Internal Medicine

Fluid and Electrolyte Management in Surgery - Part 2

Electrolyte Disorders (potassium, calcium, magnesium)

Amos Asamoah
November 2025
7 min read
Other Topics

This second part covers critical electrolyte disorders (potassium, calcium, magnesium) and acid-base disturbances. These conditions frequently complicate surgical care and require prompt recognition and management. Understanding these disorders is essential for preventing life-threatening complications in surgical patients.

🫀 5. Potassium Disorders

Normal K⁺: 3.5-5.0 mEq/L

⚠️ HIGH YIELD: Potassium disorders affect the HEART most dangerously!

📉 HYPOKALEMIA (K⁺ < 3.5 mEq/L)

Causes:

  • GI losses: Diarrhea, vomiting, NG suction (most common in surgical patients)
  • Renal losses: Diuretics (especially loop and thiazides)
  • Redistribution into cells:
    • Insulin administration
    • β-agonists (albuterol)
    • Alkalosis
  • Inadequate intake: Prolonged NPO without supplementation

Clinical Manifestations:

  • Cardiac: U waves on ECG, flattened T waves, arrhythmias
  • Neuromuscular: Weakness, cramps, paralytic ileus
  • Renal: Decreased concentrating ability

ECG Changes in Hypokalemia:

  • Flattened T waves
  • ST depression
  • Prominent U waves
  • Prolonged QT interval
  • Risk of ventricular arrhythmias!

Treatment:

  • Mild (3.0-3.5): Oral KCl 40-60 mEq/day
  • Moderate (2.5-3.0): IV KCl 10 mEq/hour (in saline, NOT dextrose alone!)
  • Severe (<2.5): ICU monitoring, faster replacement with cardiac monitoring
  • NEVER give IV bolus of K⁺ → cardiac arrest!
  • Check Mg⁺⁺ level (low Mg prevents K⁺ retention)

📈 HYPERKALEMIA (K⁺ > 5.0 mEq/L)

⚠️ MOST DANGEROUS ELECTROLYTE EMERGENCY!

Causes:

  • Renal failure (can't excrete K⁺) ← #1 cause
  • Cell breakdown: Rhabdomyolysis, tumor lysis, hemolysis, burns
  • Medications: K⁺-sparing diuretics, ACE inhibitors, ARBs
  • Metabolic acidosis (K⁺ shifts out of cells)
  • Excessive supplementation (iatrogenic)
  • Pseudohyperkalemia: Hemolysis of blood sample (check repeat!)

Clinical Manifestations:

  • Often asymptomatic until severe!
  • Muscle weakness, paresthesias
  • CARDIAC TOXICITY ← Life-threatening!

ECG Changes in Hyperkalemia (Progressive):

  1. K⁺ 5.5-6.5: Tall, peaked T waves (narrow base)
  2. K⁺ 6.5-7.5: Prolonged PR interval, flattened P waves
  3. K⁺ 7.5-8.0: Widened QRS complex
  4. K⁺ >8.0: Sine wave pattern → Ventricular fibrillation/asystole → DEATH

Treatment of Hyperkalemia:

MEMORIZE THIS SEQUENCE:
1. STABILIZE THE HEART (if ECG changes present)
  • Calcium Gluconate 10% 10 mL IV over 2-3 min
  • Doesn't lower K⁺, but protects heart membrane
  • Onset: immediate, Duration: 30-60 min
  • Repeat if ECG changes persist
2. SHIFT K⁺ INTO CELLS (temporary, buys you time)
  • Insulin 10 units IV + D50W 1 amp (50 mL)
    • Onset: 15-30 min, Duration: 4-6 hours
    • Monitor glucose! (hypoglycemia risk)
  • Sodium Bicarbonate 50 mEq IV (if acidotic)
    • Onset: 30 min, Duration: 2 hours
  • Albuterol nebulizer 10-20 mg
    • Onset: 30 min, Duration: 2 hours
    • Less reliable than insulin
3. REMOVE K⁺ FROM BODY (definitive treatment)
  • Diuretics (Furosemide 40-80 mg IV) - if kidneys work
  • Kayexalate (Sodium polystyrene sulfonate) 15-30 g PO/PR
    • Onset: 1-2 hours
    • Binds K⁺ in GI tract
  • Hemodialysis - if renal failure or severe (K⁺ >7.0)
    • Most effective
    • Onset: immediate
4. STOP K⁺ SOURCES
  • Stop K⁺ supplements
  • Stop K⁺-sparing diuretics
  • Review all medications

🦴 6. Calcium Disorders

Normal Total Ca⁺⁺: 8.5-10.5 mg/dL
Normal Ionized Ca⁺⁺: 4.5-5.5 mg/dL ← This is the active form!

⚠️ HIGH YIELD:
  • Half of calcium is bound to albumin
  • Corrected Calcium = Measured Ca + 0.8 × (4 - Albumin)
  • Always check ionized calcium if patient is hypoalbuminemic

📉 HYPOCALCEMIA (Total Ca⁺⁺ < 8.5 mg/dL)

Causes:

  • Post-thyroidectomy/parathyroidectomy (damaged parathyroid glands) ← Common in surgery!
  • Hypoparathyroidism
  • Vitamin D deficiency
  • Acute pancreatitis (Ca⁺⁺ sequestered in fat necrosis)
  • Massive blood transfusions (citrate in blood products binds Ca⁺⁺)
  • Chronic kidney disease
  • Hypomagnesemia (Mg needed for PTH secretion)

Clinical Manifestations:

  • Neuromuscular irritability:
    • Chvostek sign: Tap facial nerve → facial twitching
    • Trousseau sign: BP cuff inflated → carpopedal spasm (hand cramping)
    • Paresthesias (perioral, fingertips)
    • Tetany, seizures
  • Cardiac: Prolonged QT interval, hypotension
  • Psychiatric: Anxiety, confusion, depression

Treatment:

  • Acute symptomatic:
    • Calcium gluconate 10% 10-20 mL IV over 10 min
    • Then continuous infusion 0.5-2 mg/kg/hr
  • Chronic:
    • Oral calcium carbonate 1-2 g/day
    • Vitamin D supplementation
    • Treat underlying cause
⚠️ POST-THYROIDECTOMY PEARL:
  • Check Ca⁺⁺ every 6 hours for 24 hours
  • Give prophylactic oral Ca⁺⁺ and vitamin D
  • Have calcium gluconate at bedside!

📈 HYPERCALCEMIA (Total Ca⁺⁺ > 10.5 mg/dL)

Mnemonic for causes: CHIMPANZEES

  • Calcium supplementation
  • Hyperparathyroidism (primary) ← #1 outpatient cause
  • Immobilization
  • Malignancy ← #1 inpatient cause (breast, lung, multiple myeloma)
  • Paget's disease
  • Addison's disease
  • Neoplasm (see malignancy)
  • Zollinger-Ellison syndrome
  • Excess vitamin D
  • Excess vitamin A
  • Sarcoidosis

Clinical Manifestations: "Stones, Bones, Groans, Psychiatric Overtones"

  • Stones: Kidney stones
  • Bones: Bone pain, osteoporosis
  • Groans: Abdominal pain, constipation, nausea
  • Psychiatric: Confusion, depression, lethargy
  • Cardiac: Shortened QT interval, bradycardia
  • Polyuria, polydipsia
  • Weakness, fatigue

Treatment:

Mild (Ca⁺⁺ 10.5-12):
  • Increase oral fluids
  • Stop Ca⁺⁺ and vitamin D supplements
  • Treat underlying cause
Moderate to Severe (Ca⁺⁺ >12):
  1. IV Hydration: Normal Saline 200-300 mL/hr
    • Dilutes Ca⁺⁺ and increases renal excretion
  2. Loop diuretics (Furosemide 20-40 mg IV) after adequate hydration
    • Increases Ca⁺⁺ excretion
  3. Calcitonin 4 units/kg SC/IM q12h
    • Onset: 4-6 hours
    • Tachyphylaxis (stops working after 48 hrs)
  4. Bisphosphonates (Zoledronic acid 4 mg IV)
    • Onset: 2-4 days (slow but sustained)
    • For malignancy-associated hypercalcemia
  5. Dialysis if renal failure
Hypercalcemic Crisis (Ca⁺⁺ >14):
  • Medical emergency!
  • Confusion, coma
  • Cardiac arrhythmias
  • Aggressive IV hydration + calcitonin + bisphosphonates

🧲 7. Magnesium Disorders

Normal Mg⁺⁺: 1.5-2.5 mEq/L (1.8-3.0 mg/dL)

⚠️ HIGH YIELD: Magnesium is the "forgotten electrolyte" but critically important!

📉 HYPOMAGNESEMIA (Mg⁺⁺ < 1.5 mEq/L)

Causes:

  • GI losses: Diarrhea, NG suction, malabsorption, pancreatitis
  • Renal losses: Diuretics, alcohol, aminoglycosides, cisplatin
  • Redistribution: Refeeding syndrome
  • Endocrine: Diabetes, hyperaldosteronism

Clinical Manifestations:

  • Similar to hypocalcemia (neuromuscular irritability)
  • Chvostek and Trousseau signs
  • Cardiac: Arrhythmias (torsades de pointes), prolonged QT
  • Associated with: Hypokalemia and hypocalcemia
    • Can't correct K⁺ or Ca⁺⁺ until you correct Mg⁺⁺!

Treatment:

  • Asymptomatic: Oral magnesium oxide 400 mg TID
  • Symptomatic/Severe (<1.0):
    • Magnesium sulfate 1-2 g IV over 15 min
    • Then 4-6 g IV over 24 hours
    • Monitor for bradycardia, hypotension
    • Check K⁺ and Ca⁺⁺ and replace as needed

📈 HYPERMAGNESEMIA (Mg⁺⁺ > 2.5 mEq/L)

Causes:

  • Renal failure (decreased excretion) ← #1 cause
  • Excessive administration (magnesium-containing antacids, laxatives)
  • Rhabdomyolysis
  • Tumor lysis syndrome

Clinical Manifestations (Progressive with increasing levels):

  • 4-6 mg/dL: Nausea, flushing, hypotension
  • 6-10 mg/dL: Loss of deep tendon reflexes, drowsiness
  • 10-12 mg/dL: Respiratory depression
  • >12 mg/dL: Complete heart block, cardiac arrest

Treatment:

  • Stop Mg⁺⁺ sources
  • IV hydration + Furosemide (increases excretion)
  • Calcium gluconate 1-2 g IV (antagonizes Mg⁺⁺ at neuromuscular junction)
  • Hemodialysis if severe or renal failure

⚗️ 8. Acid-Base Disorders

Normal Arterial Blood Gas (ABG) Values:

  • pH: 7.35-7.45 (normal = 7.40)
  • PaCO₂: 35-45 mmHg (normal = 40)
  • HCO₃⁻: 22-26 mEq/L (normal = 24)
  • PaO₂: 80-100 mmHg

Systematic ABG Interpretation:

Step 1: Look at pH

  • < 7.35 = Acidemia (acidosis)
  • > 7.45 = Alkalemia (alkalosis)

Step 2: Determine primary disorder

  • If pH low and CO₂ high → Respiratory acidosis
  • If pH low and HCO₃ low → Metabolic acidosis
  • If pH high and CO₂ low → Respiratory alkalosis
  • If pH high and HCO₃ high → Metabolic alkalosis

Step 3: Check for compensation

  • Body tries to normalize pH (never overcompensates)

Step 4: Calculate anion gap

  • AG = Na⁺ - (Cl⁻ + HCO₃⁻)
  • Normal: 8-12 mEq/L

METABOLIC ACIDOSIS (Low pH, Low HCO₃⁻)

High Anion Gap Metabolic Acidosis (HAGMA)

Mnemonic: MUDPILES

  • Methanol
  • Uremia (renal failure)
  • Diabetic ketoacidosis (DKA)
  • Propylene glycol
  • Isoniazid, Iron
  • Lactic acidosis ← Most common in surgical patients!
  • Ethylene glycol
  • Salicylates (aspirin overdose)
Lactic Acidosis in Surgery:
  • Type A: Tissue hypoxia (shock, sepsis, hemorrhage)
  • Type B: Impaired metabolism (liver failure, metformin)
  • HIGH YIELD: Lactate >4 = poor prognosis in sepsis/trauma

Treatment:

  • Treat underlying cause! (resuscitate, source control)
  • Sodium bicarbonate controversial (only if pH <7.1)
  • Increase ventilation (blow off CO₂)

Normal Anion Gap Metabolic Acidosis (NAGMA)

Mnemonic: HARDUPS

  • Hyperalimentation
  • Acetazolamide, Addison's disease
  • Renal tubular acidosis, Renal failure (early)
  • Diarrhea ← Most common!
  • Ureterosigmoidostomy
  • Pancreatic fistula
  • Saline administration (dilutional)
⚠️ HIGH YIELD in Surgery:
  • Large volume NS resuscitation → Hyperchloremic acidosis
  • Diarrhea loses HCO₃⁻ → acidosis
  • Use Lactated Ringer's instead of NS to prevent!

METABOLIC ALKALOSIS (High pH, High HCO₃⁻)

Causes:

  • GI losses: Vomiting, NG suction (lose H⁺ and Cl⁻) ← Very common in surgery!
  • Renal losses: Diuretics (especially loop diuretics)
  • Volume contraction: "Contraction alkalosis"
  • Hypokalemia (K⁺ and H⁺ linked)
  • Excess alkali: Antacids, blood transfusions (citrate → HCO₃⁻)
  • Mineralocorticoid excess: Conn's syndrome, Cushing's

Classification:

Saline-Responsive (Urine Cl⁻ < 20 mEq/L):
  • Vomiting, NG suction
  • Diuretics (remote use)
  • Treatment: Normal Saline + KCl
Saline-Resistant (Urine Cl⁻ > 20 mEq/L):
  • Hyperaldosteronism
  • Current diuretic use
  • Treatment: Treat underlying cause, K⁺-sparing diuretics

Treatment:

  • Correct volume deficit: NS with KCl 20-40 mEq/L
  • Correct K⁺ and Cl⁻ (both usually low)
  • Treat underlying cause
  • Severe: Acetazolamide 250-500 mg (increases HCO₃⁻ excretion)

🧠 Key Points Summary - Part 2

  • Most dangerous electrolyte abnormality = Hyperkalemia (cardiac arrest)
  • Treat hyperkalemia K⁺ >6.5: Calcium → Insulin/Dextrose → Remove K⁺
  • Can't correct K⁺ or Ca⁺⁺ if Mg⁺⁺ is low - fix Mg⁺⁺ first!
  • Post-thyroidectomy: Check calcium q6h × 24h (hypocalcemia risk)
  • Hypercalcemia: "Stones, Bones, Groans, Psychiatric Overtones"
  • Most common cause of metabolic acidosis in surgery: Lactic acidosis from shock/sepsis
  • Large volume NS resuscitation causes hyperchloremic metabolic acidosis
  • SBO patient: Volume depleted with metabolic alkalosis and hypokalemia → Give NS with KCl
  • Anion gap formula: AG = Na⁺ - (Cl⁻ + HCO₃⁻) (Normal: 8-12 mEq/L)
  • Metabolic alkalosis classification: Saline-responsive vs saline-resistant
🎯 Clinical Memory Aids:
  • Hypokalemia ECG: Flat T, Prominent U waves
  • Hyperkalemia ECG: Peaked T → Wide QRS → Sine wave
  • Hypocalcemia ECG: Prolonged QT
  • Hypercalcemia ECG: Shortened QT
  • HAGMA: MUDPILES
  • NAGMA: HARDUPS
  • Hypercalcemia symptoms: Stones, Bones, Groans, Psychiatric Overtones
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