It is also quite common at this level to find the

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is the only vessel seen at this level. It is also quite common at this level to find the artery quite close to the vein, or even overlying it. The femoral vein is most reliably medial to the artery at a point just distal to the inguinal ligament; ultrasound evaluation of the vessels distal to this site will reveal how variable the anatomy can be as one travels caudally. As described in Chapter 8 , patent veins will completely compress to a thin line with probe pressure. If they do not, a clot is present, and cannulation should be attempted on another vessel. Although it is helpful to have color flow to show patency and spectral Doppler to distinguish flow patterns, it is not necessary. In fact, it can sometimes be misleading because partial vein occlusion will still demonstrate flow, and transmitted pulsations can affect spectral wave forms. The most important distinguishing characteristic is that veins have thinner walls, and are therefore easily and completely compressible. If the vein is not completely compressible, a clot or thrombosis should be suspected and another vessel selected (see Chapter 8 ). Moreover, it is instructive to observe a vessel throughout the respiratory cycle before attempting cannulation, because the level of respiratory variation and change in caliber or diameter that is observed is quite surprising. This is even more marked in dehydrated or septic patients and thus, if observed, may require more reverse Trendelen- burg positioning. Figure 15.2 Common femoral artery (A) and vein (V). The greater saphenous vein (S) is visible at this level as well. Procedural ultrasound 275 Vascular access
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Figure 15.4 shows the same patient with and without Valsalva and increased Trendelenburg positioning. These techniques make the internal jugular vein much easier to visualize, and the increased caliber will improve cannulation success. Figure 15.3 Superficial femoral vein (V), superficial (S) and deep (D) femoral arteries. Figure 15.4 Internal jugular vein and carotid artery. When the patient is not lying flat ( left ), the carotid is seen as a round anechoic structure but the jugular vein is collapsed. When the Valsalva maneuver and Trendelenburg position are applied ( right ), the highly distensible internal jugular vein fills with blood and is easily seen to the right of the carotid. 276 Procedural ultrasound Vascular access
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Technique Probe selection Generally, a high-frequency (5–10 MHz) linear probe is used for vascular access ( Figure 15.5 ). The higher frequency generates higher-resolution pic- tures, and the linear image display makes needle guidance and identification somewhat more intuitive. Special equipment A sterile probe cover ( Figure 15.6 ), typically packaged with sterile conducting gel, should be used when performing the venous access with maximal sterile barrier technique. Sterile gloves can be used as a substitute probe cover if these packages are not available.
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