2.5 Fluid Balance Charts & Cumulative Intake/Output

2.5 Fluid Balance Charts & Cumulative Intake/Output

Accurate fluid balance monitoring is a vital nursing intervention required across acute and subacute UK healthcare settings. The National Institute for Health and Care Excellence (NICE Clinical Guideline CG174) and NMC standards emphasize that inadequate fluid monitoring contributes significantly to avoidable patient morbidity, acute kidney injury (AKI), and circulatory overload.

A Fluid Balance Chart is a 24-hour legal document used to track all fluid entering and leaving a patient's body. The ultimate clinical goal is to calculate the 24-hour cumulative net fluid balance and identify fluid volume status early.

Categorising Fluid Intake & Output

To ensure complete charting, nurses must account for every clinical source of fluid intake and output.

Components of Fluid Intake

• Oral Intake: Water, tea, juice, soup, ice cream (measured as liquid volume once melted), and oral liquid medications.
• Intravenous (IV) Infusions: Continuous crystalloid/colloid bags, blood product transfusions, total parenteral nutrition (TPN), and intravenous boluses.
• IV Drug Flushes & Diluents: Intermittent antibiotic mini-bags (e.g., 50 mL or 100 mL infusions) and saline flush volumes (e.g., 10 mL post-medication flushes).
• Enteral Intake: Nasogastric (NG), nasojejunal (NJ), or Percutaneous Endoscopic Gastrostomy (PEG) tube feeds, including prescribed water flushes before and after feed administration.

Components of Fluid Output

• Urinary Output: Catheterized urine volume, voided urine measured via urinal/bedpan, or diaper/pad weight (1 gram increase = 1 mL fluid).
• Gastrointestinal Output: Vomitus, nasogastric aspirate, stoma fluid (e.g., high-output ileostomy), and liquid faeces.
• Surgical & Wound Drains: Vacuum drain contents (e.g., Redivac, Jackson-Pratt), chest drain exudate, and wound aspirates.
• Insensible Losses: Unmeasurable physiological fluid loss occurring through skin evaporation (~500 mL/day) and exhalation from the respiratory tract (~400 mL/day). Net insensible loss is generally estimated at 500–1,000 mL/24hr, increasing with fever, hyperventilation, or extensive burns.

Calculating 24-Hour Cumulative Net Fluid Balance

At the end of each 24-hour chart period (typically 06:00 or 08:00 in NHS wards), the nurse totals the cumulative intake and cumulative output to calculate net fluid balance.

$\text{Net 24-Hour Balance (mL)} = \text{Total 24-Hour Intake (mL)} - \text{Total 24-Hour Output (mL)}$

Clinical Interpretation of Fluid Balance Results

1. Positive Fluid Balance (Intake > Output)

Definition: The volume of fluid administered exceeds the volume excreted (e.g., Intake 3,500 mL − Output 2,000 mL = +1,500 mL). Clinical Risk: Hypervolaemia and fluid retention. Key Causes: Heart failure, renal failure, liver cirrhosis, excessive IV fluid administration, SIADH. Clinical Signs: Peripheral pitting oedema, sacral oedema, pulmonary crackles (crepitations), dyspnoea, tachypnoea, raised Jugular Venous Pressure (JVP), hypertension, and sudden weight gain.

2. Negative Fluid Balance (Output > Intake)

Definition: The volume of fluid lost exceeds the volume intake (e.g., Intake 1,200 mL − Output 2,400 mL = −1,200 mL). Clinical Risk: Hypovolaemia, dehydration, and pre-renal acute kidney injury. Key Causes: Severe vomiting, profuse diarrhoea, excessive diuresis, haemorrhage, high-output stomas, severe fever/sweating, inadequate oral fluid provision. Clinical Signs: Tachycardia, hypotension, dry mucous membranes, prolonged capillary refill time (>2 seconds), poor skin turgor, sunken eyes, dark concentrated urine, thirst, and confusion.

Oliguria, Anuria & Clinical Escalation Protocols

Monitoring urine output is the primary non-invasive clinical indicator of renal perfusion and end-organ oxygenation.

Critical Renal Output Thresholds

$\text{Hourly Urine Output Rate (mL/kg/hr)} = \frac{\text{Urine Volume in 1 Hour (mL)}}{\text{Patient Weight (kg)}}$

• Normal Adult Urine Output: $\ge 0.5\text{ mL/kg/hr}$ (or at least 30–35 mL/hr in an average adult).
• Oliguria: Urine output of < 0.5 mL/kg/hr for 2 consecutive hours (or < 400 mL in 24 hours for adults).
• Anuria: Complete absence of urine output or < 50 mL in 24 hours (a medical emergency requiring immediate senior intervention).

Step-by-Step Worked Fluid Balance Scenario

Worked Example 2.5.1: Comprehensive NHS Chart Review

Patient Profile: A 70 kg male patient recovering from abdominal surgery. 24-Hour Intake Recorded:

• Oral water/tea: 600 mL
• IV 0.9% NaCl infusion: 1,500 mL
• IV Antibiotics (2 × 100 mL mini-bags + flushes): 250 mL
• NG tube water flushes: 150 mL
• Total Intake: 600 + 1500 + 250 + 150 = 2,500 mL

24-Hour Output Recorded:

• Catheterized urine volume: 560 mL
• Redivac wound drain output: 240 mL
• NG aspirate: 200 mL
• Total Output: 560 + 240 + 200 = 1,000 mL

Step 1 (Calculate Net Balance):$2500\text{ mL (Intake)} - 1000\text{ mL (Output)} = \mathbf{+1,500\text{ mL (Positive Balance)}}$

Step 2 (Calculate Hourly Urine Output per kg):\text{Average Hourly Urine Output} = \frac{560\text{ mL}}{24\text{ hours}} = 23.33\text{ mL/hr}$$$$\text{Hourly Rate per kg} = \frac{23.33\text{ mL/hr}}{70\text{ kg}} = \mathbf{0.33\text{ mL/kg/hr}}

Clinical Interpretation & Nursing Action: Even though the patient has a overall positive fluid balance of +1,500 mL, his urine output of 0.33 mL/kg/hr is below the critical 0.5 mL/kg/hr threshold, meeting the clinical definition of oliguria. Fluid is sequestrating in third spaces or wound areas rather than perfusing the kidneys. The nurse must calculate NEWS2, perform a clinical cardiovascular assessment, and escalate immediately to the medical team for fluid responsiveness evaluation.