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Saline (medicine)
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In medicine, saline (also saline solution) is a general term referring to a sterile solution of sodium chloride (table salt) in water. It is used for intravenous infusion, rinsing contact lenses, and nasal irrigation. Saline solutions are available in various formulations for different purposes. Salines are also used in cell biology, molecular biology and biochemistry experiments.
entrations vary from low to normal to high.
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Encyclopedia
In medicine, saline (also saline solution) is a general term referring to a sterile solution of sodium chloride (table salt) in water. It is used for intravenous infusion, rinsing contact lenses, and nasal irrigation. Saline solutions are available in various formulations for different purposes. Salines are also used in cell biology, molecular biology and biochemistry experiments.
Concentrations
Concentrations vary from low to normal to high. High concentrations are used rarely in medicine but frequently in molecular biology.
Normal
In medicine, normal saline (NS) is the commonly-used term for a solution of 0.9% w/v of NaCl, about 300 mOsm/L. Less commonly, this solution is referred to as physiological saline or isotonic saline, neither of which is technically accurate. NS is used frequently in intravenous drips (IVs) for patients who cannot take fluids orally and have developed or are in danger of developing dehydration or hypovolemia. NS is typically the first fluid used when hypovolemia is severe enough to threaten the adequacy of blood circulation and has long been believed to be the safest fluid to give quickly in large volumes. However, it is now known that rapid infusion of NS can cause metabolic acidosis. NS is 9g NaCl dissolved in 1 liter water. The mass of 1 milliliter of NS is 1.009 grams. The molecular weight of sodium chloride is approximately 58 g/mole, so 58g NaCl is 1 mole. Since NS contains 9 grams NaCl, the concentration is 9g/L divided by 58g/mole = 0.154 mole/L.
Since NaCl dissociates into two ions – sodium and chloride – 1 molar NaCl is 2 osmolar.
Thus, NS contains 154 mEq/L of Na+ and Cl-. It has a slightly higher degree of osmolarity (i.e. more solute per litre) than blood (hence, though it is said to be isotonic with blood in clinical contexts, this is a technical inaccuracy).
Note that in chemistry, a one normal of NaCl (see normality) is 0.5 molar (see molarity) NaCl assuming complete dissociation. Physiological dissociation is approximately 1.7 ions per mole, so one normal of NaCl is 1/1.7 = 0.588 molar. This is roughly 4 times more concentrated than medical "normal salline" of 0.154 molar. Even with the incorrect assumption of complete dissociation, one normal of NaCl is 3 times more concentrated than medical "normal saline."
Other
Other concentrations commonly used include
- Half-normal saline (0.45% NaCl), often with "D5" (5% dextrose), contains 77 mEq/L of Na and Cl and 50 g/L glucose.
- Quarter-normal saline (0.22% NaCl) has 39 mEq/L of Na and Cl and always contains 5% dextrose for osmolality reasons.
- Hypertonic saline is used in treating hyponatremia and cerebral edema. Due to hypertonicity, administration may result in phlebitis and tissue necrosis. As such, concentrations greater than 2% NaCl should only be administered via a central venous catheter. It is commonly available in two strengths:
- 3% NaCl has 468 mEq/L of Na and Cl.
- 5% NaCL has 856 mEq/L of Na and Cl.
- Dextrose (glucose) 4% in 0.18% saline is used sometimes for maintenance replacement.
Solutions of saline with added ingredients
In medicine, common types of salines include:
And in cell biology, in addition to the above the following are used:
- Phosphate buffered saline (PBS) (recipes from Dulbecco = D-PBS, Galfre, Kuchler, Ausubel etc.)
- TRIS-buffered saline (TBS) (recipes from Goldsmith, Ausubel etc.)
- Hank's balanced salt solution (HBSS)
- Earle's balanced salt solution (EBSS)
- Standard saline citrate (SSC)
- HEPES-buffered saline (HBS) (recipes from Dittmar, Liu, Ausubel etc.)
Usage
The amount of normal saline infused depends largely on the needs of the patient (e.g. ongoing diarrhea or heart failure) but is typically between 1.5 and 3 litres a day for an adult.
Other concentrations of saline are frequently used for other medical purposes, such as supplying extra water to a dehydrated patient or supplying the daily water and salt needs ("maintenance" needs) of a patient who is unable to take them by mouth. Because infusing a solution of low osmolality can cause problems, intravenous solutions with reduced saline concentrations typically have dextrose (glucose) added to maintain a safe osmolality while providing less sodium chloride. As the molecular weight (MW) of dextrose is greater, this has the same osmolality as normal saline despite having less sodium. Because the dextrose used in these preparations is dextrose monohydrate (a commercial form having MW 198 in contrast to MW 180 for glucose), 5% dextrose is equivalent to 4.5% glucose.
History
Saline was believed to have originated during the Indian Blue Cholera pandemic that swept across Europe in 1831. William Brooke O'Shaughnessy, a recent graduate of Edinburgh Medical School, proposed in an article to medical journal The Lancet to inject cholera patients with highly-oxygenated salts to treat the "universal stagnation of the venous system and rapid cessation of arterialisation of the blood" seen in severely dehydrated cholera patients. He found his treatment harmless in dogs, and his proposal was soon adopted by the physician Thomas Latta in treating cholera patients to beneficial effect. In the following years and decades, variations and alternatives to Latta's solution were tested and used in treating cholera patients. These solutions contained a range of concentrations of sodium, chloride, potassium, carbonate, phosphate, and hydroxide. The breakthrough in achieving physiological concentrations achieved by Ringer in 1831, when he determined the optimal salt concentrations to maintain the contractility of frog heart muscle tissue. Normal saline is considered a descendant of the pre-Ringer solutions, as Ringer's findings were not adopted and widely used until decades later. The term "normal saline" itself appears to have little historical basis for its routine use, except for Hartog Jakob Hamburger's 1882-83 in vitro studies of red cell lysis that incorrectly suggested that 0.9% was the concentration of salt in human blood (rather than 0.6%, the true concentration).
See also
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