IV fluids and electrolytes

The five Rs of fluid prescribing

Resuscitation, Routine maintenance, Replacement, Redistribution and Reassessment.

Resuscitation

• Assess heart rate, BP, capillary refill, urine output, lactate, mental state. Lactate rises in shock because tissue hypoxia forces anaerobic glycolysis, turning pyruvate into lactate.
• Give a balanced crystalloid (Hartmann's, Plasma-Lyte) 500 mL over under 15 minutes (250 mL in heart failure or the frail), reassess, repeat to about 2 L, then seek senior help.
• Hartmann's does not worsen lactic acidosis in a perfused patient: its lactate is taken up by the liver and converted to bicarbonate, so it is an alkalinising fluid. It contains a little potassium but is usually safe in mild hyperkalaemia.
• Large volumes of 0.9% saline (154 mmol/L chloride) cause hyperchloraemic normal-anion-gap acidosis: the chloride load lowers bicarbonate (strong ion effect). Saline remains the right fluid for vomiting with hypochloraemic alkalosis.
• Starch colloids (hydroxyethyl starch) increase acute kidney injury and death in sepsis and are no longer used; dextrose is never a resuscitation fluid.
• Haemorrhage: blood and products early, tranexamic acid, permissive hypotension in penetrating trauma (not head injury), stop the bleeding.

• Hypotension appears only in class III.

Maintenance

• Adults: 25–30 mL/kg/day water, about 1 mmol/kg/day sodium, potassium and chloride, 50–100 g glucose.
• Children (Holliday–Segar): 100 mL/kg/day for the first 10 kg, 50 mL/kg/day for the next 10 kg, 20 mL/kg/day beyond. A 22 kg child needs 1,000 + 500 + 40 = 1,540 mL/day. Use isotonic fluid (0.9% saline with 5% glucose).
• Why hypotonic fluids cause hyponatraemia: pain, nausea, stress and surgery trigger non-osmotic ADH release, so the kidney retains free water; children and post-operative patients have died from it. Headache and vomiting after surgery in a child: check the sodium.
• Fever raises insensible losses from skin and lungs (add about 10–15% per °C); burns and open abdomens add large evaporative losses.

Replacement and redistribution

• Acute pancreatitis needs large early volumes because inflamed tissue and bowel leak fluid into the retroperitoneum and gut lumen; a rising haematocrit and urea mean under-resuscitation. Burn oedema comes from capillary leak with loss of albumin.
• Large-volume paracentesis (over 5 L, here 8 L) in cirrhosis: give IV albumin (about 8 g per litre removed) to prevent post-paracentesis circulatory dysfunction and hepatorenal AKI.

Post-operative oliguria

• Urine under 0.5 mL/kg/h. First check the catheter is not blocked (flush it). If it flushes and the patient is tachycardic with a low-normal BP, the cause is hypovolaemia (prerenal): give a fluid challenge. Never give furosemide to a dry, oliguric patient.

Electrolytes

• Potassium: IV no faster than 10 mmol/h peripherally (20 mmol/h or more centrally with ECG), concentration about 40 mmol/L peripherally, never as a bolus.
• Correct magnesium first: low magnesium lets potassium leak through renal ROMK channels, so hypokalaemia will not correct until magnesium is replaced. Both prolong ileus.
• Hypocalcaemia (after thyroid or parathyroid surgery, massive transfusion, pancreatitis): symptomatic, 10 mL of 10% calcium gluconate IV over 10 minutes with ECG monitoring, then infusion.
• Hyperkalaemia: calcium gluconate stabilises the myocardium; insulin (10 units) with 50 mL of 50% dextrose drives potassium into cells by stimulating the Na⁺/K⁺-ATPase (salbutamol does the same); then remove potassium (dialysis, binders). Monitor glucose for hours, especially in renal failure.
• Hyponatraemia: correct by no more than 8–10 mmol/L in 24 hours. Thiazide-induced hypovolaemic hyponatraemia (sodium 119, low osmolality, clinically dry): stop the thiazide and give 0.9% saline. Hypertonic saline only for severe symptoms. SIADH: fluid restriction.
• Chyle leak after neck dissection (usually left, thoracic duct): low-fat diet with medium-chain triglycerides, which are absorbed straight into the portal blood rather than lymph.

Diuretics: where they act

• Mannitol and the brain: it raises plasma osmolality, drawing water out of brain tissue across an intact blood–brain barrier (0.25–1 g/kg). Where the barrier is broken it can leak in and worsen local oedema.
• Mannitol in heart failure: before it causes diuresis it pulls water from cells into the blood, expanding circulating volume, which can precipitate pulmonary oedema; it is also contraindicated in anuria.
• Acute pulmonary oedema with high BP and hypoxia: sit up, oxygen, IV furosemide and a nitrate (venodilator), plus CPAP if needed.
• Potassium-sparing diuretics with ACE inhibitors, or in diabetics with type 4 renal tubular acidosis, cause dangerous hyperkalaemia.

Nutrition in surgical patients

• Screen everyone (MUST: BMI, weight loss, no intake over 5 days). Severely malnourished patients benefit from 7–14 days of feeding before major surgery.
• Enteral first: it keeps the gut mucosa intact, reduces bacterial translocation and infection, and is cheaper. Routes: oral supplements, nasogastric (check aspirate pH 5.5 or less, otherwise chest X-ray), nasojejunal, PEG, feeding jejunostomy.
• Parenteral nutrition when the gut cannot be used for over 5–7 days: prolonged ileus, high-output fistula, short bowel, obstruction, severe pancreatitis intolerant of enteral feeds. Give through a dedicated central line or PICC. About 25–30 kcal/kg/day and 1–1.5 g/kg protein.

• High-output stoma (over 1.5–2 L/day): oral rehydration solution with sodium 90 mmol/L or more, restrict hypotonic drinks, loperamide, PPI.

Remember Give the fluid that matches what was lost, check the catheter before blaming the kidney, and never give a diuretic to a dry patient.

Exam trap Large volumes of saline cause hyperchloraemic acidosis; Hartmann's lactate becomes bicarbonate in the liver.

Exam trap Mannitol expands blood volume before it causes diuresis, so avoid it in heart failure.

How FCPS asks about this

• Loop diuretics: Na⁺/K⁺/2Cl⁻ co-transporter; ototoxic with high doses and gentamicin.
• Thiazides: reduce lithium excretion; cause hyponatraemia, treated with stopping the drug and giving saline.
• Spironolactone: gynaecomastia from anti-androgen action; first diuretic for cirrhotic ascites; amiloride blocks ENaC.
• Child of 22 kg: 1,540 mL/day by Holliday–Segar; hypotonic fluids risk hyponatraemia through non-osmotic ADH.
• Oliguria after surgery: check the catheter first, then treat hypovolaemia.
• Hyperkalaemia: insulin–dextrose moves potassium into cells via Na⁺/K⁺-ATPase; correct magnesium before potassium.
• TPN: hyperglycaemia commonest metabolic problem; cholestasis after weeks; excess glucose raises CO₂.