Section 6.4: Abdominal Trauma (FAST Exam), Solid Organ Injury, and Postoperative / Transplant Assessment
Key Takeaways
- The FAST protocol utilizes four standard acoustic windows (subxiphoid, RUQ, LUQ, pelvic) to rapidly detect free fluid in trauma patients.
- Acutely, solid organ lacerations and hematomas appear hyperechoic or isoechoic, becoming progressively complex and hypoechoic over time as clots lyse.
- Postoperative fluid collections are differentiated by clinical timeline and features: urinomas are perinephric with high creatinine; abscesses present with fever and gas.
- Normal transplant renal arteries exhibit low resistance with an RI between 0.60 and 0.80; an RI > 0.80 indicates transplant dysfunction.
Abdominal Trauma, Postoperative Collections, and Organ Transplants
Abdominal trauma, postoperative complications, and transplant evaluations are critical components of the ARDMS Abdomen exam. Sonography is heavily utilized for rapid emergency assessments and ongoing postoperative monitoring.
The FAST Exam Protocol
The Focused Assessment with Sonography for Trauma (FAST) is a rapid bedside ultrasound protocol used in the emergency department to detect free intraperitoneal or pericardial fluid (hemoperitoneum or hemopericardium) in patients who have sustained blunt or penetrating abdominal trauma. The primary goal is to identify candidates for emergency laparotomy.
The FAST protocol consists of four primary acoustic windows:
- Subxiphoid Pericardial View: Directed through the left lobe of the liver to visualize the heart. It evaluates for pericardial effusion or cardiac tamponade, which appears as an anechoic fluid space between the visceral and parietal pericardium.
- Right Upper Quadrant (RUQ) View: Evaluates the hepatorenal recess (Morrison's pouch), which is the most dependent space in the upper abdomen of a supine patient and the most sensitive location for detecting free fluid. It also evaluates the right subdiaphragmatic space and the right pleural space (to rule out hemothorax).
- Left Upper Quadrant (LUQ) View: Evaluates the splenorenal recess, the left subdiaphragmatic space, and the left pleural space. Unlike Morrison's pouch, fluid around the spleen often accumulates in the subdiaphragmatic space before the splenorenal space.
- Suprapubic (Pelvic) View: Evaluates the rectovesical space (pouch) in males and the rectouterine space (pouch of Douglas) in females, posterior to the bladder. The bladder must be partially filled with urine or saline to serve as an acoustic window.
Solid Organ Trauma: Liver and Spleen
Traumatic injuries to the liver and spleen include lacerations, contusions, and hematomas.
- Lacerations: Sonographically, acute lacerations appear as irregular, linear hypoechoic or hyperechoic tracts within the organ parenchyma.
- Hematomas: The sonographic appearance of a hematoma changes dynamically over time.
- Acute (<24 hours): Highly echogenic (hyperechoic) because of fibrin and cell clotting, which can make it blend in with the surrounding normal parenchyma (isoechoic).
- Subacute (1 to 2 weeks): The clot lyses, and the collection becomes progressively more hypoechoic, complex, and heterogeneous with internal septations.
- Chronic (>2 weeks): Becomes completely anechoic and liquefies, resembling a simple seroma or cyst.
- Location: Hematomas can be subcapsular (a crescent-shaped fluid collection that compresses and flattens the underlying organ parenchyma) or intraparenchymal (round or irregular, within the organ tissue).
Postoperative Fluid Collections
Differentiating postoperative fluid collections requires correlating sonographic features with the patient's clinical history and postoperative timeline:
- Hematoma: Blood collection resulting from active bleeding or vascular injury. Timing is immediate postop. Sonographic appearance is highly variable, changing from echogenic to heterogeneous to anechoic as it ages.
- Seroma: Liquefied serum collection that occurs within weeks after surgery at a surgical site or lymph node dissection site. It appears as a simple, thin-walled, anechoic fluid collection with posterior acoustic enhancement.
- Urinoma: A collection of extravasated urine resulting from ureteral injury or anastomotic leak. It typically develops within 1 to 2 weeks postoperatively. Sonographically, it appears as a rapidly expanding, simple, anechoic fluid collection, usually located in the pelvis or perinephric space. Diagnosis can be confirmed by needle aspiration demonstrating high creatinine levels.
- Abscess: A localized infection (pus collection) that typically occurs 1 to 4 weeks postoperatively. The patient presents with fever, leukocytosis, and local tenderness. Sonographically, an abscess is a poorly defined, thick-walled, complex fluid collection containing low-level echoes, debris, septations, and occasionally echogenic gas bubbles that cause dirty posterior shadowing.
Transplant Assessment (Renal and Hepatic)
Renal Transplant
Renal transplants are placed extraperitoneally in the iliac fossa (usually the right). The renal artery is anastomosed to the external or internal iliac artery, and the renal vein is anastomosed to the external iliac vein.
- Normal Waveform: The transplant renal artery should demonstrate a low-resistance waveform with continuous diastolic flow.
- Resistive Index (RI): Measured at the intralobular or segmental arteries. Normal RI is between 0.60 and 0.80. An RI greater than 0.80 indicates transplant dysfunction, which can result from acute tubular necrosis (ATN), acute rejection, renal vein thrombosis, or ureteral obstruction.
- Vascular Complications:
- Renal Arterial Stenosis (RAS): Most common vascular complication, usually at the anastomosis. Doppler findings include a PSV > 250 cm/s, spectral broadening, and a parvus-tardus waveform distally.
- Renal Artery Thrombosis: Occlusion of the renal artery, showing complete absence of intrarenal arterial and venous flow.
- Renal Vein Thrombosis: Shows absent venous flow and a classic reversed diastolic flow (reversed diastolic spike or "diastolic flow reversal") in the transplant renal artery.
Hepatic Transplant
Hepatic transplants require evaluation of four main vessels: the hepatic artery, portal vein, hepatic veins, and IVC.
- Hepatic Artery: The hepatic artery is the sole blood supply to the transplant's biliary system. Hepatic artery thrombosis is the most common and serious vascular complication, leading to biliary ischemia, necrosis, and strictures.
- Normal Hepatic Artery Waveform: Low-resistance flow with a rapid systolic upstroke (acceleration time < 0.08 s) and continuous diastolic flow. The normal RI is between 0.50 and 0.70.
- Hepatic Artery Stenosis/Thrombosis: An RI less than 0.50 or an acceleration time greater than 0.08 seconds (tardus-parvus waveform) is highly suggestive of hepatic artery stenosis or thrombosis with collateral formation.
Summary of Postoperative Fluid Collections
| Fluid Collection | Typical Postoperative Timing | Sonographic Features | Clinical / Laboratory Correlations |
|---|---|---|---|
| Hematoma | Immediate (first 24-48 hours) | Variable: hyperacute is anechoic; acute is hyperechoic/clotted; chronic is liquefying | Drop in hematocrit, localized pain |
| Seroma | Subacute (1 to 4 weeks) | Anechoic, thin-walled, simple fluid collection with acoustic enhancement | Relieved by simple aspiration (clear fluid) |
| Urinoma | Subacute (1 to 2 weeks) | Anechoic, rapidly enlarging fluid collection, usually perinephric | Anastomotic leak; aspirate has high creatinine levels |
| Abscess | Subacute to chronic (1 to 4 weeks) | Thick irregular walls, internal echoes, septations, gas bubbles (dirty shadowing) | Fever, leukocytosis, localized warmth and pain |
A trauma patient is brought to the emergency department after a motor vehicle collision. Under the FAST protocol, which of the following spaces is considered the most sensitive for detecting free intraperitoneal fluid in a supine patient?
A patient who underwent a renal transplant 10 days ago presents with decreased urine output and rising serum creatinine. Ultrasound reveals an anechoic, rapidly enlarging fluid collection surrounding the lower pole of the transplant kidney. Diagnostic aspiration is performed. Which of the following findings would confirm that this collection is a urinoma rather than a seroma?
During a routine Doppler evaluation of a renal transplant 3 days post-surgery, you obtain a spectral Doppler waveform from the interlobar arteries showing an elevated Resistive Index (RI) of 0.88. What is the clinical significance of this finding, and what waveform pattern would you expect if the complication was renal vein thrombosis?