M5Fluids, responsiveness, vasopressors and haemodynamic targets

Clinical overview

Fluid is a drug: it requires an indication, type, dose or rate, target, reassessment and stop rule. The phrase “give fluids” is incomplete. In haemorrhage, blood and haemostatic products replace what is being lost while source control stops the loss. In septic vasodilatation, a limited amount of crystalloid may improve preload, but early vasopressor support is often needed. In pre-eclamptic pulmonary oedema or cardiogenic shock, more fluid may worsen respiratory failure. In DKA or hyperemesis, fluid and electrolytes are essential but must be coordinated with insulin, glucose and potassium.

Pregnancy does not make the circulation uniformly “underfilled”. Plasma volume is expanded, colloid oncotic pressure is lower, the pulmonary reserve is smaller and hypertensive disease or inflammation can increase permeability. The correct question is not “is she pregnant?” but “what is the haemodynamic phenotype, is stroke volume likely to rise with preload, and would that rise be beneficial?”

Core knowledge

Compartments and fluid composition

Crystalloids distribute through extracellular water; only a fraction remains intravascular after equilibration. Isotonic balanced crystalloids have electrolyte composition closer to plasma and reduce chloride load compared with large-volume 0.9% sodium chloride. Saline remains appropriate for selected indications, including some patients with brain injury, severe hypochloraemia or particular compatibility needs. Fluid choice follows the clinical problem rather than a slogan.

Synthetic starches and gelatins are not appropriate routine resuscitation fluids in maternal sepsis; SMFM recommends against them. Albumin is a blood-derived colloid with selected uses, not a default substitute for assessment or blood. Dextrose solutions supply water and carbohydrate after metabolism and are not volume-resuscitation fluids; they have specific roles in hypoglycaemia, DKA treatment and free-water replacement.

Every carrier volume counts. Antibiotics, magnesium, oxytocin, maintenance fluid, nutrition and blood-product flushes can create large unrecognised totals. Chart cumulative input, output, bleeding and weight, then interpret with congestion and perfusion.

The phases of fluid therapy

1. Rescue: small, rapid intervention for life-threatening hypoperfusion while the cause is addressed.
2. Optimisation: targeted additional volume only if it improves flow/perfusion and does not create unacceptable congestion.
3. Stabilisation: enough fluid/electrolyte to meet ongoing needs; discontinue resuscitation boluses.
4. Evacuation/de-resuscitation: remove accumulated excess through spontaneous diuresis, diuretics or kidney support when perfusion is stable.

Failing to move between phases produces the common ICU sequence of initial appropriate fluid followed by days of unnecessary positive balance.

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Figure M5.1 — Fluid therapy is a staged prescription: match the product to the deficit, count every carrier, transition from rescue through optimisation and stabilisation, then remove excess when perfusion is stable.