Published: February 2013
Ultrasonography uses sound waves to create images that can aid clinicians in diagnosis and act as a visual guide during procedures. Early ultrasonography machines were bulky and their use was confined to imaging laboratories. More recently, the advent of compact and portable ultrasound machines that provide excellent image quality has resulted in an profusion of bedside applications and the concept of an ultrasound stethoscope is becoming a reality.1 Ultrasonography has been widely used in cardiology, radiology, obstetrics, and emergency medicine. More recently, its use has become more widespread in pulmonary and critical care medicine, which will be the focus of this chapter.
An ultrasonography machine consists of a transducer probe, a display, a central processing unit, a key/control board, a data storage medium, and/or printer. The transducer probe contain piezoelectric crystals that vibrate when they conduct an electric current. These vibrations create ultrasonic waves that interact with skin, fluid, solid organs, bone, and air are reflected to the probe in varying intensities. These reflected waves are processed by the sonograph to generate an image that is displayed on the monitor.
Simple fluid appears anechoic (black) whereas solid organs appear in various shades of gray. Different probes emit ultrasound waves at different frequencies. Probes generating low-frequency waves (1 to 5 megahertz [MHz]) enable deeper penetration but lower image resolution. High-frequency probes (10 to 15 MHz) have shallow penetration but generate high-resolution images. Hence, high-frequency probes are used to scan superficial structures such as vessels, nerves, and tendons. Low-frequency probes are used to scan deeper structures such as the heart, abdomen, and lungs. In pulmonary and critical care medicine, low-frequency probes are used to diagnose pleural effusions and ascites, and for echocardiography. High-frequency probes are predominantly used to guide vascular access.
Ultrasonography conducted at the bedside by a clinician, known as point-of-care ultrasonography, has 2 primary uses in pulmonary and critical care medicine: procedural guidance and rapid bedside diagnosis in critically ill patients. The advantage of a point-of-care ultrasound machine in the critical care unit is its portability. The machine can be quickly moved to the critically ill patient's bedside in order to assess his or her response to an intervention.
Ultrasonography in the ICU can be extremely valuable as a diagnostic tool, as has been demonstrated by the number of bedside echocardiograms, abdominal ultrasounds and chest ultrasounds that are performed because of the difficulty associated with transporting a critically ill patient to a radiology department or facility. In radiology, technicians and other non-clinicians often perform these studies and the send the results to an interpreting physician to generate a report that is conveyed back to the primary team. While this approach works in some situations, especially in a stable critically ill patient, its use in an unstable critically ill patient is not practical or feasible due to time constraints. Point-of-care ultrasound, in contrast, is performed by the clinician who is currently caring for the patient and who has complete knowledge of the patient's current clinical status. Interpretation of the ultrasound images and immediate clinical decisions are made by the clinician conducting the imaging study, thereby enabling rapid intervention and assessment.
Some of the common ultrasound-guided procedures performed in the critical care unit include establishing vascular access and monitoring catheters, thoracentesis and pleural catheter placement, paracentesis, lumbar punctures, arthrocentesis, and pericardiocentesis. Two of the most commonly used procedures are described below.
Ultrasound is used to guide the placement of central venous catheters, peripherally inserted central catheters, and arterial lines. Real-time ultrasound-guided central venous catheter placement within the internal jugular has been associated with fewer complications, fewer attempts before successful cannulation, fewer failed procedures, and shorter procedure times compared to the traditional technique.2-4 The utility of ultrasound guidance for central venous catheter placement is supported by the Agency for Healthcare Research and Quality (AHRQ) and the British National Institute of Clinical Excellence (NICE).5,6 AHRQ also listed the "use of real-time ultrasound guidance during central line insertion to prevent complications" as one of the 12 most highly rated patient safety practices designed to reduce medical errors.7
The evidence supporting the utility of ultrasound guidance of subclavian access is less robust. A recent prospective study suggested that ultrasound-guided cannulation of the subclavian vein in critical care patients was superior to the traditional method.8 More study would be required before this approach could be uniformly applied, however. The authors of a recent meta-analysis of studies of ultrasound-guided radial artery catheter placement concluded that the use of real-time ultrasound guidance improved the first-pass success rate.9 A brief overview of ultrasound-guided vascular access procedures is given below.
A high-frequency probe (10-15 MHz) is used for vascular access. When real-time ultrasound guidance is used to approach the internal jugular vein, generally 1 of 2 techniques is used: in-plane or out-of-plane (Figure 1a-b). The advantage of the in-plane technique is that the operator can see the entire needle as it enters the vessel. This method requires more experience and is difficult for a beginner. It may also be difficult in a patient with a short neck because the operator cannot align the probe along the vessel. The out-of-plane technique may facilitate placement because the vein and the artery lie side by side. The disadvantage is that the operator cannot see the entire needle during the procedure and the depth of needle insertion may be underestimated, resulting in puncture of the posterior wall of the vessel.
For subclavian access, however, the in-plane technique should always be used because it lets the clinician see the entire needle, judge the depth of insertion, and thereby avoid pneumothorax. An important point to note is that the ultrasound beam is approximately 1 mm thick and a slight change in angulation can cause the needle to go out of view. As a general rule, the direction and angle of the needle should be the same as that of the ultrasound beam. Another advantage of ultrasound guidance is that a thrombus in the vein can be easily detected, enabling the clinician to avoid the vein altogether (Figure 2). These same principles can be applied for arterial lines and peripherally inserted central catheters.
Thoracentesis is commonly performed in the pulmonary office setting and in the critical care unit for diagnosis and drainage of pleural effusions. Some of the common adverse events associated with the procedure include pneumothorax, subdiaphragmatic puncture of the solid organs (liver and spleen), and bleeding. The incidence of pneumothorax has been reported to be as high as 20% to 39% in these procedures.10 Several studies have shown lower pneumothorax rates when ultrasound guidance was used: 0% vs 29%10 and 3% vs 10%.11 More importantly, the rate of pneumothorax requiring tube drainage was significantly reduced when ultrasound guidance was used.11,12
Ultrasound-guided thoracentesis has also been shown to be safe in mechanically ventilated patients,13,14 although no direct comparison of the 2 techniques has been performed in this population. Ultrasound also improves the success rate of thoracentesis after a clinically directed failed tap.15 Additionally, after an unsuccessful clinically directed thoracentesis, fluid can be obtained in 88% of patients with ultrasound guidance.15,16 A brief overview of ultrasound-guided thoracentesis is provided below.
A low-frequency probe (3-5 MHz) should be used to diagnose a pleural effusion, an important first step before performing thoracentesis. Identification of the static boundaries, an echo-free space, and dynamic changes confirms the diagnosis of pleural effusion. The static boundaries include the diaphragm, chest wall, and lung (Figure 3). It is vital to identify the diaphragm because fluid above the it represents pleural effusion and fluid below it indicates ascites (Video 1). If the clinician cannot identify the diaphragm, he or she should not conduct the procedure, especially in a critically ill patient where the diaphragm can be high when the patient is supine. It is important that the clinician avoid the hepatorenal and splenorenal recesses, which can mimic the diaphragm in appearance (Video 2). Confined within the static boundaries, the echo-free space represents pleural effusion. While simple fluid would appear anechoic (black), complex fluid would demonstrate septations and/or dense cellular material (Video 3). Real-time guidance is almost never necessary, and is not recommended because the near resolution of the low-frequency probe is often poor, resulting in difficult needle placement.
It is important to minimize the time between fluid visualization and needle insertion, and to not change the patient's position because this can result in shifting of the fluid. The ultrasound operator must scan the chest to match the angle and direction of the needle with that of the probe. This is especially vital in a patient who has small effusions or who is being mechanically ventilated. For loculated effusions, ultrasound guidance can be used to find and access the optimal compartment. These general principles for ultrasound use in thoracentesis can be applied to placement of pleural catheters and chest tubes as well.
Some of the common diagnostic applications of bedside ultrasound performed by the clinician are lung ultrasonography, goal-directed echocardiography, goal-directed vascular studies, and goal-directed abdominal exam. These are further discussed in the following sections.
In radiology, the application of lung ultrasonography had been limited to pleural effusion detection. However, Daniel A. Lichtenstein, MD, has pioneered its use in detecting pneumothorax, alveolar interstitial syndrome, and alveolar consolidation, and has described its use in acute respiratory failure.17
Lung ultrasonography is predominantly based on the careful analysis of artifact because the waves poorly penetrate inflated lungs. In normal individuals, the visceral and parietal pleura can be seen sliding against one another during respiration (Video 4), giving rise to the basic finding of lung sliding. Lung sliding indicates intact visceral and parietal pleura. Because air is a relatively poor conductor of ultrasound waves, in normal individuals the parietal pleural reflections can be seen and are called A-Lines or reverberation artifacts. When there is fluid or thickening of the interstitium, the pleural surface gives rise to B-lines (Video 5). B-Lines are again artifacts that arise when an ultrasound wave interacts with a small air-fluid interface. These are also called lung comets and are seen in alveolar interstitial syndrome.
The presence of lung sliding and B-lines rules out a pneumothorax with a negative predictive value of 100% at the site of the ultrasound probe.18 It is important to examine multiple points, especially the non-dependent areas of the chest where air would accumulate. While ultrasound is a good tool to rule out a pneumothorax, the diagnosis of pneumothorax with lung ultrasound depends on the identification of the lung point (Video 6). The lung point is the point where the partly inflated lung touches the chest wall and, when observed, has 100% specificity for pneumothorax.19 In a completely collapsed lung where no part is touching the chest wall, no lung point can be identified and the sensitivity is decreased. A recent meta-analysis concluded that ultrasound was superior to chest radiography in detecting pneumothorax.20 While the evidence is much more robust in trauma patients, the same principles of lung ultrasonography can be also applied to critically ill patients.
Lung ultrasound has also been shown to be of value in detecting alveolar interstitial syndrome by detecting B-lines.21 Multiple B-lines signify pulmonary edema due to cardiogenic or non-cardiogenic causes and may also be seen in patients with lung infections or atelectasis.17 B-lines are dynamic and disappear as the underlying process is treated usually either by treating heart failure, removing fluid by dialysis, or treating pneumonia.22,23 B-lines have also been shown to have clinical significance in distinguishing acute cardiogenic pulmonary edema from acute chronic obstructive pulmonary disease (COPD) exacerbation.24 ‘The BLUE protocol’ described by Lichtenstein uses lung ultrasound to establish a rapid diagnosis in a patient with acute respiratory failure.17
In the critical care unit, goal-directed echocardiography is performed by the intensivist with the intention of gathering specific information. Appropriate studies include assessment of preload and fluid responsiveness; assessment of left ventricular function and contractility; assessment of the pericardium; assessment of the right ventricular function; and assessment of major valve abnormalities. Goal-directed echocardiography can be performed in hemodynamically unstable patients and information obtained can be used immediately to improve the therapy.25 The common ICU conditions where goal-directed echocardiography is used include shock, acute respiratory failure, and assessment of preload sensitivity.26
An intensivist with skill in basic critical care echocardiography can rapidly determine the nature of shock using bedside echocardiography. It is vital to diagnose obstructive shock early because doing so would alter therapeutic management, and delay in the diagnosis may lead to a worse outcome. One can diagnose pericardial effusion and look for echocardiographic signs of cardiac tamponade, assess left and right ventricular function, and assess for severe valvular regurgitation using color Doppler ultrasound. One can also rapidly identify acute cor pulmonale and assess the inferior vena cava (IVC) for signs of fluid responsiveness. An IVC diameter <1 cm in a hypotensive patient generally indicates preload responsiveness.27 In a patient with sepsis who is passive on mechanical ventilation and in regular cardiac rhythm, IVC variability >12% indicates fluid responsiveness. IVC variability is calculated as follows:28
maximum IVC diameter - minimum IVC diameter
mean IVC diameter
Non-cardiology intensivists should achieve competence in basic critical care echocardiography. The technique favors specificity over sensitivity, and uncertain findings should prompt consultation while definite findings should aid with patient management.29 A competence statement for the critical care physician has been published.29 Training in critical care echocardiography is not part of a critical care fellowship in the US. However, it is offered in France where there is a structured pathway for training in advanced critical care echocardiography that leads to certification; 39% of fellowship programs are certified to provide the training.30 Efforts are underway to provide a similar certification process in the US.
Goal-directed vascular ultrasonography is often used to identify an acute thrombo-embolic event as the cause of acute respiratory failure. It is particularly useful when combined with lung and goal-directed cardiac ultrasound in a hemodynamically unstable patient. The clinician can rapidly detect deep venous thrombosis (DVT) in a bedside exam.31-34 Thrombi may be directly visualized (Figure 2) and an inability to compress the vein completely is considered a cardinal feature for diagnosis of intraluminal thrombus. While a complete and detailed scan of both lower extremities is time consuming, it is often not needed and a 2-point (common femoral vein and popliteal vein) exam has similar sensitivity and specificity to detect clinically important DVT in the proximal veins.32,34 While a formal vascular study involves compression ultrasonography combined with color Doppler and augmentation maneuvers, a clinician-performed bedside study involves only compression ultrasonography, which will detect the vast majority of cases of DVT. The addition of color Doppler or augmentation maneuvers does not increase the detection rate of asymptomatic DVT over compression ultrasonography alone.34,32 In one study, critical care physicians using compression ultrasonography alone achieved equal accuracy as a sonographer/radiologist who performed compression ultrasound, color Doppler ultrasound with augmentation maneuvers, and Doppler spectral analysis.35
Bedside ultrasonography has applications in exploration of the abdomen as well. It is used to detect hydronephrosis and urinary obstruction in a patient with acute renal failure. It is crucial to rule out obstructive uropathy as a cause of acute renal failure; this condition is easily reversible. The clinician can use ultrasonography in patients with acute abdominal conditions to rule out free air, observe peristalsis, and assess the abdominal aorta to detect abdominal aortic aneurysm rupture or abdominal aortic dissection.
The limitations of bedside ultrasonography performed by a critical care physician are mainly related to the clinician and the particular patient. Patient factors such as obesity, the presence of edema or subcutaneous emphysema, and suboptimal patient position can reduce image quality and make interpretation difficult. This is especially limiting in performing bedside echocardiography where appropriate patient positioning cannot be achieved and mechanical ventilation frequently hampers image quality.
The clinician-related limitations pertain mostly to training and education, and the fact that there are no formal training processes in place in the US. This situation has the obvious potential to cause harm. It is therefore important for the performing physician to know his or her limitations. There is an urgent need in the US for the critical care community to initiate a formal training process like those followed in Europe and Australia.
While clinician performed bedside ultrasonography in pulmonary critical care medicine is in the early stages of development, the evidence for its potential benefits continues to accumulate. Ultrasound appears to be a safe, non-invasive, and non-ionizing imaging modality that can be taken to the bedside of a hemodynamically unstable patient to facilitate therapy and to evaluate its efficacy.