Surgical Fluid and Electrolyte Management

The compartments, and where fluid goes

Prescribing fluid without knowing where it will end up is the source of most fluid-related harm. The distribution is entirely predictable from the tonicity of the fluid and the anatomy of the compartments.

• Total body water is roughly 60% of body weight in an adult man and 55% in a woman - about 42 litres in a 70 kg man. It is lower in the obese, because fat contains little water, and in the elderly.
• Intracellular fluid is about two thirds of total body water, roughly 28 litres. Its principal cation is potassium.
• Extracellular fluid is about one third, roughly 14 litres. Its principal cation is sodium. It divides into interstitial fluid (about 10.5 litres) and plasma (about 3.5 litres).
• Only about 3.5 litres of the 42 is inside blood vessels - which is why replacing a litre of blood loss requires considerably more than a litre of crystalloid.

The five Rs

NICE CG174 frames every fluid decision around five questions, and answering them in order prevents most errors.¹

The commonest failing on a surgical ward is a maintenance regimen prescribed on admission and continued for four days without reassessment, in a patient who has been eating and drinking since day two.

Assessing fluid status

No single sign is reliable; the assessment is a synthesis.

• History - thirst, reduced oral intake, vomiting, diarrhoea, stoma output, bleeding, fever, bowel preparation, duration of fasting, and diuretic use
• Observations and trends - heart rate, blood pressure including postural drop, respiratory rate, temperature and NEWS2 trend
• Signs of hypovolaemia - tachycardia, hypotension, cool peripheries, capillary refill over 2 seconds, dry mucous membranes, reduced skin turgor, a low JVP, sunken eyes, confusion and reduced urine output
• Signs of overload - a raised JVP, peripheral and sacral oedema, basal crackles, a third heart sound, ascites, and increasing oxygen requirement
• Urine output - the single most useful bedside measure of renal perfusion. Aim for at least 0.5 mL/kg/hour.
• A properly completed fluid balance chart and a daily weight - the most reliable and most neglected tools. A weight change of 1 kg is approximately 1 litre of fluid.
• Bloods - urea and creatinine (a disproportionately raised urea suggests hypovolaemia), sodium, potassium, chloride, bicarbonate, lactate, haematocrit and albumin
• Dynamic assessment - a passive leg raise or a 250 mL fluid challenge, observing the response in blood pressure, heart rate and urine output

Replacing gastrointestinal losses

Surgical patients lose fluid through routes that healthy people do not, and each has a characteristic electrolyte composition. Replacement should approximate that composition.

Third space losses deserve a note. Fluid sequestered into the bowel lumen and wall in obstruction, into the retroperitoneum in pancreatitis, or into the peritoneal cavity in peritonitis is physiologically outside the circulation, and the volumes can be several litres. It is invisible on the fluid balance chart, which is why patients with these conditions are so consistently under-resuscitated.

Electrolyte problems in surgical patients

Sodium

• Post-operative hyponatraemia is usually dilutional. Surgery, pain, nausea and opioids all stimulate antidiuretic hormone, so free water is retained. Giving hypotonic fluid such as 5% glucose or 0.18% saline in this setting causes the sodium to fall. Prevention is to avoid unnecessary hypotonic fluid and to reassess daily.
• Correct chronic hyponatraemia slowly - no more than 8 to 10 mmol/L in 24 hours - because rapid correction risks osmotic demyelination syndrome. Symptomatic or severe hyponatraemia with seizures or reduced consciousness requires hypertonic saline under senior supervision.
• Hypernatraemia is usually a water deficit - inadequate intake in a patient who cannot drink, insensible losses with fever, or osmotic diuresis. Correct with enteral water where possible, or 5% glucose, again slowly.
• Excess 0.9% saline produces a hyperchloraemic metabolic acidosis with a normal anion gap, and is associated with worse renal outcomes in large volumes. Balanced crystalloids such as Hartmann's or Plasma-Lyte are preferred for both resuscitation and replacement in most surgical patients.

Potassium and magnesium

• Hypokalaemia is very common, from vomiting, nasogastric aspiration, diarrhoea, stoma output, diuretics and the alkalosis that shifts potassium intracellularly. It perpetuates ileus and provokes arrhythmia.
• Intravenous potassium must not exceed 10 mmol/hour peripherally, and any faster rate requires cardiac monitoring and central access. Never give a concentrated potassium bolus.
• Correct magnesium alongside potassium - hypokalaemia is refractory to replacement while magnesium is low, and this is a common reason for repeated unsuccessful correction
• Hyperkalaemia in surgical patients arises from acute kidney injury, tissue necrosis, rhabdomyolysis, reperfusion after limb ischaemia, massive transfusion and potassium-sparing drugs. Treat with calcium gluconate for cardiac protection, insulin with glucose, salbutamol nebulisers, and treatment of the underlying cause.

Calcium and refeeding

• Hypocalcaemia after total thyroidectomy or parathyroidectomy - from inadvertent removal of or injury to the parathyroid glands, typically at 24 to 72 hours. Look for perioral paraesthesiae, carpopedal spasm, Chvostek and Trousseau signs, and a prolonged QT interval. Check calcium routinely after these operations.
• Refeeding syndrome - in a patient malnourished or with minimal intake for more than five days, restarting nutrition drives an insulin surge with intracellular shift of phosphate, potassium and magnesium, causing arrhythmia, cardiac failure, rhabdomyolysis and seizures. Identify patients at risk using NICE criteria, reintroduce nutrition slowly, give thiamine and vitamin B before feeding, and monitor electrolytes daily.²
• Hypophosphataemia more generally, which causes muscle weakness and impairs weaning from ventilation

Prescribing in practice

A worked maintenance prescription

For a 70 kg adult who is nil by mouth with no abnormal losses and normal renal function:

• Water: 25 to 30 mL/kg/day = 1750 to 2100 mL/24 hours
• Sodium, potassium and chloride: 1 mmol/kg/day = about 70 mmol of each
• Glucose: 50 to 100 g/day
• A common prescription meeting this is 1 L of 0.18% sodium chloride with 4% glucose plus 27 mmol KCl over 8 hours, three bags in 24 hours - though many units now prefer a lower-volume regimen using a balanced crystalloid with added potassium, alternating with glucose
• In the elderly, frail, malnourished, or in cardiac or renal impairment, prescribe 20 to 25 mL/kg/day to avoid overload
• Weight-based calculations use ideal body weight in obese patients, and total maintenance should rarely exceed about 3 litres per day

Points that prevent harm

• Write the indication on the chart - resuscitation, maintenance or replacement - so the next person knows why the fluid is running
• Prescribe for 24 hours at a time and review daily, rather than writing 72 hours of fluid on admission
• Stop intravenous fluids as soon as the patient is drinking, and encourage oral intake actively
• Include drain, stoma and nasogastric losses in the calculation, not just urine
• Weigh the patient daily where fluid balance is a clinical issue
• Check U&Es daily in anyone on intravenous fluids, and before adding potassium
• Do not use starch-based colloids, which are associated with increased mortality and renal injury and are no longer recommended. Gelatins have largely fallen out of favour, and crystalloids are the default.
• Albumin has a specific role in spontaneous bacterial peritonitis and in large-volume paracentesis in cirrhosis, not as a general resuscitation fluid

Red flags

Why this matters

Fluid prescribing is one of the few tasks performed almost exclusively by the most junior members of the team, and one of the few where a routine decision can cause serious harm. The NCEPOD report that prompted NICE CG174 found that a fifth of hospital patients on intravenous fluids suffered complications attributable to their prescription, in both directions - dehydration and acute kidney injury on the one hand, pulmonary oedema and hyponatraemia on the other.

Excess fluid is not benign. Perioperative fluid overload delays return of bowel function, contributes to anastomotic oedema and leak, impairs wound healing, prolongs ventilation and lengthens hospital stay. This is why enhanced recovery programmes emphasise goal-directed rather than liberal fluid therapy, and why avoiding prolonged preoperative fasting matters - a patient who has drunk clear fluids until two hours before theatre does not arrive depleted.

The transferable habit is simple: before writing any fluid, decide which of the five Rs you are addressing, state the indication on the chart, and set a time to reassess. That single discipline prevents most of the harm described above.