabolic System) for full acute hypercalcemia treatment; Hyperkalemia and Tumor Lysis Syndrome (Renal System) for the related electrolyte emergencies; Subarachnoid Hemorrhage (Neurology System) for the cerebral salt wasting clinical context.