Fluid balance

1️⃣ Introduction

Fluid balance assessment is one of the most fundamental skills in critical care medicine.It is often the most neglected part and widely misunderstood.

MYTH 1-assuming positive/negative balance just by 24 hour input/output balance 

MYTH2- Using  holiday-segar formula for calculating daily fluid reqirements 

2️⃣ Physiology of Fluid Compartments

Total Body Water (TBW)

• 60% body weight (adult male)
• 50–55% (female)
• ↓ in elderly, obesity

Compartments

Starling Forces & Fluid Shifts

Fluid movement depends on:

• Hydrostatic pressure
• Oncotic pressure
• Capillary permeability
• Lymphatic drainage

In critical illness, glycocalyx damage + capillary leak = third spacing.

3️⃣ What is Input–Output Balance?

Input–Output balance = Total fluid intake – Total fluid loss

It is assessed:

• Hourly in ICU
• Every 4–6 hours in wards
• Daily cumulative

4️⃣ Components of Input (Fluid Intake)

A. Intravenous Fluids

• Crystalloids (NS, RL, Plasma-Lyte)
• Colloids (Albumin)
• Blood products
• Drug infusions (vasopressors, antibiotics)
• TPN

 Often forgotten:
Drug diluents and line flushes add significant volume

B. Enteral Intake

• Oral fluids
• Enteral feeding (tube feeds)
• Oral medications in liquid form

C. Metabolic Water

Produced during oxidation (~300 mL/day)
Clinically negligible in ICU documentation

5️⃣ Components of Output (Fluid Loss)

A. Measurable (Obligatory)

B. Insensible Losses

Not directly measurable.

Increased In:

• Fever (↑ 10% per °C rise)
• Tachypnea
• Mechanical ventilation (dry gases)
• Burns
• Open abdomen

6️⃣ Methods of Monitoring I/O

1️⃣ Urine Output (UO)

Most important real-time marker.

Targets:

• ≥0.5 mL/kg/hr (general ICU)
• ≥1 mL/kg/hr (burns)
• ≥2 mL/kg/hr (rhabdomyolysis)

According to KDIGO AKI criteria, oliguria:

• <0.5 mL/kg/hr for 6 hours

2️⃣ Fluid Balance Charts

Hourly ICU chart

Daily cumulative balance must be calculated.

3️⃣ Body Weight

1 kg weight gain = ~1 liter fluid retention

Best indicator of cumulative fluid balance.

4️⃣ Central Venous Pressure (CVP)

Limited value in predicting fluid responsiveness.
No longer recommended as sole guide (modern critical care guidelines).

5️⃣ Dynamic Indices (Better Than Static)

• Pulse pressure variation (PPV)
• Stroke volume variation (SVV)
• Passive leg raise (PLR)
• IVC ultrasound

8️⃣ Positive vs Negative Fluid Balance

Positive Fluid Balance

Risks:

• Pulmonary edema
• ARDS
• Delayed weaning
• AKI
• Increased mortality (shown in septic shock, ARDS, trauma)

Excess >5–10% body weight → worse outcomes.

Negative Fluid Balance

Goal in recovery phase:

• Improve lung function
• Improve oxygenation
• Reduce ICU stay

FACTT trial showed conservative strategy improved ventilator-free days.

🔟 Complications of Improper I/O Monitoring

• Under-resuscitation → shock, AKI
• Over-resuscitation → pulmonary edema
• Abdominal compartment syndrome
• Delayed wound healing

Fluid Overload Definition

Fluid accumulation >10% of baseline body weight

Associated with:

• Increased ICU mortality
• Need for RRT

Fluid Stewardship (Modern Concept)

Just like antibiotic stewardship:

Ask daily:

• Does patient need more fluids?
• Can we remove fluids?