Normal saline, and why it is anything but "normal"

Normal saline, NaCl 0.9%

An eminent intensive care physician I have had the pleasure of orbiting has at one stage remarked that normal saline is the most dangerous drug in use by the ward staff. It is most certainly not a physiological resuscitation fluid.

Yes, many are confused by the fact that it has water in it. However, it contains ridiculous amounts of chloride and sodium. Nobody should be left for hours to marinade in this brine; least of all, an already acidotic patient on their way to the ICU.

If you were for whatever reason compelled to mix normal saline from sterile salt and sterile water…

...Or more likely, if some sort of sadistic viva asked of you, "how would you make normal saline", you would be forced to say that sodium chloride has a molar weight of 58.44 grams, and so 150mmol of it weighs 8.77 grams.

Thus, you would pull out your meth scales, and you would carefully weigh out 8.77g of this sterile sodium chloride, and you would mix it with the litre of sterile water. This (generously rounded up) would make about 0.9g per 100ml, or a 0.9% solution.

Difference between calculated and measured osmolality

The calculated osmolality of normal saline is 300, and yet its measured osmolality is 286, which resembles the measured plasma osmolality.

Why, you ask? Well.

The measurement of osmolality is performed with an osmometer, and it involves extrapolating osmolality on the basis of the freezing point of the liquid (see about the colligative properties of liquids). The freezing point of normal saline would suggest that its osmolality is actually 286.

Why do we call this horrible fluid "normal" saline?

Well. There is an excellent article detaling the history of this solution, co-authored by the great champion of fluid physiology D.N Lobo. I will not make an attempt to improve on the already excellent coverage of the topic which is afforded by this article. Instead, a brief summary and explation is being offered. Apparently, the whole concept of infusing electrolyte-laden water into patients arose during the great cholera epidemics of the 1830s. Authors published on the effects of resuscitation fluids which were composed of "two drachms of muriate, and two scruples of carbonate, of soda, to sixty ounces of water". Rather than go into details of the old apothecary system of weights, I will refer the reader to the same article, which contains within it a table detailing the electrolyte concentration of the early resuscitation solutions.

The term "normal saline" entered the popular medical vernacular after a 1888 Lancet paper lauded its effects in resuscitating a "prostrate and pulseless’’ patient. However the content of the fluid used in that case (or most subsequent cases) was totally unlike the normal content of extracellular fluid. The first description of a 0.9% NaCl solution as "isotonic" had come from Hartog Jacob Hamburger, who performed freezing point experiments and observed haemolysis, concluding that 0.9% NaCl was isotonic with mammalian body fluids. Lobo and co-authors demonstrate obvious disappointment: "The scientific evidence supporting the use of 0.9% saline in clinical practice seems to be based solely on this in vitro study".