Saline solution for intravenous infusion.
|Intravenous, topical, subcutaneous|
|Defined daily dose||Not established|
|AHFS/Drugs.com||FDA Professional Drug Information|
|Chemical and physical data|
|3D model (JSmol)|
Saline, also known as saline solution, is a mixture of sodium chloride in water and has a number of uses in medicine. Applied to the affected area it is used to clean wounds, help remove contact lenses, and help with dry eyes. By injection into a vein it is used to treat dehydration such as from gastroenteritis and diabetic ketoacidosis. It is also used to dilute other medications to be given by injection.
Large amounts may result in fluid overload, swelling, acidosis, and high blood sodium. In those with long-standing low blood sodium, excessive use may result in osmotic demyelination syndrome. Saline is in the crystalloid family of medications. It is most commonly used as a sterile 9 g of salt per litre (0.9%) solution, known as normal saline. Higher and lower concentrations may also occasionally be used. Saline has a pH of 5.5 (mainly due to dissolved carbon dioxide) making it acidic.
The medical use of saline began around 1831. It is on the World Health Organization's List of Essential Medicines. The wholesale cost in the developing world is about US$0.60–4.20 per liter of normal saline. In 2017, sodium salts were the 225th most commonly prescribed medication in the United States, with more than two million prescriptions.
For medical purposes, saline is often used to flush wounds and skin abrasions. Normal saline will not burn or sting when applied.
Saline is also used in intravenous therapy, intravenously supplying extra water to rehydrate people or supplying the daily water and salt needs ("maintenance" needs) of a person who is unable to take them by mouth. Because infusing a solution of low osmolality can cause problems such as hemolysis, intravenous solutions with reduced saline concentrations (less than 0.9%) typically have dextrose (glucose) added to maintain a safe osmolality while providing less sodium chloride.
There is tentative evidence that saline nasal irrigation may help with long term cases of rhinosinusitis. Evidence for use in cases of rhinosinusitis of short duration is unclear. The solution exerts a softening and loosening influence on the mucus to make it easier to wash out and clear the nasal passages in babies. In this case "home-made" saline may be used: this is made by dissolving approximately half a teaspoon of table salt into 240ml (approx. 8 ounces) of clean tap water. In very rare instances, amoeba Naegleria fowleri infection can occur if amoeba enters the body through the nose, therefore water used for nasal irrigation should be sterile.
Normal saline (NSS, NS or N/S) is the commonly used phrase for a solution of 0.90% w/v of NaCl, 308 mOsm/L or 9.0 g per liter. Less commonly, this solution is referred to as physiological saline or isotonic saline (because it closely approximates isotonic, that is, physiologically normal, solution); although neither of those names is technically accurate (because normal saline is not exactly like blood serum), they convey the practical effect usually seen: good fluid balance with minimal hypotonicity or hypertonicity. NS is used frequently in intravenous drips (IVs) for people who cannot take fluids orally and have developed or are in danger of developing dehydration or hypovolemia. NS is also used for aseptic purpose. 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.
The solution is 9 grams of sodium chloride (NaCl) dissolved in water, to a total volume of 1000 ml (weight per unit volume(w/v)). The mass of 1 millilitre of normal saline is 1.0046 gram at 22 °C. The molecular weight of sodium chloride is approximately 58.5 grams per mole, so 58.5 grams of sodium chloride equals 1 mole. Since normal saline contains 9 grams of NaCl, the concentration is 9 grams per litre divided by 58.5 grams per mole, or 0.154 mole per litre. Since NaCl dissociates into two ions – sodium and chloride – 1 molar NaCl is 2 osmolar. Thus, NS contains 154 mEq/L of Na+ and the same amount of Cl−. This points to an osmolality of 154 + 154 = 308, which is higher (i.e. more solute per litre) than that of blood (approximately 285). However, if the osmotic coefficient (a correction for non-ideal solutions) is taken into account, then the saline solution is much closer to isotonic. The osmotic coefficient of NaCl is about 0.93, which yields an osmolality of 0.154 × 1000 × 2 × 0.93 = 286.44. Therefore, the osmolarity of normal saline is a close approximation to the osmolarity of blood.
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 dextrose.
- Quarter-normal saline (0.22% NaCl) has 39 mEq/L of Na and Cl and almost always contains 5% dextrose for osmolality reasons. It can be used alone in Neonatal Intensive Care Units.
- Hypertonic saline may be used in perioperative fluid management protocols to reduce excessive intravenous fluid infusions and lessen pulmonary complications. Hypertonic saline is used in treating hyponatremia and cerebral edema Rapid correction of hyponatremia via hypertonic saline, or via any saline infusion > 40 mmol/L (Na+ having a valence of 1, 40 mmol/L = 40 mEq/L) greatly increases risk of central pontine myelinolysis (CPM), and so requires constant monitoring of the person's response. Water privation combined with diuretic block does not produce as much risk of CPM as saline administration does; however, it does not correct hyponatremia as rapidly as administration of hypertonic saline does. Due to hypertonicity, administration may result in phlebitis and tissue necrosis. As such, concentrations greater than 3% NaCl should normally be administered via a central venous catheter, also known as a 'central line'. Such hypertonic saline is normally available in two strengths, the former of which is more commonly administered:
- 3% NaCl has 513 mEq/L of Na and Cl.
- 5% NaCl has 856 mEq/L of Na and Cl.
- NaCl solutions that are less commonly used are 7% (1200 mEq/L) and 23.4% (approx 4000 mEq/L), both of which are used (also via central line), often in conjunction with supplementary diuretics, in the treatment of traumatic brain injury.
- Dextrose (glucose) 4% in 0.18% saline is used sometimes for maintenance replacement.
Solutions with added ingredients
In medicine, common types of salines include:
- Lactated Ringer's solution
- Acetated Ringer's solution
- Intravenous sugar solutions
- 5% dextrose in normal saline (D5NS)
- 10% dextrose in normal saline (D10NS)
- 5% dextrose in half-normal saline (D5HNS)
- 10% dextrose in half-normal saline (D10HNS)
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.)
- Gey's balanced salt solution (GBSS)
Replacing fluids intravenously was believed to have originated during the 1826–1837 cholera pandemic. After analysing the blood and stool of people with cholera, William Brooke O'Shaughnessy, a recent graduate of Edinburgh Medical School working in India, proposed in an article to The Lancet to inject severely dehydrated cholera affected people with highly oxygenated salts. He found his treatment harmless in dogs, and his proposal was soon adopted by the physician Thomas Aitchison Latta in treating people with cholera to beneficial effect. In the following decades, variations and alternatives to Latta's solution were tested and used in treating people with cholera. These solutions contained a range of concentrations of sodium, chloride, potassium, carbonate, phosphate, and hydroxide. The breakthrough in achieving physiological concentrations was accomplished by Sydney Ringer in the early 1880s, 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, except for studies done in 1882–83 by Dutch physiologist Hartog Jacob Hamburger; these in vitro studies of red cell lysis suggested incorrectly that 0.9% was the concentration of salt in human blood (rather than 0.6%, the true concentration).
Normal saline has become widely used in modern medicine, but due to the mismatch with real blood, other solutions have proved better. The 2018 publication of a randomized, controlled trial with 15,000 people showed that lactated Ringer's solution reduced mortality risk of people in intensive care unit by 1% compared to normal saline, which given the large number of people is a significant reduction.
Society and culture
The wholesale cost in the developing world is about US$0.60–4.20 per liter of normal saline. In 2017, sodium salts were the 225th most commonly prescribed medication in the United States, with more than two million prescriptions. Although inexpensive, the accompanying equipment required to administer saline can escalate the cost.
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