A midline catheter is 3 inches (7.5 cm) to 8 inches (20 cm) in length, and it’s inserted into the basilic, cephalic, or brachial veins. The tip is advanced no farther than the distal axillary vein in the upper arm. Because veins increase in diameter as they travel proximally up the arm, a midline directs infusate into much larger veins. A midline offers significantly better hemodilution than a peripheral IV placed in smaller, more distal veins because of the placement of the tip in one of the larger veins of the upper arm.
A single venipuncture for a midline catheter can meet the infusion therapy needs of a patient who requires more than 5 days of intravenous therapy, has poor or limited peripheral access, or needs reliable access in a care setting.
The Infusion Nurses Society (INS) classifies midlines as peripheral I.V. catheters, they are managed somewhat differently than other peripheral catheters. For example, while a short peripheral catheter may dwell for up to 72 hours, a midline is appropriate for therapy lasting 2 to 4 weeks. This means a midline and PIV should have only medications with an osmolarity btween 250-350 mEq/L or a pH greater than 5 and less than 9. If the medication being considered is outside of these parameters a PICC should be considered.