9.2 Severe Electrolyte Emergencies & Continuous Renal Replacement Therapy

9.2 Severe Electrolyte Emergencies & Continuous Renal Replacement Therapy

Clinical Scenario: A 56-year-old male with ESRD missed his last two hemodialysis sessions. He presents with profound muscular weakness, nausea, and lethargy. Vital signs: BP 82/46 mmHg, HR 36 bpm, SpO2 95% on room air. Continuous telemetry shows a wide-complex bradycardia with absent P waves, QRS widening to 184 ms, and peaked T waves coalescing into a sine-wave morphology. Point-of-care venous blood gas reveals pH 7.12, HCO3 10 mEq/L, and serum potassium 8.4 mEq/L.

Severe Hyperkalemia: The 3-Step Emergency Protocol

Hyperkalemia ($K^+ > 6.5$ mEq/L or ECG changes) is an immediate life threat. High extracellular potassium depolarizes cardiac myocytes, blunting Phase 0 depolarization and slowing intraventricular conduction. ECG changes progress from tall peaked T waves to PR prolongation, loss of P waves, QRS widening, sine waves, and asystole.

Step 1: Membrane Stabilization

• Intravenous Calcium: Does not lower serum potassium. It raises the cardiac action potential threshold, restoring the normal gradient between resting and threshold potentials to prevent fatal arrhythmias.
• Calcium Gluconate: 1–2 g IV (10–20 mL of 10% solution) over 2–5 minutes. First choice for peripheral IV access due to minimal tissue necrosis risk.
• Calcium Chloride: 1 g IV (10 mL of 10% solution) over 2–5 minutes. Delivers three times more elemental calcium (~270 mg vs ~90 mg). Preferred in cardiac arrest or through central lines.
• Kinetics: Onset is 1–3 minutes; duration is 30–60 minutes. Repeat dose in 5–10 minutes if ECG abnormalities persist.

Step 2: Intracellular Potassium Shifting

• Regular Insulin + Dextrose: 10 units IV regular insulin with 25–50 g 50% dextrose (D50W). Omit dextrose if blood glucose $\ge 250$ mg/dL. Activates $Na^+/K^+$-ATPase pumps, shifting potassium into skeletal myocytes within 15–30 minutes (lowers $K^+$ by 0.5–1.2 mEq/L for 4–6 hours). Check blood glucose hourly for 4–6 hours to monitor for hypoglycemia.
• Nebulized Albuterol: 10–20 mg in 4 mL normal saline (4–8 times standard bronchodilator dose). Beta-2 agonism stimulates $Na^+/K^+$-ATPase via cAMP. Onset is 30 minutes, with an additive effect to insulin.
• IV Sodium Bicarbonate: 50–100 mEq IV push. Indicated strictly for concurrent severe metabolic acidosis (pH < 7.15). Ineffective as monotherapy in non-acidotic hyperkalemia; risks volume overload and tetany from sudden hypocalcemia.

Step 3: Total Body Potassium Elimination

• Loop Diuretics: Furosemide 40–80 mg IV induces kaliuresis in patients with residual kidney function.
• GI Potassium Binders: Sodium zirconium cyclosilicate (Lokelma) 10 g PO TID or Patiromer 8.4 g PO daily. Avoid sodium polystyrene sulfonate (Kayexalate) with sorbitol due to the risk of intestinal necrosis and bowel perforation.
• Emergent Hemodialysis: Definitive clearance for refractory hyperkalemia and anuric ESRD.

Severe Hyponatremia: Neuro-Crisis & Correction Ceilings

Severe hyponatremia (serum sodium $< 120$ mEq/L) requires tailored therapy based on acuity and symptoms:

• Pathophysiology: Extracellular hypo-osmolality drives water into brain astrocytes. In acute hyponatremia (<48h), rapid cerebral edema produces seizures, coma, and tentorial herniation. In chronic hyponatremia (>48h), astrocytes extrude ions and organic osmolytes to restore brain volume.
• Emergency Management: For active seizures, coma, or herniation, administer 3% hypertonic saline as a 100 mL IV bolus over 10 minutes. Repeat up to two additional times every 10 minutes until symptoms resolve. The goal is an initial rise of 4–6 mEq/L to arrest brain swelling.
• Safe Correction Limits: The 24-hour correction ceiling is strictly 8–10 mEq/L (or 6–8 mEq/L in high-risk patients: alcoholism, malnutrition, advanced cirrhosis, baseline $Na^+ < 105$ mEq/L).
• Osmotic Demyelination Syndrome (ODS): Overly rapid correction ($> 10–12$ mEq/L/24h) shrinks adapted glial cells, causing blood-brain barrier disruption and pontine myelinolysis. Manifestations appear 2–7 days later: dysarthria, dysphagia, spastic quadriparesis, or locked-in syndrome.
• Overcorrection Reversal: If sodium rises too rapidly, halt hypertonic fluids immediately, infuse Dextrose 5% in water (D5W) at 3–10 mL/kg/h, and administer desmopressin (DDAVP 1–2 mcg IV/SC q6–8h) to clamp free water diuresis.

Indications for Urgent Renal Replacement Therapy: AEIOU

CRRT Modalities & Operational Mechanics

While Intermittent Hemodialysis (IHD) clears solutes rapidly over 3–4 hours, its osmotic shifts often provoke hypotension and elevated ICP. Continuous Renal Replacement Therapy (CRRT) provides continuous 24-hour clearance for hemodynamically unstable ICU patients:

• CVVH (Continuous Veno-Venous Hemofiltration): Relies on convection (solvent drag). Hydrostatic pressure pushes plasma water across a semipermeable membrane, dragging solutes along. Large replacement fluid volumes ($20–35$ mL/kg/h) are infused pre- or post-filter. Superior for removing middle/larger molecules (cytokines, myoglobin).
• CVVHD (Continuous Veno-Venous Hemodialysis): Relies on diffusion. Blood and dialysate flow counter-currently down concentration gradients, clearing small solutes (urea, creatinine, potassium) efficiently.
• CVVHDF (Continuous Veno-Venous Hemodiafiltration): Combines both convective replacement fluid and diffusive dialysate flow for maximal broad-spectrum clearance.

CRRT Circuit Anticoagulation: Citrate vs Heparin

• Regional Citrate Anticoagulation (RCA): Guideline-preferred. Citrate infused pre-filter chelates ionized calcium, reducing circuit ionized calcium to $< 0.35$ mmol/L to inhibit clotting. A post-filter systemic calcium infusion replenishes systemic ionized calcium to $1.1–1.3$ mmol/L, keeping anticoagulation regional and reducing bleeding risk compared with systemic heparin; bleeding remains possible.
• Citrate Toxicity ("Citrate Lock" / "Calcium Gap"): Impaired hepatic clearance (cirrhosis, severe shock) leads to citrate accumulation. Hallmarks include a total-to-ionized calcium ratio $>2.5$ using matching units, widening anion gap metabolic acidosis, and refractory systemic hypocalcemia requiring escalating calcium infusions. Management: immediately stop or reduce citrate infusion.
• Systemic Heparin: Targeted to aPTT 45–60s or anti-Xa 0.25–0.40 IU/mL; carries bleeding and HIT risks.