Confusion at the End of a TURP
A 72-year-old man (82 kg) undergoes transurethral resection of the prostate under a single-shot spinal anesthetic. Resection lasts 78 minutes with 1.5% glycine irrigant hung 60 cm above the bladder and 14 L used. Twenty minutes before the end he becomes restless and then confused; blood pressure is 172/94 mmHg, heart rate 56 in sinus rhythm, SpO2 95% on room air, and there are bibasilar crackles. Serum sodium is 118 mmol/L (138 mmol/L preoperatively), serum osmolality 262 mOsm/kg, glucose 108 mg/dL, and hemoglobin 11.2 g/dL (13.4 g/dL preoperatively). Creatinine and blood urea nitrogen are unchanged from baseline. He is not known to have had a seizure and has no focal neurologic deficit.
1. Explain the mechanism linking the resection to the sodium, the osmolality, and the vital signs. Why is this not SIADH?
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Mechanism: open prostatic venous sinuses absorb large volumes of hypotonic irrigant. In this case the hemoglobin fall of 2.2 g/dL indicates roughly 2 to 3 L of absorption, which produces acute dilutional hyponatremia with volume expansion - the serum osmolality of 262 mOsm/kg is low, confirming true hypo-osmolality rather than pseudohyponatremia. Water moves into brain tissue down the osmotic gradient, producing cerebral edema, which explains the confusion and the hypertension with bradycardia (a Cushing-type response to raised intracranial pressure). Glycine itself is an inhibitory neurotransmitter, and its metabolism generates ammonia, so the neurologic picture can be more profound than the sodium value alone would predict.
Why not SIADH: SIADH is euvolemic, with inappropriately concentrated urine, normal or low-normal osmolality developing over hours to days, and no source of free water loading. Here there is a clear exogenous source, a rapid onset within the case, hypervolemia with crackles, and a drop in hemoglobin from absorbed fluid. Calling this SIADH would lead to fluid restriction, which is exactly wrong in a hypervolemic, hypo-osmolar patient who needs the sodium corrected and the free water removed.
2. Outline your immediate treatment and show the arithmetic for how much 3% saline would be needed to raise the sodium by 6 mmol/L. What correction limits apply?
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Immediate steps: stop the irrigation, give 100% oxygen, secure monitoring, protect the airway if consciousness is declining, and give hypertonic saline. Because he is symptomatic with acute hyponatremia, 3% saline 100 to 150 mL over 10 to 20 minutes is appropriate, repeated until symptoms improve, with a goal rise of 4 to 6 mmol/L in the first hour.
Arithmetic: total body water approximates 0.6 x 82 = 49 L. A 6 mmol/L rise requires 49 x 6 = 295 mmol of sodium. 3% saline contains about 513 mmol/L, so 295/513 = 0.58 L, roughly 575 mL. A useful bedside shortcut is that 3% saline at about 1 mL/kg/h raises serum sodium by approximately 1 mmol/L/h in a patient this size, so the formula and the shortcut agree reasonably.
Limits: do not exceed a rise of 8 to 10 mmol/L in 24 hours. Acute hyponatremia carries a lower osmotic demyelination risk than chronic, but the risk is not zero. Once the acute symptoms resolve, stop the hypertonic saline and consider a loop diuretic to remove the absorbed free water, with serial sodium measurements every 2 to 4 hours.
3. What would you have monitored intraoperatively to detect this earlier, and what would you change for a similar case next time?
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Earlier detection:
- Track irrigant use against bag weight or volume; 14 L for a 78-minute resection is a red flag in itself.
- Keep resections under about 60 minutes and use bipolar resection with continuous low-pressure irrigation where the equipment allows.
- Check serum sodium every 20 to 30 minutes once the resection passes 45 minutes or the irrigant volume exceeds 6 to 8 L.
- Watch for early surrogates: restlessness, nausea, a rising blood pressure with falling heart rate, and an unexplained fall in hemoglobin.
Systems changes: make sodium sampling and a stop-time trigger part of the room protocol, designate a person to call out irrigant volume, and rehearse hypertonic saline availability. The judgement lesson is that the spinal anesthetic masked the usual warning signs and the patient was alert until he was not; the value of the monitors lies in their trend, and none of them replaces a scheduled laboratory check when the surgical exposure is one that absorbs free water.