Caution must be taken to ensure the probe does not slip from being on top of the vein to above the artery Once the vein is positioned appropriately in the center of the screen, the needle can be inserted, along the center of the probe In cross section, the needle appears as a single bright dot, with or without artifact ( Video 131.25 ) When the needle encounters the vein, tenting of the vessel wall will be seen and then the wall will “pop” back after the needle tip punctures it At this point, blood should be aspirated, and the ultrasound probe can be set aside as the procedure continues in normal fashion Once the wire is threaded into the vein, ultrasound can be used again to visualize the wire in the vessel prior to dilating the skin and inserting the catheter (Fig 131.23 and Videos 131.25 and 131.26 ) Pitfalls While ultrasound can certainly enhance placement of a CV catheter, there are certain caveats Puncturing the skin either too close or too far from the transducer may be problematic If the needle is inserted too close to the transducer, it will pass under the probe (i.e., through the plane of sound waves) before encountering the vessel The point at which the needle contacts the vessel will not be visualized unless the probe is repositioned If the needle is inserted in the skin too far from the transducer, it will encounter the vessel before being visualized In the shortaxis view the transducer should always be repositioned in order to follow the tip of the needle In the long-axis view, the opposite is true Once the transducer is correctly centered over the target vessel, the probe should not move Moving the transducer from side to side could lead to cannulation of the artery rather than the vein