KEY POINTS
Ultrasonography has multiple applications in critical care medicine. The development of high-quality portable bedside machines now allows the frontline intensivist to perform the ultrasonographic examination at the bedside of the critically ill patient. The results are applied for diagnostic purposes, to aid in the ongoing management of the patient, and for procedural guidance.
The frontline intensivist who is in charge of the management of the patient in the intensive care unit (ICU) personally performs and interprets the ultrasound scan at the patient bedside. This requires mastery of image acquisition and interpretation as well as the cognitive elements of the field.
Conceptually, ultrasonography is an extension of the standard physical examination, as it allows the clinician to directly assess the anatomy and function of the body in a manner that complements the traditional bedside physical examination. The examination may be limited or goal-directed in scope and repeated whenever there is clinical indication. The information derived from the scan is then integrated into the overall management plan.
Ultrasonographic examination of the heart (goal-directed echocardiography), thorax (lung and pleura), abdomen (limited scope), and venous anatomy (deep vein thrombosis) are key elements of critical care ultrasonography. In addition, ultrasonography has major utility for guidance of vascular access, thoracentesis, paracentesis, and pericardiocentesis.
INTRODUCTION
Ultrasonography has multiple applications in critical care medicine. The development of high-quality portable bedside machines now allows the frontline intensivist to perform the ultrasonographic examination at the bedside of the critically ill patient. The results are applied for diagnostic purposes, to aid in the ongoing management of the patient, and for procedural guidance. The emphasis is on limited or goal-directed examination, with serial examinations performed as indicated. This chapter will review some important aspects of critical care ultrasonography.
GENERAL PRINCIPLES
The intensivist uses observation, palpation, percussion, and auscultation as key tools in their assessment of the critically ill patient. Conceptually, ultrasonography is an extension of the standard physical examination, as it allows the clinician to directly assess the anatomy and function of the body in a manner that complements the traditional bedside physical examination. In accepting this simple principle, the intensivist uses ultrasonography at point of care whenever it is indicated, just as they would evaluate the patient with standard physical examination methods.
Critical care ultrasonography is performed at the bedside. The frontline intensivist who is in charge of the management of the patient in the intensive care unit (ICU) personally performs and interprets the scan. The results are then promptly integrated into the management plan. This is very different from the standard radiology or cardiology- guided approach to ultrasonography in the ICU. In this latter circumstance, the intensivist orders the test. Following some period of time, often many hours, the test is performed. Sometime later, a radiologist or cardiologist interprets the scan in a reading room without a clear understanding of the clinical situation. The combination of time delay and clinical disassociation degrades the utility of the results compared to the scan performed by the intensivist at the bedside. To compound the problem, resource allocation and economic pressures combine to limit the ability of radiologists and cardiologists to perform serial examinations. Critical illness implies instability and evolution of illness, such that serial examinations are an implicit requirement for effective management in the ICU. The concept of a limited or goal-directed ultrasonographic examination is different than the standard radiology and cardiology approach.
Intensivists use ultrasonography within a different paradigm. They do not order the test and wait for a delayed result. They do not rely on a technician or specialist to perform the examination. They do not try to integrate a delayed reading into the immediate clinical management of the critically ill patient. Instead, they do everything personally: image acquisition, image interpretation, and the application of the results to the clinical situation of the moment.
The radiology and cardiology community have been responsible for the development of the field of diagnostic ultrasonography. Through their work, the technology and validation of the field is fully established. The responsibility of the intensivist is to adapt a fully developed tool to the peculiar demands of the ICU. The issue for the intensivist does not so much relate to the utility of ultrasonography, but rather to the question of how to achieve competence in its use. The intensivist must have definitive skill in all components of bedside ultrasonography: image acquisition, image interpretation, and the cognitive elements required for effective clinical applications. There is no expert radiologist or cardiologist involved; the intensivist is solely responsible for all aspects of the examination.
SCOPE OF PRACTICE AND TRAINING IN CRITICAL CARE ULTRASONOGRAPHY
The scope of practice of critical care ultrasonography includes all aspects of modalities that have utility for diagnosis and management of the critically ill patient. A recent Consensus Statement summarizes the important elements that are required for competence in the field1 and describes a reasonable scope of practice for the field. These include thoracic, abdominal, vascular, and cardiac ultrasonography, with the latter being subcategorized into basic and advanced echocardiography. Advanced echocardiography is not a necessary part of competence in critical care ultrasonography for the intensivist, whereas mastery of basic echocardiography is a key component of competence.
A recent Consensus Statement summarizes the important elements of training that are required to achieve competence.2 This document represents the opinion of a working group comprised of 17 national critical care societies including the three societies from the United States. When combined with the Competence Statement, it serves to guide intensivists in planning their training for which there are three interrelated parts.
Mastery of image acquisition: This includes knowledge of ultrasound physics, machine controls, transducer manipulation, ultrasound anatomy, and scanning tactics that are specific to each organ system. Skill in image acquisition is a mandatory component of competence, as the intensivist personally performs the scan. Skill in image acquisition can only be achieved with hands-on training. It best starts with deliberate practice on normal human subjects followed by supervised scanning of patients. The training process may be supervised by a local expert who is responsible for ensuring the quality of training. It is recommended that the trainee keeps a logbook of scanning activity and develops an image portfolio for review.
Mastery of image interpretation: This includes the ability to identify the wide variety of normal variants of ultrasound anatomy, as well as to recognize a wide range of pathology. This may be achieved by scanning of actual patients, but primarily through review of a comprehensive image collection.
Mastery of the cognitive elements: These are required to integrate ultrasonography with clinical management. This may be achieved in blended fashion using textbook, articles, lecture material, and Internet-based learning programs. Cognitive training includes review of the limitations of intensivist performed ultrasonography, in particular when to ask for review of the results by an advanced-level ultrasonographer, and when to use alternative imaging modalities.
Training in advanced critical care echocardiography requires a major time commitment, a large number of scans both performed and interpreted, and comprehensive knowledge of the cognitive elements of the field. Most intensivists neither need nor are interested in this level of training for typical ICU function. For those who seek this type of training, the American Heart Association/American College of Cardiology has applicable recommendations that can be combined with the optional requirement of taking the echocardiography boards.3 La Société de Réanimation de Langue Française in France has very specific guidelines for training for advanced-level critical care echocardiography that include a board-type examination. Training in critical care echocardiography includes mastery of transesophageal echocardiography.
EQUIPMENT REQUIREMENTS
The ICU must be equipped with a fully capable ultrasound machine on site 24/7 that is under the complete control of the intensivist staff. The machine should be equipped with both a standard cardiac transducer and an additional probe that is designed specifically for vascular ultrasonography. A separate abdominal transducer, while desirable, adds significant cost to the machine. It is not required as the cardiac probe has multipurpose utility and is capable of good-quality thoracic, abdominal, and cardiac imaging. There are many types of machines on the market. The size and portability of the machine has major implication for ICU use. A large high-end machine used for cardiology-type echocardiography is impractical in a busy ICU. The industry has designed portable machines they may be easily positioned, by virtue of their small footprint, around the crowded ICU bed. The machine may be rapidly detached from the cart to become a handheld unit that is ideal to carry to cardiac arrest or rapid response events outside of the ICU. These modern units have excellent image quality as well as the mandatory memory capability that is required to capture image clips in digital format. For those interested in advanced echocardiography, they may be configured with full Doppler and TEE capability. While a recent generation portable machine is desirable, many older generation machines have excellent imaging capability. In fact, many of the key elements of critical care ultrasonography were fully defined using a machine built in 1990.4 Use of a capable older machine results in substantial cost savings. Because modern portable machines have excellent image quality, other qualities are important to consider in making a purchase decision. Durability, reliability, ease of operation, and the manufacturer’s reputation for service-related matters are important considerations when making a purchase decision.
APPLICATIONS OF CRITICAL CARE ULTRASONOGRAPHY
Vascular access is a common procedure for the intensivist. Central venous access, arterial line insertion, and challenging peripheral venous access are routine in the ICU. Considerations such as unusual body habitus, obesity, or bleeding risk may present special challenges. Peripheral venous access may be difficult in patients due to intravenous drug use, obesity, or repeated hospitalization. Ultrasound is very useful for guidance of all forms of vascular access.
Ultrasonography allows the clinician to identify contraindications to access that are not apparent on physical examination. A thrombus in the internal jugular vein will not be detected on physical examination. It contraindicates venous access at that site, and it is readily detected with ultrasonography. The volume depleted patient with respiratory distress may have marked intrathoracic pressure swings that completely obliterate the lumen of the internal jugular or subclavian vein during inspiration. This precludes safe venous access, and yet can only be detected with ultrasonography. The intensivist who uses landmark technique assumes that the carotid artery lies medial to the internal jugular vein, and that the vein is of normal caliber. In fact, there is risk of variant position of the vein relative to the artery, as well as of a narrowed venous caliber.5,6 Ultrasonography is able to identify variant anatomy, which is not detectable with physical examination. In addition to identification of dangerous anatomy, ultrasonographic guidance of central venous access improves success rate and decreases complication rate at the internal jugular,7 subclavian,8 and femoral site.9 Ultrasonographic guidance of arterial access has benefit,10 and in difficult peripheral venous access cases, ultrasound improves success rate as well.11
The evidence so favors ultrasound guidance for vascular access that major quality organizations recommend its use,12,13 and is now a requirement for critical care fellowship training in the United States (USA) as of July 1, 2012.14
From the point of view of a pragmatic frontline intensivist, it is hard to argue against the evident advantage of being able to see the target vessel, as opposed to guessing where it is. An argument against ultrasonography is that it might degrade the practitioner’s ability to perform access using landmark technique when ultrasonography is not available. The counter argument is that it may actually improve the landmark approach, as the clinician learns the anatomy from ultrasonographic examination. Another argument is that it complicates setup for line insertion. Compared to the complexity of setup required for prevention of central-line infection, the addition of a transducer with sterile probe cover is inconsequential. A benefit of ultrasonography is that it greatly decreases the number of attempts required for successful insertion in difficult cases; while this decreases the risk of mechanical complication, it may also reduce the risk of disrupting the sterile field.
Ultrasonographic guidance of vascular access is performed with a transducer of higher frequency (typically 7.5 MHz) than that used for general body ultrasonography. Most transducers are of linear design. Microconvex types are available as well, and are useful for small area scanning. Compared to a cardiac transducer of lower frequency, the vascular transducer has superior resolution but reduced penetration. Most major vessels of interest are close to the surface of the body, so are within the depth range of a vascular transducer.
The ultrasound machine should be positioned for maximal ergometric efficiency. This may require repositioning ICU equipment and the patient bed, but it is well worth the effort. Optimal machine position is such that the operator can look at the insertion site and then the screen with minimal head movement. Gain, depth, and screen orientation must be optimized. Real-time guidance of needle insertion improves success rate, so that the sterile field must include the transducer covered with a purpose built full-length sterile cover. It is inappropriate to improvise using a sterile glove as a substitute for a full-length sterile probe cover.
The operator may choose a two-person method, where one person holds the transducer, while the other inserts the needle. Alternatively, a single operator holds the transducer in one hand and performs the needle insertion under real-time guidance with the other. This is the preferred technique of most operators.
Thoracic, abdominal, and vascular ultrasonography is performed with the orientation marker placed on the left of the screen and the transducer indicator pointed toward the right side of the patient when scanning in transverse plane. In this manner, structures on the left side of the screen will correspond to the right side of the body. This is identical to the projection used with computerized tomography (CT). When performing internal jugular venous access from the head of the bed, the operator will need to decide on how to orientate the transducer indicator. When scanning from the head of the bed, most operators hold the transducer such the indicator that it points toward the patients left side, when scanning in transverse plain. In any case, the operator should standardize their approach, so as to be able direct the needle in predictable fashion during real-time guidance of needle insertion.
For central venous access, it is important to scan both sides of the body in order to select the best target. In the internal jugular position, there can be significant variation of vessel size. The presence of a thrombus prohibits cannulation on the ipsilateral side, and relatively contraindicates insertion contralaterally due to the risk of bilateral thrombus formation.
In using ultrasonography for real-time needle guidance, the operator must choose between transverse and longitudinal scanning planes. This is by personal preference, as there is no literature that favors one or the other approach. The transverse method requires that the operator be able to track the needle tip as it advances toward the target vessel. This requires moving the needle tip forward in tandem with the movement of the transducer scanning plane. The longitudinal method requires that the operator keep the entire needle in clear view throughout the insertion. This is difficult, as the thickness of the scanning plane may be only 1 to 2 mm. Even minimal deviation of the needle from this plane causes loss of tip visualization. With either method, repeated practice on an ultrasound mannequin model is an essential part of skill acquisition. It is not intuitively obvious how to track a needle during insertion, and multiple passes on a well-designed task trainer greatly increase success rate at the bedside. Veins are surprisingly compressible, so that a frequent problem is that the needle compresses the vein to the extent that the lumen is completely effaced without blood return. This is especially common in the internal jugular position. The operator may pass through the back wall of the vessel, and obtain blood return only upon slow withdrawal of needle as it passes through the now open lumen. This should be avoided in the subclavian position, due to the close proximity of the pleural surface.
A key element for safe venous access is to distinguish the vein from its paired artery. The vein is easily compressible and thin walled compared to the adjacent artery. Veins may have mobile thin valves and exhibit respirophasic size variation. Attention to image orientation and scanning technique is helpful in identifying the vessels, but is not sufficient to be certain. Unusual positional relationship of the artery and vein are particularly common in the internal jugular position. Sometimes it may be difficult to differentiate between the artery and the vein. For example, severe hypotension may cause the artery to be easily compressible. Massive obesity and edema, wounds, and dressings may impair definitive ultrasonographic visualization. In obese or very muscular individuals, the subclavian vessels may be difficult to image. Color Doppler imaging is always an option, but is usually not required.
Following needle access and insertion of the wire, a standard safety precaution is to reimage the vein before dilation. The requirement is to identify the wire lying in longitudinal axis within the vein. This is straightforward in the internal jugular and femoral position, but more difficult for the subclavian vein. To check wire placement in the subclavian vein, the transducer may need to be placed in the supraclavicular fossa and angled downward in coronal scanning plane to identify the wire as it passes into the great veins of the thorax.
Internal Jugular Vein: As part of the initial ultrasound scan to determine which side is best for line insertion, the operator should examine the anterior chest in order to rule out preprocedure pneumothorax. This precaution holds for the subclavian position as well. This may be done with the vascular transducer by identifying sliding lung (see discussion below). In thick-chested patients, the vascular transducer may have insufficient penetration, so that the cardiac transducer is required to identify sliding lung. Following insertion of the line, the anterior chest is again examined. Loss of lung sliding when it was present beforehand is strong evidence for procedure-related pneumothorax.
In our experience, the internal jugular vein is best accessed using a transverse scanning plane. Optimally, the needle is introduced through the skin at a point above the point of vessel penetration and advanced forward with simultaneous forward movement of the transducer such that the needle tip is guided into the vessel. This is often difficult to do, and many operators rely on watching for movement of the vessel wall as evidence of appropriate needle trajectory. This entails the risk of needle insertion outside of the scanning plane. Practice on a task trainer is required to refine needle tip control. The proper catheter tip position may be documented by ultrasonography.15,16
Subclavian Vein: When using ultrasonography for guidance, the subclavian vein is best accessed from lateral chest wall location. The insertion site may be as far lateral as the proximal portion of the axillary vein. The landmark expert is used to a more medial approach with the clavicle as a definitive anatomic feature used to guide needle trajectory. With ultrasonographic guidance, the operator does not use the clavicle as a primary guide, but relies on the ultrasound image. Imaging the needle in its longitudinal axis allows the operator to insert the needle in real time with safety,8 but care must be taken to visualize the entire needle throughout the insertion. An oblique scanning plane may cause the operator to lose control of the needle tip, as the barrel of the needle may be misinterpreted as the needle tip. Loss of needle tip control may result in a pneumothorax.
Femoral Vein: Femoral venous access under ultrasound guidance is straightforward. One benefit for the operator is that ultrasound examination allows the operator to insert the needle into the common femoral vein where it lies medial to the artery. Immediately caudad to the inguinal ligament, the vein (now the superficial femoral vein) rotates to become deep to the artery. Blind insertion at this point risks arterial injury. Ultrasonographic guidance avoids this pitfall of blind insertion technique.
