From the FDA Drug Label
Sodium Bicarbonate Injection, USP is a sterile, nonpyrogenic, hypertonic solution of sodium bicarbonate (NaHCO3) in water for injection Bicarbonate solutions are hypertonic and may produce an undesirable rise in plasma sodium concentration
The solution of bicarbonate itself is hypertonic. When mixed with half normal saline, the resulting solution will still be hypertonic because half normal saline is isotonic or slightly hypotonic and will not be enough to dilute the bicarbonate solution to make it isotonic. Therefore, bicarb with half normal saline is not isotonic 1 1.
From the Research
Bicarbonate with half normal saline is not isotonic. When sodium bicarbonate is added to half normal saline (0.45% NaCl), the resulting solution becomes hypertonic rather than isotonic. Half normal saline alone is hypotonic with an osmolarity of approximately 154 mOsm/L, while sodium bicarbonate solutions (typically 8.4% or 4.2%) are highly hypertonic. When combined, the sodium content from both components raises the overall tonicity above the physiological range of 275-295 mOsm/L. This hypertonic nature is important to consider clinically because administering hypertonic solutions can cause fluid shifts, potentially leading to cellular dehydration as water moves out of cells into the extracellular space following the osmotic gradient. For patients requiring both bicarbonate and fluid therapy, the tonicity should be carefully considered based on the patient's electrolyte status, acid-base balance, and volume needs. In situations requiring isotonic solutions, normal saline (0.9% NaCl) might be more appropriate as a base solution, though the addition of bicarbonate would still increase its tonicity.
According to the study by 2, each liter of half saline-bicarbonate, which is relatively isoosmotic to human plasma, was composed of 70 mEq bicarbonate, 77 mEq chloride, and 147 mEq sodium, indicating that the combination of half normal saline and bicarbonate results in a solution that is close to isotonic but still may have varying effects on tonicity depending on the exact composition and the patient's condition. However, the most recent and highest quality study, 3, suggests that the use of half saline plus 75 mEq/L sodium bicarbonate does not significantly differ from normal saline in preventing acute kidney injury, implying that the tonicity of the solution may not be the primary concern in this context, but rather the overall fluid and electrolyte management.
Key points to consider:
- The tonicity of the solution is crucial in clinical settings to avoid fluid shifts and cellular dehydration.
- Half normal saline alone is hypotonic, while sodium bicarbonate solutions are highly hypertonic.
- The combination of half normal saline and bicarbonate may result in a solution that is close to isotonic but can still cause fluid shifts.
- Normal saline might be a more appropriate base solution for patients requiring isotonic solutions, but the addition of bicarbonate would increase its tonicity.
- Recent studies, such as 3, suggest that the use of half saline plus sodium bicarbonate does not significantly differ from normal saline in preventing acute kidney injury, highlighting the importance of considering the overall fluid and electrolyte management rather than just the tonicity of the solution.