49.3 Electrolyte Abnormalities: Mild-to-Moderate Disturbances

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49.3 Electrolyte Abnormalities: Mild-to-Moderate Disturbances

Key Takeaways Explain This Topic AI

Evaluation of hyponatremia (serum sodium <135 mEq/L) begins with measuring serum osmolality: hypertonic (>295 mOsm/kg, seen in severe hyperglycemia with corrected sodium = measured Na + [1.6 to 2.0 x (glucose - 100)/100]), isotonic (275-295 mOsm/kg, pseudohyponatremia from hypertriglyceridemia or paraproteins), or hypotonic (<275 mOsm/kg, true hyponatremia).

Hypotonic hyponatremia is categorized by extracellular volume status: hypovolemic (urine Na <20 mEq/L indicates extrarenal losses, >20 mEq/L indicates renal losses; treated with 0.9% normal saline), hypervolemic (CHF, cirrhosis, nephrotic syndrome; urine Na <20 mEq/L; treated with water/sodium restriction and loop diuretics), and euvolemic (SIADH [urine Osm >100, urine Na >20-40 mEq/L; treated with fluid restriction <800-1000 mL/day, oral salt tabs, urea], hypothyroidism, adrenal insufficiency, psychogenic polydipsia [urine Osm <100]).

To prevent irreversible, catastrophic Osmotic Demyelination Syndrome (ODS / central pontine myelinolysis), the rate of correction for chronic hyponatremia MUST NOT EXCEED <=8 mEq/L in any 24-hour period (<=4-6 mEq/L in high-risk patients with cirrhosis, malnutrition, or alcoholism); accidental overcorrection is treated emergently with desmopressin (dDAVP) and free water (D5W).

Hyperkalemia (potassium >5.0-5.5 mEq/L) causes characteristic ECG changes (peaked symmetric T waves -> PR prolongation -> QRS widening -> sine wave -> VFib) and requires the emergency stabilization triad: 1) membrane stabilization with IV Calcium Gluconate (1 g over 2-5 min), 2) intracellular shifting with IV Regular Insulin (10 units) + 25 g D50W, nebulized albuterol, and IV bicarbonate, and 3) potassium elimination via loop diuretics, gastrointestinal binders (patiromer, Lokelma), or hemodialysis.

In hypokalemia (serum potassium <3.5 mEq/L), clinicians must ALWAYS check and replete serum magnesium first; hypomagnesemia impairs Na+/K+-ATPase and disinhibits renal ROMK channels, causing refractory urinary potassium wasting.

Last updated: September 2026

Algorithmic Diagnostic Approach to Hyponatremia

Hyponatremia, defined as a serum sodium concentration < 135 mEq/L , is the most common electrolyte disturbance in clinical practice. The clinical evaluation must follow a disciplined, stepwise diagnostic algorithm based on serum osmolality and clinical extracellular volume status .

Step 1: Determine Serum Osmolality

Normal serum osmolality ranges from 275 to 295 mOsm/kg. Hyponatremia is categorized into three distinct pathophysiologic states:

Hypertonic Hyponatremia (Serum Osmolality > 295 mOsm/kg) :

Caused by the presence of unmeasured, impermeant osmoles in the intravascular space that draw water out of cells into the extracellular fluid, diluting measured sodium. Prototypic cause: severe hyperglycemia (or IV mannitol/glycerol administration).

Corrected Sodium Formula : Corrected [ Na + ] = Measured [ Na + ] + 1.6 × ( Glucose − 100 100 ) \text{Corrected } [\text{Na}^+] = \text{Measured } [\text{Na}^+] + 1.6 \times \left(\frac{\text{Glucose} - 100}{100}\right) Corrected [ Na + ] = Measured [ Na + ] + 1.6 × ( 100 Glucose − 100 ​ )

Isotonic Hyponatremia (Serum Osmolality 275 to 295 mOsm/kg) — Pseudohyponatremia :

An in vitro laboratory artifact occurring when conventional indirect ion-selective electrodes (ISE) are used. In states of severe hypertriglyceridemia (triglycerides > 1,000 to 1,500 mg/dL) or profound hyperproteinemia / paraproteinemia (multiple myeloma, Waldenström macroglobulinemia), the non-aqueous (lipid/protein) phase of plasma expands from its normal 7% to >20%. The measured sodium per total volume appears artificially low, even though true aqueous sodium concentration and plasma osmolality are normal. Confirmed by measuring sodium via direct ISE (blood gas analyzer).

Hypotonic Hyponatremia (Serum Osmolality < 275 mOsm/kg) :

Represents true physiological hypotonicity and excess of free water relative to sodium. Proceed immediately to Step 2.

Step 2: Determine Extracellular Fluid (ECF) Volume Status

In hypotonic hyponatremia, the patient's clinical volume status and urinary electrolytes narrow the differential diagnosis: DIFFERENTIAL OF HYPOTONIC HYPONATREMIA (< 275 mOsm/kg)

• Volume Status Clinical Signs Urine Osm / Urine Na Etiologies & First-Line Therapy
• ═════════════════════════════════════════════════════════════════════════════════════════════════
• Hypovolemic Dry mucosa, poor • Extrarenal Losses: Extrarenal: Vomiting, diarrhea,

turgor, orthostasis, Urine Na < 20 mEq/L sweating, burns, pancreatitis.

tachycardia, flat JVP • Renal Losses: Renal: Diuretics, hypoaldosterone,

Urine Na > 20 mEq/L cerebral salt wasting.

Urine Osm > 300 mOsm/kg Rx: 0.9% Normal Saline bolus.

Hypervolemic Peripheral edema, • Urine Na < 20 mEq/L Congestive heart failure, liver

ascites, elevated (avidity from low ECV) cirrhosis, nephrotic syndrome.

JVP, bibasilar rales • Urine Na > 20 mEq/L Acute or chronic kidney injury.

(in renal failure) Rx: Fluid/Na restriction + Loops.

• Euvolemic Normal skin turgor, • SIADH: SIADH (malignancy, CNS, drugs),
• moist mucosa, no Urine Osm > 100 mOsm/kg severe hypothyroidism, secondary

edema, normal JVP Urine Na > 20-40 mEq/L adrenal insufficiency.

• Psychogenic Polydipsia: Rx: Fluid restriction (<800 mL/d),

Urine Osm < 100 mOsm/kg oral salt tabs, urea, vaptans.

• ═════════════════════════════════════════════════════════════════════════════════════════════════
• Syndrome of Inappropriate Antidiuretic Hormone (SIADH)

SIADH is the most common cause of euvolemic hypotonic hyponatremia. Persistent, non-physiologic ADH (arginine vasopressin) release causes water reabsorption in the collecting ducts via V2 receptors and aquaporin-2 channels.

Diagnostic Criteria : Hypotonic hyponatremia (serum Osm <275 mOsm/kg), inappropriately concentrated urine ( urine Osm > 100 mOsm/kg , often >300 mOsm/kg), natriuresis ( urine Na > 20 to 40 mEq/L reflecting euvolemic sodium excretion), and normal adrenal, thyroid, and renal function in the absence of diuretic therapy.

Etiologies : Central nervous system disorders (stroke, subdural hematoma, trauma, infection), pulmonary disease (small cell lung cancer [ectopic ADH], pneumonia, tuberculosis), and medications ( SSRIs/SNRIs , carbamazepine , oxcarbazepine , cyclophosphamide, NSAIDs).

Management : Fluid restriction to < 800 to 1,000 mL/day is the cornerstone. Oral sodium chloride tablets (2-3 g TID) combined with low-dose loop diuretics (furosemide 20 mg daily to disrupt the medullary concentration gradient), oral urea packets (induces osmotic water diuresis), or vasopressin receptor antagonists (tolvaptan) for refractory cases.

Acute Severe Hyponatremia & Correction Limits

Acute Symptomatic Hyponatremia (<48 hours duration) :

Presents with severe cerebral edema: seizures, lethargy, respiratory arrest, brainstem herniation.

Emergency Protocol : Administer 3% Hypertonic Saline 100 mL IV bolus over 10 minutes . This can be repeated up to two times (maximum 300 mL) at 10-minute intervals if severe neuro symptoms persist, aiming for a rapid 4 to 6 mEq/L rise in serum sodium to halt cerebral herniation.

Chronic Hyponatremia (>48 hours duration) & The ODS Mandate :

In chronic hyponatremia, cerebral astrocytes adapt by extruding intracellular organic osmolytes (taurine, myo-inositol, glutamate) to normalize brain cell volume.

Caution

THE ABSOLUTE SAFETY RULE: PREVENTING OSMOTIC DEMYELINATION SYNDROME (ODS) In chronic hyponatremia, rapid elevation of extracellular sodium shrinks brain cells and causes catastrophic Osmotic Demyelination Syndrome (ODS) (formerly central pontine myelinolysis).

Patients typically improve initially, but 2 to 6 days later develop irreversible dysarthria, dysphagia, horizontal gaze paralysis, spastic quadriparesis, and locked-in syndrome.

MAXIMUM CORRECTION LIMIT : Serum sodium elevation must NOT EXCEED <= 8 mEq/L in any 24-hour period (and <= 4 to 6 mEq/L in 24 hours in high-risk patients: liver cirrhosis, severe malnutrition, advanced alcoholism, hypokalemia). If accidental overcorrection occurs, immediately discontinue saline and administer IV free water (5% Dextrose) and Desmopressin (dDAVP 1 to 2 mcg IV/SC) to re-lower sodium into safe limits.

Hyperkalemia: Mechanisms, ECG Progression & Stabilization

Hyperkalemia ( serum potassium > 5.0 to 5.5 mEq/L ) is a life-threatening arrhythmogenic emergency. Serum potassium is normally maintained between 3.5 and 5.0 mEq/L, with >98% sequestered intracellularly.

Etiology

Impaired Renal Excretion : Acute kidney injury, chronic kidney disease (GFR <30 mL/min), hypoaldosteronism (type 4 renal tubular acidosis).

Pharmacologic Inhibition of the RAAS : ACE inhibitors, Angiotensin Receptor Blockers (ARBs), Mineralocorticoid Receptor Antagonists ( spironolactone , eplerenone ), direct renin inhibitors, and NSAIDs (blunt renin release).

Tubular Potassium Blockade : Trimethoprim (inhibits apical epithelial sodium channels [ENaC] in the collecting tubule, mimicking amiloride), calcineurin inhibitors (tacrolimus, cyclosporine).

Extracellular Potassium Shifts : Severe metabolic acidosis (H+/K+ exchange), insulin deficiency, beta-blocker therapy, cell breakdown ( rhabdomyolysis , tumor lysis syndrome, massive hemolysis).

Diagnostic Electrophysiologic Sequence on ECG

As extracellular potassium rises, resting membrane potential depolarizes (becomes less negative), partially inactivating voltage-gated sodium channels and accelerating repolarization:

Peaked, Narrow-Based, Symmetric T Waves : The earliest ECG manifestation (K+ ~5.5 to 6.5 mEq/L), most prominent in precordial leads V2-V4.

PR Interval Prolongation & P Wave Flattening : Progressive delay in atrioventricular conduction (K+ ~6.5 to 7.5 mEq/L); P waves eventually disappear entirely (sinoventricular rhythm).

QRS Complex Widening : Intraventricular conduction delay (K+ ~7.5 to 8.0 mEq/L), merging with the ST segment and T wave.

"Sine Wave" Pattern : Extreme, terminal conduction delay where widened QRS complexes blend directly into T waves. Imminent harbinger of ventricular fibrillation or asystole . ECG PROGRESSION IN HYPERKALEMIA

• Serum K+ Level Classic ECG Finding Clinical Danger
• ─────────────────────────────────────────────────────────────────────────────
• 5.5 - 6.5 mEq/L Tall, peaked, symmetric T waves Early repolarization disturbance
• 6.5 - 7.5 mEq/L Prolonged PR, flattened P waves AV conduction slowing
• 7.5 - 8.0 mEq/L Marked QRS widening, lost P waves Severe intraventricular delay
• > 8.0 mEq/L "Sine wave" pattern ──► VFib Lethal pre-terminal arrest
• ─────────────────────────────────────────────────────────────────────────────
• The Emergency Stabilization Triad

Managing acute severe hyperkalemia (K+ >=6.5 mEq/L or any hyperkalemic ECG change) demands immediate execution of three distinct steps: EMERGENCY PHARMACOTHERAPY FOR HYPERKALEMIA

• Action / Step Drug & Dosage Onset & Duration Mechanism of Action
• ═════════════════════════════════════════════════════════════════════════════════════════════
• 1. Membrane Calcium Gluconate 10% Onset: 1-3 minutes Does NOT lower K+;
• Stabilization 10 mL (1 g) IV over 2-5 min Duration: 30-60 min restores cardiac resting
• (Repeat in 5-10 min if ECG membrane potential threshold,
• changes persist) preventing lethal arrhythmia

2. Intracellular Regular Insulin 10 units IV Onset: 15-30 minutes Drives K+ into cells via

Shifting PLUS D50W 25 g (50 mL) IV Duration: 4-6 hours Na+/K+-ATPase stimulation.

(Omit D50W if glucose >250) Lowers K+ by ~0.5-1.2 mEq/L.

Nebulized Albuterol Onset: 30 minutes Stimulates beta-2 receptors,

10 to 20 mg in 4 mL saline Duration: 2-4 hours shifting K+ intracellularly.

Sodium Bicarbonate Onset: 30-60 minutes Promotes H+/K+ exchange;

50 mEq IV over 5 minutes Duration: 1-2 hours effective in severe acidosis.

3. Potassium Furosemide 40 to 80 mg IV Onset: 30-60 minutes Induces renal kaliuresis

Elimination in functional kidneys.

• Sodium Zirconium Onset: 1 hour Inorganic crystal binding K+
• Cyclosilicate (Lokelma) in GI tract in exchange

10 g PO TID for sodium and hydrogen.

Patiromer (Veltassa) Onset: 4-7 hours Non-absorbed polymer binding

8.4 g PO once daily K+ in exchange for calcium.

Emergent Hemodialysis Immediate Definitive elimination for

anuric ESRD or refractory K+.

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• Hypokalemia & The Critical Magnesium Link

Hypokalemia ( serum potassium < 3.5 mEq/L ) is triggered by gastrointestinal losses (vomiting, diarrhea, laxatives), renal losses (loop or thiazide diuretics, hyperaldosteronism, renal tubular acidosis), or intracellular shifts (alkalosis, beta-agonists, insulin therapy).

Clinical Manifestations & ECG Findings

Neuromuscular : Generalized muscle weakness, muscle cramps, hyporeflexia, ascending flaccid paralysis, constipation, and paralytic ileus.

Cardiovascular : Flattened or inverted T waves, prominent U waves (positive wave immediately following T wave), ST-segment depression, prolonged QT/QU intervals, and high risk of ventricular ectopy and torsades de pointes.

Why Magnesium Must Be Checked and Repleted First

Important

THE CARDINAL PHYSIOLOGIC PEARL: REFRACTORY HYPOKALEMIA AND MAGNESIUM Up to 40% to 50% of hypokalemic patients possess concurrent hypomagnesemia. Clinicians must ALWAYS CHECK AND REPLETE MAGNESIUM (target > 2.0 mg/dL) in any patient with hypokalemia.

Intracellular magnesium exerts a crucial physiologic brake on renal potassium excretion:

Magnesium normally binds to and inhibits Renal Outer Medullary Potassium (ROMK) channels in the apical membrane of the cortical collecting duct. In hypomagnesemia, this inhibition is removed, permitting unchecked, torrential potassium leakage into the urine.

Magnesium is an essential cofactor for Na+/K+-ATPase pumps . Hypomagnesemia impairs pump activity, preventing potassium uptake into cells.

Consequently, hypokalemia is completely refractory to massive oral or IV potassium repletion until serum magnesium is normalized!

Calcium Derangements: Hypercalcemia & Hypocalcemia

Calcium homeostasis is tightly orchestrated by parathyroid hormone (PTH) , calcitriol (1,25-dihydroxyvitamin D) , and calcitonin acting on bone, kidneys, and intestines.

Albumin Binding & Corrected Calcium

Approximately 40% to 45% of total serum calcium is bound to plasma proteins, primarily albumin. In hypoalbuminemia (cirrhosis, nephrotic syndrome, malnutrition), measured total calcium appears deceptively low while active ionized calcium remains normal.

Corrected Calcium Formula : Corrected Total Calcium (mg/dL) = Measured Total Calcium + 0.8 × ( 4.0 − Serum Albumin [g/dL] ) \text{Corrected Total Calcium (mg/dL)} = \text{Measured Total Calcium} + 0.8 \times (4.0 - \text{Serum Albumin [g/dL]}) Corrected Total Calcium (mg/dL) = Measured Total Calcium + 0.8 × ( 4.0 − Serum Albumin [g/dL] )

In critically ill or complex patients, measuring ionized calcium directly (normal 1.15 to 1.30 mmol/L) is the definitive gold standard.

Hypercalcemia (Total Calcium > 10.2 mg/dL or Ionized Ca > 1.30 mmol/L)

Over 90% of all hypercalcemia cases are attributable to two major conditions:

Primary Hyperparathyroidism : The predominant cause in ambulatory outpatients (>80%). Characterized by autonomous PTH secretion from a solitary parathyroid adenoma (85%) or multiglandular hyperplasia. Laboratory findings: elevated or "inappropriately normal" intact PTH, hypercalcemia, hypophosphatemia, and hypercalciuria.

Hypercalcemia of Malignancy : The predominant cause in hospitalized inpatients (>75%). Characterized by rapid onset, severe symptoms, and suppressed intact PTH (<10 pg/mL) . Mechanisms include:

Humoral Hypercalcemia of Malignancy (80%) : Tumor secretion of PTH-related peptide (PTHrP) , seen in squamous cell carcinomas (lung, head/neck, esophagus), renal cell carcinoma, and bladder cancer.

Osteolytic Metastases (20%) : Extensive osteoclastic bone destruction, seen in multiple myeloma and metastatic breast cancer.

Calcitriol-Mediated (Ectopic 1-alpha-hydroxylase) : Uncontrolled 1,25-(OH)2D production in lymphomas and granulomatous disorders (sarcoidosis, tuberculosis). ACUTE MEDICAL MANAGEMENT OF SEVERE HYPERCALCEMIA

• Therapeutic Modality Dosage & Route Onset / Mechanism
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1. Aggressive Isotonic 0.9% Normal Saline Onset: Immediate (within hours).

Hydration 200 to 300 mL/hr Restores intravascular volume; promotes

(Titrated to urine output) urinary calciuresis via proximal tubule.

2. Calcitonin 4 units/kg SC or IM Onset: 2 to 4 hours.

• (Salmon) every 12 hours Inhibits osteoclastic bone resorption and
• enhances calciuresis; acts as a rapid

bridge; limited by tachyphylaxis at 48h.

3. Intravenous Zoledronic Acid 4 mg IV Onset: 2 to 4 days (Nadir: 4 to 7 days).

Bisphosphonates over 15 minutes Potent, sustained osteoclast apoptosis;

OR Pamidronate 60-90 mg IV drug of choice for malignancy hypercalcemia.

4. Denosumab 120 mg SC weekly x 3 weeks Onset: 2 to 4 days.

then monthly Monoclonal antibody against RANKL; ideal

for bisphosphonate-refractory or CKD.

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• Hypocalcemia (Total Calcium < 8.5 mg/dL or Ionized Ca < 1.15 mmol/L)

Etiologies : Hypoparathyroidism (post-surgical thyroidectomy or parathyroidectomy), severe vitamin D deficiency, acute pancreatitis (fat saponification binding calcium), chronic kidney disease (impaired calcitriol synthesis), and profound hypomagnesemia (suppresses PTH synthesis and induces PTH end-organ resistance).

Neuromuscular Hyperexcitability & Physical Signs :

Chvostek Sign : Tapping the facial nerve anterior to the ear tragus produces involuntary twitching of ipsilateral facial muscles (angle of mouth, nose, eyelid).

Trousseau Sign : Inflation of a blood pressure cuff to 20 mmHg above systolic blood pressure for 3 minutes precipitates painful carpal spasm (flexion of wrist and metacarpophalangeal joints with hyperextension of fingers and adduction of thumb — "main d'accoucheur"). Trousseau sign is significantly more sensitive (94%) and specific (99%) than Chvostek sign .

Cardiovascular : Prolongation of the QTc interval , predisposing to ventricular arrhythmias and torsades de pointes.

Treatment : Acute symptomatic hypocalcemia is treated with IV Calcium Gluconate (1 to 2 g in 50 mL D5W infused over 10 to 20 minutes) under continuous cardiac monitoring.

Loading diagram... Diagnostic Algorithm for Hypotonic Hyponatremia and Volume Triage