Pediatric emergency medicine trisk 1129
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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
