In a patient with a low blood urea nitrogen (<5 mg/dL) and mild hypernatremia (serum sodium 147 mEq/L) who has a non‑healing wound, what are the likely etiologies and how should fluid, electrolyte, and nutritional management be performed?

Low BUN with Mild Hypernatremia and Non-Healing Wound: Etiologies and Management

In a patient with BUN <5 mg/dL, sodium 147 mEq/L, and a non-healing wound, the most likely etiology is severe protein-energy malnutrition causing both the low BUN (from inadequate protein intake/synthesis) and impaired wound healing, while the mild hypernatremia suggests either inadequate free water intake or ongoing losses in the setting of volume depletion.

Likely Etiologies

Low BUN (<5 mg/dL)

• Severe malnutrition or protein depletion is the primary cause, reflecting inadequate protein intake, hepatic synthetic dysfunction, or severe catabolic states 1
• Overhydration with hypotonic fluids can dilute BUN, though this would typically cause hyponatremia rather than hypernatremia 1
• Severe liver disease reducing urea synthesis, though this would be evident clinically 1

Mild Hypernatremia (147 mEq/L)

• Inadequate free water intake in a malnourished patient with impaired thirst mechanism 2
• Ongoing insensible losses (fever, tachypnea) without adequate water replacement 2
• Renal concentrating defect from chronic illness or medications 1
• In the context of malnutrition, this represents hypovolemic hypernatremia from water deficit exceeding sodium deficit 2

Non-Healing Wound Connection

• Protein-energy malnutrition directly impairs wound healing through reduced collagen synthesis and immune dysfunction 1
• Metabolic instability (reflected in the electrolyte abnormalities) further compromises tissue repair 1
• Possible diabetic foot infection should be considered if the patient has diabetes, as metabolic derangements including azotemia (or in this case, low BUN) can complicate wound healing 1

Management Approach

Fluid Management

For mild hypernatremia (147 mEq/L) with suspected hypovolemia:

• Calculate corrected sodium to guide fluid selection: Corrected Na = Measured Na + [(Glucose - 100)/100 × 1.6] 3
• If corrected sodium is normal or elevated: Use 0.45% NaCl at 4-14 mL/kg/h 3
• If corrected sodium is low: Use 0.9% NaCl at similar rates 3
• Target correction rate: Do not decrease osmolality more than 3 mOsm/kg/h to prevent cerebral edema 3
• Monitor serum sodium every 2-4 hours initially during correction 2

Critical considerations:

• In malnourished patients, avoid rapid fluid resuscitation as they are at risk for refeeding syndrome and fluid overload 1
• Restrict sodium intake to ≤2 g daily once euvolemia is achieved to prevent volume overload 1
• Fluid restriction to 2 liters daily may be needed if persistent volume issues develop 1

Electrolyte Management

• Measure BUN, creatinine, and electrolytes every 1-2 days during hospitalization 1
• Assess for concurrent hypokalemia, hypophosphatemia, and hypomagnesemia (refeeding syndrome risk in malnourished patients) 1
• Add potassium supplementation (20-40 mEq/L) to IV fluids once renal function is confirmed and potassium is known 1

Nutritional Management

This is the cornerstone of treatment given the low BUN and non-healing wound:

• Immediate protein repletion: Target 1.5-2.0 g/kg/day of high-quality protein to restore nitrogen balance and support wound healing 1
• Caloric support: Provide adequate calories (25-30 kcal/kg/day) to prevent protein catabolism 1
• Micronutrient supplementation: Zinc, vitamin C, and vitamin A are critical for wound healing 1
• Monitor for refeeding syndrome: Check phosphorus, potassium, and magnesium daily during initial refeeding 1
• Enteral nutrition preferred over parenteral when gastrointestinal tract is functional 1

Wound-Specific Management

For the non-healing wound:

• Debride necrotic tissue and obtain specimens from the debrided base for culture if infection is suspected 1
• Assess wound severity: Determine depth, tissue involvement, and presence of infection (purulence, erythema >2 cm, warmth, induration) 1
• Metabolic stabilization is essential: Correction of fluid, electrolyte imbalances, hyperglycemia (if diabetic), and azotemia must precede or accompany wound treatment 1
• Glycemic control: If diabetic, improved glucose control aids both infection eradication and wound healing 1

Antibiotic considerations (if infection present):

• Mild infection: Oral agents covering aerobic gram-positive cocci are usually sufficient 1
• Moderate-to-severe infection: Broad-spectrum parenteral antibiotics covering gram-positive cocci (including MRSA if prevalent locally), gram-negatives, and anaerobes 1
• Do not treat uninfected ulcers with antibiotics as this promotes resistance without benefit 1

Common Pitfalls to Avoid

• Do not correct hypernatremia too rapidly: Risk of cerebral edema if osmolality drops >3 mOsm/kg/h 3
• Do not overlook refeeding syndrome: In severely malnourished patients (BUN <5 suggests severe depletion), aggressive nutritional repletion without electrolyte monitoring can be fatal 1
• Do not use hypotonic fluids initially without calculating corrected sodium, as this may worsen hyponatremia if present 3
• Do not discharge until stable diuretic/fluid regimen established and metabolic parameters are improving 1
• Do not assume wound is infected without clinical signs; avoid unnecessary antibiotics 1