What is the most appropriate management for a patient with a history of Chronic Obstructive Pulmonary Disease (COPD) presenting with severe thirst, polyuria, hypernatremia, mild hyperglycemia, and orthostatic hypotension while on doxycycline (doxycycline)?

Management of Polyuria and Severe Thirst in COPD Patient on Doxycycline

The most appropriate management is C: Tell him to drink at least 4 litres of water per day. This patient has hypernatremic dehydration with orthostatic hypotension requiring aggressive free water replacement.

Clinical Presentation Analysis

This patient presents with classic signs of hypernatremic dehydration:

• Severe thirst and polyuria (cardinal symptoms of hypernatremia) 1
• Hypernatremia (sodium 145 mmol/L, at upper limit of normal) 1
• Orthostatic hypotension (BP drop of 20/10 mmHg indicating significant volume depletion) 1
• Mild hyperglycemia (7.2 mmol/L, which can contribute to osmotic diuresis) 2

The clinical picture suggests hypovolemic hypernatremia from inadequate free water intake relative to losses, not diabetes insipidus (which would present with more severe hypernatremia and inappropriately dilute urine) 3.

Why Oral Free Water Replacement is Correct

Aggressive oral hydration (≥4 liters/day) is the appropriate first-line treatment for this patient because:

• He is conscious, has intact thirst mechanism (complaining of severe thirst), and can drink orally 1
• Hypernatremia reflects an imbalance in water balance with increased loss of free water, requiring hypotonic fluid replacement 2
• The orthostatic hypotension indicates significant volume depletion requiring substantial fluid repletion 1
• Oral hydration is safer than IV therapy for chronic hypernatremia (likely >48 hours given the clinical presentation), as it allows gradual correction and reduces risk of overly rapid sodium correction 2

Why Other Options Are Incorrect

A. Stop Doxycycline - INCORRECT

• Doxycycline is not causing this presentation 4
• The patient needs antibiotic therapy for COPD exacerbation prevention, and tetracyclines (including doxycycline) are first-line choices for COPD patients 4
• Stopping appropriate antibiotic therapy would increase exacerbation risk without addressing the actual problem 4

B. Fluid Restriction - INCORRECT

• This is dangerous and contraindicated 1
• Fluid restriction is used for hyponatremia (low sodium), not hypernatremia 5, 6
• This patient needs MORE fluid, not less, given his hypernatremia and volume depletion 1
• Fluid restriction would worsen his hypernatremia and orthostatic hypotension 2

D. BD Vasopressin - INCORRECT

• Vasopressin (or desmopressin) is used for diabetes insipidus when there is inadequate ADH secretion 3
• This patient does not have diabetes insipidus—he has appropriate thirst response and likely adequate ADH function 3
• His polyuria is likely secondary to osmotic diuresis from mild hyperglycemia and/or inadequate fluid intake 1
• Giving vasopressin without diabetes insipidus could cause dangerous water retention and hyponatremia 3

E. High-Dose Bendroflumethiazide - INCORRECT

• This is extremely dangerous and contraindicated 4
• Thiazide diuretics would worsen volume depletion and hypernatremia by causing further sodium and water loss 4
• Diuretics in COPD patients should only be used cautiously when there is evidence of fluid overload (cor pulmonale with peripheral edema and elevated JVP), which this patient does not have 4
• The European Respiratory Society specifically warns that diuretics should be used carefully to avoid reducing cardiac output and renal perfusion and creating electrolyte imbalance 4

Management Algorithm

Immediate management:

• Instruct patient to drink at least 4 liters of water daily, spread throughout the day 1
• Monitor for improvement in thirst, urine output normalization, and resolution of orthostatic hypotension 1

Follow-up within 24-48 hours to assess:

• Resolution of symptoms (thirst, polyuria) 1
• Normalization of orthostatic vital signs 1
• Repeat sodium level to ensure gradual correction (should not decrease >8-10 mmol/L/day if chronic hypernatremia) 2

If patient cannot tolerate oral intake or worsens:

• Consider IV hypotonic fluids (0.45% saline or 5% dextrose in water) 1
• Avoid rapid correction to prevent cerebral edema 2

Critical Pitfalls to Avoid

• Never restrict fluids in hypernatremic patients—this is the opposite of correct management and can be life-threatening 1
• Do not use diuretics in volume-depleted patients—this will worsen hypernatremia and hypotension 4
• Do not give vasopressin without confirming diabetes insipidus—inappropriate use causes severe hyponatremia 3
• Monitor correction rate carefully—chronic hypernatremia (>48 hours) should not be corrected faster than 8-10 mmol/L/day to avoid cerebral edema 2