Saline solutions are mixtures of salt and water used in many medical treatments. They are categorized by their sodium chloride concentration relative to the body’s normal fluid concentration, which is approximately 0.9% salt. Hypertonic saline is a specialized solution containing a salt concentration significantly higher than this physiological baseline, ranging from 3% to 23.4% sodium chloride. This high concentration makes it a powerful therapeutic tool, primarily functioning to manipulate the balance of fluid within and around the body’s cells and tissues.
How Hypertonic Saline Works
The therapeutic action of hypertonic saline depends on the principle of osmosis, the natural movement of water across a semipermeable membrane to equalize solute concentrations. When a high salt concentration solution is introduced, it creates a powerful osmotic gradient. Since water moves from lower to higher solute concentration, the hypertonic saline actively draws water out of surrounding cells and tissues.
This fluid shift causes cells to temporarily shrink as they release water to dilute the concentrated solution. Clinically, this mechanism is deliberately employed to pull excess fluid out of swollen tissues and into the bloodstream, where it is circulated and excreted by the kidneys. Common concentrations used for intravenous administration include 3% or 5% sodium chloride, while higher concentrations, such as 7% or 23.4%, are reserved for specific, acute applications.
Use in Respiratory Therapy
Hypertonic saline is commonly used as a nebulized treatment for lung conditions characterized by thick, sticky mucus. In conditions such as cystic fibrosis (CF), the airways suffer from a defect in ion transport, which leads to insufficient hydration of the airway surface liquid. The resulting mucus becomes highly viscous and difficult for the body’s natural clearance mechanisms to move, leading to chronic infections and lung damage.
When a 7% hypertonic saline solution is inhaled as a fine mist, it deposits directly onto the airway lining and the mucus layer. The high salt concentration creates osmotic pressure, pulling water from underlying airway cells and tissue into the mucus itself. This influx of water rehydrates the secretions, thinning the mucus and restoring the depth of the airway surface liquid.
This thinning action reduces the viscosity of the mucus, making it easier for the cilia—the hair-like structures lining the airways—to sweep the secretions upward. By improving mucociliary clearance, the treatment helps patients with CF or bronchiectasis to cough up tenacious sputum and clear their airways. Studies show that regular use of nebulized hypertonic saline can improve lung function and reduce pulmonary exacerbations in people with cystic fibrosis.
A lower 3% concentration of nebulized hypertonic saline is also sometimes used to treat acute bronchiolitis in hospitalized infants. Bronchiolitis causes inflammation and swelling of the small airways, leading to thick secretions and mucosal edema that obstruct airflow. The osmotic effect draws water into the secretions to loosen them and may also pull excess fluid out of the swollen airway walls, reducing localized edema. This application is generally reserved for inpatient settings where the infant can be closely monitored.
Application in Neurological Conditions
Hypertonic saline is a powerful intervention used in critical care settings to manage elevated intracranial pressure (ICP), a condition that can result from traumatic brain injury (TBI), stroke, or severe cerebral edema. The confined space within the skull means that any swelling of the brain tissue can rapidly increase pressure, leading to secondary injury and potentially fatal brain herniation.
Administered intravenously, the concentrated saline solution, often 3% or 7%, creates a strong osmotic force within the blood vessels of the brain. This force pulls excess water directly out of swollen brain cells and interstitial tissue and into the intravascular space, a process referred to as “osmotic therapy.” By rapidly shifting this excess fluid out of the brain parenchyma, the volume inside the skull is reduced, which effectively lowers the intracranial pressure.
This rapid reduction in pressure helps restore adequate blood flow to the brain, which is often compromised when ICP is too high. Hypertonic saline is sometimes preferred over other osmotic agents because it can also help maintain the patient’s blood pressure, which is often necessary in trauma patients. The goal in this high-stakes scenario is to quickly stabilize the patient and prevent further neurological damage until the underlying cause of the swelling can be addressed.
Other Clinical Uses and Safety
Beyond its primary uses in respiratory and neurological care, hypertonic saline serves a purpose in managing acute, severe hyponatremia, which is a dangerously low level of sodium in the blood. In this application, a concentrated solution, typically 3% sodium chloride, is infused intravenously to rapidly increase the serum sodium concentration. This systemic intervention is necessary to prevent neurological symptoms like seizures and coma that occur when low sodium levels cause brain cells to swell.
The solution is also utilized topically in certain types of wound care. Hypertonic saline dressings or ointments work by creating an osmotic gradient on the wound surface, which draws excess fluid, bacteria, and necrotic tissue (slough) out of the wound bed. This process assists in debridement, helping to cleanse the wound and promote the formation of healthy granulation tissue. The localized osmotic effect helps reduce wound edema and manage heavily exudating wounds.
The use of hypertonic saline is not without significant risks, and it requires careful monitoring by medical professionals. A primary concern is the potential for rapid or excessive shifts in fluid and electrolyte balance. If severe hyponatremia is corrected too quickly, it can lead to a devastating neurological condition known as osmotic demyelination syndrome, which causes severe damage to nerve cells. Other risks include hypernatremia (high sodium levels) and hyperchloremic metabolic acidosis from the high chloride load. When nebulized, the solution can sometimes cause irritation or bronchospasm, which can be mitigated by combining the treatment with a bronchodilator.