Is normal isotonic saline a 0.9% sodium chloride solution for a hyponatraemic patient (serum sodium 126 mmol/L, serum osmolality 259 mOsm/kg) on hydralazine?

Is Normal Saline 0.9% Isotonic?

Yes, normal saline is 0.9% sodium chloride solution, and it is isotonic with a sodium concentration of 154 mmol/L. 1

Definition and Composition

• Normal saline (0.9% NaCl) contains 154 mmol/L of sodium, which is equivalent to the sodium concentration found in colloid solutions like gelofusine, haemaccel, and 4.5% albumin. 1
• This solution is considered isotonic because its osmolality approximates that of plasma. 1

Application to Your Patient (Serum Sodium 126 mmol/L)

For your hyponatremic patient with serum sodium of 126 mmol/L and serum osmolality of 259 mOsm/kg, the management approach depends critically on volume status:

If Hypovolemic Hyponatremia (Most Likely Given Hydralazine Use)

Discontinue diuretics and administer normal saline (0.9% NaCl) for volume expansion. 1

• The 2021 Gut guidelines specifically recommend that hypovolaemic hyponatraemia during diuretic therapy should be managed by discontinuation of diuretics and expansion of plasma volume with normal saline. 1
• Hydralazine can cause postural hypotension and may contribute to effective hypovolemia. 2

Dosing Considerations

• Aim for 23-30 mL/kg/24h of isotonic saline to achieve safe and effective correction while minimizing overcorrection risk. 3
• An isotonic saline dose ≥30 mL/kg/24h carries a 16-fold increased odds ratio for overcorrection (≥8 mmol/L/24h). 3
• Do not exceed 12 mmol/L correction per 24 hours to avoid osmotic demyelination syndrome. 1

Important Caveats

• Avoid fluid restriction at this sodium level (126 mmol/L) unless the patient is clinically hypervolemic. 1
• Fluid restriction should be reserved only for those who are clinically hypervolemic with severe hyponatremia (<125 mmol/L). 1
• Monitor serum sodium every 2-4 hours initially during active correction to prevent overcorrection. 1

When Normal Saline May Worsen Hyponatremia

• In euvolemic SIADH patients with urine osmolality >530 mOsm/kg, isotonic saline can paradoxically worsen hyponatremia because the kidneys excrete the sodium while retaining free water. 4
• However, in patients with urine osmolality <530 mOsm/kg, isotonic saline typically improves serum sodium. 4

Hydralazine-Specific Considerations

• Hydralazine should be used with caution in patients with advanced renal damage and can affect renal hemodynamics. 2
• The drug may reduce pressor responses and cause postural hypotension, potentially contributing to hypovolemia. 2