Saline (medicine)

Saline solution for intravenous infusion. The white port at the base of the bag is where additives can be injected with a hypodermic needle. The port with the blue cover is where the bag is spiked with an infusion set. Child receiving an intravenous infusion Saline solution for irrigation. This solution is used for irrigating wounds, tissues, body cavities, and bladders. Saline solution for irrigation should not be administered intravenously.

In medicine, saline (also saline solution) is a general term referring to a sterile solution of sodium chloride (NaCl, more commonly known as salt) in water but is only sterile when it is to be placed intravenously, otherwise, a saline solution is a salt water solution. The sterile solution is typically used for intravenous infusion, rinsing contact lenses, nasal irrigation, and often used to clean a new piercing. 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

Normal saline (NS) is the commonly-used term for a solution of 0.91% w/v of NaCl, about 300 mOsm/L.^[1] 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.^[2]

The solution is 9 grams of sodium chloride (NaCl) dissolved in 1 liter of water. The mass of 1 milliliter of normal saline is 1.0046 grams at 22°C ^[3][4]. The molecular weight of sodium chloride is approximately 58 grams per mole, so 58 grams of sodium chloride equals 1 mole. Since normal saline contains 9 grams of NaCl, the concentration is 9 grams per liter divided by 58 grams per mole, or 0.154 moles per liter. 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 technically inaccurate). Nonetheless, the osmolarity of normal saline is a close approximation to the osmolarity of NaCl in blood.

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, also known as a 'central line.' It is commonly available in two strengths:
  • 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 available are 7% (1200 mEq/L) and 23.4% (approx 4000 mEq/L), both of which are used (also via central line) in the treatment of traumatic brain injury.^[5]
• 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:

• Lactated Ringer's solution
• Acetated Ringer's solution

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)

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.^[6] 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 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, 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).^[7]

See also

• Intravenous therapy
• Oral rehydration therapy
• Hypertonic Saline-Dextran
• Salinometer

References