It is important to check compressibility and vessel

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It is important to check compressibility and vessel patency. Smaller veins and arteries often do not have sufficient flow to generate a brisk Doppler signal, but differences in compressibility often highlight the veins from the arteries. The brachial and basilic veins readily compress, but the brachial artery remains patent ( Figure 15.24 ). In real time, even small arteries can be seen to pulsate when pressure is placed on them. Once the target vein is identified, the process of intravenous catheter placement is largely unchanged (except for the use of a longer catheter). Tips Peripheral veins are more superficial, inconstant, and fragile than central veins. All the care and finesse one usually employs in cannulating a difficult peripheral vein should still be used with ultrasound. Use the least amount of pressure possible with the transducer when imaging the veins. Often, no veins are visible because they have all been collapsed by too much pressure. Pitfalls Difficult veins are difficult veins. Ultrasound can help find veins that would otherwise be invisible, but it cannot prevent veins from rolling or collapsing, and nor does it melt away valves. Figure 15.23 The brachial artery (1) and adjacent superficial (2) and deep (3) brachial veins are visualized. The basilic vein (4) is seen to the right. 292 Procedural ultrasound Vascular access
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Do not approach peripheral veins with a catheter at an acute angle. Remember that getting to the vessel is only the first step – you must thread the catheter into the vessel. If you would normally place an IV at a 15- to 30- degree angle to the skin, ultrasound cannot make a 60-degree angle of approach feasible. A commonly used guide to catheter length is that two- thirds of the length of the catheter should reside within the vessel for it to (A) (B) Figure 15.24 ( A ) The brachial artery (1) and adjacent superficial (2) and deep (3) brachial veins are visualized. The basilic vein (4) is seen to the right. ( B ) With slight compression, the veins collapse entirely and only the brachial artery remains patent. In real time, the artery would visibly pulsate with compression. Procedural ultrasound 293 Vascular access
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remain stable in its position. Catheters typically used for peripheral venous access are designed for vessels just beneath the skin surface; it is better to overestimate the length needed rather than underestimate when approaching a deeper vessel. Occasionally, thrombophlebitis of the peripheral veins is encountered ( Figure 15.25 ). As with DVT of the central veins, it is not recommended that cannulation is attempted in thrombosed vessels. Literature review Ultrasound use in central venous access was first described in the early 1990s. Since then, dozens of studies have sought to assess the efficacy of the tech- nique. In 2005, Milling et al . randomized patients to landmark-based, static ultrasound, or dynamic ultrasound guidance for central venous cannu- lation [ 1 ]. The study found that ultrasound guidance was associated with a higher success rate. In 1997, Hilty et al . found a reduction in the number of
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