Normal saline is one of the most ubiquitous medications one can find in medicine. 1 liter of solution contains 154 mEq of sodium and 154 mEq of chloride with a pH ~ 5.5. The fact that the solution is acidic is actually UNRELATED to how it CAUSES non-anion gap acidosis in large volumes.
How does a simple salt solution have a pH of 5.5 instead of being totally neutral at 7? Atmospheric carbon dioxide (CO2) dissolves into solution producing carbonic acid (H2CO3) which readily splits into bicarbonate (HCO3–), carbonate (CO3-2), and hydrogen ions (H+). Interestingly, the concentrations of hydrogen and bicarb are roughly equal and the equilibrium H2CO3 ⇆ HCO3– + H+ is actually pushed forward due to other ions stabilizing bicarbonate and hydrogen.
In solutions like water where hydrogen bonding occurs in a vast network between adjacent molecules, hydrogen ions (protons) can move freely between neighboring molecules. This movement is disrupted in saline theoretically altering the pH as well. Furthermore, degradation products of PVC packaging may play a role in dropping the pH.
So how does saline create a non-gap acidosis? To keep things simple, it acutely “dilutes” out bicarbonate without really changing the pCO2. Your body wants to maintain electroneutrality. By giving such a large chloride load (154 mEq/L) relative to your normal chloride concentration (~100 mEq/L), something else that is negatively charged is reduced – in this case, bicarbonate.
Fun fact – the word “normal” in “normal saline” is actually the result of a miscalculation from back in the 1800s when red blood cell lysis experiments overestimated human plasma tonicity to be 0.9% (in reality, we’re closer to 0.6%).
When do I use saline in my ICU/OR practices? Unless the patient is hyponatremic AND hypochloremic, I can’t think of an example. Whew, that was easy.
Drop me a comment below with questions! 🙂