Focused critical care ultrasound (CCU) answers specific clinical questions and is an extension of the physical examination.
Bedside ultrasound can benefit patients by decreasing ionizing radiation exposure and improving patient safety by decreasing the need for transport out of the intensive care unit (ICU).
Bedside ultrasound is better for ruling in diagnoses than ruling out diagnoses.
If the diagnosis is unclear after a focused ultrasound, expert consultation or additional imaging is advised.
Bedside ultrasound is shown to decrease complications and improve success rates for many common critical care procedures.
INTRODUCTION TO BEDSIDE ULTRASOUND
Ultrasound technology was introduced to the medical field in the early 1950s. It was not until 1970s that multiple specialties in medicine adopted ultrasound as a useful diagnostic tool. Slasky et al mentioned the importance of ultrasound in the ICU. He reported a 2-year retrospective study of ultrasound indications in the ICU and found that 64.4% of patients had abnormal findings and 52% of these patients' clinical course and therapy were modified by ultrasonography.1 Other authors reported similar findings.2,3,4
Bedside ultrasonography differs from the ultrasound studies done by consult services (ie, cardiology, radiology, etc). The intensivist uses real-time (image acquisition and interpretation are done at the same time) point-of-care ultrasonography (POC-US) to answer simple clinical questions that will change patient's management. One clear example of POC-US frequently used by emergency physicians and trauma surgeons is the focused assessment by sonography for trauma (FAST). This became standard of care and it is fully integrated into advanced trauma life support teaching. For intensivists, POC-US helps diagnose, evaluate response to treatment and guide procedures safely.
In this chapter we will cover the basics of image acquisition and clinical applications of POC-US.
BASICS OF ULTRASONOGRAPHY
Understanding the basic physics of how ultrasound images are generated will help interpreting images and identifying artifacts. The ultrasound frequency ranges within millions of Hertz (MHz). It is generated when electric current is applied to a piezoelectric crystal. These mechanical vibrations are transmitted through tissues; each tissue will have different impedance, generating echoes throughout. The same transducer, then receives and transduces into a grayscale image that is generated at 20 to 40 frames per second. This is perceived as continuous images on the screen. For deeper structures, low-frequency waves such as 1 to 5 MHz are preferred and for superficial structures, high-frequency waves such as 5 to 15 MHz are preferred. By convention, tissues like the liver and kidneys are isoechoic. Bone, stones, and other tissues that reflect more echoes are hyperechoic and fat, fluid, and blood have low impedance generating hypoechoic or anechoic images.
Basic understanding of acoustic artifacts can help with image interpretation; these are described in Table 12–1.
Table 12–1Important artifacts in ultrasonography.