Ultrasound in the Intensive Care Unit

Published on 07/03/2015 by admin

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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?

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