What are the possible explanations for an elderly female patient with ischemic heart disease (IHD) and poor left ventricular (LV) function, controlled on furosemide (Lasix), experiencing weight gain despite having a urine output that exceeds fluid intake?

Weight Gain Despite Positive Fluid Balance in Heart Failure

The most likely explanation is inaccurate measurement of fluid intake/output or weight, followed by occult fluid retention in third spaces (ascites, pleural effusions) that is not being adequately mobilized by current diuretic therapy. 1

Primary Explanations to Investigate

Measurement Error (Most Common)

• Fluid intake is likely being underestimated - hidden sources include medications mixed in fluids, ice chips, foods with high water content, and fluids consumed outside of meal times 1
• Urine output may be overestimated - collection errors, spillage, or incomplete measurement of all voids are common in elderly patients 1
• Weight measurement inconsistencies - different scales, different times of day, clothing variations, or bladder fullness can create false impressions of weight changes 1, 2

Diuretic Resistance

• The patient has developed diuretic resistance, defined as failure to increase fluid and sodium output sufficiently despite escalating loop diuretic doses 3
• This is a major cause of recurrent hospitalizations and predicts mortality in chronic heart failure 3
• Mechanisms include:
  • Tubular tolerance developing even during single-dose exposure 3
  • Enhanced proximal tubule reabsorption limiting delivery to the loop of Henle 3
  • Adaptive increase in distal tubule reabsorption offsetting loop blockade 3
  • Inappropriately high dietary sodium intake exceeding diuretic-induced losses 3

Occult Third-Space Fluid Accumulation

• Fluid may be redistributing into third spaces (pleural effusions, ascites, peripheral edema in dependent areas) rather than being mobilized 1, 2
• This represents "silent overhydration" where patients have fluid excess without gross clinical evidence of volume expansion 1
• Jugular venous distention is the most important examination for volume status and should be assessed systematically 1

Inadequate Diuretic Dosing

• The current furosemide dose may be insufficient for her degree of heart failure and renal perfusion 1
• As heart failure advances, declining renal perfusion limits kidney response to diuretics, requiring progressive dose increments 1
• A spot urine sodium <50-70 mEq/L at 2 hours after loop diuretic administration indicates insufficient diuretic response 1

Immediate Diagnostic Steps

Verify Measurements

• Weigh patient at same time daily, same scale, after voiding, in similar clothing 1, 2
• Implement meticulous fluid intake/output documentation by trained staff 1
• Account for all fluid sources including IV medications, oral medications with water, and food moisture content 1

Assess Volume Status Clinically

• Measure jugular venous pressure systematically - this is the single most important examination 1
• Examine for peripheral edema, ascites, pleural effusions 1, 2
• Check orthostatic blood pressures to distinguish true hypovolemia from congestion with fluid redistribution 2

Laboratory Assessment

• Obtain spot urine sodium 2 hours after morning furosemide dose - values <50-70 mEq/L indicate diuretic resistance 1
• Check serum electrolytes, BUN, creatinine to assess for hyponatremia, hypokalemia, or metabolic alkalosis contributing to resistance 1, 3
• Monitor daily during active management 2

Management Algorithm

If Diuretic Resistance Confirmed

• Increase furosemide dose progressively - the initial dose should equal or exceed 2-2.5 times the current daily dose 4, 5
• Target urine output >100-150 mL/hour in first 6 hours or 3-5 L in 24 hours 5
• Add a thiazide-like diuretic (metolazone) for sequential nephron blockade if inadequate response to increased loop diuretic alone 1, 4

If Severe Resistance Persists

• Consider hospitalization for IV diuretic therapy with possible addition of low-dose dopamine or dobutamine to enhance renal perfusion 1
• Ultrafiltration or hemofiltration may be needed for diuretic-resistant fluid retention 1

Optimize Sodium Restriction

• Restrict dietary sodium to ≤2 grams daily - this greatly assists maintenance of volume balance 1
• Excessive sodium intake can exceed acute diuretic-induced salt loss, perpetuating resistance 3

Critical Pitfalls to Avoid

• Do not assume positive fluid balance is accurate without verifying measurement technique - this is the most common error 1
• Do not delay diuretic escalation due to mild azotemia - small elevations in BUN/creatinine should not minimize therapy intensity if renal function stabilizes 1
• Do not discharge until euvolemia is achieved and stable diuretic regimen established - unresolved edema increases early readmission risk 1, 6
• Do not overlook non-cardiac causes - hypothyroidism, hypoalbuminemia, venous insufficiency, or medication effects (NSAIDs, calcium channel blockers) can contribute to fluid retention 1