FIGURE 131.22 Femoral artery and vein When using ultrasound dynamically, a one- or two-person approach can be used The two-person technique may be easier for the inexperienced ultrasound user In this scenario, one person is responsible for holding the probe and keeping the vein centered on the screen The proceduralist can then focus his or her attention on the patient, the site of puncture, and the ultrasound screen, without having to hold the probe In the one-person approach, the proceduralist holds the probe in the nondominant hand and inserts the needle with the dominant hand This requires more technical skill, but ultimately allows for improved awareness regarding needle tip location and more fine-tuned adjustments As the procedure begins, the vein is reidentified and centered in the screen In the dynamic technique, the transverse view is commonly used because it allows visualization of not only the intended vein, but also the nearby arteries and nerves in cross section (Fig 131.22 ) This short-axis approach is more successful for the novice user and should be the method of choice for beginners Alternatively, the vasculature can be identified in the longitudinal plane The long-axis approach is more difficult and requires greater skill; however, for the experienced sonographer, this view offers the advantage of visualizing the vein (and thus needle and catheter) along its entire course of cannulation ( Video 131.24 ) 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 FIGURE 131.23 Wire in vessel When differentiating arteries from veins using compressibility, it should be noted that if the sonographer pushes with extreme pressure, the walls of the artery could touch one another, especially in hypotensive patients Using ultrasound statically should always be performed after the patient has been positioned Repositioning the patient after identification can lead to changes in anatomic relationships and may result in failed attempts at catheterization When inserting the needle, proceduralists need to be cautious about not inserting the needle too slowly If the needle is inserted too slowly, tenting will occur, the walls of the vein will be pushed together and the needle can transverse both walls Furthermore, it is important that the vein and artery are alongside each other in the transverse view If not, and tenting occurs, it is easy to cannulate the underlying artery instead of the vein The proceduralist must pay attention to both the ultrasound image on the screen and the site of the procedure Inexperienced sonographers may focus too heavily on the screen and a flash of blood in the hub will go unnoticed A methodical approach, along with experience, can help minimize this occurrence Although ultrasound-guided catheter placement of the subclavian vein has been described in the literature, it is much more difficult due to the shadows created by the clavicle and should only be undertaken by the experienced sonographer Peripheral Venous Cannulation Peripheral IV catheter placement can be challenging, especially in infants and obese children Research suggests that bedside ultrasound may improve success rates of peripheral catheter placement in children with difficult access or after two failed attempts The most common anatomical sites attempted under ultrasound guidance are the brachial, cephalic, and basilic veins of the upper arm In obese children, ultrasound can be useful to locate the antecubital veins when palpation proves difficult Technique Cannulation of the peripheral veins uses the same principles as CV catheter placement The desired vein for cannulation should be identified in cross section and long axis Once the relevant anatomy has been reviewed, compressibility of the vein should again be noted Color Doppler may be used; however, in smaller veins, the velocity of the blood flow may not be adequate to generate a color change or Doppler signal The vein imaged should be centered on the screen and catheter placement should be placed with the same technique as CV catheter placement ( Videos 131.25 and 131.26 ) Pitfalls There are several pitfalls when placing an ultrasound-guided peripheral catheter First, standard IV catheters are usually not long enough to reach the deeper veins of the upper arm; therefore, longer catheters should be used Second, peripheral veins are much easier to compress than central veins and it is not uncommon for the inexperienced sonographer to apply too much pressure with the probe, thereby compressing the vein A simple way to avoid this is to place a generous amount of gel, providing a “step-off,” so that the transducer does not touch the skin Third, the angle of insertion of a peripheral catheter may be shallower than that of a central catheter One must avoid the temptation to take a steeper approach when using ultrasound guidance because this often leads to inability to advance the catheter over the needle Finally, novice sonographers may have difficulty following the needle tip Whenever possible, simulation should be used to practice prior to performing the procedure on a patient Thoracentesis Of the “centesis” procedures, thoracentesis is the most commonly performed in the ED Ultrasound is helpful because it differentiates pathology such as