# Introducer sheath

An introducer sheath (or simply sheath) is a sheath through which fluids or tools can be introduced (inserted) into a blood vessel. It is a surgical tool consisting of a tube inserted into the body to provide an access point and allow the insertion of other instruments into an artery or vein and a seal to maintain hemostasis. It is used during percutaneous procedures such as angiography, angioplasty and stent placement. After the procedure, the introducer sheath is removed and the puncture site is managed to control bleeding.

## Insertion

Insertion of a percutaneous introducer sheath is typically performed using the Seldinger technique with or without the use of vascular ultrasound. Sheaths include a removable dilator that dilates from the size of a wire (eg, 0.018" or 0.035") to the size of the sheath.
Insertion steps:
* Lumen access of the blood vessel using a needle
* Wire inserted through the needle into the vessel lumen
* If a micropuncture is used, then a micropuncture sheath is used to allow increasing the wire size
* Insertion of the sheath + dilator over the wire
* Removal of the wire and dilator
* Flush the sheath to remove any air
Once the sheath is inserted, it can be used to introduce larger sheaths using dilator(s) inserted over wires.

## Benefits of sheaths

The key benefit of using a sheath is the ability to insert and remove catheters from the blood vessel at will. Without the use of a sheath, removing the catheter from the blood vessel would lose access to the vessel.
In some instances, the tip of the sheath may be in a desired location and allow different catheters to be inserted & removed from that desired location. For example, during a transseptal puncture the gaol is to access the left atrium from the right atrium, and using a sheath to cross that puncture site allows different catheters to be used in the left heart.

## Types of sheaths

Sheaths come in various types and sizes. Size is commonly measured in French units (1 Fr = 1/3 mm in diameter). If the size is not specified further, then it refers to the inner diameter of the sheath, whereas the French size of a catheter refers to the outer diameter of the catheter. For example, an 8 Fr sheath can accommodate catheter sizes up to 8 Fr.
Sheaths also vary in length from a few centimeters to tens of centimeters in length.
Sheaths use a valve to permit insertion and removal of catheters to minimize air entry into sheath and minimize blood loss from the sheath. For large sheaths, there may be a minimize catheter size to allow the valve to work appropriately.
Sheaths may have a "side arm" port that allows fluid access to the lumen of the sheath. This can be used to flush the sheath with saline, remove air from the lumen, and introduce medications to the circulation.
Simple introducer sheaths have a straight shape to them. Some are pre-formed curves, and some use mechanical linkages to allow the curve to be manipulated externally.
Most are designed to be removed intact at the end of their use, but special "peel away" sheaths are used when inserting permanent pacemaker leads to allow the lead to remain intravascular.

## Uses of sheaths
### Cardiology

A large number of "percutaneous" procedures are done in cardiology for diagnostic and therapeutic reasons. In interventional cardiology, this can be for angiograms and angioplasty, transcatheter aortic valve replacement (TAVR), and more. In cardiac electrophysiology, this can be for electrophysiology studies, permanent pacemaker leads, and more.

## Closure

When the use of the sheath is complete, there are two ways to achieve hemostasis. The first is manual pressure by simple compression of the blood vessel at the site of access with hands and pushing into the vessel to keep blood from leaking. The second method is to use a vascular closure device. Small venous sheaths readily achieve hemostasis with manual pressure. However, larger sheaths leave bigger holes in the vessel wall and may require prolonged inactivity to achieve durable hemostasis. Sheaths in the artery will have a much higher pressure to bleed and manual pressure takes longer. Dysfunctional clotting or use of anticoagulants can increase the time further if using manual pressure.