A midline is a type of intravenous (IV) catheter that sits in a vein in your upper arm, longer than a standard IV but shorter than a PICC line. It’s typically 8 to 20 centimeters long (roughly 3 to 8 inches) and is designed for people who need IV therapy lasting one to four weeks, or whose veins are too difficult to access with a regular IV.
Where a Midline Sits in Your Body
A midline catheter is inserted into one of the larger veins in your upper arm, most commonly the basilic vein on the inner side of the arm. The brachial and cephalic veins are also used. The key distinction is where the tip ends up: it stays in the peripheral veins of the arm, typically reaching the axillary vein near the armpit, roughly 3 centimeters below the collarbone. It does not enter the large central veins near the heart.
This is what separates a midline from a PICC line. A PICC (peripherally inserted central catheter) is inserted in a similar spot on the arm, but it’s much longer and threads all the way into the large vein just above the heart. That difference in tip location changes what each catheter can do and what risks come with it.
Why You Might Need One
Midlines fill a gap between two common options. A standard IV in your hand or forearm works well for short treatments, but it often needs replacing every few days and fails frequently from dislodging, swelling, or vein irritation. A PICC line handles long-term therapy but carries higher risks of blood clots and bloodstream infections.
Your care team may recommend a midline when:
- IV therapy is expected to last more than six days but less than about four weeks
- Your veins are difficult to access, meaning nurses can’t find a visible or palpable vein for a standard IV
- You’ve had repeated IV failures, with multiple restarts causing discomfort and delays
In clinical studies, midlines lasted an average of about 6 days, though they can remain functional for up to 17 days or longer depending on the patient and the therapy. They’re considerably more durable than standard IVs, which often fail within 2 to 3 days.
How It’s Inserted
A trained nurse or clinician places a midline using ultrasound guidance. You’ll lie with your arm extended and turned palm-up while the clinician uses an ultrasound probe to locate a suitable vein in your upper arm. They look for veins that are at least 3 millimeters wide and less than 30 millimeters deep beneath the skin.
Once a good vein is identified, the area is cleaned and numbed with a local anesthetic. The clinician punctures the vein under direct ultrasound visualization and threads the catheter into position. The external end is secured to your arm with an adhesive dressing. The whole process is relatively quick, and ultrasound guidance makes it significantly more reliable than blind insertion, especially for patients with hard-to-find veins.
What Can and Can’t Go Through a Midline
Because the catheter tip sits in a peripheral vein rather than a large central vein, there are limits on what can be infused through it. Midlines work well for antibiotics, fluids, pain medications, and many other standard IV therapies. However, certain substances are off-limits.
You cannot receive parenteral nutrition (IV feeding solutions), continuous infusions of vesicant medications (drugs that damage tissue if they leak), or solutions with extreme acidity, alkalinity, or concentration through a midline. These substances need the rapid blood flow of a large central vein to dilute them safely. If they were infused into a smaller peripheral vein, they could damage the vessel lining. There is no universally agreed-upon cutoff for pH or concentration limits, so clinicians make judgment calls based on the specific medication.
How a Midline Compares to a PICC Line
The most important difference is infection risk. Bloodstream infections occur at a rate of about 0.4% for midlines compared to 2.4% for PICC lines, a sixfold difference. Standard peripheral IVs have the lowest infection rate at 0.1%, but they fail so often that the repeated restarts create their own problems.
Midlines also carry a lower risk of deep vein thrombosis (blood clots) compared to PICC lines, since the catheter doesn’t extend into the central veins where clots are more dangerous. However, midlines are limited in what they can deliver and how long they stay in. If you need therapy lasting months, need certain high-concentration medications, or need frequent blood draws, a PICC line is the better choice.
Possible Complications
Midlines are generally considered safe, but complications can occur. The most common issues include vein inflammation (phlebitis), catheter blockage, accidental dislodgement, and, less frequently, blood clots or infection. One thing to be aware of is that because the catheter sits in a deeper vein than a standard IV, early signs of problems like swelling, redness, or leaking can be harder to spot on the skin’s surface.
You should pay attention to pain, swelling, warmth, or redness in the arm where the midline is placed, as well as any fever or chills. These could signal a clot or an infection that needs prompt attention.
Taking Care of Your Midline
Midline maintenance is straightforward but important. The catheter needs to be flushed with normal saline before and after every medication or fluid infusion. This keeps the line clear and prevents blood from clotting inside it. The standard flush volume is 10 milliliters, delivered in a pulsing rhythm (short bursts rather than one continuous push) to create turbulence that cleans the catheter walls.
When the midline isn’t actively being used, it’s “locked” with a small amount of fluid, typically about 1.5 milliliters, to keep the line open. This lock may be refreshed every 8 to 24 hours depending on your care setting. The dressing over the insertion site needs to stay clean and dry, and it’s usually changed on a regular schedule by a nurse. You’ll want to keep the arm dry during showers, often by wrapping it in plastic, and avoid submerging it in water.
Who Should Not Get a Midline
Certain situations make a midline a poor choice for a particular arm. If you have a history of blood clots in the upper arm veins or have had vascular surgery on that side, the affected arm should be avoided. Arms with significant contractures or limited mobility are also unsuitable. If your blood’s ability to clot is severely impaired, either from very low platelet counts or from blood-thinning medications, insertion may need to wait until those values improve.