3.3 Electrolyte Disorders: Potassium, Sodium, Calcium & Phosphorus

Electrolyte Disorders: Potassium, Sodium, Calcium & Phosphorus

VTS Core Concept: Electrolytes dictate cellular resting membrane potentials, neuromuscular excitability, cardiac conduction, and fluid osmolality. In the emergency room and ICU, acute electrolyte derangements are rapidly fatal if the veterinary technician fails to recognize their electrocardiographic hallmarks and execute strict, rate-controlled stabilization protocols.

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1. Potassium ($K^+$) Homeostasis & Derangements

Potassium is the primary intracellular cation ($[K^+]{ICF} \approx 140-150\text{ mEq/L}$ vs. $[K^+]{ECF} \approx 3.5-5.5\text{ mEq/L}$). The high intracellular-to-extracellular ratio ($[K^+]{in} / [K^+]{out}$) established by the $Na^+/K^+$ ATPase pump determines the resting membrane potential ($RMP$) of excitable cardiac and neuromuscular cells via the Nernst equation.

Hyperkalemia ($K^+ > 5.5\text{ mEq/L}$, Severe/Critical $> 7.0-8.0\text{ mEq/L}$)

Etiologies

1. Urinary Obstruction & Uroabdomen: Feline urethral obstruction (FLUTD), ruptured urinary bladder/ureter, severe oliguric/anuric Acute Kidney Injury (AKI).
2. Hypoadrenocorticism (Addison's Disease): Aldosterone deficiency leads to renal sodium wasting and potassium retention; $Na^+:K^+$ ratio $< 27:1$ (highly suggestive) or $< 20:1$ (classic).
3. Severe Acidemia: Transcellular shift where extracellular $H^+$ enters cells to be buffered in exchange for intracellular $K^+$ shifting outward ($0.1\text{ unit drop in pH}$ increases serum $K^+$ by $\approx 0.2-0.6\text{ mEq/L}$).
4. Reperfusion Injury & Massive Tissue Necrosis: Feline aortic thromboembolism (FATE) following ischemic limb reperfusion, severe crush injuries, massive tumor lysis syndrome.

Progressive Electrocardiographic (ECG) Changes

As extracellular potassium rises, the resting membrane potential becomes less negative (partially depolarized), inactivating voltage-gated fast $Na^+$ channels and impairing myocardial conduction velocity.

Normal ECG  ==>  K+ 5.5-6.5 mEq/L  ==>  K+ 6.5-7.5 mEq/L  ==>  K+ 7.5-8.5 mEq/L  ==>  K+ > 8.5 mEq/L
  (Normal)      (Tall, peaked     (Prolonged P-R,      (Lost P waves,       (Sine-wave, V-Fib,
                 narrow T waves)   Flattened P waves)    Wide QRS: SVR)       Asystole/PEA)

1. $5.5 - 6.5\text{ mEq/L}$: Tall, narrow, peaked ("tented") T waves with shortened Q-T interval.
2. $6.5 - 7.5\text{ mEq/L}$: Prolongation of the P-R interval, decreased P wave amplitude (flattening), widening of the QRS complex, ST segment depression or elevation.
3. $7.5 - 8.5\text{ mEq/L}$: Complete loss of P waves with widened, bizarre QRS complexes. The sinoatrial node continues to drive the ventricles through specialized internodal tracts without generating atrial depolarization—termed Sinoventricular Rhythm.
4. $> 8.5 - 10.0\text{ mEq/L}$: Development of a smooth, undulating Sine-Wave pattern, degrading into Ventricular Fibrillation, Pulseless Electrical Activity (PEA), or Ventricular Asystole.

Emergency Four-Tiered Hyperkalemia Stabilization Protocol

Hypokalemia ($K^+ < 3.5\text{ mEq/L}$, Severe $< 2.5\text{ mEq/L}$)

• Etiologies: Prolonged anorexia, aggressive fluid diuresis with potassium-deficient fluids, GI loss (vomiting, diarrhea), DKA (osmotic diuresis + insulin therapy), hyperaldosteronism (Conn's syndrome in cats), post-obstructive diuresis.
• Clinical Signs: Generalized skeletal muscle weakness, flat/curved posture, lethargy, hypoventilation due to diaphragmatic weakness. In cats, severe hypokalemia presents with pathognomonic Cervical Ventroflexion (inability to raise the head due to lack of nuchal ligament and weak neck extensor muscles).

⚠️ Critical Safety Rule: Potassium Infusion Ceiling ($K_{max}$)

Maximum IV Potassium Administration Rate:

$$K_{max} = 0.5\text{ mEq/kg/hr}$$

Never exceed $0.5\text{ mEq/kg/hr}$ under any circumstances. Infusing concentrated potassium faster than this rate produces sudden, lethal hyperkalemic cardiac arrest. Always ensure thorough mixing in fluid bags (never inject $KCl$ directly into an existing running line or patient catheter).

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2. Sodium ($Na^+$) Disorders & Osmotic Shifts

Sodium is the major extracellular cation ($[Na^+] = 140-155\text{ mEq/L}$ dog, $145-160\text{ mEq/L}$ cat) and the primary determinant of effective serum osmolality ($Posm = 2[Na^+] + [Glucose]/18 + [BUN]/2.8$).

Hypernatremia ($Na^+ > 155\text{ mEq/L}$ dog, $> 162\text{ mEq/L}$ cat)

• Mechanisms: Pure water loss (diabetes insipidus, heatstroke, lack of water access), hypotonic fluid loss (renal, GI), or sodium gain (salt ingestion, hypertonic saline administration).
• Cerebral Adaptation & Idiogenic Osmoles: Acute hypertonicity shrinks brain cells. Within $24-48\text{ hours}$, brain cells synthesize and accumulate intracellular organic osmoles (idiogenic osmoles: taurine, myo-inositol, glutamine) to restore brain cell volume.

Hyponatremia ($Na^+ < 135\text{ mEq/L}$ dog, $< 140\text{ mEq/L}$ cat)

• Mechanisms: Hypovolemic sodium loss (Addison's, pancreatitis, severe diarrhea), hypervolemic dilution (CHF, severe liver failure, nephrotic syndrome), or SIADH.

🛑 The 0.5 mEq/L/hr Rule of Sodium Correction

Never change serum sodium concentration faster than $0.5\text{ mEq/L/hr}$ (maximum $10-12\text{ mEq/L}$ change in a $24\text{ hour}$ period).

• Rapid correction of chronic hypernatremia with hypotonic fluids causes water to rush into idiogenic osmole-laden brain cells, producing Fatal Cerebral Edema, brain herniation, and coma.
• Rapid correction of chronic hyponatremia with hypertonic/isotonic fluids draws water violently out of brain cells, producing Myelinolysis / Central Pontine Myelinolysis (Osmotic Demyelination Syndrome), leading to irreversible quadriplegia, pseudobulbar palsy, and death.

Clinical Protocol: Choose fluids with a $[Na^+]$ within $10-15\text{ mEq/L}$ of the patient's current serum sodium. For severe chronic hyponatremia, add sterile water or formulate custom fluids to ensure slow, controlled correction.

3. Calcium ($Ca^{2+}$) & Phosphorus ($PO_4^{3-}$) Derangements

Calcium Dynamics: Ionized vs. Total Calcium

• Distribution: $50%$ Ionized Calcium ($iCa^{2+}$) (the only physiologically active fraction), $40%$ protein-bound (principally to albumin), and $10%$ complexed to anions (citrate, phosphate, bicarbonate).
• Acid-Base Influence: Acidemia displaces calcium from albumin, increasing $iCa^{2+}$; alkalemia increases calcium binding to albumin, decreasing $iCa^{2+}$.

Hypocalcemia ($iCa^{2+} < 1.0\text{ mmol/L}$)

• Etiologies: Puerperal tetany / Eclampsia (post-partum nursing bitches), primary hypoparathyroidism, acute pancreatitis (fat saponification binding calcium), ethylene glycol toxicity (calcium oxalate precipitation), severe sepsis/SIRS, massive citrated blood transfusion.
• Clinical Signs: Facial pruritus and rubbing, muscle twitches/fasciculations, stiff gait, tetany, hyperthermia, seizures, prolonged Q-T interval on ECG.
• Emergency Therapy: 10% Calcium Gluconate ($0.5-1.5\text{ mL/kg}$ slow IV over $15-20\text{ minutes}$ with continuous ECG monitoring). Once stabilized, initiate oral calcitriol and calcium carbonate.

Phosphorus & Refeeding Syndrome / DKA Hemolysis

• Hypophosphatemia ($PO_4 < 2.0\text{ mg/dL}$, Critical $< 1.0-1.5\text{ mg/dL}$):
  • Pathophysiology: In chronically starved patients (Refeeding Syndrome) or untreated Diabetic Ketoacidosis (DKA), reintroducing nutrition or administering insulin triggers massive cellular uptake of glucose and phosphorus for intracellular phosphorylation.
  • Clinical Crisis: Severe intracellular phosphate depletion exhausts erythrocyte ATP and 2,3-diphosphoglycerate (2,3-DPG). Red blood cells lose membrane deformability, resulting in Acute Intravascular Hemolytic Anemia, severe Heinz body formation, metabolic encephalopathy, and diaphragmatic muscle weakness.
  • Treatment: Intravenous potassium phosphate or sodium phosphate CRI ($0.01-0.03\text{ mmol/kg/hr}$) titrated to serum phosphorus levels.