20.3 Acute Kidney Injury, Indications for Dialysis, and Continuous Renal Replacement Therapy

20.3 Acute Kidney Injury, Indications for Dialysis, and Continuous Renal Replacement Therapy

Acute Kidney Injury (AKI) occurs in up to $50\%$ of critically ill patients and is an independent predictor of intensive care mortality. Mastery of the staging systems, physiological differentiation between pre-renal and intrinsic injury, emergent indications for renal replacement therapy (RRT), and physical principles of Continuous Renal Replacement Therapy (CRRT) is essential for EDAIC Part I candidates.

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The KDIGO Classification of AKI (2012)

The Kidney Disease: Improving Global Outcomes (KDIGO) criteria harmonize earlier RIFLE and AKIN definitions, staging AKI by either serum creatinine elevation or hourly urine output criteria (whichever metric is more severe determines the stage).

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Etiological Classification and Diagnostic Differentiation of AKI

AKI is categorized anatomically into pre-renal, intrinsic (renal), and post-renal causes:

1. Pre-Renal Azotemia ($55-60\%$ of cases): Functional response to renal hypoperfusion without structural parenchymal damage. Etiologies include intravascular volume depletion (hemorrhage, gastrointestinal fluid loss, third-spacing), systemic vasodilation (distributive shock), cardiogenic pump failure, and drug-induced disruption of renal autoregulation (NSAIDs inhibit prostaglandin-mediated afferent arteriolar vasodilation; ACE inhibitors and ARBs inhibit angiotensin II-mediated efferent arteriolar vasoconstriction).
2. Intrinsic / Parenchymal AKI ($35-40\%$ of cases): Structural cellular damage involving the renal parenchyma. Most commonly Acute Tubular Necrosis (ATN) resulting from severe, prolonged ischemia or nephrotoxins (exogenous: aminoglycosides, vancomycin, amphotericin B, iodinated radiocontrast; endogenous: myoglobin from rhabdomyolysis, hemoglobin from intravascular hemolysis, Bence Jones proteins). Other intrinsic etiologies include acute interstitial nephritis (AIN) and rapidly progressive glomerulonephritis.
3. Post-Renal / Obstructive Uropathy ($<5\%$ of ICU cases): Mechanical obstruction to urinary flow anywhere from the renal pelvis to the urethral meatus (benign prostatic hyperplasia, bilateral ureteral calculi, retroperitoneal fibrosis, neurogenic bladder, or an obstructed Foley catheter).

Biochemical Indices: Pre-Renal Azotemia vs Acute Tubular Necrosis

$\text{FE}_{\text{Na}} = \frac{U_{\text{Na}} \times P_{\text{Cr}}}{P_{\text{Na}} \times U_{\text{Cr}}} \times 100\%$ $\text{FE}_{\text{Urea}} = \frac{U_{\text{Urea}} \times P_{\text{Cr}}}{P_{\text{Urea}} \times U_{\text{Cr}}} \times 100\%$

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Absolute Emergent Indications for Dialysis: The AEIOU Mnemonic

When medical management fails to stabilize homeostasis, emergent renal replacement therapy is indicated:

• A — Acidosis: Severe metabolic acidosis with $pH < 7.15-7.20$ refractory to fluid resuscitation, source control, and conservative medical management, particularly when sodium bicarbonate infusion is contraindicated by hypernatremia or fluid overload.
• E — Electrolytes: Severe, refractory hyperkalemia ($[K^+] > 6.5\text{ mmol/L}$) or rapidly rising potassium accompanied by electrocardiographic manifestations (peaked T waves, PR prolongation, loss of P waves, QRS widening, sine wave pattern) unresponsive to temporary shifting therapy (intravenous calcium gluconate/chloride for membrane stabilization, regular insulin with hypertonic dextrose, inhaled $\beta_2$-agonists).
• I — Ingestions / Intoxications: Poisoning with dialyzable toxins characterized by low molecular weight ($<500\text{ Da}$), low volume of distribution ($V_d < 1-2\text{ L/kg}$), low plasma protein binding, and high water solubility. Classic examples include methanol, ethylene glycol, lithium, salicylates (aspirin), and theophylline.
• O — Overload: Refractory volume overload (e.g., severe pulmonary edema, anasarca, intra-abdominal hypertension) unresponsive or resistant to high-dose intravenous loop diuretics (furosemide, bumetanide).
• U — Uremia: Symptomatic organ dysfunction directly attributable to accumulation of uremic toxins, specifically: uremic pericarditis (friction rub, risk of hemorrhagic tamponade), uremic encephalopathy (asterixis, lethargy, seizures, coma), or uremic bleeding / platelet dysfunction.

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Modalities of Renal Replacement Therapy: CRRT vs IHD

CRRT Subtypes

• Continuous Veno-Venous Hemofiltration (CVVH): Clearance driven exclusively by convection (solvent drag). Requires large volumes of sterile replacement fluid.
• Continuous Veno-Venous Hemodialysis (CVVHD): Clearance driven exclusively by diffusion across a semipermeable membrane down a concentration gradient generated by counter-current dialysate flow. Does not require replacement fluid.
• Continuous Veno-Venous Hemodiafiltration (CVVHDF): Combines convection and diffusion simultaneously. Utilizes both replacement fluid and dialysate fluid.
• Slow Continuous Ultrafiltration (SCUF): Isolated fluid removal via ultrafiltration without replacement or dialysate fluids (fluid removal rate usually $100-300\text{ mL/hour}$ in fluid-overloaded patients without severe uremia or electrolyte crises).

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Physical Principles of Solute Clearance: Diffusion vs Convection

DIFFUSION (Hemodialysis)                  CONVECTION (Hemofiltration)
Dialysate Flow (Counter-Current)          Hydrostatic Pressure Gradient (TMP)
     ↓                                         ↓
[Concentration Gradient]                  [Bulk Solvent Drag]
     ↓                                         ↓
High Clearance of Small Molecules        High Clearance of Middle Molecules
(< 500 Da: Urea, K+, Creatinine)         (500 Da - 30-40 kDa: Cytokines, Myoglobin)

1. Diffusion

• Mechanism: Passive movement of solute molecules down their concentration gradient across a semipermeable membrane, governed by Fick's first law of diffusion.
• Determinants: Driven by counter-current dialysate flow relative to blood flow. Clearance is inversely proportional to the square root of solute molecular weight.
• Molecular Selectivity: Highly efficient for small molecules ($<500\text{ Da}$) such as urea ($60\text{ Da}$), potassium ($39\text{ Da}$), sodium ($23\text{ Da}$), and creatinine ($113\text{ Da}$). Inefficient for larger solutes above $1{,}000-5{,}000\text{ Da}$.

2. Convection ("Solute Drag")

• Mechanism: Solutes are swept across a high-flux, large-pore membrane along with bulk solvent (plasma water) filtration driven by a transmembrane hydrostatic pressure gradient (TMP):

$\text{TMP} = \frac{P_{\text{filter, in}} + P_{\text{filter, out}}}{2} - P_{\text{effluent}}$

• Determinants: Solute clearance equals ultrafiltration rate ($Q_{\text{uf}}$) multiplied by the sieving coefficient ($S$) of the membrane ($S = C_{\text{filtrate}} / C_{\text{plasma}}$). For solutes freely permeable across the membrane, $S = 1.0$.
• Molecular Selectivity: Highly effective for middle and large molecules ($500\text{ Da} - 30-40\text{ kDa}$) such as $\beta_2$-microglobulin ($11.8\text{ kDa}$), myoglobin ($17\text{ kDa}$), and inflammatory cytokines (IL-6, TNF-$\alpha$).

Pre-Dilution vs Post-Dilution Replacement in Hemofiltration

• Pre-Dilution: Replacement fluid is infused into the blood circuit before it enters the hemofilter. Decreases circuit hematocrit and viscosity, prolonging filter life and reducing clotting, but lowers clearance efficiency by $10-15\%$ because the entering solute concentration is diluted.
• Post-Dilution: Replacement fluid is infused after the hemofilter. Maximizes concentration gradients and solute clearance efficiency, but causes hemoconcentration within the filter capillaries (higher hematocrit and protein concentration), increasing the risk of filter clotting.

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Anticoagulation in CRRT: Regional Citrate vs Systemic Heparin

Circuit clotting is the primary technical cause of interrupted CRRT, blood loss, and reduced clearance efficacy.

1. Regional Citrate Anticoagulation (RCA) — The Gold Standard

KDIGO guidelines recommend Regional Citrate Anticoagulation as first-line therapy for CRRT in patients without contraindications.

• Mechanism of Action: Trisodium citrate is infused continuously into the arterial (pre-filter) access line of the extracorporeal circuit. Citrate binds and chelates free ionized calcium ($Ca^{2+}$), forming soluble calcium-citrate complexes. Ionized calcium is an essential cofactor (Factor IV) required for multiple stages of the clotting cascade (tenase and prothrombinase complex activation). By lowering circuit ionized calcium to $<0.35\text{ mmol/L}$, the clotting cascade inside the filter is completely arrested.
• Post-Filter Reversal: A substantial fraction of calcium-citrate complexes is removed in the effluent dialysate/ultrafiltrate. To prevent systemic hypocalcemia, calcium chloride or calcium gluconate is continuously infused into the venous return line (or via a separate central venous catheter) to restore systemic ionized calcium to normal ($1.10-1.30\text{ mmol/L}$).
• Advantages: Prolonged filter circuit life ($>72\text{ hours}$), significantly reduced systemic bleeding risks compared to heparin, and avoidance of Heparin-Induced Thrombocytopenia (HIT).
• Citrate Metabolism and Citrate Toxicity ("Citrate Accumulation"):
  • Unfiltered calcium-citrate complexes entering the systemic circulation are metabolized predominantly by hepatocytes in the liver, skeletal muscle, and renal cortex via the Krebs cycle, with each citrate molecule generating 3 molecules of bicarbonate ($HCO_3^-$).
  • In severe hepatic failure, severe shock with tissue hypoperfusion, or mitochondrial dysfunction, systemic citrate metabolism fails, leading to citrate toxicity.
  • Diagnostic Hallmark: A Total Calcium to Ionized Calcium ratio ($[\text{Total Ca}] / [\text{iCa}]$) $>2.5$ (both measured in the same units, $\text{mmol/L}$). Accompanied by refractory systemic ionized hypocalcemia, widening high anion gap metabolic acidosis (due to circulating citrate anions), and an escalating calcium infusion requirement.

2. Systemic Unfractionated Heparin

• Mechanism: Binds antithrombin III, accelerating neutralization of thrombin (Factor IIa) and Factor Xa.
• Monitoring: Titrated to achieve an activated partial thromboplastin time (aPTT) of $1.5-2.0\times$ baseline or anti-Xa activity of $0.3-0.5\text{ IU/mL}$.
• Limitations: Substantial risk of systemic hemorrhage ($10-15\%$), unpredictable pharmacokinetics in critical illness, risk of Heparin-Induced Thrombocytopenia (HIT), and premature circuit clotting in patients with antithrombin III deficiency (common in sepsis).

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Clinical Pearls and Exam Traps

• The Diuretic FE-Na Trap: In patients who have received loop diuretics within $24-48\text{ hours}$, the $\text{FE}_{\text{Na}}$ is falsely elevated because loop diuretics block the $Na^+/K^+/2Cl^-$ cotransporter in the thick ascending limb of Henle, forcing urinary sodium excretion even in true pre-renal states. Always calculate the Fractional Excretion of Urea ($\text{FE}_{\text{Urea}}$) in patients taking diuretics: an $\text{FE}_{\text{Urea}} < 35\%$ confirms pre-renal azotemia.
• Citrate Toxicity vs Normal Citrate Metabolism: Candidates often confuse citrate toxicity with normal citrate metabolism. Normal citrate metabolism yields an excess of bicarbonate, causing metabolic alkalosis. Citrate toxicity (accumulation without hepatic metabolism) results in high anion gap metabolic acidosis, profound ionized hypocalcemia, and a total-to-ionized calcium ratio $>2.5$.
• AEIOU Timing in Sepsis: Early observational trials suggested benefit from early initiation of RRT, but large multicenter randomized trials (AKIKI, IDEAL-ICU, STARRT-AKI) demonstrated that an accelerated initiation strategy does not improve survival compared to a watchful waiting strategy that initiates RRT only upon developing conventional absolute indications (the AEIOU criteria).