Ultrasound in the Intensive Care Unit

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Chapter 12 Ultrasound in the Intensive Care Unit

Daniel W. Johnson, MD

1 What is point-of-care ultrasound, and how is it used in the intensive care unit (ICU)?

Point-of-care ultrasound is the application of ultrasound by the bedside clinician for the purpose of answering diagnostic questions or guiding procedures. Traditionally, after consultation was requested by the bedside clinician, ultrasound examination was performed and interpreted by radiologists, cardiologists, and their sonographers. With the advent of point-of-care ultrasonography, the clinician can apply the ultrasound device himself or herself to answer specific questions without the inherent inefficiencies of consulting imaging services. Point-of-care ultrasound is of particular value in the ICU, where the patient’s status changes rapidly and diagnostic and therapeutic applications of ultrasound are necessary immediately.

2 What are the main variables that determine ultrasound image quality?

The most important variables are frequency, depth, and gain. Ultrasound frequency is largely determined by transducer selection. High-frequency transducers provide excellent resolution but limit the depth at which structures can be viewed. When viewing structures close to the body’s surface, a high-frequency transducer should be used. When viewing deep structures, a low-frequency transducer must be used. Increasing the depth allows the user to view more structures, but with increased depth comes a reduction in resolution. Increasing the gain increases the intensity, or brightness, of the image.

3 How can the ultrasound transducer be manipulated to optimize image quality?

The transducer can be moved to a different location on the body, it can be tilted, or it can be rotated about an axis. When manipulating an ultrasound transducer, the user must take care to alter only the location or the tilt or the rotation at one time. Novice users frequently attempt to alter two or three of these at the same time, which makes image acquisition more difficult.

4 Where is the ultrasound transducer placed for cardiac imaging?

Because bone effectively blocks the transmission of ultrasound waves and creates an acoustic shadow on the ultrasound image display, the sonographer must use windows that allow ultrasound waves to avoid the sternum and ribs. The main echocardiographic windows are depicted in Figure 12-1.

image

Figure 12-1 Main echocardiographic windows. A, Parasternal; B, apical; C, subcostal.

(Modified from Ferrada P, Murthi S, Anand RJ, et al: Transthoracic focused rapid echocardiographic examination: real-time evaluation of fluid status in critically ill trauma patients. J Trauma 70:56-62, 2011.)

5 What questions can be answered by focused transthoracic echocardiography?

Focused transthoracic echocardiography is used to answer specific clinical questions, the answers to which impact immediate medical decision making. Examples of such questions:

image Is the left ventricular systolic function hyperdynamic, normal, depressed, or severely depressed?

image Is the right ventricle dilated?

image Is right ventricular systolic function normal or depressed?

image Is a pericardial effusion present, and, if so, is there evidence of tamponade?

image Do any of the valves appear grossly abnormal on two-dimensional imaging?

image Does the inferior vena cava (IVC) appear underfilled, normal, or overloaded?

With advanced training, the intensivist can perform comprehensive transthoracic and/or transesophageal examinations. Figure 12-2 is an example of a normal subcostal four-chamber view obtained in the ICU.

image

Figure 12-2 Normal subcostal four-chamber view.

6 What does severe hypovolemia look like on echocardiography?

In the setting of severe hypovolemia, the left ventricle appears hyperdynamic with near or complete effacement of the small left ventricular cavity (in other words, the ejection fraction approaches 100%). In the setting of shock, subcostal imaging of the IVC is also useful for determining a patient’s volume status. A large IVC with minimal respiratory variation suggests that the patient will not be fluid responsive. A small IVC with marked respiratory variation suggests that the patient will likely be fluid responsive.

7 How can pneumothorax be ruled out with pleural ultrasound?

The presence of lung sliding effectively rules out pneumothorax at the site of the ultrasound transducer. The transducer is applied to the chest in the sagittal plane. The user identifies rib shadows with a bright pleural line in between the ribs. In the normal subject, movement of the visceral pleura against the parietal pleura creates a characteristic pattern known as “lung sliding” on the ultrasound display. By applying the probe in multiple locations on the right and left chest, the user can detect lung sliding and rule out pneumothorax. Because a variety of lung and pleural pathologies can cause a lack of lung sliding, it is important to know that, although lung sliding can rule out pneumothorax, a lack of lung sliding requires the user to work through a differential diagnosis that includes pneumothorax, acute respiratory distress syndrome, atelectasis, pleural disease, and contralateral main-stem intubation.

8 If “air is the enemy of ultrasound,” how can ultrasound examination of the lung be useful?

A number of normal and pathologic states can be detected by lung ultrasound examination, often by interpretation of stereotypical patterns of artifact. Normal aerated lung exhibits a reverberation artifact with horizontal lines known as A lines across the ultrasound display. When fluid accumulates in the lung interstitium or alveoli, artifacts known as B lines appear on the ultrasound display. B lines are bright vertical lines that extend from the most superficial aspect of the display to the edge of the display. In general, the number of B lines correlates with the quantity of fluid accumulation in the lung.

9 How does fluid in the pleural space appear on ultrasound?

Pleural effusion and hemothorax, like all free fluid, appear black on the ultrasound display.

Although it is impossible to determine the nature of free fluid by imaging alone, the history and physical examination provide the context that allows the clinician to narrow the differential diagnosis.

When sufficient pleural fluid has accumulated to collapse nearby lung, the atelectatic lung can be seen floating in pleural fluid. Ultrasound imaging of pleural fluid can help the intensivist to determine whether thoracentesis or thoracostomy tube placement would be possible and can be used for procedural guidance.

10 What is the FAST examination?

FAST stands for “focused abdominal sonography for trauma.” It is performed to evaluate for free fluid (blood) and cardiac activity in trauma patients, particularly those with shock. The FAST examination consists of:

image Low-frequency ultrasound examination of the right upper quadrant (for blood between the diaphragm and liver, between liver and kidney, or around the kidney)

image Left upper quadrant (for blood between diaphragm and spleen, between spleen and kidney, or around the kidney)

image Heart and pericardial space (for evidence of cardiac activity and pericardial fluid)

image Pelvis

The trauma surgeon uses the positive (free fluid present) or negative (free fluid not present) result of the FAST examination to aid in decision making regarding need for immediate surgery versus ongoing diagnostic evaluation.

11 What is the smallest amount of abdominal free fluid detectable by the FAST examination?

The sensitivity of the FAST examination depends on the location of the free fluid, position of the patient, and quality of imaging and interpretation. In general, the FAST examination is capable of detecting free fluid in quantities greater than 250 mL.

12 Which ultrasound transducer is appropriate for vascular access?

A high-frequency transducer is essential for ultrasound-guided vascular access. High-frequency imaging results in excellent resolution and a high-quality image with limited depth. Low frequency results in less resolution with greater depth.

13 What are some of the ways ultrasound can be used for central venous access?

The most common application involves imaging of the internal jugular vein in short axis (in cross section). Ultrasound examination allows the operator to view the relationship between the internal jugular vein and the carotid artery and thus optimize needle placement and trajectory during the procedure. When used properly, ultrasound reduces the risk of carotid artery puncture and reduces the number of attempts necessary for successful cannulation of the internal jugular vein. A similar technique can be applied to the femoral vein. Increasingly, intensivists are using ultrasound to visualize the subclavian-axillary vein before placement of central lines at this site.

14 Are there other applications of ultrasound for vascular access?

Ultrasound can facilitate placement of arterial catheters in both superficial (e.g., radial) and deep (e.g., femoral, axillary, brachial) vessels. Placement of arterial catheters by landmark and palpation alone is sometimes impossible, and in some of these cases ultrasound guidance leads to successful placement. Ultrasound can also be used to facilitate placement of peripheral intravenous lines. Critically ill patients’ peripheral veins are frequently difficult to access by traditional techniques because of prior lines, repeated phlebotomy, and soft-tissue edema. Ultrasound allows the intensivist to visualize and access veins that would be undetectable by inspection and palpation alone.

15 How can ultrasound be used to screen for deep venous thrombosis (DVT)?

Because most lower-extremity DVTs are located at the branch points of the venous system, ultrasound examination of these sites is useful in ruling out DVT. The venous system is viewed with a high-frequency transducer in cross section at the proximal thigh (where the common femoral vein joins with the greater saphenous vein) and in the popliteal fossa. A distance of 5 to 10 cm of vein should be examined in both the femoral and popliteal regions. If the common femoral vein and popliteal vein are fully compressible and no thrombus is visualized, the patient is unlikely to have a significant DVT.

16 Where can I learn critical care ultrasound techniques?

The best way to learn these techniques is to train with someone who is already an expert in point-of-care ultrasound. Local, regional, and national courses in critical care ultrasound are available throughout the year and are open to all health care professionals and trainees. Several high-quality texts and articles describe these ultrasound techniques in detail. Online resources including full curricula in critical care ultrasound are also available. Many critical care fellowships currently include training in point-of-care ultrasound. After training in point-of-care ultrasound, the clinician must use ultrasound examination regularly to continue to improve in imaging and interpretation.

Key Points Ultrasound in the intensive care unitimage

1. High frequency provides excellent image resolution with poor depth. Low frequency allows deep imaging at the expense of image resolution.

2. Free fluid appears black on the ultrasound display.

3. Left ventricular systolic function can be accurately estimated by focused echocardiography.

4. Pneumothorax can be ruled out by ultrasound detection of lung sliding in multiple sites over the chest.

5. Ultrasound imaging of full compression of the common femoral vein and popliteal vein can be used to screen for lower-extremity DVT.

Bibliography

1 Blaivas M. Ultrasound in the detection of venous thromboembolism. Crit Care Med. 2007;35(5 Suppl):S224–S234.

2 Feissel M., Michard F., Faller J.P., et al. The respiratory variation in inferior vena cava diameter as a guide to fluid therapy. Intensive Care Med. 2004;30:1834–1837.

3 Lichtenstein D.A., Menu Y. A bedside ultrasound sign ruling out pneumothorax in the critically ill: lung sliding. Chest. 1995;108:1345–1348.

4 Mayo P.H., Beaulieu Y., Doelken P., et al. American College of Chest Physicians/La Société de Réanimation de Langue Française statement on competence in critical care ultrasonography. Chest. 2009;135:1050–1060.

5 Moore C.L., Copel J.A. Point-of-care ultrasonography. N Engl J Med. 2011;364:749–757.

6 Noble V.E., Nelson B. Manual of Emergency and Critical Care Ultrasound, 2nd ed. New York: Cambridge University Press; 2011.

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