The ultrasound-guided technique allows to insert (into the deep veins of the arm) not only short cannulas, but also long peripheral devices, such as mini-midlines (6-15 cm) or traditional midlines (15-25 cm). While midlines have little role in the emergency setting, mini-midlines, although having an expectation of shorter duration (few weeks) compared to midlines, are ideal in the emergency/urgency setting since they are rapid and easy to insert (simple Seldinger's technique) and inexpensive.
The opportunity to remove emergency mini-midlines within 24-48 hours is controversial: removal is mandatory if the precautions recommended by the infection prevention guidelines have not been adopted. On the other hand, a mini-midline inserted with an appropriate technique (skin antisepsis with chlorhexidine 2% in alcohol, maximum barrier precautions, sutureless securement, cover with transparent dressing) can be left over 48 hours. Mini-midlines can be made of different materials; in the case of expected long duration, polyurethane rather than polyethylene mini-midlines are preferred (the latter being associated with greater risk of thrombosis). The difference in material obviously has little relevance if the mini-midline is to be removed within 48 hours.
In the child, the caliber of a mini-midline should be 3Fr (which corresponds approximately to 20G).
Some mini-midlines are designed to be positioned with simple Seldinger technique (needle puncture - insertion of the metal guidewire into the needle - needle removal - catheter insertion above the guide), while others have a more complex design, in which needle, cannula and metal guide are assembled in an all-in-one coaxial device. At present there is no evidence to recommend the latter type of mini-midline, characterized by a higher cost and less ease of use.