Interventional Ultrasound
Gisela I. Banauch
Paul H. Mayo
Introduction
Ultrasonography has major applications in critical care medicine. When used at the bedside by the intensivist who is in charge of the clinical management of the case, it allows for immediate diagnosis and management decisions to be made at the point of care. Bedside, intensivist-performed ultrasound differs substantially from standard radiology or cardiology performed ultrasonography in that the intensivist acquires the image, interprets the image, and promptly applies the results to the clinical situation. This avoids the time delay and clinical disassociation implicit to ultrasonography that is performed on a consultative basis by radiology or cardiology services.
The scope of practice of critical care ultrasonography encompasses those aspects of the discipline that have utility to diagnosis and management of the critically ill patient. A summary of the important elements that are required for competence in the field have been presented in a recent consensus statement [1]. Ultrasonography may be divided into two general categories of application in critical care management: (i) to guide diagnosis and management and (ii) for purposes of procedural guidance. The two are often related. For example, ultrasonography may be used to diagnose a pleural effusion. Ultrasonography is then used to guide thoracentesis, which in turn is useful in identifying the cause of the pleural effusion and therefore its management. This chapter reviews the use of ultrasonography for procedural guidance in the intensive care unit (ICU). For detailed review of critical care ultrasonography, the reader is referred to comprehensive texts on the subject [2,3].
A major responsibility of the intensivist is to safely perform a wide variety of invasive procedures that may be associated with significant complications. The proceduralist has a specific target, such as a vascular structure or body compartment (e.g., pleural, peritoneal, or pericardial), and seeks to avoid injury to adjacent structures while assuring accurate placement of the needle. Inaccurate placement of the needle may injure adjacent structures with potential major morbidity or even life-threatening complication, as well as lead to failure of either diagnostic effort or essential vascular or body cavity access.
This discussion assumes that the reader is fully trained in physical tasks of the procedure (proper sterile technique, needle manipulation, wire insertion, dilation etc.). These are reviewed in other chapters of this text specific to each procedure. Ultrasonography is used to augment the safety and success rate of the operator who is fully competent in the mechanical aspects of the procedure.
The use of ultrasonography for procedural guidance is based on a simple principle. The safety and success of needle insertion is augmented by the ability to image the target; to identify and therefore avoid adjacent structures; and if required, to guide real-time needle insertion. The alternative is to rely on off-line analysis of standard radiography images and/or on landmark technique. Intuitively, ultrasound guidance is an attractive alternative to traditional technique. It is now in widespread use in the critical care community. This chapter reviews the use of ultrasonography for the guidance of a variety of procedures that are commonly performed by the intensivist.
General Principles
To maximize the utility of ultrasonography, the operator should have basic knowledge of ultrasound physics, machine control, transducer manipulation, image acquisition, ultrasound anatomy, image orientation, and image interpretation. In addition, the intensivist must have full capability in all the mechanical aspects of the procedure.
The machine should be carefully positioned such that the operator may view the screen and the procedure site without untoward head movement; this often requires rearrangement of cluttered equipment that typically surrounds the patient bed in the critical care unit. Machine position for ergonomic efficiency is particularly important when using ultrasonography for real-time image-guided needle insertion. Room lighting and angle of the ultrasound machine’s screen should be adjusted to minimize screen glare. Before starting the procedure, machine settings should be set for optimal image quality with attention to gain, depth, and image orientation. Many modern machines are designed such that the structure of interest is best visualized if it is placed in the center of the screen. Some machines have automated image optimization software so that the operator does not need to adjust controls beyond pushing a single control button. The resulting image may not, in fact, be optimal, and it may need further readjustment.
In situations where real-time guidance is required (e.g., vascular access) or when there is need for scanning while maintaining a sterile field, ultrasound procedure guidance requires that the operator use a purpose-designed sterile probe cover. The use of covers made from sterile gloves or sterile intravenous skin covers is strongly discouraged. They frequently fail during the procedure, while the operator’s attention is focused on the sonographic image or on needle direction and insertion on the sterile field. Well-designed sterile transducer covers are low cost and come with sterile ultrasound coupling medium.
By standard convention, guidance of thoracic, abdominal, and vascular procedures requires that the screen orientation marker be placed on the left of the screen. Guidance of procedures related to the heart, such as pericardiocentesis or transvenous pacemaker insertion, is performed with the screen orientation marker placed on the right of the screen. This convention relates purely to common usage patterns. When scanning from the head of the patient, as with internal jugular venous (IJV) access, the operator needs to decide on how to orient the screen marker in reference to the transducer. We suggest that the orientation marker be
on the left of the screen and that the corresponding marker on the transducer always be held such that it is pointed toward the left side of the examiner (unless scanning the vessel in longitudinal axis when the transducer marker is directed cephalad). It is important to understand and standardize orientation and transducer marker position so that the operator can direct the needle in predictable fashion during real-time guidance of needle insertion.
Whenever planning an ultrasound-guided procedure, the operator should explore the structure of interest before prepping and draping the patient. This allows for optimal site selection before site preparation. If the procedure aims to cannulate a vessel (e.g., central venous or arterial catheterization), the potential target should be evaluated on both sides of the body unless absolute contraindications exist on one side (e.g., arteriovenous fistula in the upper extremity would preclude radial arterial catheterization on that side). Multiple studies have documented significant anatomic variability in vascular lumina, positioning, and location with respect to adjacent structures for both venous and arterial targets [4,5,6,7,8,9].
Initially, vascular structures should be imaged in their transverse axis, as this approach is best to differentiate the artery and the vein [10]. Features such as compressibility, pulsation, luminal variation with respiratory effort, and/or respiratory maneuvers can all be used to help distinguish arterial from venous vessels. The cross-sectional ultrasonographic view usually displays the vein in close proximity to its accompanying artery, thus facilitating comparison of vessel changes with dynamic maneuvers, such as compression and Valsalva. Detection of vessel pulsatility requires a steady imaging plane for at least a few seconds. Pulsatility is sometimes diminished with hypotension. Differentiation of arterial from venous structures is challenging especially when the patient’s perfusion is maintained with a nonpulsatile ventricular assist device (impeller device). The much less compressible, thicker arterial walls, as well as the lack of vessel lumen variability with respiratory effort and/or respiratory maneuvers, provide the most reliable features that differentiate arterial from venous structures in this situation. Color and spectral Doppler analysis may occasionally be required to distinguish the vein from the artery in situations of difficult anatomy or in the subclavian position.
For pleural or abdominal access, initial orientation should always be achieved in the longitudinal image plane. The variable position of the diaphragm in the critically ill patient makes is easiest to differentiate intrathoracic from intra-abdominal fluid collections using longitudinal image planes.
Whenever possible, the operator should document relevant ultrasound images during the procedure. This may be as simple as capturing a frozen video image that can be placed in the chart. Depending on system capability, video clips may be captured and stored off line. Image documentation is important for quality review and billing purposes. However, it may not be practical in all situations, particularly during hectic resuscitation efforts.
Ultrasound guidance of procedures requires specific training. The cognitive aspects of the field are straightforward, and can be easily learned from books, audiovisual sources, courses, or via e-learning program. Image interpretation and acquisition require a component of hands-on scanning under the supervision of a skilled bedside instructor. Real-time guidance of needle insertion is a complex psychomotor skill that requires practice. Unfortunately, this is often achieved with the experiential approach; that is, the inexperienced operator is expected to perform the procedure the first time on an actual patient. To avoid this, we strongly recommend that training in real-time needle insertion take place on an ultrasound manikin. Ultrasound-capable vascular access manikins of excellent design are now commercially available [11]. Trainees may practice ultrasound control of the needle and targeted vascular access multiple times before their first effort at the patient bedside. This is imperative for patient safety and comfort as well as for operator confidence.
Ultrasound Guidance of Vascular Access
Vascular access is a major responsibility of the intensivist. Insertion of catheters of varying size and function requires central venous cannulation, accurate ongoing measurement of arterial pressure and waveform requires arterial line insertion, whereas peripheral venous (PV) access is a routine requirement of patient care. Considerations such as obesity or unusual body habitus (e.g., kyphoscoliosis or genetic disease) and coagulopathy may present special challenges. PV access may be difficult in patients due to obesity, intravenous drug use, or chemotherapy. Ultrasound is uniquely useful for guidance of all forms of vascular access.
A benefit of ultrasound guidance of vascular access is that it allows the operator to identify contraindications to vascular access that are not apparent by simple physical examination. For example, marked respiratory effort may completely obliterate internal jugular and subclavian vein lumina during inspiration in the volume-depleted patient. Such intermittent luminal collapse precludes successful vascular access and cannot be identified, except with ultrasonography. The presence of a thrombus in the femoral vein (FV) frequently cannot be detected by physical examination, but it is readily identified ultrasonographically and contraindicates cannulation at that site. Ultrasonography thus warns the operator to redirect attention to less complication-prone sites.
Specific Procedures
Internal Jugular Venous Access
Several studies report that ultrasound guidance of IJV access is superior to landmark technique, with lower complication and higher success rate [12]. The reasons for this are obvious. Landmark technique may be straightforward in a slender subject, but much less so in an obese subject. Asymmetric IJV size and variation in IJV position relative to the carotid occur in up to 30% of the normal population and cannot be appreciated by surface physical examination [13,14]. A national quality organization has stated that ultrasound guidance of IJV access is required for patient safety purposes [15]. The Residency Review Committee has stated that training in this technique is highly recommended during critical care fellowship training; this will likely be followed by it becoming a mandatory requirement.
In guiding IJV access, ultrasonography should be used in a methodical fashion in order to maximize its utility as follows:
Vascular access requires the use of a linear ultrasound transducer typically of 7.5 MHz frequency. This allows for adequate resolution of structures that are relatively near the surface of the body. Lower frequency transducers, which penetrate more deeply at the cost of reduced resolution, are not suitable for guidance of vascular access. The patient should be placed in Trendelenburg in order to distend the vein as much as possible.
The operator should perform a preliminary scan of both sides of the neck before the sterile preparation. This allows for identification of aberrant anatomy and/or thrombus, and determination of the best site, angle, and depth of needle penetration. The IJV is usually lateral to the carotid
artery when scanning the anterior neck, and is differentiated from the carotid artery by its larger size, thin wall, and lack of characteristic pulsation, easy compressibility, size fluctuation with respiration or respiratory maneuvers, and the presence of thin mobile venous valves. Color Doppler may be used to confirm, but it is not generally required. The examination of the vein starts with a two-dimensional (2-D) study to examine the anatomy and observe for visible echogenic thrombus. The 2-D examination is followed by compression of the vessel to exclude isoechoic thrombus not visible on 2-D imaging. A fully compressible IJV indicates that there is no thrombus at the site of the examination. In order to ensure patency of the vessel along the length that will be traversed by the central venous catheter, several sites along the course of the vessel must be examined and then compressed. The presence of an ipsilateral thrombus contraindicates line insertion, whereas the presence of a contralateral thrombus is of concern, as the proposed IJV insertion may itself predispose to thrombus. This may yield bilateral IJV thrombus, which is undesirable.
The preprocedure scan should include examination of the anterior lung (with the patient in supine position) in order to rule out pneumothorax before the procedure. The transducer is held perpendicular to the chest wall in order to examine the rib interspaces of the upper anterior chest. The pleural interface is identified between the rib shadows. Presence of lung sliding, lung pulse, or B-lines rules out pneumothorax with a high level of certainty [16]. The examination may be accomplished with similar result, using a low-frequency abdominal or cardiac transducer, or using a high-frequency vascular transducer that is used to guide vascular access. Following the procedure, the operator again examines the anterior chest for pneumothorax. The finding of pneumothorax following the procedure, when none existed before, is strong evidence for procedural mishap. The preprocedure chest examination should include both lungs to cover the very rare eventuality that the patient has a contralateral pneumothorax before the procedure.
Before the sterile field is established, the ultrasound machine must be positioned to allow optimal hand–eye coordination for the operator. Because the operator normally stands on the side of the IJV to be cannulated, next to the patient’s head and facing the patient’s feet, the optimal position for the ultrasound machine is on the operator’s side of the patient, immediately adjacent to the patient’s lower chest or upper abdomen. Inadequate placement of the ultrasound screen makes efficient hand–eye coordination very difficult. With an inappropriately placed ultrasound screen, the operator needs to rotate his or her head in order to compare changes in the ultrasound image with changes in needle insertion depth and angle. With a well-placed ultrasound screen, the operator needs to only move his or her head up or down in order to compare ultrasonographic image changes with changes in needle angle and insertion depth on the sterile field. The chance of accidentally changing the ultrasonographic imaging plane (thus losing the ultrasound image essential for real-time guidance) during head rotation is much greater than the chance of an inadvertent change in scanning plane during a simple up-and-down movement of the head.
Following preliminary scanning and appropriate placement of the ultrasound unit, the patient is prepared with standard sterile technique. The transducer is covered with a purpose-designed sterile transducer cover. The operator has a choice at this point. A helper may hold the transducer while the operator introduces the needle, or the operator may hold the transducer in one hand while guiding the needle with the other. The latter is the preferred technique. The ability to manipulate the transducer and needle in tandem is advantageous. A variation of ultrasound guidance of vascular access is the “mark-and-stick” technique wherein the operator identifies the vessel and marks an appropriate site for line insertion. The needle is introduced without the benefit of real-time guidance. Although this yields higher success rate than traditional landmark method, it is inferior to real-time guidance [12] and so is not discussed further.
The operator needs to decide whether to use transverse or longitudinal scanning plane for real-time needle guidance. This is based on personal preference and training background. Some skilled operators prefer longitudinal approach, as they maintain that it is easier to identify the needle in long axis and therefore to guide it into the vessel. Many operators prefer the transverse plane. In either case, maximal safety is achieved by maintaining clear identification of the needle tip throughout the procedure [17].Related posts:
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