280 procedural ultrasound vascular access muscle

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280 Procedural ultrasound Vascular access
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muscle, carotid artery for the internal jugular approach; femoral artery for the femoral vein approach). Check compressibility of the vein. This serves to distinguish artery from vein and reduces the risk of attempting catheter placement at the site of a deep vein thrombus. At this point, the center of the vessel should be held in the center of the screen. This means that the vessel is beneath the center of the probe. For a short-axis approach Center the cross-section of the vein on the screen. One simple way to assess the proper distance from the transducer is to use the geometry of a 45 ± –45 ± –90 ± triangle, or the Pythagorean theorem. As shown in Figure 15.10 , measure the depth from the surface to the vessel ( D 2 ). This is equal to the distance from the transducer to where the skin puncture should be made ( D 1 ), as long as the needle enters the skin at a 45-degree angle. When a more shallow angle is desired, the distance from the transducer (for a given vessel depth) must be increased. For a 45-degree approach the hypotenuse ( H ) is roughly equal to 1.4 ² D 2 . Thus, if the vessel is centered 1 cm beneath the skin, puncture the skin 1 cm toward the operator from the transducer and the vein will be punctured by 1.4 cm of needle depth. If the vein is 2 cm deep, puncture 2 cm from the transducer and hit the vessel when the needle has traveled 2.8 cm. It is useful to make this calculation before attempting can- nulation, to avoid complications. If the needle is at distance H and the vessel has not been cannulated, then the trajectory is not correct and the needle should be repositioned before injuring deeper structures such as the carotid or femoral artery. Puncturing the skin at a point too close to the transducer position will yield a steep trajectory and will make cannulation more difficult. Figure 15.9 Holding the ultrasound probe in the non-dominant hand. Procedural ultrasound 281 Vascular access
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In the short-axis approach, the needle is only visible at the point where it crosses the ultrasound plane perpendicular to it. The needle will be seen as a dot, often with either a faint shadow (black) or reverberation artifact (white) deep to the needle. However, often the needle itself will not be visualized. This is because the needle width is quite small, and during the initial portion of the path, the needle has not yet crossed the plane of the ultrasound beam. It is possible to angle the probe toward the needle to ensure that it is traveling along the correct trajectory. Signs of the needle pushing through tissue will be seen (muscle displacement, tenting in of the vein when the needle is attempting to pierce the wall) even if the needle itself is not well visualized. Note that the appearance of the needle tip and needle shaft will be identi- cal on ultrasound. The cross-section of a line (the needle) is a dot, no matter which part of the line is cut by a plane (the ultrasound beam). Figure 15.11 demonstrates an approach to ensuring it is the tip, and not the shaft, of the needle that is visualized in the short-axis approach. The tip of the needle is
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