What is the significance of positive and negative balance in input and output (I&O) in terms of fluid balance and patient management?

Fluid Balance: Positive and Negative Balance in Patient Management

Definitions

Positive fluid balance occurs when fluid intake exceeds output, while negative fluid balance occurs when output exceeds intake. 1 In clinical practice, fluid balance is typically calculated as the difference between all fluid inputs (IV fluids, medications, blood products, oral intake) and all outputs (urine, drains, stool, insensible losses). 1, 2

• Positive balance is clinically defined as fluid accumulation ≥5-10% of body weight or a cumulative positive balance of >2-2.5 liters 3, 4
• Negative balance means net fluid removal, typically achieved through diuresis or renal replacement therapy 5
• Near-zero or even balance (0-5% body weight change) represents the optimal target state 1, 6

Clinical Significance for Morbidity and Mortality

Positive Fluid Balance: Major Risks

Patients managed with positive fluid balance (>2.5L excess) have a 59% increased risk of complications and 3.4-day longer hospital stays compared to those maintained at near-zero balance. 1

Specific complications of fluid overload include: 1, 7, 2

• Pulmonary complications: Impaired gas exchange, tissue hypoxia, increased ventilator dependence, and pulmonary edema 1, 2
• Renal dysfunction: Decreased renal blood flow, reduced glomerular filtration rate, and increased acute kidney injury risk (RR 1.98 for >2L positive balance) 1, 4
• Gastrointestinal complications: Splanchnic edema, ileus, delayed GI recovery, increased gut permeability, intestinal failure, and anastomotic dehiscence 1, 7, 2
• Abdominal compartment syndrome: From ascites and increased intra-abdominal pressure 1
• Cardiovascular effects: Compromised microvascular perfusion and increased arterio-venous shunting 1
• Wound healing: Poor wound healing from tissue edema 2

In critically ill patients, positive fluid balance on day 3 of ICU stay is an independent risk factor for 30-day mortality (OR 1.26 per liter, 95% CI 1.07-1.46). 5 Among trauma patients, each liter of positive fluid above zero balance incrementally increases AKI risk by 22% (RR 1.22,95% CI 1.11-1.34). 4

In ECMO patients, positive fluid balance by day 3 is associated with increased mortality. 1

Negative Fluid Balance: Nuanced Risks

While negative fluid balance achieved through deresuscitation on day 3 is associated with lower mortality in the short term, excessive negative balance carries its own risks. 5, 6

Complications of fluid deficit include: 1

• Cardiovascular: Decreased venous return, reduced cardiac output, and hypotension (especially problematic during anesthesia induction) 1
• Tissue perfusion: Diminished oxygen delivery and increased blood viscosity 1
• Pulmonary: Increased mucus viscosity leading to mucous plugs and atelectasis 1
• Gastrointestinal: Mucosal acidosis and poorer outcomes from inadequate resuscitation 1

Critically, patients with negative fluid balance compared to even balance show lower short-term mortality (HR 0.81,95% CI 0.68-0.96) but paradoxically higher long-term mortality at 1 year (HR 1.16-1.22). 6 This suggests that while aggressive deresuscitation may help acutely ill patients, excessive negative balance may harm long-term survival.

Optimal Management Strategy

The goal is to maintain near-zero or even fluid balance (0-5% body weight change), avoiding both extremes. 1, 6

Perioperative Context

Target a mildly positive balance of 1-2 liters by end of surgery, then transition immediately to minimizing IV fluids postoperatively. 7, 2 This approach protects kidney function better than stringently restrictive regimens, which significantly increase AKI rates. 7, 2

Critical Care Context

After initial resuscitation and hemodynamic stabilization, aim for daily negative fluid balance to remove accumulated fluid. 1, 5 Deresuscitation using diuretics or renal replacement therapy on day 3 of ICU stay is associated with improved outcomes. 5

Special Populations

In short bowel syndrome patients, maintain positive fluid and sodium balance to prevent dehydration, tiredness, and masked hypokalemia. 1 Gastrointestinal fluid balance should exceed 1.4 kg/day to avoid parenteral fluid dependence. 1

For micronutrients and minerals (zinc, magnesium, vitamins), positive balance is required to promote nitrogen retention and prevent deficiency states. 1

Monitoring Requirements

Essential monitoring includes: 1, 2

• Daily weights measured at the same time each day 1
• Accurate intake and output records (all IV fluids, medications, blood products, oral intake vs. urine, drains, stool) 1, 2
• Supine and standing vital signs 1
• Daily electrolytes, BUN, and creatinine during active fluid management 1
• Clinical assessment of volume status and perfusion 1

Critical Pitfalls to Avoid

Never administer large volumes of 0.9% saline as it causes hyperchloremic acidosis, renal vasoconstriction, decreased gastric blood flow, and impaired GI motility. 1, 7, 2 Use buffered crystalloids (Ringer's lactate, Plasmalyte) instead. 7, 2

Avoid fluid overload >2.5 kg perioperative weight gain, which dramatically increases complications. 7, 2 However, stringently restrictive regimens also increase AKI risk. 7, 2

Patients with heart failure, chronic kidney disease, and lung disease have significantly lower fluid tolerance and require more conservative strategies. 2

Do not assume patients are adequately deresuscitated based solely on symptom improvement—they often remain congested despite feeling better and may be discharged after losing only a few pounds. 1