What is a suitable starting rate for hypertonic saline in a critically ill adult patient with impaired renal function undergoing Continuous Renal Replacement Therapy (CRRT) to maintain permissive hypernatremia?

Starting Rate for Hypertonic Saline During CRRT to Maintain Permissive Hypernatremia

Begin with 3% hypertonic saline at an initial rate calculated as: body weight (kg) × 0.5 mEq/L/hr ÷ 0.5 = approximately 1 mL/kg/hr, then titrate using a sliding-scale protocol to maintain serum sodium in the target range of 145-155 mEq/L.

Rationale for Initial Dosing

• The starting rate of approximately 1 mL/kg/hr of 3% saline provides a controlled sodium delivery of roughly 0.5 mEq/L/hr, which aligns with safe correction rates while accounting for concurrent sodium removal by CRRT 1, 2

• This calculation is derived from the formula: infusion rate (mL/kg/hr) = body weight (kg) × desired rate of sodium increase (mEq/L/hr), which has been validated for hypertonic saline administration 2

• A sliding-scale protocol allows for systematic adjustments based on measured sodium levels every 2-4 hours, minimizing both undershoot and overshoot 1

CRRT-Specific Considerations

• Account for dialysate sodium concentration: Standard CRRT dialysate contains 140 mEq/L sodium, which will continuously remove sodium and oppose your hypertonic saline infusion 3

• Calculate net sodium balance: Your hypertonic saline delivery must exceed the sodium removal by CRRT to achieve net positive sodium balance and maintain hypernatremia

• Adjust for effluent rate: Higher CRRT effluent rates (20-25 mL/kg/hr is typical) will require proportionally higher hypertonic saline rates to maintain target sodium 3

Practical Implementation Algorithm

Step 1: Calculate baseline requirements

• For a 70 kg patient: Start 3% saline at 70 mL/hr (1 mL/kg/hr)
• This delivers approximately 35 mEq sodium/hr (3% saline = 513 mEq/L)

Step 2: Adjust for CRRT losses

• If CRRT effluent is 2000 mL/hr (approximately 28 mL/kg/hr) with dialysate sodium 140 mEq/L, you're removing approximately 280 mEq sodium/hr
• Net sodium balance = 35 mEq/hr (delivered) - 280 mEq/hr (removed) = -245 mEq/hr deficit
• This means you need to increase the 3% saline rate substantially or use higher concentration saline

Step 3: Consider using higher concentration saline

• 23.5% hypertonic saline (4000 mEq/L) at 15-20 mL/hr provides 60-80 mEq/hr with minimal volume 3, 1
• This is more practical during CRRT as it delivers concentrated sodium without excessive fluid administration

Step 4: Implement sliding-scale adjustments

• Check sodium every 2-4 hours initially 1
• If sodium <145 mEq/L: increase rate by 25-50%
• If sodium 145-155 mEq/L (goal range): continue current rate
• If sodium >155 mEq/L: decrease rate by 25-50% or hold temporarily 1

Critical Safety Parameters

• Maximum correction rate: Do not exceed 0.5 mEq/L/hr increase in serum sodium, though evidence suggests faster rates may be safe in critically ill patients 4

• Osmolality monitoring: Ensure osmolality changes do not exceed 3 mOsm/kg/hr to prevent neurologic complications 5, 6

• Avoid hyperchloremic acidosis: Monitor chloride levels and acid-base status, as both hypertonic saline and CRRT can contribute to hyperchloremia 7

• Volume considerations: 3% saline delivers significant volume; consider 23.5% saline if fluid restriction is needed 3

Common Pitfalls to Avoid

• Underestimating CRRT sodium removal: Failing to account for continuous sodium losses will result in inability to maintain hypernatremia despite hypertonic saline infusion 3

• Using standard protocols without CRRT adjustment: Protocols designed for patients without CRRT will be inadequate when concurrent renal replacement is removing sodium 1

• Inadequate monitoring frequency: Check sodium every 2 hours initially until stable, then every 4 hours 1

• Ignoring chloride accumulation: Both 3% saline and standard CRRT dialysate are chloride-rich; monitor for hyperchloremic metabolic acidosis and consider bicarbonate-buffered dialysate if acidosis develops 7

Monitoring Requirements

• Serum sodium every 2-4 hours until stable in target range 1
• Serum osmolality every 6-8 hours 5, 6
• Chloride and acid-base status every 6-12 hours 7
• Neurologic examination for signs of osmotic complications 1, 4
• Fluid balance accounting for both hypertonic saline input and CRRT effluent 3

Alternative Approach for Severe Cases

• If standard 3% saline at practical infusion rates cannot overcome CRRT sodium losses, consider 23.5% saline at 10-30 mL/hr, which delivers 400-1200 mEq sodium/hr in minimal volume 3

• This concentrated approach is particularly useful when permissive hypernatremia is being used for cerebral edema management in traumatic brain injury patients on CRRT 3