5.3 FAST Exam, Diagnostic Peritoneal Lavage (DPL) & Abdominal CT Indications

FAST Exam, Diagnostic Peritoneal Lavage (DPL) & Abdominal CT Indications

Clinical Core: Timely diagnostic selection in abdominal trauma hinges on hemodynamic stability. The Focused Assessment with Sonography for Trauma (FAST) exam rapidly identifies free hemoperitoneum at the bedside in unstable patients. Diagnostic Peritoneal Lavage (DPL) serves as a sensitive secondary tool when FAST is equivocal. Abdominal CT is the gold standard but is strictly restricted to hemodynamically stable patients.

Evaluating the abdomen during trauma resuscitation requires rapid, accurate identification of intra-abdominal hemorrhage or hollow viscus injury. Trauma teams rely on three primary diagnostic modalities: the Focused Assessment with Sonography for Trauma (FAST), Diagnostic Peritoneal Lavage (DPL), and Abdominal Computed Tomography (CT). Selecting the correct modality depends entirely on the patient's hemodynamic status, mechanism of injury, and clinical presentation.

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Focused Assessment with Sonography for Trauma (FAST)

The FAST exam is a rapid, point-of-care ultrasound examination performed during the primary or secondary survey in the resuscitation bay. Its primary purpose is to identify pathological free fluid (usually blood) in the pericardial, peritoneal, or pleural spaces in under 5 minutes.

The Four Core Anatomical Views

The standard FAST exam evaluates four distinct acoustic windows:

1. Pericardial View (Subxiphoid / Parasternal Window): Evaluates the pericardial sac for pericardial effusion or cardiac tamponade. An anechoic (black) strip separating the parietal and visceral pericardium indicates free fluid.
2. Right Upper Quadrant (RUQ / Morison's Pouch): Examines the hepatorenal space (interface between the liver and right kidney) and the right subphrenic space. Morison's pouch is the most sensitive dependent anatomical area for detecting free intraperitoneal fluid in a supine trauma patient. As little as 100 to 200 mL of fluid can be detected here.
3. Left Upper Quadrant (LUQ / Splenorenal Window): Examines the splenorenal space (interface between the spleen and left kidney) and the left subdiaphragmatic space. Fluid accumulation here suggests splenic injury or left retroperitoneal disruption.
4. Pelvic Window (Suprapubic View): Placed superior to the pubic symphysis to visualize the rectovesical pouch in males or the rectouterine pouch (Pouch of Douglas) in females. The bladder must contain urine to serve as an acoustic window.

Extended FAST (E-FAST)

E-FAST extends the exam to the anterior thoracic pleura (2nd–4th intercostal spaces at the midclavicular line) to assess for pneumothorax (detecting absence of lung sliding or absence of "comet-tail" artifacts) and hemothorax (costophrenic angle view).

Diagnostic Advantages & Limitations

• Advantages: Non-invasive, portable, extremely rapid (<3–5 minutes), repeatable, zero radiation, safe in pregnancy.
• Limitations: Operator dependent; cannot distinguish blood from urine, bile, or ascites; cannot evaluate solid organ parenchymal injury grading; poor visualization in severe obesity or subcutaneous emphysema; low sensitivity for retroperitoneal hemorrhage and hollow viscus perforation.

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Diagnostic Peritoneal Lavage (DPL)

Diagnostic Peritoneal Lavage is an invasive procedure historically considered the gold standard for detecting intra-abdominal hemorrhage. While largely replaced by FAST and CT, DPL remains a vital diagnostic tool in specific clinical scenarios: a hemodynamically unstable patient with blunt trauma when FAST ultrasound is equivocal, non-diagnostic, or unavailable.

Procedure Technique

1. A catheter is inserted into the peritoneal cavity via a closed (Seldinger) or open infraumbilical incision (suprapubic approach if pelvic fracture is present to avoid hematoma tracking).
2. Aspiration Phase: The catheter is attached to a syringe and aspirated. If >10 mL of gross blood is aspirated immediately, the test is positive, aspiration ceases, and the patient is transported immediately to the operating room for exploratory laparotomy.
3. Lavage Phase: If <10 mL of blood is aspirated, 1,000 mL of warm 0.9% Normal Saline (10 mL/kg in pediatric patients) is instilled into the peritoneal cavity. The abdomen is gently agitated to mix fluid, and the IV bag is placed on the floor to allow fluid to drain by gravity.

Positive Quantitative Criteria for DPL

Laboratory analysis of the returned lavage fluid defines a positive result:

• Contraindications: Absolute: Existing clear indication for laparotomy (e.g., evisceration, gunshot wound, severe peritonitis). Relative: Prior abdominal surgeries (adhesions), advanced pregnancy, severe obesity, uncorrected coagulopathy.

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Abdominal Computed Tomography (CT) with IV Contrast

Abdominal CT with intravenous contrast is the definitive diagnostic modality for abdominal trauma. It provides exquisite anatomical detail, allowing precise grading of solid organ injuries (AAST I–V), identification of active arterial bleeding ("contrast blush"), visualization of retroperitoneal structures, and assessment of pelvic architecture.

The Cardinal Rule of CT Scanning in Trauma

CRITICAL MANDATE: Abdominal CT scanning is strictly indicated ONLY for hemodynamically STABLE trauma patients. Transporting an unstable patient with ongoing occult hemorrhage into the CT scanner ("the death box") is a catastrophic clinical error.

If a patient becomes hemodynamically unstable during CT transport or scanning, the scan must be aborted immediately, and the patient must be transferred directly to the resuscitation bay or operating room.