Saline (medicine)

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For other uses, see Saline.

In medicine, saline is a solution of sodium chloride (a substance also commonly known as table salt) in sterile water, used frequently for intravenous infusion, rinsing contact lenses, and nasal irrigation (or the yogic practice called jala neti). Saline solutions are available in various concentrations for different purposes.

Normal saline is the solution of 0.9% w/v of NaCl (this nomenclature is confusing - "normal sodium chloride" to a chemist means a concentration of 5.85% w/v, also expressed as 1N NaCl_(aq) ). It contains 154 mEq/L of Na⁺ and Cl⁻. It has a slightly higher degree of osmolality (i.e. more solute per liter) compared to blood (hence, though it is referred to as being isotonic with blood in clinical contexts, this is a technical inaccuracy), about 300 mOsm/L. Normal saline (NS) is therefore used frequently in intravenous drips (IVs) for patients who cannot take fluids orally and have developed severe dehydration. Normal saline is typically the first fluid used when dehydration is severe enough to threaten the adequacy of blood circulation and is the safest fluid to give quickly in large volumes.

Other concentrations of saline are frequently used for other 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 but contributes less sodium to the circulation. Because dextrose monohydrate (MW 198 in contrast to MW 180 for glucose) is the commercial form of dextrose used in these preparations, 5% dextrose actually contains only 4.5 g/dL of glucose.

Concentrations commonly used include

Half-normal saline (0.45% NaCl), often with "D5" (5% dextrose), contains 77 mEq/L of Na and Cl and 4.5 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.
Dextrose (glucose) 4% in 0.18% saline is used sometimes for maintenance replacement.

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.