Catherine Erickson
FUNDAMENTALS
Ultrasound waves that are completely reflected (eg, bone) appear white and are referred to as hyperechoic.
A perfect transmitter of ultrasound waves (eg, fluid) appears black and is referred to as anechoic.
Structures on the left side of the monitor correspond to the marker on the probe. This marker is generally oriented toward the patient’s head in sagittal planes and toward the patient’s right in transverse planes. Visualize structures in both the long and the short axis.
Structures toward the top of the monitor correspond to structures closest (more superficial) to the probe. Hash marks on the screen correspond to increasing depths.
ULTRASONOGRAPHY IN THE EMERGENCY DEPARTMENT
PRIMARY INDICATIONS
ULTRASONOGRAPHY IN TRAUMA
The focused assessment with sonography in trauma (FAST) examination has a sensitivity of 90%, a specificity of 99%, and an accuracy rate of 99% for detecting free intraperitoneal fluid. It is the ideal imaging modality of choice in patients who are too unstable for CT because of its rapid and noninvasive features.
The standard FAST views include (1) the sub-xiphoid view for the evaluation of pericardial fluid; (2) Morison’s pouch (hepatorenal recess); (3) left subphrenic and splenorenal recess views; and (4) the pouch of Douglas and rectovesicular space.
In addition, the upper abdominal views are useful in the evaluation of the patient for hemothorax.
Hemodynamically unstable blunt trauma patients with a positive FAST examination for free intraperitoneal fluid should be taken directly to the operating room for exploratory laparotomy.
Anteromedial lung windows can be visualized to detect pneumothorax in the extended FAST (EFAST) examination. An absence of the normal “lung sliding” has a sensitivity of 86% to 98% for detecting pneumothorax and is superior to the supine chest radiograph.
CARDIAC ULTRASONOGRAPHY
The major applications for ED cardiac ultrasonography are in the evaluation of pulseless electrical activity, cardiac trauma, and pericardial tamponade. Key sonographic findings are pericardial fluid collections and myocardial wall activity.
Pericardial effusions appear as echo-free areas within the pericardial sac. A small pericardial effusion (<100 mL)will occupy a dependent position, while a large effusion (>300 mL) will present circumferentially.
Sonographic localization of the pericardial sac is the best approach for performing pericardiocentesis.
ABDOMINAL AORTIC ANEURYSMS
Ultrasonography is as accurate as CT in detecting and measuring the diameter of an abdominal aortic aneurysm.
When the aorta is imaged from the diaphragm to its distal bifurcation (at the level of the umbilicus), it is nearly 100% accurate in the evaluation for an abdominal aortic aneurysm.
An abdominal aortic diameter >3 cm is abnormal. Transverse images measured horizontally from outside wall to outside wall are the most reliable in determining the true size of the aorta.
Potential indications for performing a screening aortic ultrasound in the ED include age >50 years with unexplained back, flank, abdominal, or groin pain and in patients with hypotension, syncope, or dizziness.
EVALUATION OF FIRST-TRIMESTER PREGNANCY
In the ED, ultrasound detection of an intrauterine pregnancy (IUP) greatly reduces the possibility of ectopic pregnancy. The prevalence of spontaneous heterotopic (simultaneous intrauterine and extrauterine pregnancies) is less than 1 in 30,000. However, the incidence greatly increases with fertility assistance.
First-trimester pregnant patients presenting to the ED with any abdominal or pelvic pain, vaginal bleeding, or risk factors for ectopic pregnancy should have an ultrasound evaluation to confirm the presence of an intrauterine pregnancy.
The earliest sonographic finding of a pregnancy is the gestational sac. This appears as a round or oval anechoic area within the uterus surrounded by two concentric echogenic rings (double decidual sign).
The first reliable sign of an IUP is a yolk sac within the gestational sac. Transvaginal ultrasound can detect a yolk sac between 5 and 6 weeks’ gestational age.
An IUP should be detectable on endovaginal scanning if the beta-human chorionic gonadotropin (β-hCG) is >1000 mlU/mL (termed the discriminatory zone).
Patients with a β-hCG greater than the discriminatory zone and who do not have evidence of an IUP on ultrasound should be presumed to have an ectopic pregnancy until proven otherwise.
VASCULAR ACCESS
Ultrasound use in central venous catheter placement decreases failure rates and complications.
Additionally, in difficult access patients, ultrasound-guided peripheral vascular access decreases time to cannulation and the number of puncture attempts.
URGENT INDICATIONS
GALLBLADDER DISEASE
Ultrasound is the imaging modality of choice in evaluating biliary disease.
Gallstones appear as echogenic structures with posterior shadowing lying within the gallbladder. They will move with positional changes unless embedded in the gallbladder neck.
A sonographic Murphy’s sign is positive when the point of maximal tenderness to transducer pressure is directly over the visualized gallbladder. This sign, in the presence of gallstones, is reported to have a 92% positive predictive value for cholecystitis.
Gallbladder wall thickening, defined as proximal gallbladder wall thickness >3 mm, occurs in 50% to 75% of patients with acute cholecystitis. It is not pathognomonic for cholecystitis; it is also seen in patients with ascites and other hypoproteinemic states.
RENAL COLIC
The renal sinus appears as an extremely echogenic region within the center of the kidney and includes the collecting system. The renal cortex surrounds the sinus and appears slightly less echogenic than that of the liver or spleen.
Both longitudinal and transverse images should be obtained of both kidneys. The identification of hydronephrosis, an anechoic fluid collection within the renal sinus, is a marker for renal calculi. One method of grading hydronephrosis is from mild (minimal separation of the sinus) to severe (extensive communicating anechoic regions in the sinus with cortical thinning).
In patients with flank pain and hematuria, bedside ultrasound has a sensitivity >85% in diagnosing a ureteral stone.
Ureteral calculi typically lodge at the ureterovesicular junction, the ureteropelvic junction, or the pelvic brim, and are only visualized on ultrasound in 19% of patients with documented kidney stones.
MISCELLANEOUS EMERGENCY DEPARTMENT APPLICATIONS
Compression ultrasound for the diagnosis of deep venous thrombosis (DVT) has a sensitivity of 95%. The femoralpopliteal region is evaluated for complete vessel collapse to rule out thrombus. Because calf ultrasound is less sensitive, repeat ultrasound should be performed within 1 week when a calf thrombus is suspected.
Ultrasonography can guide the emergency physician in performing procedures such as thoracentesis and paracentesis.
Ultrasound evaluation of soft tissues can help to differentiate cutaneous abscesses from cellulitis, confirm need for drainage, and avoid unnecessary procedures. Soft tissue can also be evaluated for radiolucent foreign bodies, such as wood and plastic.
For further reading in Tintinalli’s Emergency Medicine: A Comprehensive Study Guide, 7th ed., see Chapter 299.4, “Emergency Ultrasonography,” by O. John Ma, Robert F. Reardon, and Alfredo Sabbaj.