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.
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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.
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Bedside Applications in Pulmonary and Critical Care Medicine
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.
Procedural Guidance using Ultrasound
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.
Establishing Vascular Access with Ultrasound Guidance
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
Internal Jugular vein with a guidewire
Figure 1a: Click to Enlarge
Internal Jugular vein and Carotid artery
Figure 1b: Click to Enlarge
Left Internal Jugular
Figure 2: Click to Enlarge
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.
Overview of procedure technique:
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.
Overview of Procedure Technique:
Figure 3: Click to Enlarge
Video 1: Click to View
Hepatorenal and Splenorenal Recesses
Video 2: Click to View
Pleural Effusion with Septations
Video 3: Click to View
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.