An intravenous (IV) cannula is a thin, flexible tube temporarily inserted into a patient’s vein, typically in the arm or hand. This device provides direct access to the bloodstream for administering medications, fluids, or drawing blood samples. To ensure the access point remains functional and safe, flushing is performed regularly by injecting a small volume of fluid into the catheter both before and immediately after any substance is introduced through the line.
Maintaining Clear Flow
The primary purpose of flushing the cannula is to maintain its patency, meaning the catheter must remain open and unobstructed. The internal surface of the plastic catheter can easily trigger the body’s natural clotting cascade when it contacts blood. Even a tiny amount of blood residue left inside the narrow lumen can quickly coagulate.
This residual blood can lead to the formation of fibrin sheaths or small clots that adhere to the inner wall of the tube. These blockages, known as occlusions, prevent the flow of subsequent medications or fluids, rendering the IV line useless. Flushing with normal saline provides hydrostatic pressure to push any blood or fibrin deposits out of the catheter’s tip and into the larger vein where they can be harmlessly diluted.
The standard flushing technique often involves a “push-pause” method, where the fluid is injected in short, rapid bursts rather than a continuous stream. This turbulent flow is more effective at dislodging material sticking to the catheter walls than a smooth, steady push. Regular flushing ensures the catheter remains available for immediate access when needed. A patent catheter also reduces the risk of vein lining irritation caused by the pressure of trying to force fluid past an obstruction.
Preventing Medication Interactions
A second reason for using a normal saline flush is to act as a buffer between two different substances given sequentially through the same IV line. Hospitalized patients often receive multiple intravenous medications over a short period. Without an intervening flush, a small amount of the first drug remains inside the catheter and its hubs, creating the potential for chemical interaction with the second drug.
This chemical mixing, known as drug incompatibility, can lead to dangerous outcomes. The two residual drugs may react to form a precipitate, which is a solid particle that could travel into the bloodstream and cause a harmful embolism. Other combinations may chemically inactivate one or both therapeutic agents, making them ineffective for the patient’s treatment.
The flush serves as a simple “rinse cycle” to clear the entire volume of the catheter and associated connections before the next medication is administered. By physically removing all traces of the first drug, the normal saline ensures that the second medication enters a clean environment and is delivered in its intended, active form. This practice is particularly important when administering drugs with known incompatibilities, such as certain antibiotics or chemotherapy agents.
Why Normal Saline is the Standard Solution
Normal saline is the specific fluid of choice for flushing because of its precise chemical composition: a 0.9% concentration of sodium chloride (salt) dissolved in sterile water. This concentration is classified as an isotonic solution, meaning it has the same osmotic pressure as the patient’s blood plasma and the red blood cells within the vein. This isotonic nature is required for any solution placed directly into the bloodstream.
Introducing a fluid with a different concentration can have immediate and damaging effects on the red blood cells near the catheter tip. If a hypotonic solution, such as plain sterile water, were used, the lower salt concentration outside the cell would cause water to rush into the red blood cells via osmosis. This influx of water would cause the cells to swell and potentially rupture, a process known as hemolysis, leading to pain and cellular damage.
Conversely, a hypertonic solution with a higher salt concentration than blood would cause water to rush out of the red blood cells. This loss of water causes the cells to shrink and shrivel, a process called crenation, which is painful and can damage the vein wall, potentially causing phlebitis or irritation. Normal saline’s isotonic nature ensures there is no net movement of water across the cell membrane, allowing red blood cells to maintain their normal shape and volume, making the flush safe and comfortable.