Infusion osmotic value

As a measure of the tonicity of blood one can calculate with the osmotic value because active substances and additives cannot pass the membrane of the erythrocyte (see Sect. 18.5.2). The osmotic value of blood is around 290 mOsm/kg. Some parenteral fluids however contain substances that can pass the membrane fast ethanol, glycerol, urea. Hyperosmotic solutions of these substances may cause haemolysis so they are hypotonic. The iso-osmotic concentration of ethanol is for example 1.39 % m/m. Ethanol 5 % v/v infusion fluid is therefore hyperosmotic but appears to be practically isotonic. 

The alcoholic nitroglycerin solution 10 mg/g contains 90 % / ethanol (= 86.5 % m/m). The nitroglycerin infusion concentrate thus contains 86.5 mg ethanol per ml. It has an osmotic value of 1,800 mosmol/1 and is thus hyper osmotic, but at the same time hypotonic because ethanol can pass the erythrocyte membrane freely. This solution can only be administered when it is mixed with NaCl 0,9 % or glucose 5 %. 

Note For concentrated solutions, the calculated values of mOsm/L may not be accurate because of factors such as solvation and interionic forces which influence the osmotic pressure. Thus, results from the above calculations should be referred to as theoretical or approximate osmolarities. However, since most intravenous infusions are dilute solutions, results obtained from the above calculations are accurate enough to be clinically meaningful. 

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