Acute Kidney Injury in Critical Care: KDIGO Criteria, CRRT, Electrolyte Management

Acute Kidney Injury in Critical Care: KDIGO Criteria, CRRT, Electrolyte Management

A high-density, practical guide for clinicians in critical care.

What Is This?

Acute kidney injury (AKI) is a sudden decline in kidney function, common in critically ill patients. This guide covers KDIGO criteria (for diagnosis and staging), continuous renal replacement therapy (CRRT) (for severe AKI), and electrolyte management (to prevent life-threatening imbalances). Use this to rapidly assess, intervene, and stabilize patients in the ICU.

Why It Matters

• AKI occurs in 30–60% of ICU patients, with mortality >50% in severe cases.
• Delayed recognition leads to irreversible damage, fluid overload, and multi-organ failure.
• CRRT is the gold standard for hemodynamically unstable patients, but electrolyte derangements (e.g., hyperkalemia, hypocalcemia) can be fatal if mismanaged.
• KDIGO criteria standardize diagnosis, enabling early intervention and research consistency.

Core Concepts

1. KDIGO Criteria for AKI

Definition: A staging system based on creatinine rise and urine output to classify AKI severity. Key thresholds: | Stage | Serum Creatinine | Urine Output | |-----------|----------------------|------------------| | 1 |-?0.3 mg/dL in 48h OR 1.5–1.9× baseline | <0.5 mL/kg/h for 6–12h | | 2 | 2.0–2.9× baseline | <0.5 mL/kg/h for ?12h | | 3 | 3.0× baseline OR-to ?4.0 mg/dL OR RRT initiation | <0.3 mL/kg/h for ?24h OR anuria for ?12h |

Why it matters: - Stage 1-Early intervention (e.g., fluid optimization, nephrotoxic drug avoidance). - Stage 3-Likely needs CRRT (especially if oliguric/anuric or fluid-overloaded).

2. Continuous Renal Replacement Therapy (CRRT)

Definition: A slow, continuous dialysis method for hemodynamically unstable patients who can’t tolerate intermittent hemodialysis (IHD). Key modalities: | Modality | Mechanism | Use Case | |--------------|---------------|--------------| | CVVH (Continuous venovenous hemofiltration) | Convection (solutes dragged by fluid) | High-volume removal (e.g., sepsis, rhabdomyolysis) | | CVVHD (Continuous venovenous hemodialysis) | Diffusion (solute gradient) | Uremia, electrolyte imbalances | | CVVHDF (Continuous venovenous hemodiafiltration) | Convection + diffusion | Mixed indications (e.g., uremia + fluid overload) |

Key settings: - Blood flow rate (Qb): 100–200 mL/min (lower than IHD to avoid hypotension). - Dialysate flow rate (Qd): 1–2 L/h (adjust based on clearance needs). - Replacement fluid: Pre- or post-filter (post-filter reduces clotting risk). - Anticoagulation: Citrate (preferred for bleeding risk) or heparin (if citrate contraindicated).

3. Electrolyte Management in AKI/CRRT

Critical electrolytes to monitor: | Electrolyte | Normal Range | AKI/CRRT Risks | Management | |-----------------|------------------|--------------------|----------------| | Potassium (K?) | 3.5–5.0 mEq/L | Hyperkalemia (?K?-arrhythmias) | Insulin + glucose, calcium gluconate, CRRT (if refractory) | | Phosphate (PO?³?) | 2.5–4.5 mg/dL | Hypophosphatemia (?PO?³?-muscle weakness, respiratory failure) | IV phosphate (e.g., sodium phosphate) | | Calcium (Ca²?) | 8.5–10.2 mg/dL | Hypocalcemia (citrate toxicity in CRRT) | Calcium chloride/gluconate, reduce citrate | | Magnesium (Mg²?) | 1.7–2.2 mg/dL | Hypomagnesemia (?Mg²?-arrhythmias, seizures) | IV magnesium sulfate | | Sodium (Na?) | 135–145 mEq/L | Dysnatremias (rapid shifts-cerebral edema) | Adjust dialysate Na?, avoid rapid correction |

Key principles: - Avoid rapid shifts (e.g., correcting Na? >10 mEq/L/day-osmotic demyelination). - Citrate anticoagulation-monitor ionized Ca²? (target: 1.0–1.2 mmol/L). - CRRT removes K? and PO?³?-supplement aggressively (e.g., add K? to dialysate/replacement fluid).

How It Works

1. KDIGO Criteria Workflow

1. Baseline creatinine: Use the lowest value in the past 7 days (or estimate with MDRD equation if unknown).
2. Monitor urine output: Hourly in ICU (oliguria = <0.5 mL/kg/h).
3. Stage AKI: Compare current creatinine/urine output to baseline.
4. Intervene:
5. Stage 1: Optimize fluids, stop nephrotoxins (e.g., NSAIDs, contrast, aminoglycosides).
6. Stage 2/3: Consider CRRT if:
   • Oliguria/anuria (despite diuretics).
   • Fluid overload (e.g., pulmonary edema).
   • Metabolic acidosis (pH <7.2).
   • Hyperkalemia (>6.5 mEq/L refractory to medical therapy).

2. CRRT Circuit (Simplified)

[Patient]? (Blood pump)-[Hemofilter]? (Effluent pump)-[Waste bag]
      -                    ?
[Anticoagulant]       [Dialysate/Replacement fluid]

Key steps:
1. Vascular access: Double-lumen catheter (e.g., 13.5 Fr Mahurkar) in internal jugular or femoral vein.
2. Blood flow: 100–200 mL/min (slower than IHD to avoid hypotension).
3. Filtration: - Convection (CVVH): Solutes dragged by fluid (ultrafiltration). - Diffusion (CVVHD): Solutes move down concentration gradient (dialysate).
4. Replacement fluid: Added pre- or post-filter to maintain volume.
5. Anticoagulation: Citrate (regional) or heparin (systemic).
6. Effluent: Waste fluid (contains solutes, excess volume).

3. Electrolyte Management in CRRT

Example: Hyperkalemia on CRRT
1. Medical therapy first: - Calcium gluconate (10%, 10 mL IV)-stabilizes myocardium. - Insulin + glucose (10 units regular insulin + 50 mL D50W)-shifts K? intracellularly. - Albuterol (10–20 mg nebulized)-?-agonist effect drives K? into cells.
2. CRRT adjustments: - Increase dialysate flow rate (? clearance). - Add K? to replacement fluid (if hypokalemic post-CRRT). - Monitor ionized Ca²? (if using citrate).

Hands-On / Getting Started

Prerequisites

• Knowledge:
• Basic renal physiology (GFR, creatinine, urine output).
• ICU monitoring (invasive BP, CVP, urine output).
• Skills:
• Central line placement (for CRRT access).
• ABG interpretation (for acidosis/electrolytes).
• Equipment:
• CRRT machine (e.g., Prismaflex, NxStage).
• Double-lumen dialysis catheter.
• Point-of-care lab (e.g., i-STAT for K?, Ca²?, pH).

Step-by-Step: Initiating CRRT for AKI

Scenario: 65M with sepsis, oliguric AKI (Stage 3), K? 6.8 mEq/L, pH 7.15.

1. Assess need for CRRT:
2. Oliguria (UOP <0.3 mL/kg/h for 24h).
3. Metabolic acidosis (pH <7.2).
4. Hyperkalemia (K? >6.5 mEq/L refractory to medical therapy).

5. Fluid overload (CVP >12 mmHg, pulmonary edema).

6. Place vascular access:

7. Internal jugular vein (preferred; lower infection risk than femoral).

8. 13.5 Fr Mahurkar catheter (for high flow rates).

9. Prime CRRT circuit:

10. Machine: Prismaflex (or equivalent).
11. Modality: CVVHDF (for mixed uremia + fluid overload).

12. Settings:

    • Blood flow (Qb): 150 mL/min.
    • Dialysate flow (Qd): 2 L/h.
    • Replacement fluid: Post-filter, 1 L/h.
    • Anticoagulation: Citrate (target ionized Ca²? 1.0–1.2 mmol/L).

13. Start CRRT:

14. Connect patient to circuit.

15. Monitor:

    • BP every 15 min (hypotension risk).
    • Ionized Ca²? hourly (citrate toxicity).
    • Effluent volume (target: 20–30 mL/kg/h for clearance).

16. Adjust electrolytes:

17. Hyperkalemia: Add K? 4 mEq/L to dialysate if K? <4.0 mEq/L post-CRRT.
18. Hypophosphatemia: Add phosphate 1.2 mmol/L to replacement fluid.

19. Hypocalcemia: Reduce citrate rate or give calcium chloride.

20. Wean CRRT:

21. Criteria for stopping:
    • Urine output >0.5 mL/kg/h.
    • Stable electrolytes (K? 3.5–5.0 mEq/L, pH >7.3).
    • No fluid overload.

Common Pitfalls & Mistakes

1. Delaying CRRT in Stage 3 AKI

• Mistake: Waiting for "absolute indications" (e.g., K? >7.0 mEq/L, pH <7.0).
• Why it happens: Fear of CRRT complications (e.g., bleeding, hypotension).
• Fix: Start early (Stage 3 with oliguria/fluid overload) to avoid irreversible damage.

2. Incorrect KDIGO Staging

• Mistake: Using current creatinine instead of baseline (e.g., calling a creatinine of 2.0 mg/dL "Stage 1" when baseline was 0.8 mg/dL).
• Why it happens: Not checking old labs or assuming "normal" creatinine.
• Fix: Always compare to baseline (lowest in past 7 days).

3. Citrate Toxicity in CRRT

• Mistake: Not monitoring ionized Ca²?-hypocalcemia (arrhythmias, hypotension).
• Why it happens: Assuming citrate is "safe" without checks.
• Fix:
• Check ionized Ca²? hourly (target: 1.0–1.2 mmol/L).
• Reduce citrate rate if Ca²? <1.0 mmol/L.
• Give calcium chloride if severe.

4. Overcorrecting Sodium

• Mistake: Rapidly correcting hyponatremia (>10 mEq/L/day)-osmotic demyelination.
• Why it happens: Using hypertonic saline without monitoring.
• Fix:
• Max correction: 8–10 mEq/L/day.
• Use 3% saline only if Na? <120 mEq/L with symptoms.

5. Ignoring Phosphate in CRRT

• Mistake: Not supplementing phosphate-hypophosphatemia (respiratory failure, weakness).
• Why it happens: Assuming CRRT only removes K?.
• Fix:
• Add phosphate 1.2 mmol/L to replacement fluid.
• Monitor PO?³? daily.

Best Practices

1. KDIGO & AKI Management

• Check creatinine/urine output daily in ICU patients.
• Avoid nephrotoxins (e.g., NSAIDs, contrast, aminoglycosides) in AKI.
• Use balanced crystalloids (e.g., Plasma-Lyte) instead of normal saline (? risk of hyperchloremic acidosis).

2. CRRT Optimization

• Start with CVVHDF (covers convection + diffusion) unless specific indication (e.g., sepsis-CVVH).
• Target effluent dose: 20–30 mL/kg/h (higher in sepsis).
• Use citrate anticoagulation (lower bleeding risk than heparin).
• Monitor for clotting (? transmembrane pressure-change filter).

3. Electrolyte Management

• Hyperkalemia:
• Medical therapy first (calcium, insulin, albuterol).
• CRRT if refractory (K? >6.5 mEq/L).
• Hypophosphatemia:
• Add phosphate to replacement fluid (1.2 mmol/L).
• Check PO?³? daily.
• Citrate toxicity:
• Monitor ionized Ca²? hourly.
• Reduce citrate rate if Ca²? <1.0 mmol/L.

Tools & Frameworks

Real-World Use Cases

1. Septic Shock with AKI

• Scenario: 50F with E. coli bacteremia, hypotension (MAP 55 mmHg), oliguric AKI (UOP 0.2 mL/kg/h), K? 6.2 mEq/L.
• Intervention:
• Start CVVH (high-volume removal for cytokine clearance).
• Citrate anticoagulation (bleeding risk from sepsis).
• Monitor ionized Ca²? hourly (citrate toxicity risk).
• Outcome: Stabilized BP, cleared K?, weaned off CRRT after 48h.

2. Post-Cardiac Surgery AKI

• Scenario: 70M s/p CABG, fluid-overloaded (CVP 18 mmHg), creatinine 3.5 mg/dL (baseline 1.0 mg/dL).
• Intervention:
• Start CVVHDF (uremia + fluid removal).