Chapter 4. Ultrasound-Assisted Procedures

Emergency Physicians commonly perform invasive procedures. These procedures have traditionally been taught using surface landmarks, with the assumption that anatomy is reliably similar from patient to patient. The increasing use of ultrasound (US) to assist in procedural guidance has demonstrated that this is not the case. Utilizing US to assist with procedures has numerous benefits.1–9 It is safer for patients as it has been shown to reduce complications. US improves patient comfort and satisfaction. This is due primarily to decreased attempts at the procedure. The use of US usually decreases the duration of the procedure.

This chapter reviews the basic information regarding the use of US to assist or guide procedures in the Emergency Department (ED). US can assist in many commonly performed ED procedures (Table 4-1). The specific US technique for a procedure is described in the chapter for the particular procedure.

The sonographer must make several decisions prior to beginning a procedure using US. Will the procedure be performed under real-time US guidance or will US only be used to map the anatomy? Will one person (the sonographer) or two people (the sonographer and an assistant) be necessary? Which US probe is the most appropriate for the procedure? Should the instrument or needle be imaged in the long-axis view or the short-axis view? Are needle guides necessary? What is the ideal location of the US machine in relation to the sonographer and patient? These general questions must be kept in mind when using US for invasive procedures and are discussed below.

US can be used in one of the two ways for procedural assistance: the dynamic technique or the static technique. The dynamic technique is also known as US guidance. The sonographer uses US guidance in real time during the procedure to survey the anatomy, to confirm a diagnosis, and to visualize the needle or instrument as it enters tissue and reaches the target. The static technique is also known as US mapping. The sonographer uses US mapping prior to starting the procedure to map the local anatomy, to confirm a diagnosis, and to mark the site of needle entry. The US probe is then put away and the procedure performed in the traditional fashion without real-time US.

The decision between US guidance versus US mapping is influenced primarily by the degree of inherent danger of the procedure. Perform procedures that carry a higher risk or have greater technical difficulty (e.g., central venous access, pericardiocentesis, or foreign body removal) under US guidance. Perform lower risk procedures (e.g., thoracentesis, paracentesis, or abscess I&D) with US mapping. Another factor is sonographer experience. The experienced sonographer is more technically adept and comfortable doing procedures under real-time US guidance. Novice sonographers who do not have as much psychomotor training and experience may find it easier to use US mapping.

The decision to perform a procedure with one or two people is based on the sonographers experience and the availability of an assistant. Sonographers with more experience often prefer to guide the US probe and manipulate the instrument themselves. Their experience and expertise allows them to work with a higher efficiency without an assistant. Assistance during the procedure may not be an option during a busy ED shift, in an ED with single physician coverage or in an ED with limited personnel. For one-person US guidance, the sonographer holds the US probe with the nondominant hand and guides the needle or instrument with the dominant hand. This requires a degree of hand-and-eye coordination to maintain continuous alignment of the needle with the US probe.

Novice sonographers may find it less daunting to use the two-person technique and work with an assistant. The assistant holds the US probe while the sonographer guides the needle or instrument. The assistant's responsibility is to maintain alignment of the US probe with the needle during the procedure. The sonographer must maintain good communication with the assistant throughout the procedure in order to maintain sight of both the needle tip and the target organ on the US screen.

The following general principle will be helpful in determining which US probe is the most appropriate for a given procedure. The higher the frequency of the US probe, the better the resolution of the structures visualized, but the shallower the maximum depth of view. In other words, use a high-frequency US probe for superficial structures and a low-frequency US probe for deep structures. Low-frequency US probes include the curvilinear probe and the phased-array probe (Table 4-2). High-frequency US probes include the linear probe and the endocavitary probe (Table 4-2). More than one probe may be required for some procedures. Another factor in selecting the US probe is its footprint or surface area. A larger footprint is recommended to get a larger scan image unless specific anatomic barriers (e.g., ribs) dictate a smaller footprint. The types of US probes recommended for specific procedures are listed in Table 4-2.

Two orientations or approaches are used for US guidance during procedures. The longitudinal or long-axis approach refers to placing the long axis of the needle in-line with the long axis of the US probe (Figure 4-1). The transverse or short-axis approach refers to placing the long axis of the needle 90° to the long axis of the US probe (Figure 4-2). It is paramount for the sonographer to keep sight of the needle regardless of which approach is used during the procedure. It is helpful to have the US probe indicator facing the same side as the marker on the top of the US screen to maintain left-to-right alignment.

The long-axis approach allows the entire length of the needle to be visualized as it approaches the target (Figure 4-3). Depth perception is better with this approach. This is a more intuitive approach for some ultrasonographers. The main disadvantage of the long-axis approach is the poor lateral resolution. A needle located to the side of a structure may appear in the same plane as the structure when actually it is not. This approach requires precise alignment of the US probe with the needle, otherwise the sonographer can lose sight of the needle. The one procedure best performed with the long-axis approach is a peripheral nerve block.

The short-axis approach requires the sonographer to center the target of interest on the US screen. The long axis of the needle is positioned 90° to the long axis of the US probe, at the center of the long axis of the US probe (Figure 4-2). The short-axis view of the needle is visible on the US monitor, resulting in better lateral resolution (Figure 4-4). One disadvantage of this approach is the challenge to maintain sight of the needle tip. The sonographer must move the US probe forward along with the needle tip as it is advanced. The depth of the needle tip can be misjudged if one is not careful. This can sometimes result in the inadvertent puncture of the anterior and/or posterior wall of the target structure or vessel, or of another adjacent structure.

Needle guides are attachments to the US probe that keep the needle in a predictable path during the procedure. They are often utilized by radiologists for aspiration or biopsy of deep structures. The benefit of using a needle guide is that less hand-to-eye coordination is required to keep the needle aligned with the US probe during the procedure. The disadvantage of a needle guide is that the path angle is fixed and cannot be changed in the midst of the procedure. A needle guide is not necessary for the vast majority of Emergency Medicine procedures. It may actually be disadvantageous since it does not allow “on-the-fly” corrections of the needle path.

Always ensure that there is a direct line of sight from the procedural field to the US machine (Figure 4-5). The sonographer should not need to turn their head to the side during the procedure to view the US screen. This will greatly improve the sonographer's comfort during the procedure and increase the likelihood of success. Having to turn your head back-and-forth can result in the needle or the US probe moving, an unsuccessful procedure, puncture of incorrect structures, and potential morbidity. Hold the US probe comfortably in the nondominant hand using a pencil-like grip and maintaining light contact with the patient. It is best to use short controlled movements to maintain visualization when manipulating a needle or other instrument under US guidance.

Misalignment of the US probe and the needle is the most common reason for the needle tip not being visualized during the procedure. Stop advancing the needle if it is not visible on the US screen. Reposition the US probe by dragging or fanning it back and forth over the area of the needle until the needle can be seen. Gently rocking or bouncing the needle within the soft tissue can sometimes assist in determining the location of the needle tip.

Indirect clues can help determine the location of the needle tip if it cannot be seen directly. These include the ring-down artifact (Figure 4-6) and the needle shadow artifact (Figure 4-7). The ring-down artifact is a bright hyperechoic streak due to reverberation of the US beam from the highly reflective interface of the needle. The shadow artifact is a hypoechoic line below the needle due to the needle blocking the US beam. Follow either artifact until the needle can be visualized on the US screen. Continue to follow the needle until the tip is found.

Other factors can affect the ability to visualize the needle. Consider these when preparing to perform a procedure. Use the appropriate probe and depth of field for the procedure. Smaller gauge needles produce smaller artifacts and are more difficult to visualize, especially in cross section. It is easier to visualize needles perpendicular, or in the short axis, to the US beam. Consider using needles specific for ultrasonography that are extra reflective. The costs of these needles are hard to justify, but may be acceptable for teaching. Normal needles are appropriately reflective and it is difficult to justify these specific needles clinically. The beveled tip of the needle is easier to visualize than the shaft. The irregular surface of the bevel will reflect the US beam better than the shaft. Insert and advance the needle with a slight “to-and-fro” motion to easily follow the needle path.

The use of US assistance for procedures is an important skill set in the armamentarium of the Emergency Physician. It allows potentially dangerous procedures (e.g., incision and drainage of a peritonsillar abscess) to be performed in a safer manner due to visualization of the surrounding anatomy. It allows for the rapid confirmation of a diagnosis at the bedside, resulting in faster therapeutic interventions (e.g., pericardiocentesis). It usually results in greater patient satisfaction because of fewer attempts being required to successfully complete the procedure. ED ultrasonography can truly be a lifesaving modality when used by trained Emergency Physicians.

The authors would like to thank Linda J. Kesselring, MS, ELS, for copyediting the manuscript and incorporating our revisions into the final document; and Angela Taylor for manuscript preparation.