Following furosemide for chest pain with edema, a patient is suspected of SIADH. Which finding would support this diagnosis?

• A. Hypertension and weight gain
• B. General edema and pulse of 110 beats per minute
• C. Seizure disorder and serum sodium level of 118 mEq/L
• D. Minimal responsiveness to furosemide and small cell lung cancer

Answer: (D)

Syndrome of inappropriate antidiuretic hormone secretion (SIADH) causes dilutional hyponatremia from excess ADH, leading to a euvolemic state where the body retains water but shows little signs of volume overload. The hyponatremia in SIADH is often resistant to diuretics because the ADH-driven water reabsorption continues despite loop diuretics. A classic clue is an ectopic source of ADH, with small cell lung cancer being a well-known cause.

So, finding minimal improvement in hyponatremia with furosemide and having small cell lung cancer strongly supports SIADH, since the cancer provides the ADH source and the diuretic response is blunted by ongoing water reabsorption.