Selecting the right fluid requires understanding what is actually inside the bag. (Values approximate per Litre.)
| Fluid Type | Na+ (mmol) | Cl- (mmol) | K+ (mmol) | Buffer / Other | Calories / Osmolarity |
|---|---|---|---|---|---|
| 0.9% Normal Saline | 154 | 154 | 0 | None | 0 kcal / 308 mOsm/L |
| Ringer's Lactate | 130 | 109 | 4 | Lactate 28, Ca 1.4 | 0 kcal / 273 mOsm/L |
| Plasma-Lyte 148 | 140 | 98 | 5 | Acetate 27, Gluconate 23 | 0 kcal / 294 mOsm/L |
| 0.45% Saline + 5% Dextrose | 77 | 77 | 0 | Dextrose 50 g | 200 kcal / 406 mOsm/L |
| 5% Dextrose (D5W) | 0 | 0 | 0 | Dextrose 50 g | 200 kcal / 252 mOsm/L |
| 5% Dextrose in 0.9% NS (DNS) | 154 | 154 | 0 | Dextrose 50 g | 200 kcal / 560 mOsm/L |
Patients kept NPO for more than 12 hours require obligatory glucose to suppress lipolysis and protein catabolism. 50 to 100 grams of Dextrose per day is the minimum required (200 to 400 kcal). Pure crystalloids (NS, RL, Plasmalyte) contain ZERO calories. Standard maintenance should incorporate Dextrose-containing fluids (e.g., 0.45% Saline + 5% Dextrose). Each litre of 5% Dextrose provides 50 grams of glucose (200 kcal). Two litres of Dextrose-containing fluid meet the minimum obligatory glucose requirement.
Potassium: The Silent Killer in Maintenance FluidsThe kidney obligatorily excretes potassium even in hypokalaemic states. Daily maintenance requires roughly 1 mmol/kg/day (typically 40 to 80 mmol/day, capped at 80 mmol for routine maintenance). Adding 20 mmol KCl per litre is the standard safe concentration for peripheral IV infusion. At typical maintenance volumes (2 to 2.5 L), this provides 40 to 50 mmol, which may not fully meet the daily requirement. The shortfall should be covered by oral supplements or additional IV KCl if the patient remains NPO.
Critical Safety Rule: NEVER add KCl to IV fluids without a documented serum K+ value and confirmed urine output (> 0.5 mL/kg/hr). Do not add potassium if serum K+ is above 5.0 mmol/L. If K+ is above 5.5, treat the hyperkalaemia first.
Serum Sodium and Fluid TonicityThe choice between hypotonic (0.45% NS) and isotonic (0.9% NS) maintenance fluid depends critically on serum sodium. In hyponatraemia (Na+ < 130), giving hypotonic fluid worsens the dilutional state and can cause fatal cerebral oedema. These patients need isotonic fluids or fluid restriction depending on the cause. In hypernatraemia (Na+ > 145), the patient needs free water (D5W or 0.45% NS) to correct the deficit. This synthesiser adjusts its fluid recommendation based on the entered serum sodium.
Obesity and Fluid DosingAdipose tissue has lower metabolic water requirements than lean mass. Using actual body weight in obese patients (BMI > 30) leads to dangerous overestimation. The Adjusted Body Weight formula accounts for this: ABW = IBW + 0.4 × (Actual Weight - IBW). This synthesiser automatically detects obesity and uses ABW as the dosing weight when BMI exceeds 30.
Why Not Ringer's Lactate for Maintenance?RL is an excellent resuscitation fluid but a poor maintenance fluid. It contains 130 mmol/L Na+ (daily need is only ~1 mmol/kg), provides zero calories (starvation ketosis risk), and contains calcium which is incompatible with blood products and certain drugs in the same IV line. For maintenance, hypotonic dextrose-saline combinations are preferred.
Temperature: Fahrenheit vs Celsius at the Indian BedsideMost clinical thermometers in Indian hospitals display in Fahrenheit. Normal body temperature is 98.6°F (37°C). The fever threshold for fluid adjustment is 100.4°F (38°C). This tool accepts temperature in Fahrenheit and converts internally. A common resident error is entering a Celsius-ranged number (e.g., 38.5) in a Fahrenheit field, or vice versa. The tool validates the range (95 to 107°F) to prevent this.
AMA Style:
Umakanth S. Maintenance IV Fluid & Caloric Synthesiser. MEDiscuss. Published 2026. Accessed .
Vancouver Style:
Umakanth S. Maintenance IV Fluid & Caloric Synthesiser [Internet]. MEDiscuss.org; 2026 [cited ]. Available from:
