Postoperative Hyponatremia: Free Water, SIADH, and Why Normal Saline Is Not the Fix
A 70-year-old woman weighing 60 kg is on postoperative day 2 after hip repair. Admission sodium was 138 mmol/L. She has received 5% dextrose in water at 125 mL/h for the past 24 hours (3 L) and has been sipping water to relieve dry mouth. This morning she is drowsy and confused but has not seized. Serum sodium is 122 mmol/L, serum osmolality 258 mOsm/kg, urine output 50 mL/h with urine osmolality 600 mOsm/kg and urine sodium 40 mmol/L. Pain and nausea have been poorly controlled. The surgeon says, "Just give normal saline - it's isotonic." Estimated total body water is 0.5 × body weight.
1. Compute total body water, predict the sodium from the free water alone, and explain why the measured value is lower.
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Total body water: 0.5 × 60 kg = 30 L
Predicted sodium from 3 L of retained free water:
- Na ≈ (138 mmol/L × 30 L) ÷ 33 L = 4140 ÷ 33 ≈ 125.5 mmol/L
Why 5% dextrose acts as free water: once the glucose is metabolized, the bag leaves essentially pure water behind. That water distributes across all 30 L of body water, not just the plasma, so 3 L dilutes the whole compartment.
Why she is at 122, not 125.5: the prediction assumes all 3 L were retained and no additional water was taken. She is also drinking, and her urine osmolality of 600 mOsm/kg exceeds her serum osmolality of 258 - her kidneys are concentrating urine and cannot excrete free water. With continued intake and impaired excretion, net free water is higher than the infusion alone predicts. The picture (concentrated urine, urine Na 40, low serum osmolality, euvolemia after surgery, pain and nausea) fits SIADH, which is the reason the dilution progressed.
2. Evaluate the surgeon's plan quantitatively: calculate the sodium deficit to reach 130 mmol/L, and determine whether 0.9% saline will raise or worsen her sodium given a urine osmolality of 600 mOsm/kg.
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Sodium deficit to reach 130 mmol/L:
- Deficit = TBW × (target − current) = 30 L × (130 − 122) = 240 mmol of sodium
Will normal saline deliver it usefully? 0.9% saline has an osmolality of about 308 mOsm/L and 154 mmol/L of sodium. Her urine is 600 mOsm/kg - nearly twice the infusate. In this setting the kidney retains the infused salt in a small volume and excretes the water poorly; the net effect can be no improvement, or a further fall in sodium (the "desalination" phenomenon of SIADH). Isotonic saline is only reliably corrective when the infused fluid is more concentrated than the urine.
Judgment: the surgeon's reasoning ("it's isotonic") confuses isotonicity with the patient's plasma with effectiveness in a patient who cannot excrete water. Appropriate steps: stop the D5W, restrict fluid (roughly 800–1000 mL/day, below urine output plus insensible losses), treat pain and nausea, and use 3% saline (513 mmol/L) only for severe symptoms - 240 mmol ÷ 513 mmol/L ≈ 470 mL, which is a small volume precisely because it is so potent.
3. She is confused but not seizing, and her sodium has been low for at least a day. Set a safe correction target for the first 24 hours and explain the monitoring needed, including the risk of correcting too fast.
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Classify first: confusion without seizures, in a patient whose sodium fell over ≥24–48 hours, is chronic, moderately symptomatic hyponatremia - not the emergency that mandates immediate 3% boluses. (Seizure, obtundation, or coma would change this.)
Safe target: raise sodium by no more than 8 mmol/L in 24 hours - so from 122 toward 128–130, not to 138. Rate ≈ 0.5 mmol/L/h.
Why the limit exists: brain cells have already extruded osmolytes to avoid swelling in the hyponatremic state. Rapid correction reverses the gradient faster than the brain can re-accumulate solute, causing osmotic demyelination syndrome (pseudobulbar palsy, quadriparesis, locked-in state) that may appear days later and is largely irreversible. Risk is highest with chronic hyponatremia, low body weight, malnutrition, alcohol use, and liver disease - several of which apply to a 70-year-old postoperative patient.
Monitoring plan:
- Serum sodium every 4–6 hours during active correction, and again after any change in therapy.
- Strict intake/output with daily weight; a sudden diuresis can overcorrect within hours - if that happens, the fix is to re-lower the sodium (D5W with or without desmopressin), not to watch and hope.
- Neurologic checks each shift for confusion, dysarthria, dysphagia, or new weakness.
- Potassium and magnesium (correction drives K⁺ intracellularly), and glucose, since hyperglycemia shifts sodium.
- Treat the cause: analgesia, antiemetics, and removal of the free-water source; recheck the medication list for other contributors.