In a hypertensive adult undergoing surgery, which is more appropriate for pre‑operative and postoperative fluid management: isotonic normal saline (0.9% sodium chloride) or hypotonic half‑normal saline (0.45% sodium chloride)?

Fluid Management in Hypertensive Surgical Patients

Direct Recommendation

Use isotonic normal saline (0.9% NaCl) or balanced crystalloids for perioperative fluid management in hypertensive surgical patients; avoid hypotonic 0.45% saline entirely. Hypotonic solutions have no role in routine perioperative fluid management and risk causing hyponatremia and inadequate volume expansion 1.

Rationale and Evidence-Based Approach

Primary Fluid Choice: Balanced Crystalloids Preferred

Balanced crystalloids (such as lactated Ringer's or Plasma-Lyte) should be the first-line choice over 0.9% saline for most surgical patients, including those with hypertension 1. The evidence strongly supports this recommendation:

• Balanced crystalloids reduce major adverse kidney events compared to 0.9% saline, with the SMART trial (15,802 patients) demonstrating significantly lower composite kidney injury outcomes (14.3% vs 15.4%) 1

• Lower mortality and reduced need for renal replacement therapy were demonstrated in the SALT trial with balanced crystalloids 1

• 0.9% saline causes hyperchloremic metabolic acidosis, electrolyte derangements, decreased renal blood flow, increased vasopressor requirements, and acute kidney injury 1, 2

• Saline-induced acidosis impairs gastric blood flow, decreases gastric intramucosal pH, and may contribute to delayed gastrointestinal recovery 1

When 0.9% Saline Is Appropriate

The only specific exception where 0.9% saline is preferred is traumatic brain injury (TBI), where current data support its use over buffered solutions, though the mechanism (tonicity vs. salt load) remains unclear 1. For hypertensive surgical patients without brain injury, this exception does not apply.

Why Hypotonic 0.45% Saline Is Contraindicated

Hypotonic saline (0.45% NaCl) should never be used for perioperative resuscitation or maintenance in surgical patients for multiple critical reasons:

• Inadequate intravascular volume expansion: Hypotonic solutions distribute primarily to the intracellular space rather than maintaining intravascular volume 1

• Risk of hyponatremia: Particularly dangerous in the perioperative setting where antidiuretic hormone (ADH) is elevated due to surgical stress 1

• No evidence base: None of the major perioperative guidelines or high-quality studies support hypotonic saline use in adult surgical patients 1

• Maintenance fluid guidelines specify isotonic solutions: Even for postoperative maintenance (25-30 mL/kg/day), guidelines recommend isotonic fluids with appropriate electrolyte supplementation 1

Hypertension-Specific Considerations

Hypertension itself does not alter the choice between isotonic crystalloids; the key is avoiding fluid overload while maintaining adequate perfusion:

• Target near-zero fluid balance to minimize complications, as both fluid excess and deficit worsen outcomes 1

• Use vasopressors for epidural-induced hypotension rather than excessive fluid boluses, which is particularly relevant in hypertensive patients who may have underlying cardiovascular disease 1

• Monitor for fluid accumulation: Hypertensive patients may have reduced cardiac reserve, making them more susceptible to fluid overload complications 1

Practical Implementation Algorithm

Preoperative fluid management:

• Balanced crystalloid at maintenance rates (1-2 mL/kg/h) if NPO prolonged 1
• Avoid preoperative fluid loading unless specific indication exists 1

Intraoperative fluid management:

• First choice: Balanced crystalloid (lactated Ringer's or Plasma-Lyte) 1
• Alternative: 0.9% saline only if balanced crystalloids unavailable or contraindicated 1
• Never use: 0.45% saline for any indication 1
• Goal-directed fluid therapy using stroke volume monitoring when available for high-risk patients 1

Postoperative fluid management:

• Continue balanced crystalloids at maintenance rates (25-30 mL/kg/day) with potassium supplementation (up to 1 mmol/kg/day) 1
• Transition to oral intake as soon as tolerated, typically morning after surgery 1
• Discontinue IV fluids when adequate oral intake achieved 1

Common Pitfalls to Avoid

Do not use hypotonic saline based on outdated teaching that hypertensive patients need "less sodium" - this confuses chronic dietary sodium restriction with acute perioperative fluid management 1

Avoid treating epidural-induced hypotension with excessive crystalloid in hypertensive patients; use vasopressors (phenylephrine or norepinephrine) instead 1

Do not assume 0.9% saline is "safer" in hypertension - the chloride load and acidosis are harmful regardless of baseline blood pressure 1, 2

Recognize that "normal saline" is neither normal nor physiologic - it contains supraphysiologic chloride (154 mEq/L vs. plasma 100 mEq/L) and causes predictable metabolic derangements 1, 2

Summary of Fluid Tonicity

• Isotonic (0.9% saline, balanced crystalloids): Appropriate for perioperative use 1
• Hypotonic (0.45% saline): No role in adult surgical patients 1
• Hypertonic (3%, 7.5% saline): Limited to specific indications like TBI with elevated intracranial pressure, not routine perioperative care 1, 3