Focused Abdominal Sonography for Trauma

Abdominal CT scanning remains an important tool in evaluating trauma, with an accuracy of more than 90 percent in detecting intraabdominal injuries. However, CT is expensive and requires a hemodynamically stable, cooperative patient. It also involves contrast-medium administration and ionizing radiation. Diagnostic peritoneal lavage (DPL) has sensitivities greater than 90 percent for hemoperitoneum. However, up to one-third of laparotomies performed on the basis of positive DPL findings are unnecessary. As little as 20 mL of blood mixed with the standard liter of peritoneal lavage fluid will result in a positive DPL (100,000 red blood cells per cubic millimeter). In addition, this invasive procedure requires significant time and causes complications in up to 5 percent of patients.

Focused abdominal sonography for trauma (FAST) has proven to be a valuable tool in the evaluation of trauma victims. More than two dozen studies in European and North American centers have demonstrated that ED sonography has an accuracy similar to that of DPL for the detection of hemoperitoneum but has several advantages: it is more rapid (generally completed in under 5 min), requires no preparations (e.g., nasogastric tube and Foley catheter), has no contraindications, and is noninvasive. While ultrasound cannot reliably identify intraabdominal organ injuries (as can CT), it is accurate in predicting the need for laparotomy in trauma patients. Large studies have found the sensitivity of ultrasound to be 85 percent or higher with specificities of 96 percent or higher, 5 depending on the gold standards employed. Some studies have used the need for a therapeutic laparotomy as the gold standard, whereas others have used the presence of any intraabdominal injury or blood on CT as the gold standard, regardless of need for laparotomy (see Chap.25.2 for further discussion.) Subcutaneous emphysema and marked obesity can impair the ability of ultrasound to image the abdomen, but it is uncommon that they prevent an adequate FAST examination. Training for as little as 2 to 8 has been sufficient for emergency physicians and surgeons to learn the FAST examination6 and in many centers it has completely replaced DPL.

SONOGRAPHIC CONSIDERATIONS The primary finding in the FAST examination is anechoic fluid collections (blood) within the peritoneal cavity. The four standard views of the trauma ultrasound examination are depicted in Fig 2.9.5.-10.. The right upper quadrant view, the easiest of the FAST examination views to visualize, is obtained by imaging over the lower rib cage in the area of the anterior to midaxillary line with the marker dot pointed cephalad. The liver, kidney, and

Morison pouch are examined (Fig 2.9.5.-1.1). The Morison pouch is a potential space between the Gerota fascia of the kidney and the Glisson capsule of the liver and is usually devoid of fluid. Because the Morison pouch is one of the most posterior compartments of the supine abdomen, blood tends to accumulate in this space, creating an easily identified anechoic stripe ( Fig.295-12). Organ lesions, while not reliably identified on ultrasound, may appear as anechoic areas within the organ or as echogenic foci. The left upper quadrant view examines the potential space between the spleen and the kidney, the splenorenal space ( Fig..295-13). This view is obtained by placing the transducer in the mid to posterior axillary line over the lower left costal margin. Here again, blood will appear as an anechoic stripe between the spleen and kidney. While not routine, placing the patient into a Trendelenburg position may increase the amount of blood in the upper abdomen, facilitating sonographic identification of blood in both the right and left upper quadrant. Both upper abdominal views are also capable of identifying hemothorax, where the anechoic fluid collection appears above the diaphragm. Studies have found ultrasound to be at least as accurate as chest x-ray in identifying hemothorax. 78 The pelvis is the most dependent location within the intraperitoneal cavity in a supine patient, explaining why hemoperitoneum commonly collects in this location. The pelvic view seeks to identify intraperitoneal blood in the potential space between the rectum and uterus (pouch of Douglas) or its homologue in the male, the rectovesicular space. As with the other views, blood here appears as a fluid collection between two adjacent soft-tissue structures ( Fig 29.5.-14). Unclotted intraperitoneal blood is anechoic, with sharp borders against the peritoneal confines. The pelvic view is facilitated by a full bladder, which can be produced by instilling 250 mL of normal saline solution via a Foley catheter. The full bladder acts as an acoustic window, optimizing the view of the pelvic structures. As little as 250 mL of intraperitoneal blood should be routinely noted with ultrasound; even smaller collections can often be identified.

FIG. 295-10. Standard four views of the FAST examination. (From Sisley AC et al,7 with permission.)

FIG. 295-11. Normal ultrasound image of the right upper quadrant demonstrating an absence of fluid in Morison's pouch (mp).

FIG. 295-12. Positive right upper quadrant FAST examination findings. Note "stripe" of fluid in Morison's pouch.

FIG. 295-13. Normal findings on left upper quadrant ultrasound. The potential space between the kidney and spleen is devoid of anechoic fluid.

FIG. 295-14. Positive pelvic FAST examination findings. An anechoic fluid collection is seen distal to the large fluid-filled bladder.

The last view, that of the subcostal area, examines the heart for evidence of pericardial fluid collections, which would suggest cardiac injury ( Fig 295-15). Rapid ultrasound evaluation of the heart has resulted in faster times to diagnosis and to surgical intervention, as well as improved survival rates and neurologic outcomes in patients with penetrating cardiac injuries.9 It is occasionally necessary to differentiate pleural from pericardial fluid. Any fluid collection that follows the contours of the heart and is surrounded by the echogenic pericardium is pericardial fluid. There is no pleural reflection between the liver and heart; the subcostal view will therefore demonstrate pericardial, but not pleural, fluid. Blood within the pericardial space can appear anechoic but may be partially echogenic, depending on the degree of clotting and defibrination that occurs.

FIG. 295-15. Subcostal echocardiogram demonstrating pericardial hemorrhage (PH). Hep, hepatic parenchyma; RV, right ventricle; LV, left ventricle. (From Heller M, Jehle D, eds: Ultrasound in Emergency Medicine. Philadelphia, Saunders, 1995, with permission.)

The great majority of research in the use of trauma sonography has focused on blunt abdominal trauma. The studies that have utilized ultrasound in penetrating abdominal trauma have found that sonography has a sensitivity and specificity for predicting need for laparotomy similar to those found for blunt trauma. Several studies have examined trauma sonography in blunt pediatric trauma and have demonstrated it to be as accurate in detecting hemoperitoneum in this setting as in the adult population.

A number of centers routinely repeat the ultrasound examination in all patients not taken immediately to surgery. The fact that ultrasound is inexpensive, rapid, and noninvasive makes this a practical approach to minimizing the chance of missing intraabdominal injuries that develop more slowly. Repeat scans are performed within 30 min to 6 h, depending on the stability of the patient.

Ultrasound can be easily incorporated into routine trauma care. Sonography is often performed at the completion of the primary survey, allowing rapid identification of hemoperitoneum. Figure295-16. illustrates an algorithm depicting how ultrasound can be used in a clinical setting. A hemodynamically unstable patient with positive trauma ultrasound findings should be taken immediately to laparotomy. A stable patient with positive sonogram findings can be evaluated with CT to identify the specific organ lesion and to determine the feasibility of nonoperative intervention.

FIG. 295-16. Ultrasound utilization in a blunt abdominal trauma patient.
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