Saline (medicine)

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

For other uses, see Saline.

Saline solution for intravenous infusion

Child receiving an intravenous infusion

Saline solution for irrigation

Overview

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 is a solution of 0.9% w/v of NaCl, 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. Physiological saline is 9g NaCl dissolved in 1 liter water. The molecular weight of sodium chloride is approximately 58 g/mole, so 58g NaCl is 1 mole. Since saline 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. 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 said to be isotonic with blood in clinical contexts, this is a technical inaccuracy).

Note that in chemistry, a normal concentration of sodium chloride is 5.85% w/v (also expressed as 1M NaCl_(aq)), which is 6.5 times more concentrated than medically normal saline.

Other

Other concentrations commonly used include

1. 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.
2. Quarter-normal saline (0.22% NaCl) has 39 mEq/L of Na and Cl and always contains 5% dextrose for osmolality reasons.
3. 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.)

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 cholera pandemic that swept across Europe in 1831. However, an examination of the composition of the fluids used by the pioneering physicians of that era reveals solutions that bear no resemblance to 0.9% or so-called 'normal' saline which appears to have very little scientific or historical basis for its routine use, except for Hamburger's in vitro studies of red cell lysis. The composition of 0.9% sodium chloride is in no way 'normal' or 'physiological', our current practice may be based on historical fallacy and misconception

See also

• Intravenous therapy
• Oral rehydration therapy