Nutrition, Fluids & Electrolytes

Body-Fluid Compartments and IV Fluids: Where Does the Bag Go?

Two patients get a litre of fluid, fast, into a vein. One gets 5% dextrose, the other 0.9% saline. An hour later almost none of the dextrose is left in the circulation — it has scattered through every cell in the body — while roughly a third of the saline is still holding up the blood pressure. Same volume, same vein, completely different destinations. Nobody chose that; the chemistry did. Prescribing fluid without knowing where it lands is like posting a letter with no address — and the physiology that writes the address is the most quietly important thing in acute medicine.

Prescribing IV Fluids: The Five Rs and the Daily Maintenance

It is the commonest prescription on a surgical ward and one of the most carelessly written. A bag of fluid is signed off in seconds, often copied from the last chart, rarely reconsidered on the ward round. Yet a litre of the wrong fluid at the wrong rate can drown a failing heart, tip the sodium off a cliff, or acidify the blood — real harm from something everyone treats as harmless salt water. Intravenous fluid is a drug: it has a dose, an indication, and side effects. Prescribing it well means answering five questions before you sign the bag.

Acid–Base Balance and the Anion Gap: Reading the Gas

The body defends its pH with almost religious devotion: every enzyme, every ion channel, every drug receptor is tuned to a narrow band around 7.4, and drifting far outside it is lethal within hours. A blood gas is the bedside window onto that defence — four numbers that tell you whether the lungs or the kidneys are failing, whether a poison is loose in the blood, and whether the patient in front of you is compensating or crashing. Learning to read it is less about arithmetic than about a way of thinking: pH first, then who caused it, then who is trying to fix it. And the moment you find a high anion gap, the question stops being "how acidotic?" and becomes "what unmeasured acid is in there?"

Assessing Fluid Status and Osmolality: The Prescriber's Compass

Every fluid prescription and every sodium result begins with a single question that the blood tests cannot answer for you: is this patient dry, wet, or just right? Get that wrong and the safest-looking bag of fluid becomes a poison — you drown the overloaded patient or leave the dehydrated one to spiral. Osmolality then tells you not how much water there is, but how the water is distributed and where it wants to move. Master volume assessment and osmolality first, and the frightening electrolyte number on the chart suddenly reads like a sentence you can finish.

Hyponatraemia: A Water Problem, Not a Salt Problem

It is the commonest electrolyte abnormality on the ward, and one of the few where the intuitive treatment can kill. The name says "low sodium," so the reflex is to reach for salt. But a low serum sodium almost never means the body is short of salt — it means there is too much water diluting the salt that is there. Get the reasoning backwards and you either flood a patient already drowning in water, or you correct them so fast that you destroy the brain you were trying to protect. This chapter is about thinking in the right currency: water and volume, not salt.