A 62-year-old male patient, admitted to the critical care unit following subarachnoid hemorrhage, is currently on assist-control ventilation with a central venous catheter in place for hemodynamic monitoring. During evening rounds, the critical care pharmacist notes new-onset hyponatremia. The patient has no signs of volume overload or depletion (no peripheral edema, ascites, orthostatic hypotension, or dry mucous membranes) and has been on a regular diet with normal salt intake (~2–3 g sodium daily). Renal, adrenal, and thyroid function tests are within normal limits, and he has not received any diuretics or other medications known to cause SIADH recently.

Relevant laboratory values:
– Serum sodium: 122 mEq/L (normal 135–145)
– Plasma osmolality: 255 mOsm/kg (normal 275–295)
– Urine osmolality: 410 mOsm/kg (normal 50–1200)
– Urine sodium: 75 mEq/L (normal 20–220)

Based on the clinical presentation and laboratory findings, which diagnosis is MOST consistent with the cause of his hyponatremia and should be prioritized in his management?

A 58-year-old man with chronic back pain on long-term opioids is in the ICU when routine labs reveal a serum sodium of 124 mEq/L. He is afebrile, heart rate 95 bpm, blood pressure 90/60 mmHg. Physical examination shows euvolemia (no edema or jugular venous distension). His home medications are methadone and sertraline. Laboratory studies demonstrate serum osmolality 262 mOsm/kg (normal 275–295), urine osmolality 540 mOsm/kg (normal 50–1200), and urine sodium 55 mEq/L. He has no history of heart failure, liver disease, or renal dysfunction. Which of the following is the most appropriate next diagnostic step to differentiate the cause of his hypotonic, euvolemic hyponatremia?

A 62-year-old woman (65 kg, 165 cm) is admitted for confusion and nausea. Vital signs are blood pressure 130/78 mm Hg, heart rate 82 bpm, respiratory rate 16/min, and temperature 36.8 °C. On exam she is euvolemic with no edema or orthostatic changes. Labs show serum sodium 122 mEq/L, serum osmolality 260 mOsm/kg, potassium 4.2 mEq/L, creatinine 0.8 mg/dL. Urine studies reveal sodium 75 mEq/L, potassium 50 mEq/L, osmolality 550 mOsm/kg, and total urine output 2000 mL over 24 hours. Other electrolytes and renal function are normal. What is the MOST appropriate interpretation of her electrolyte-free water clearance (EFWC) and its implication for fluid management in SIADH?

A 68-year-old man with small cell lung cancer is admitted to the ICU with community-acquired pneumonia. On hospital day 3, he develops nausea and mild confusion. Laboratory studies reveal serum sodium 122 mmol/L, serum osmolality 260 mOsm/kg, urine osmolality 550 mOsm/kg, and urine sodium 60 mmol/L. He is euvolemic on exam and has been on a 1,000 mL/day fluid restriction for 48 hours without improvement in serum sodium. Which of the following is the MOST appropriate next step in managing his SIADH?

A 70-kg patient in the ICU presents with new‐onset generalized seizure within the past 6 hours. Laboratory evaluation reveals a serum sodium of 112 mmol/L, low serum osmolality, and euvolemia on physical exam. The diagnosis of syndrome of inappropriate antidiuretic hormone secretion (SIADH) is confirmed. Given the severity and acute onset of symptoms, which of the following represents the MOST appropriate initial pharmacotherapy plan, considering both immediate symptom control and safe sodium correction?

A 65-year-old man (70 kg) is in the ICU with euvolemic hyponatremia (serum Na+ 125 mEq/L for 4 days) due to SIADH. He has no neurologic symptoms, vital signs are stable (BP 120/70 mm Hg, HR 80 bpm), and physical exam shows no edema or volume depletion. He has COPD and takes an SSRI. After 48 hours of strict fluid restriction (800 mL/day), serum sodium remains at 125 mEq/L. Laboratory studies: serum osmolality 260 mOsm/kg, urine osmolality 600 mOsm/kg, urine sodium 45 mEq/L. Which of the following is the most appropriate next step in management?

A 58-year-old man with subarachnoid hemorrhage develops hyponatremia. Laboratory studies show serum sodium 125 mEq/L, serum osmolality 260 mOsm/kg, urine osmolality 350 mOsm/kg, and urine sodium 50 mEq/L, consistent with SIADH. He is asymptomatic and has received strict fluid restriction (1 L/day) for 48 hours without improvement in serum sodium. Which of the following pharmacotherapy plans is most appropriate as second-line treatment?

A 65-year-old man (weight 70 kg) is admitted to the ICU with euvolemic hyponatremia secondary to syndrome of inappropriate antidiuretic hormone secretion (SIADH). Despite strict fluid restriction of 800 mL/day for 48 hours, his serum sodium remains 120 mmol/L and he has mild confusion. Laboratory evaluation shows low serum osmolality and inappropriately concentrated urine. Which is the MOST appropriate next step in management?

A 62-year-old man in the ICU with pneumonia has developed SIADH despite 48 hours of strict fluid restriction. His serum sodium is 122 mEq/L and he is confused with headache but has no seizures or focal neurologic deficits. Which initial pharmacologic therapy is most appropriate based on pharmacokinetics and clinical urgency?

A 62-year-old man in the ICU for community-acquired pneumonia is on mechanical ventilation. He has euvolemic hyponatremia (serum Na+ 122 mEq/L) with urine osmolality 650 mOsm/kg and urine Na+ 85 mEq/L. Thyroid-stimulating hormone and morning cortisol are normal. He is hemodynamically stable without neurologic symptoms. After 48 hours of fluid restriction (1 L/day), his serum Na+ rises only to 124 mEq/L, and he reports significant thirst limiting further restriction. As the ICU pharmacist, which pharmacologic therapy should be prioritized next, considering efficacy, safety, and cost?