The tip of the needle is the area at which the dot is

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needle that is visualized in the short-axis approach. The tip of the needle is the area at which the dot is just barely visible – moving the probe slightly should cause the dot to disappear if you are at the tip (because the beam will slide past the needle). If you are visualizing the shaft, the dot will be visible in either direction. Since one great weakness of the short-axis approach is visualizing the tip of the needle, it is important that operators become D 1 D 2 45° H Figure 15.10 Pythagorean theorem: needle orientation in a simplified 45–45–90 degree triangle where D 1 ¼ D 2 . The hypotenuse ( H ) is roughly equal to 1.4 ² D 2 . 282 Procedural ultrasound Vascular access
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comfortable with the concept of ensuring that they can see the tip of the needle by moving the ultrasound plane appropriately. As the needle approaches the vessel, the walls of the vessel will tent downward and then pop back when the wall is punctured. In Figure 15.12 , the needle cross-section is visualized as a bright point with some reverber- ation artifact. Tenting of the internal jugular vein is also seen in Figure 15.13 . Figure 15.11 Moving the probe (sliding or tilting) from the shaft of the needle ( left ) toward the tip ( center ) and beyond ( right ). When the needle is about to disappear from view, the tip is being visualized. Figure 15.12 The needle tenting the internal jugular vein ( right ) as it is about to puncture the vessel. Note that the vessel is pushed inward as compared to the image prior to cannulation ( left ). Procedural ultrasound 283 Vascular access
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After the vein is punctured, a flash should be seen in the syringe, and the tip of the needle should be visible within the vessel ( Figure 15.14 ). Proceed with normal cannulation techniques (guidewire, introducer) from this point onward. Many authors advocate the use of ultrasound to confirm venous (and not arterial) cannulation after the flash is obtained. The guidewire can be visual- ized within the vein ( Figure 15.15 ) and the catheter can often be seen as well ( Figure 15.16 ). Doppler can be used to demonstrate flow near the catheter tip Figure 15.13 Another view of a needle tenting the anterior wall of the vein (arrow). Figure 15.14 Needle tip (arrow) visible within vein lumen. 284 Procedural ultrasound Vascular access
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as a small flush of saline is infused. Saline flow can also be seen as micro- bubbles in the right heart when pushed through a subclavian or internal jugular central line ( Figure 15.17 ). For a long-axis approach Again, it is recommended that the short-axis approach be mastered before attempting the dynamic long-axis approach, because the technique is similar. Center the long axis of the vein on the screen. To ensure that you are in the center, focus on the largest diameter of the vessel. Hold the needle in line with the trajectory of the vessel, which should be in the same plane as the ultrasound beam ( Figure 15.18 ).
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