In the Seldinger technique, what is threaded through the needle after insertion?

In the Seldinger technique, after the needle is inserted into the vascular access site, a guidewire is threaded through the needle. This crucial step allows for the establishment of a pathway within the vessel. Once the guidewire is in place, the needle can then be removed, and a catheter can be advanced over the guidewire into the vessel. The use of a guidewire is fundamental because it offers a stable and narrow access point, facilitating the placement of larger devices, such as catheters, without risking damage to the blood vessel. The guidewire essentially serves as a template for ensuring accurate placement of subsequent devices.