5.3 AKI, CRRT & Pediatric Renal Disease Assessment

AKI, CRRT & Pediatric Renal Disease Assessment

Nutritional management in specialized nephrology spans high-acuity critical care environments and complex pediatric developmental stages. Managing Acute Kidney Injury (AKI) requiring Continuous Renal Replacement Therapy (CRRT) requires navigating profound hypercatabolism, massive dialytic nutrient losses, and hidden electrolyte-derived energy sources. Conversely, pediatric renal nutrition requires specialized assessment of somatic growth, epiphyseal bone maturation, and neurodevelopmental feeding challenges.

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Acute Kidney Injury: Diagnostic Staging & Catabolic Kinetics

Acute Kidney Injury (AKI) is characterized by an abrupt decline in renal filtration capacity, leading to azotemia, fluid overload, and metabolic disarray. The Kidney Disease: Improving Global Outcomes (KDIGO 2012) criteria define and stage AKI using sensitive serum creatinine and urine output parameters:

Pathophysiology of Hypercatabolic AKI

In critically ill patients with AKI secondary to sepsis, major trauma, or multi-organ dysfunction syndrome (MODS), the metabolic milieu is characterized by extreme hypercatabolism driven by neuroendocrine and immunological activation:

┌────────────────────────────────────────────────────────────────────────┐
│                     Hypercatabolic AKI Metabolic Cascades              │
├────────────────────────────────────────────────────────────────────────┤
│  1. Neuroendocrine & Cytokine Surge                                    │
│  • Massive release of epinephrine, cortisol, glucagon, TNF-alpha, IL-6. │
│  • Severe peripheral insulin resistance; suppressed lipolysis.         │
│                                                                        │
│  2. Skeletal Muscle Proteolysis                                        │
│  • Activation of ATP-ubiquitin-proteasome pathway in myocytes.         │
│  • Muscle proteolysis reaches 1.5 – 2.0 g/kg/day.                      │
│  • Massive efflux of amino acids to support hepatic gluconeogenesis.   │
│                                                                        │
│  3. Negative Nitrogen Balance & Uremic Toxicity                        │
│  • Nitrogen excretion exceeds intake: negative balance of -10 to -25 g │
│    nitrogen/day (equivalent to losing 300–750 g lean mass daily!).     │
│  • Rapid accumulation of urea, creatinine, potassium, and phosphorus.  │
└────────────────────────────────────────────────────────────────────────┘

Clinical Error: Restricting protein intake to 0.6–0.8 g/kg/day in hypercatabolic AKI to delay renal replacement therapy is dangerous and obsolete. Protein restriction does not slow renal deterioration in acute critical illness; it directly exacerbates sarcopenia, impairs immune function, and increases mortality. When RRT is initiated, protein must be provided aggressively.

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Continuous Renal Replacement Therapy: Modalities & Nutrient Losses

Hemodynamically unstable ICU patients cannot tolerate the rapid fluid and osmolar shifts of intermittent hemodialysis. Continuous Renal Replacement Therapy (CRRT) provides continuous, slow solute clearance and fluid removal over 24 hours daily.

CRRT Modality Classifications

• CVVH (Continuous Veno-Venous Hemofiltration): Pure convective transport. Large replacement fluid volumes are infused pre- or post-filter, driving solvent drag of small and middle molecules.
• CVVHD (Continuous Veno-Venous Hemodialysis): Pure diffusive transport. Countercurrent dialysate flows past blood hollow fibers, optimizing small solute clearance.
• CVVHDF (Continuous Veno-Venous Hemodiafiltration): Combines diffusion and convection simultaneously, utilizing both dialysate and replacement fluids for maximal solute extraction.
• PIRRT / SLED (Prolonged Intermittent RRT / Sustained Low-Efficiency Dialysis): Hybrid modality operating at slower blood/dialysate flow rates over 8 to 12 hours.

Nutrient Losses in CRRT Effluent

High-flux synthetic membranes continuously filter circulating nutrients into the effluent waste fluid:

1. Amino Acid Clearance: Free amino acids pass unhindered across CRRT membranes, resulting in a continuous loss of 10 to 15 grams of amino acids daily (accounting for 10% to 15% of total daily protein intake!).
2. Water-Soluble Vitamin Losses: Continuous clearance leaches thiamine ($B_1$), pyridoxine ($B_6$), folic acid, and vitamin C. Severe refractory lactic acidosis in CRRT patients frequently stems from acute dialytic thiamine depletion, which halts pyruvate dehydrogenase activity.
3. Trace Element Clearance: Effluent fluid strips essential trace elements—particularly selenium, copper, and zinc—impairing antioxidant defenses, wound healing, and thyroid hormone conversion.

Protein Prescriptions on CRRT

To compensate for effluent amino acid losses and blunt hypercatabolic proteolysis, the renal dietitian prescribes:

• KDIGO 2012 (AKI): 0.8–1.0 g/kg/day in non-catabolic AKI without RRT; 1.0–1.5 g/kg/day on RRT; up to 1.7 g/kg/day on CRRT or when hypercatabolic. Do not restrict protein to delay RRT; energy 20–30 kcal/kg/day; the enteral route is preferred.
• ASPEN/SCCM 2016 (critical care): 1.2–2.0 g/kg actual body weight/day in AKI, rising to as much as 2.5 g/kg/day on frequent or continuous RRT.

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Hidden Caloric Contributions: Citrate & Dextrose Kinetics

A critical responsibility of the renal dietitian in the ICU is quantifying non-nutritional caloric delivery from CRRT solutions to avoid overfeeding.

┌────────────────────────────────────────────────────────────────────────┐
│                     CRRT Non-Nutritional Caloric Inputs                │
├────────────────────────────────────────────────────────────────────────┤
│  1. Regional Citrate Anticoagulation (RCA)                             │
│  • Trisodium citrate or ACD-A infused pre-filter to chelate Ca2+.      │
│  • Systemic citrate enters Krebs cycle, yielding 3 HCO3- + energy.     │
│  • Caloric density: 3.0 kcal / g citrate (0.59 kcal / mmol citrate).   │
│  • Delivers: 200 – 800+ kcal/day!                                      │
│                                                                        │
│  2. Dialysate & Replacement Dextrose                                   │
│  • Formulations containing 100–110 mg/dL dextrose monohydrate.         │
│  • Net trans-membrane glucose uptake into circulation.                 │
│  • Delivers: 100 – 300 kcal/day.                                       │
└────────────────────────────────────────────────────────────────────────┘

Metabolic Hazards of Citrate Caloric Omission

Failing to account for 400 to 800 kcal/day of metabolized citrate when designing enteral or parenteral nutrition formulas causes severe overfeeding:

• Hypercapnia & Ventilator Dependence: Excessive total energy intake forces lipogenesis, driving the Respiratory Quotient (RQ) above 1.0. Carbon dioxide production ($V_{\text{CO}_2}$) surges, preventing liberation from mechanical ventilation.
• Hyperglycemia & Hepatic Steatosis: Massive carbohydrate/substrate delivery induces de novo hepatic lipogenesis, steatohepatitis, and refractory hyperglycemia.

Dietetic Duty: The RDN must calculate the 24-hour citrate infusion volume, multiply by 3.0 kcal/g, and subtract this energy from the target enteral/parenteral caloric goal.

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Pediatric Renal Disease Assessment: Somatic Growth & Bone Maturation

Pediatric renal disease presents distinct challenges: renal insufficiency directly halts physical growth and neurodevelopment.

Linear Growth Failure & Stunting

In children with CKD, linear growth velocity (height-age progression) is the single most sensitive biomarker of nutritional and metabolic adequacy. Stunting (height-for-age $< -2$ standard deviations below the mean) occurs in over 35% of pediatric ESRD patients.

The Acidosis-Growth Hormone-IGF-1 Axis

Chronic metabolic acidosis ($< 22$ mEq/L) directly suppresses pediatric linear growth through multiple molecular pathways:

1. GH-IGF-1 Axis Blunting: Acidosis downregulates hepatic growth hormone receptors and decreases circulating Insulin-Like Growth Factor-1 (IGF-1) bioavailability.
2. Epiphyseal Chondrocyte Resistance: Acidemia alters the microenvironment of epiphyseal growth plates, blunting chondrocyte proliferation and inhibiting longitudinal bone elongation.
3. Muscle Proteolysis: Metabolic acidosis activates the ubiquitin-proteasome pathway, accelerating skeletal muscle breakdown.

Intervention: Serum bicarbonate must be maintained strictly at $\ge 22$ mEq/L using oral sodium bicarbonate or sodium citrate solutions. If growth velocity remains stunted ($< -1.88$ SD) despite metabolic correction and optimized caloric intake, treatment with recombinant human growth hormone (rhGH) is indicated.

Energy & Protein Requirements in Pediatric CKD

• Energy Targets: Start at 100% of the estimated energy requirement (EER) for chronological age (KDOQI 2008 pediatric guideline), adjusted for activity and BMI. However, if severe linear stunting is present, calculate caloric intake based on height-age (the age at which the child's height would plot at the 50th percentile) to facilitate catch-up growth while preventing disproportionate obesity.
• Protein Targets (KDOQI 2008 pediatric guideline): 100%–140% of the DRI for ideal body weight in CKD stage 3 and 100%–120% in stages 4–5; on hemodialysis add 0.1 g/kg/day, and on peritoneal dialysis add 0.15 to 0.3 g/kg/day to offset dialytic losses.

Infant Feeding Challenges & Enteral Gastrostomy Tubes

Infants with CKD exhibit intractable uremic anorexia, severe gastroesophageal reflux (GERD), delayed gastric emptying, and fluid intolerance. Because standard infant formulas provide only 20 kcal/ounce, volume restrictions prevent adequate caloric intake.

• Formula Concentration: Renal dietitians systematically concentrate specialized low-mineral, low-potassium, low-phosphorus formulas (or fortified breast milk) to 24, 27, or 30 kcal/ounce using modular carbohydrates (maltodextrin) and fats (medium-chain triglycerides).
• Gastrostomy Tube (G-Tube) Dependency: Up to 60% of infants with ESRD require surgical G-tube placement. Nocturnal continuous pump infusions deliver the required concentrated calories, preventing severe failure to thrive and preserving neurodevelopment.