Flushing a midline catheter follows a simple sequence: clean the injection cap, attach a prefilled saline syringe, and push the solution through in short bursts. The standard flush volume is 10 mL of normal saline, delivered before and after every medication infusion. If your provider has also prescribed a heparin lock, that goes last. The entire process takes about a minute once you’re familiar with it.
A midline is a peripheral catheter, typically 8 to 20 cm long, placed in a vein of the upper arm (usually the basilic, cephalic, or brachial vein). Unlike a PICC line, the tip stays below the armpit and doesn’t reach the central circulation. Flushing keeps the line clear of blood and medication residue so it works reliably every time you need it.
The SASH Sequence
Most midline flushing follows the SASH protocol: Saline, Administer, Saline, Heparin. Each letter represents a step in your infusion routine:
- S (Saline): Flush with 10 mL of normal saline to confirm the catheter is open and working.
- A (Administer): Infuse your medication as directed.
- S (Saline): Flush again with 10 mL of normal saline to clear any remaining medication from the catheter.
- H (Heparin): If your provider ordered it, finish with a heparin flush to help prevent clot formation inside the line. Not everyone will have this step.
When the midline isn’t being used for medication, you still need to flush it regularly to keep it patent. Your care team will give you a specific schedule, but the same 10 mL saline flush applies. The goal is to prevent blood from sitting inside the catheter and forming a clot that blocks it.
Step-by-Step Flushing Process
Start by washing your hands thoroughly. Gather your prefilled saline syringe (and heparin syringe if prescribed), alcohol pads, and a clean workspace.
Hold the syringe upright with the tip cap still on and press the plunger firmly until you feel it move. This pushes out any trapped air. Release pressure on the plunger, then loosen the tip cap without removing it yet.
Next, scrub the injection cap on your catheter with an alcohol pad for at least 15 seconds using a twisting, back-and-forth motion. Let it air dry for about 5 seconds. Use a fresh alcohol pad before each new syringe connection throughout the SASH steps. This “scrub the hub” step is one of the most important things you can do to prevent infection.
Make sure the clamp on the catheter or extension set is open. Remove the tip cap from your syringe and attach it to the injection cap by pushing in and twisting clockwise until it’s secure. Then slowly push the plunger to deliver the flush.
Use the Push-Pause Technique
Rather than pushing the saline through in one smooth motion, use a pulsatile, or “push-pause,” technique. Push about 1 mL, pause briefly, push another 1 mL, pause, and repeat until the syringe is empty. These short bursts create turbulence inside the catheter that is significantly more effective at clearing deposits and medication residue from the inner walls than a steady, continuous push.
Research has shown that this turbulent flow reduces bacterial buildup inside catheters and does a better job maintaining patency over time. One study found that pulsatile flushing significantly reduced colonization by Staphylococcus aureus compared to continuous flushing. It’s a small technique change that makes a real difference in how long your midline stays functional.
Syringe Size Matters
Always use a 10 mL syringe or larger when flushing a midline. Smaller syringes generate higher pressure relative to their size, and that excess pressure can damage the catheter from the inside, potentially causing it to leak or rupture. Catheter manufacturers, including Teleflex, specifically warn against using syringes smaller than 10 mL for this reason. Your prefilled flush syringes should already be the correct size, but it’s worth checking if you’re ever supplied with something different.
What Normal Flushing Feels Like
When a midline is working properly, the saline flows through with little resistance. You should be able to push the plunger smoothly without needing to apply much force. The flush shouldn’t cause pain, burning, or swelling near the insertion site or anywhere along the arm.
If you notice any of the following, stop flushing immediately and contact your care team:
- Resistance on the plunger: This can signal a partial blockage from a blood clot or fibrin buildup inside the catheter.
- Swelling or leaking at the insertion site: This may mean the catheter has shifted or there’s a break in the line.
- Pain, tingling, or swelling in the arm, neck, or shoulder: These can be signs of a blood clot in the vein around the catheter.
Never force a flush against resistance. Pushing harder will not clear the blockage and risks rupturing the catheter. Instead, check visible tubing for kinks or bends, make sure the clamp is fully open, and reposition your arm to see if that helps. If the line still won’t flush, it needs professional assessment. Sometimes the issue is something simple like a kinked extension set. Other times it’s a fibrin clot that requires a specific clearing solution your provider can administer.
Saline vs. Heparin Locks
For most patients, normal saline alone is sufficient for routine flushing and locking a midline between uses. The CDC does not recommend routine use of anticoagulant therapy solely to reduce infection risk in general patient populations. However, some providers still prescribe a small heparin flush as the final locking step to help prevent clot formation, particularly if you’ve had occlusion problems before or if your midline sits unused for longer periods.
If heparin is part of your protocol, it always goes last, after the final saline flush. The saline clears medication from the line, and the heparin solution sits inside the catheter between uses. When you access the line again, the first saline flush of the next SASH cycle pushes out the old heparin lock before your medication goes in. Follow whatever your provider has specifically ordered for your situation, since practice varies between institutions and individual patient needs.
Keeping Your Midline Clean Between Flushes
Flushing technique only works if the access point stays clean. Every time you connect a syringe or tubing to the injection cap, scrub with 70% isopropyl alcohol or alcoholic chlorhexidine first. Research supports scrubbing for at least 15 seconds, though some guidelines recommend longer. The key is vigorous friction on all surfaces of the cap, not just a quick wipe.
Between flushes, keep the catheter site dry and protected. Avoid submerging the arm in water. If the dressing becomes loose, damp, or visibly soiled, it needs to be changed. A secure, clean dressing is your first line of defense against bacteria entering the site.