Saline (medicine)

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 is approximately isotonic to blood serum, which makes it a 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.[11]

The solution is 9 grams of sodium chloride (NaCl) dissolved in water, to a total volume of 1000 ml (weight per unit volume). The mass of 1 millilitre of normal saline is 1.0046 grams at 22 °C.[12][13] The molecular weight of sodium chloride is approximately 58.4 grams per mole, so 58.4 grams of sodium chloride equals 1 mole. Since normal saline contains 9 grams of NaCl, the concentration is 9 grams per litre divided by 58.4 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 osmolarity of 154 + 154 = 308, which is higher (i.e. more solute per litre) than that of blood (approximately 285).[14] 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,[15] which yields an osmolarity of 0.154 × 1000 × 2 × 0.93 = 286.44. Therefore, the osmolarity of normal saline is a close approximation to the osmolarity of blood.

For medical purposes, saline is often used to flush wounds and skin abrasions. However, research indicates that it is no more effective than potable tap water.[16] Normal saline will not burn or sting when applied.

Saline is also used in I.V. 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. The amount of normal saline infused depends largely on the needs of the person (e.g. ongoing diarrhea or heart failure).

Saline is also often used for nasal washes to relieve some of the symptoms of rhinitis and the common cold.[17] The solution exerts a softening and loosening influence on the mucus to make it easier to wash out and clear the nasal passages for both babies[18] and adults.[19][20] In very rare instances, fatal infection by the amoeba Naegleria fowleri can occur if it enters the body through the nose; therefore tap water must not be used for nasal irrigation. Water is only appropriate for this purpose if it is sterile, distilled, boiled, filtered, or disinfected.[21]

Sterile isotonic saline is also used to fill breast implants for use in breast augmentation surgery, to correct congenital abnormalities such as tuberous breast deformity, and to correct breast asymmetry. [22] [23] Saline breast implants are also used in reconstructive surgery post-mastectomy.

Eye drops are saline-containing drops used on the eye. Depending on the condition being treated, they may contain steroids, antihistamines, sympathomimetics, beta receptor blockers, parasympathomimetics, parasympatholytics, prostaglandins, non-steroidal anti-inflammatory drugs (NSAIDs), antibiotics or topical anesthetics. Eye drops sometimes do not have medications in them and are only lubricating and tear-replacing solutions.

Syringe designed saline drops (e.g. Wallace Cameron Ultra Saline Minipod) are distributed in modern needle-exchange programmes as drugs efficiently can be administered either by injection, or ophthalmic, which is compared to intravenous use; By demonstration, the elimination of latanoprost acid from plasma is rapid (half-life 17 minutes) after either ophthalmic or intravenous administration.[24] However, ophthalmic use is done with sterile filtered drugs that is potent in quite small doses, and with adjusted acidity of pH 7.0–7.5 after the drug has been added, to avoid eye damage. The human eye has a pH of approximately 7.5, water has 7.0.[25]

There is tentative evidence that saline nasal irrigation may help with long term cases of rhinosinusitis.[26] Evidence for use in cases of rhinosinusitis of short duration is unclear.[27]

Vial of 23.4% sodium chloride

Hypertonic saline—7% NaCl solutions are considered mucoactive agents and thus are used to hydrate thick secretions (mucus) in order to make it easier to cough up and out (expectorate). 3% hypertonic saline solutions are also used in critical care settings, acutely increased intracranial pressure, or severe hyponatremia.[28] Inhalation of hypertonic saline has also been shown to help in other respiratory problems, specifically bronchiolitis.[29] Hypertonic saline is currently recommended by the Cystic Fibrosis Foundation as a primary part of a cystic fibrosis treatment regimen.[30]

An 11% solution of xylitol with 0.65% saline stimulates the washing of the nasopharynx and has an effect on the nasal pathogenic bacteria. This has been used in complementary and alternative medicine.[31]

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.[32] 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.[33]
• Dextrose (glucose) 4% in 0.18% saline is used sometimes for maintenance replacement.