What is the concern with a 96-year-old female's elevated Blood Urea Nitrogen (BUN) of 98, given her recent increase in Lasix (furosemide) dose to 20 mg twice daily (BID) and impaired renal function with a Glomerular Filtration Rate (GFR) of 24?

Elevated BUN in an Elderly Patient on Increased Furosemide

The elevated BUN of 98 in this 96-year-old female is primarily due to pre-renal azotemia from excessive diuresis caused by the recent increase in furosemide dose to 20 mg BID, which requires immediate dose reduction and careful fluid management. 1, 2

Causes of Elevated BUN in This Patient

• Pre-renal azotemia from volume depletion: The recent increase in Lasix (furosemide) to 20mg BID has likely caused excessive diuresis, leading to intravascular volume depletion 2
• Impaired renal function: Baseline renal impairment (GFR 24, Cr 1.9) worsens the body's ability to handle the increased diuretic dose 1
• Age-related factors: At 96 years old, the patient has decreased renal reserve and is more susceptible to diuretic-induced azotemia 1
• Disproportionate BUN elevation: The markedly elevated BUN (98) compared to creatinine (1.9) suggests a pre-renal pattern rather than intrinsic renal failure 1

Pathophysiology of Elevated BUN with Furosemide

1. Enhanced urea reabsorption: Furosemide causes sodium and water depletion, leading to enhanced urea reabsorption in the distal nephron 3
2. Reduced urea excretion: Despite potential increases in urea filtration, overall urea excretion decreases with volume depletion 3
3. Neurohormonal activation: Volume depletion activates the renin-angiotensin-aldosterone system, further promoting sodium and water retention and urea reabsorption 1
4. Reduced renal perfusion: Volume depletion reduces effective renal blood flow, further impairing urea clearance 4

Management Algorithm

1. Reduce furosemide dose immediately:

   • Decrease to previous dose or lower (e.g., 20mg once daily or 10mg BID) 1
   • Consider temporary discontinuation if signs of significant dehydration are present 1

2. Assess volume status:

   • Check for orthostatic hypotension, dry mucous membranes, poor skin turgor
   • Evaluate jugular venous pressure and presence/absence of edema
   • Determine if patient has symptoms of congestion or dehydration 1

3. Fluid management:

   • If signs of dehydration: provide oral or IV fluid replacement 3
   • If still congested but with high BUN: consider lower diuretic dose with more frequent administration 5

4. Monitor closely:

   • Check electrolytes, BUN, creatinine within 24-48 hours 2
   • Monitor daily weights to assess fluid status 1
   • Watch for signs of worsening heart failure if diuretic reduced 1

5. Adjust medications:

   • Consider temporarily reducing doses of ACE inhibitors or ARBs if the patient is taking them 1
   • Avoid NSAIDs which can worsen renal function and diuretic resistance 1

Pitfalls to Avoid

• Don't abruptly stop diuretics if the patient has heart failure, as this may precipitate acute decompensation 1
• Don't ignore hyperkalemia which may accompany the azotemia and can be life-threatening 2
• Don't assume the elevated BUN is solely from renal failure - the disproportionate rise compared to creatinine suggests pre-renal causes 1
• Don't continue aggressive diuresis despite rising BUN without assessing volume status 1
• Don't miss other potential contributors to azotemia such as GI bleeding, high protein intake, or catabolic states 2

Special Considerations in the Elderly

• Elderly patients have reduced renal reserve and are more susceptible to diuretic-induced azotemia 1
• Thiazide diuretics are less effective when GFR is <30-40 mL/min, making loop diuretics necessary but requiring careful dosing 1
• Elderly patients are more prone to orthostatic hypotension and falls with volume depletion 2
• The BUN threshold for concern should be lower in elderly patients with baseline renal impairment 1

This patient's markedly elevated BUN with only moderate creatinine elevation, in the setting of recently increased furosemide dose and baseline renal impairment, strongly suggests pre-renal azotemia from excessive diuresis that requires prompt intervention to prevent further deterioration of renal function and associated complications.