Clinical overview
Fluid management is one of the most consequential and most poorly executed decisions in operative obstetrics and gynaecology. The patient on your list is not a passive recipient of a standing maintenance order — she is a physiological system whose intravascular volume, electrolyte balance, oncotic pressure and tissue perfusion you are actively steering through a period of surgical stress, fasting, fluid shifts and, often, blood loss. Get it right and she wakes warm, perfused, lucid and passing urine. Get it wrong in either direction and the consequences are real: under-resuscitation drives hypovolaemic shock, acute kidney injury and lactic acidosis; over-resuscitation drives bowel oedema, ileus, anastomotic and wound-healing failure, pulmonary oedema and — in the pregnant or pre-eclamptic woman — sometimes fatal cardiogenic and permeability pulmonary oedema.
The verb in this objective is consider — you are being asked to reason from principles, not to recite a single recipe. There is no universal "10 mL/kg/h" that is safe for every patient. The same 2 litres of crystalloid that rescue a septic, ruptured ectopic will kill a severe pre-eclamptic with capillary leak. A registrar who understands the principles — what compartment is depleted, what fluid distributes where, what the heart and kidneys are telling you, and which patient phenotype is in front of you — will make safe individual decisions in theatre, on the post-operative ward and in the high-care unit. This chapter builds that reasoning. It pairs naturally with ERAS principles, Fluids and electrolytes in O&G, Shock management and Arterial blood gas.
Core knowledge
Figure F5.1 — Where the litre goes: total body water across the ICF/ECF/plasma/interstitial compartments, and how balanced crystalloid, 5% dextrose and colloid distribute — distribution predicts expansion, acidosis and oedema.
Body water compartments and where fluids go
Total body water is classically ~60% of body weight in an adult (lower in the obese and elderly, higher in neonates) — standard physiology teaching. Of that, roughly two-thirds is intracellular and one-third extracellular, and of the extracellular fluid about a quarter is intravascular plasma and three-quarters interstitial. This 1:3 plasma:interstitial split is the single most important number for fluid prescribing, because it predicts where an infused fluid ends up.
- 0.9% saline and balanced crystalloids (Ringer's lactate, Plasma-Lyte) distribute across the whole extracellular space. Only about a quarter to a fifth stays intravascular after equilibration — so to expand plasma volume by 1 unit you must give roughly 3–4 units of crystalloid (standard teaching). The rest becomes interstitial oedema.
- 5% dextrose is effectively free water once the glucose is metabolised; it distributes across total body water, so only ~1/12 stays intravascular. It is a maintenance/free-water fluid, never a resuscitation fluid.
- Colloids (albumin, gelatins, starches) were designed to stay intravascular by oncotic pull. In health that logic holds; in the capillary-leak states common in our patients (sepsis, pre-eclampsia, major surgery) the barrier is leaky and colloid advantage shrinks.
Composition matters: the chloride problem
A registrar must know the difference between 0.9% "normal" saline (Na⁺ 154, Cl⁻ 154 mmol/L — both supraphysiological) and balanced solutions (Ringer's lactate / Hartmann's: Na⁺ ~131, Cl⁻ ~111, with lactate as a bicarbonate precursor; Plasma-Lyte similar with acetate/gluconate). Large volumes of 0.9% saline produce a hyperchloraemic metabolic acidosis and are associated with more renal vasoconstriction. The pragmatic landmark trials in this space — SMART and SALT-ED (balanced crystalloids vs saline in critically ill and non-critically ill adults; standard critical-care evidence) — showed a small favourable signal for balanced solutions on a composite of death, new renal replacement and persistent renal dysfunction. The practical principle: default to a balanced crystalloid for resuscitation and replacement; reserve 0.9% saline for specific indications (hypochloraemic alkalosis from vomiting, or hyponatraemia where you want the higher sodium). The major exception to "balanced by default" is the head-injured or hyponatraemic patient where the slightly hypotonic Ringer's is undesirable.
Maintenance vs resuscitation vs replacement — three different jobs
These are conceptually distinct and must never be conflated:
| Job | Question it answers | Typical fluid | Rough volume |
|---|---|---|---|
| Maintenance | What does a fasting patient need to stay in balance? | balanced crystalloid ± dextrose/K⁺ | ~25–30 mL/kg/day water, ~1 mmol/kg/day Na⁺/K⁺/Cl⁻, ~50–100 g/day glucose (standard teaching, per NICE IV-fluids principles) |
| Replacement | What ongoing abnormal losses must I match? | match the composition of the loss (e.g. balanced crystalloid for GI/third-space) | volume-for-volume |
| Resuscitation | Is there a perfusion deficit now? | balanced crystalloid bolus, then blood if bleeding | reassess after each ~250–500 mL bolus |